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Accurate radiometry from space: an essential tool
for climate studies
Nigel Fox, Andrea Kaiser-Weiss, Werner Schmutz, Kurtis Thome, Dave Young,
Bruce Wielicki, Rainer Winkler and Emma Woolliams
Phil. Trans. R. Soc. A 2011 369, doi: 10.1098/rsta.2011.0246, published 19
September 2011
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Phil. Trans. R. Soc. A (2011) 369, 4028–4063
doi:10.1098/rsta.2011.0246
Accurate radiometry from space: an essential
tool for climate studies
BY NIGEL FOX1, *, ANDREA KAISER-WEISS2 , WERNER SCHMUTZ3 ,
KURTIS THOME4 , DAVE YOUNG5 , BRUCE WIELICKI5 , RAINER WINKLER1
AND EMMA WOOLLIAMS1
1 National
Physical Laboratory, Hampton Road, Teddington,
Middlesex, TW11 0LW, UK
2 National Centre for Earth Observation, University of Reading, Earley Gate,
Reading RG6 6BB, UK
3 Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center,
Dorfstrasse 33, 7260 Davos Dorf, Switzerland
4 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
5 NASA Langley Research Center, Hampton, VA 23681-0001, USA
The Earth’s climate is undoubtedly changing; however, the time scale, consequences and
causal attribution remain the subject of significant debate and uncertainty. Detection
of subtle indicators from a background of natural variability requires measurements
over a time base of decades. This places severe demands on the instrumentation used,
requiring measurements of sufficient accuracy and sensitivity that can allow reliable
judgements to be made decades apart. The International System of Units (SI) and the
network of National Metrology Institutes were developed to address such requirements.
However, ensuring and maintaining SI traceability of sufficient accuracy in instruments
orbiting the Earth presents a significant new challenge to the metrology community.
This paper highlights some key measurands and applications driving the uncertainty
demand of the climate community in the solar reflective domain, e.g. solar irradiances and
reflectances/radiances of the Earth. It discusses how meeting these uncertainties facilitate
significant improvement in the forecasting abilities of climate models. After discussing
the current state of the art, it describes a new satellite mission, called TRUTHS, which
enables, for the first time, high-accuracy SI traceability to be established in orbit. The
direct use of a ‘primary standard’ and replication of the terrestrial traceability chain
extends the SI into space, in effect realizing a ‘metrology laboratory in space’.
Keywords: climate change; Earth observation; satellites; radiometry; solar irradiance
1. Introduction
(a) Background
Unequivocal attribution of the causes, consequential impact and effective
mitigation/adaptation of change in the Earth’s climate is arguably the greatest
challenge facing science today. The ‘Kyoto Protocol’ and ‘Copenhagen Accord’
*Author for correspondence (nigel.fox@npl.co.uk).
One contribution of 15 to a Discussion Meeting Issue ‘The new SI based on fundamental constants’.
4028
This journal is © 2011 The Royal Society
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Figure 1. From IPCC [1], the variance in forecast temperatures of the Earth for emissions scenario
A2 for a range of climate models.
exemplify the intense political, scientific and public debate associated with this
issue. This is underpinned by the enormous cost implications of policy decisions
based on forecasted impacts. However, these forecasts have significant uncertainty,
with estimates of global temperature increases ranging from, at best, 2 to 5◦ C
by 2100 (figure 1) [1]. Such a range could be the difference between the need
for major new flood defences to prevent large land losses or maintenance/minor
updates of the status quo. It is the duty of the science community to reduce this
unacceptably large uncertainty in forecasts by finding and delivering the necessary
information, with the highest possible confidence, in the shortest possible time.
The Intergovernmental Panel on Climate Change (IPCC) concludes that recent
decades have revealed hallmarks of anthropogenic climate change, but the mix of
natural variability and anthropogenic effects on decadal time scales is far from
fully understood or measured, requiring significant improvements in accuracy [1].
Unequivocal attribution and the quantification of subtle fingerprint indicators
are fundamental to our ability to reliably predict the impact and the development
of appropriate mitigation/adaptation strategies. The uncertainty in climate
prediction lies in the complexity of the models, our inadequate understanding
of the Earth system and its feedback mechanisms, and the relatively poor quality
of available data against which to test predictions on the necessary decadal
time scales [2].
The data needed to benchmark and to test these models must be global
in nature and collected over a sufficiently long time base with appropriate
uncertainty to enable the identification of ‘change’ in a particular measurand
above variance that could be caused by natural or ‘local’ effects. In general, this
means that such measurements need to be considered on decadal time scales and
collected from satellite-based instrumentation.
Terrestrial networks, collecting in situ data, will always be necessary to
provide local validation of global observations and support detailed studies of
the Earth system processes, e.g. air quality, carbon and hydrological cycles,
etc. However, it is only remote and continuous observation of the Earth
system by satellite that the key datasets, the essential climate variables (ECVs)
identified by the Global Climate Observing System (GCOS) of the United
Nations (http:// www.wmo.int/pages/prog/gcos/publications/gcos-138.pdf), can
ultimately be adequately addressed. In general, these ECVs (e.g. albedo, cloud
cover, chlorophyll, etc.) are not measured directly but are derived from more
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N. Fox et al.
fundamental and familiar physical quantities, such as radiance, reflectance,
transmittance, etc. The satellite instruments measuring these quantities utilize
the full electromagnetic spectrum and in most cases passively, for example making
use of the Sun as a source for reflectance/radiance/transmittance measurements.
A community workshop held in the USA in 2007 considered the needs of
climate [3–5] and concluded that, in the optical domain, the critical underpinning
measurands and the associated uncertainty requirements were:
—
—
—
—
total solar irradiance (TSI) 0.01% (k = 1);
solar spectral irradiance (SSI) 0.1% (k = 1);
Earth-reflected solar radiance 0.3% (k = 2); and
Earth-emitted infrared (IR) radiances (expressed in terms of resultant
temperature) 0.1 K (k = 3).
This, and their priority, was reinforced in the decadal review carried out by the
National Research Council of the USA [6].
Note that the uncertainties are expressed at different confidence levels simply
to provide more convenient values for communication. It should also be noted here
that the temperature of the Earth is expected to rise at the rate of approximately
0.2 K per decade. National Metrology Institutes (NMIs) rarely disseminate
radiation thermometer (spectral radiance) measurement uncertainties in this
spectral region at uncertainties less than approximately 0.1 K.
(b) Earth observation data quality
The advent of the European ‘Global Monitoring of Environment and Security’
(GMES, http://GMES.info) programme and that of the Group on Earth
Observation (GEO) ‘Global Earth Observation System of Systems’ (GEOSS,
http://www.earthobservations.org/geoss.shtml) have transformed space-based
Earth observation (EO). While maintaining the need for scientific endeavour on
a ‘best efforts’ basis, the drive for operationally delivered services to meet the
needs of society, e.g. disaster monitoring, pollution detection and agriculture, has
become a major commercial focus.
Fundamental to these initiatives is the recognition that the data products,
services and conclusions arising from them are unlikely to be derived from a
single source. They will instead rely upon the synergistic combination of data
from all components of the EO sector, space, in situ and airborne, in many
cases derived from instrumentation developed and operated by different teams,
countries and agencies. There are some good examples of how this process has
been, and continues to be, successful, e.g. weather prediction. In this example,
data assimilation techniques have been developed as a powerful tool to weight the
relative contributions of various data sources. However, for many applications,
such techniques cannot easily be applied, as the models are inadequate and the
source data lack sufficiently reliable ‘quality indicators’ to allow the necessary
discrimination and weighting to take place.
Coupling this with the availability of low-cost small satellites, such as those
of Surrey Satellite Technology Ltd. (SSTL; http://www.sstl.co.uk/), has led
to the emergence of new ‘space nations’ and the launch of privately financed
commercial satellites, complementing those of the big space agencies. This rapid
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Accurate radiometry from space
4031
proliferation of data, data providers and, most importantly, users (or customers)
has brought to the fore long-standing concerns over reliability and adequacy of the
data’s quality and declared accuracy. Such issues have long since been resolved
in conventional terrestrial-based industries through the adoption of rigorous
quality assurance (QA) procedures underpinned by traceability to international
standards and measures, e.g. the International System of Units (Le Système
International d’Unitès, SI). Although the space industry adheres to high levels
of QA during manufacture of satellites and instrumentation, this is not always
rigorously maintained during calibration activities or following launch.
The world’s space agencies have now identified this as a priority issue,
which must be addressed if EO and the emerging applications market that
it serves are to develop and flourish. However, the nature of space flight,
e.g. launch vibration and harsh environment, causes unpredictable drifts in
the instrumentation (particularly optical). This is further complicated by the
inability, following launch, to regularly recalibrate instrumentation in any
traceable manner. Establishing robust traceability in space has been identified
as an essential element of any long-term strategy to improve data quality to
a level where it fully meets the needs of society (see http://www.ceos.org;
http://www.star.nesdis.noaa.gov/smcd/spb/calibration/icvs/GSICS for details
and references) [3].
The most critical application area is of course ‘climate change’. Many of the
key indicators of climate change require the detection of subtle changes over
decades. In the solar-reflective (SR) domain (<2500 nm), this can be at the level
of less than 1 per cent per decade, and in the IR 0.2 K per decade [1,5]. Because
no existing EO optical sensor comes close to achieving this level of accuracy,
strategies have been devised that seek to monitor change by maintaining an
overlapping time series of measurements, wherever possible using similar heritage
instrumentation. In this way, the often-large instrumental biases can be artificially
removed from the data. It is widely recognized that the adoption of such a strategy
brings with it an extremely high level of risk:
—
—
—
—
—
—
any ‘data gap’ destroys the climate record;
drifts within a mission lifetime are hard to evaluate and remove;
innovation in technology is difficult to implement;
reduced operational flexibility;
requires long-term financial commitments; and
data reliability and user confidence are poor.
The only robust option to meet the exacting needs of the climate change
community and also the emerging operational needs of the EO community
as a whole is to ensure that all EO data and derived knowledge information
products should be traceable to SI units and have with them an associated
uncertainty estimate and/or ‘quality indicator’. Such requirements now pervade
the strategy documents and recommendations of organizations such as the
CEOS (Committee on Earth Observation Satellites), the GEOSS and the
World Meteorological Organization (WMO) and have subsequently been passed
down to the space agencies (see http://www.earthobservations.org/geoss.shtml;
http://www.ceos.org; http://www.star.nesdis.noaa.gov/smcd/spb/calibration/
icvs/GSICS for details and references). Many of these organizations and agencies
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N. Fox et al.
recognize not only that the space element needs to have exacting QA associated
with it, but also that this must be embedded within the whole validation
and data-processing chain. In response to this demand, CEOS has led the
development of a new internationally endorsed Quality Assurance Framework for
Earth Observation (QA4EO; http://QA4EO.org). This framework and its core
principle, i.e.
All data and derived products must have associated with them a Quality Indicator (QI) based
on documented quantitative assessment of its traceability to community agreed (ideally tied
to SI) reference standards
are now being implemented throughout the world’s space agencies and the
WMO. This principle is not revolutionary and is widely practised in most other
commercial/academic sectors. In fact, there are many good examples of how
this has been implemented in the EO community as well. However, rarely has it
been comprehensive or applied in an internationally harmonious manner, making
it difficult to identify and assess any differences in results. Therefore, QA4EO
provides specific guidance to aid in the implementation and interpretation of the
core ‘traceability principle’ (http://QA4EO.org).
Traceability to SI is the key underpinning requirement to enable such a QA
infrastructure to function. However, to achieve ‘climate quality data’ also requires
significant improvement in accuracy. The necessary improvement in accuracy
to meet these requirements is not contained within any of the current planned
missions of any space agency, although many have the necessary sensitivity and
resolution. In fact, no strategy exists to enable optical-based sensors to establish
and maintain traceability to SI in flight with any reliable uncertainty estimate,
as required to meet the long-term needs of the EO community, and present
sensors are an order of magnitude away from meeting the needs of the climate
community. Until recently, it was hoped that NASA would proceed with the
development of a mission called CLARREO (Climate Absolute Radiance and
Refractivity Observatory; http://clarreo.larc.nasa.gov/) [6,7], which would seek
to meet these objectives. However, budgetary reductions in 2011 have effectively
put an indefinite delay on the mission at the time of writing this paper.
(c) SI traceability from space
Benchmark measurements of the Earth’s incoming and reflected solar radiances
with unprecedented uncertainties (factor 10 improvement) tied robustly to
internationally accepted physical standards of the SI in orbit are now timecritical (but achievable) and will allow indicators of decadal climate change
and feedback processes (radiative fluxes, clouds, albedo, ocean colour) to be
extracted from a background of natural variability. This paper will ultimately
describe a small satellite-based mission called TRUTHS (Traceable Radiometry
Underpinning Terrestrial- and Helio- Studies; http://www.npl.co.uk/truths) [8],
which was first proposed to the European Space Agency (ESA) in 2002 and
again in 2010, enabling high-accuracy primary SI-traceable calibration chains to
be established directly in orbit for the first time and realizing the concept of a
‘standards laboratory in space’. TRUTHS is complementary to the CLARREO
mission in observing the Earth’s reflected solar spectrum, while the CLARREO
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Accurate radiometry from space
4033
mission adds climate-accuracy SI-traceable observations of the full IR spectrum
and global navigation satellite system radio occultation. In addition to collecting
its own datasets—spectrally resolved radiances and solar irradiances—TRUTHS
will also provide the catalyst (through provision of rigorous cross-calibration
from space) to facilitate improvements in the accuracy of current and planned
observation systems, enabling them to better meet the demanding challenges
emerging from, for example, the new ESA Climate Change Initiative.
In this paper, where appropriate, we will for convenience use TRUTHS as
the named example mission, but for many cases it could easily be substituted by
CLARREO. In fact, it should be acknowledged that much of the example scientific
justification case for the Earth viewing reflected solar spectral observations was
largely originally developed by the CLARREO science team [7] and has been
adapted for TRUTHS (§2b).
2. Uncertainty drivers for climate
The following sections provide an overview of a few of the key drivers, in terms of
uncertainty and SI traceability, for decadal climate studies in the solar-reflective
domain. These examples are illustrative of the measurement issues and are by no
means comprehensive. Similarly, each example is not intended to be a full treatise
on the issues related to the topic under discussion.
(a) Solar irradiance
(i) Introduction
Solar radiation, the driving force of the Earth’s climate, has been observed for
many centuries, with quantitative measurements of solar irradiance being made
for over 100 years. It is expected that variations in solar irradiance influence
the terrestrial climate, but we do not yet fully understand how suspected longterm variations arise or how the climate system actually responds. Monitoring
solar variations is therefore crucial, but the continuous record of space-based
measurements since 1979, which have revealed solar irradiance variations of a
few tenths of one per cent during the 11-year cycle, are as yet inadequate for the
detection of long-term variations.
(ii) Instrumental biases
Solar radiative forcing on climate was qualified by the latest IPCC report [1]
as ‘very low’ and ‘highly uncertain’. The small magnitude of the forcing is largely
based on the satellite record derived over the last three decades. The analysis of
the observation data [9] showed that, despite substantial scatter between different
instruments (figure 2) [10], following normalization, the variability of the TSI
from the minimum to the maximum of the solar activity does not exceed 0.1
per cent or approximately 1.4 W m−2 , which translates into a direct radiative
forcing of the Earth of approximately 0.25 W m−2 . In reviewing the scale of
instrumental variation in the non-normalized datasets and subtle differences that
can be obtained using differing normalization strategies (figure 2), one is drawn to
question the metrological reliability of this approach [11]. Even when accepting
the above conclusions (which are probably reasonable), it is clear that, if any
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N. Fox et al.
0
days (Epoch Jan 0, 1980)
4000
6000
8000
2000
10000
1375
original data
HF
1370
ACRIM I
(a)
DIARAD/VIRGO 1102
ACRIM III vers: 0912
VIRGO 6_002_1102
1365
ERBS
total solar irradiance (W m–2)
ACRIM II vers: 101001
0.3%
1360
TIM/SORCE vers: 11_1102
1368
(b)
1366
1364
PMOD composite
1368
(c)
1366
1364
ACRIM composite
(d)
1368
1366
1364
IRMB composite
78 80 82 84 86 88 90 92 94 96 98 00 02 04 06 08 10
year
Figure 2. Upper panel: daily averaged values of the Sun’s total irradiance TSI from radiometers
on different space platforms since November 1978: HF on Nimbus7, ACRIM I on SMM, ERBE
on ERBS, ACRIM II on UARS, VIRGO on SOHO, ACRIM III on ACRIM-Sat and TIM on
SORCE. The data are plotted as published by the corresponding instrument teams. Note that
the HF and ERBE radiometers did not have in-flight corrections for degradation. Lower panels:
the PMOD, ACRIM and IRMB composites showing small but significant differences. TSI as daily
values plotted in different colours to indicate where the data come from. Adapted from figure 1
of Fröhlich [10].
data gap were to emerge (i.e. failure to have at least two instruments operating
in parallel) due to failure of an instrument or launch (as was the case of the
recently anticipated GLORY mission of NASA), the full record would in effect
be lost, or at best have a significantly increased uncertainty.
(iii) Solar radiometers
The instruments used to measure TSI in space (and on the ground) are
all radiometers based on the principle of electrical substitution, comparing the
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Accurate radiometry from space
Sunspot number
200
160
120
80
40
0
1600
1650
1700
1750
1800
1850
year
1900
1950
2000
2050
2100
Figure 3. Sunspot group numbers (Rg, yellow) and Wolf Sunspot numbers (SSN, dark red-brown).
Predictions for the upcoming solar cycles are shown as black shading.
heating effect of optical radiation with that of electrical power, which is a concept
more than 100 years old [12]. These radiometers were first launched into space
in the late 1970s. At a similar time, a group was incorporated by the WMO to
form a World Standard Group (WSG) with a mean to be defined as the World
Radiometric Reference (WRR) [13]. Similar instruments were also used in the
NMIs to establish primary radiometric scales.
Quinn and Martin of the National Physical Laboratory (NPL) made a step
change in the performance of this type of instrument through cooling to the
temperature of liquid helium [14]. This cooling reduced many of the sources of
uncertainty significantly, leading to more than a factor of 50 improvement in
accuracy.
The significant benefit of cryogenic radiometry over optical radiometry in
general was compounded with the design of a specific instrument optimized
for spectral responsivity measurements [15] and its subsequent usage and
development in a wide range of applications, as reviewed earlier [16,17]. It
is perhaps notable that, although a preliminary design for a cryogenic solar
radiometer capable of achieving 0.01 per cent uncertainty has been in existence
since the early 1990s [18,19] and widely recommended for flight, to date, none has
yet flown in space. The Cryogenic Solar Absolute Radiometer (CSAR) described
in §4b(ii) brings the prospect of achieving this a step closer.
(iv) Solar variability on climate time scales
In climate terms, it is also important to note that solar activity during the past
30 years can be considered unusually high. A long-term proxy of solar activity
is the solar modulation function, which shows that, in the past 50 years, the
solar activity has been remarkably constant and at a 500-year maximum [20,21].
Therefore, we can clearly place no reliance, in terms of climate impact, on this
rather short observation time series of 30 years, when we know that the Sun
has varied dramatically over the past 400 years, emphasizing the importance of
monitoring with sufficient accuracy to detect relatively subtle radiometric, but
climatically dramatic, changes.
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A notable minimum of solar activity (low sunspot number) occurred in the
seventeenth century, e.g. the Maunder (1645–1715) (figure 3). Our estimate
correlates well with the ‘Little Ice Age’ in northern Europe, indicating a strong
linkage, confirmed by short-term correlations with solar irradiance observed
during the 11-year solar cycles [22]. The first half of the twentieth century was
characterized by an increase in solar activity, reaching a maximum (in terms of
sunspot numbers) in the year 1957. Moreover, after five decades of high, but
stable, solar activity, there is now growing evidence for a decline in solar activity
in the near future [23,24].
While there is general agreement about how TSI has changed over time, there
is scientific controversy about the magnitude. The TSI relative to the Maunder
minimum had been determined to range from 0.7 to 6 W m−2 [25]. The obvious
conclusion is that, despite considerable progress, we still do not have a reliable,
commonly accepted value for the amplitude of the solar forcing. On the other
hand, many attempts to simulate past climate changes and define the solar
irradiance variability from climate simulations have also failed to give a unique
and indisputable answer to the question about solar irradiance variability and its
impact on climate.
Further investigations are obviously needed, and from the observational side,
the only way is to obtain a very long-term time series (more than 30 years) of
the TSI with high precision and absolute uncertainty better than 0.01 per cent.
All estimates of the solar influence on the climate will eventually depend on
this experimental determination and can only be achieved by robust SI-traceable
calibrations.
(v) Solar spectral irradiance
Measurements of TSI as described above can give a good indication of the
overall impact on the Earth’s climate, but in reality it is of course the variation
in the spectral distribution of radiation within the TSI that controls the detail
of the climatic impact. Thus, an accurate estimate of solar radiative forcing
requires knowledge not only of changes in the incoming irradiance at the top
of the atmosphere but also of the influence of variations in UV on stratospheric
temperature and composition [26]. Much larger fractional changes take place at
shorter wavelengths than for the integrated visible portion of the spectrum [22].
Variances in solar UV influence the ozone distribution and the heating rates
in the stratosphere [26,27] and subsequently affect the radiation propagation
to the troposphere in a nonlinear manner, which varies with latitude and
season.
A recent analysis by Haigh et al. [28] showed that observed changes in SSI
collected by the Spectral Irradiance Monitor (SIM) instrument [29] (more than
four times greater reduction in UV between solar maximum and minimum than
expected but with an increase (!) in visible radiation) would lead to an overall
warming of the planet, when all existing expectations would predict the opposite.
(vi) Summary
Long-term observation of the Sun’s radiation on the Earth, total and spectrally
resolved, at uncertainties of 0.01 and 0.1 per cent, respectively, is critical to
ensure that we can fully account for the effects of all natural variability when
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Accurate radiometry from space
4037
monitoring climate and the results of any mitigation strategies. This can only
be achieved through robust traceability to SI. The uncertainty levels required,
which are commensurate with those of primary scales at NMIs, mean that
we have to take instruments based on the same concepts as the terrestrial
primary standards, i.e. cryogenic radiometry, into orbit. It is long overdue for
deployment in space to remove the issues and controversy associated with existing
climate datasets. Robust SI-traceable instruments can, in principle, be flown
on occasions to provide an anchor for simpler lower-mass instruments, in this
way reducing the high risk and cost associated with guaranteeing overlapping
missions.
(b) Reducing uncertainty in climate forecasting
(i) Constraining climate models
Currently, there is significant variability between the predictions of different
models of the effects and consequences of climate change (figure 1), mainly
driven by a factor of 3 uncertainty in climate sensitivity of the equilibrium
response of the climate system to a given level of anthropogenic radiative forcing
from greenhouse gases, aerosols and surface albedo changes [1]. Climate change
feedbacks (processes that amplify or retard the planetary response to a radiative
forcing) are the biggest source of uncertainty in the current understanding of
climate, and in determining future climate change [30]. The uncertainty in
climate feedback directly impacts the uncertainties associated with the projected
temperature change [31].
The largest single uncertainty component is the cloud feedback, and
particularly the climate feedback from low-level cloud [32]. The second largest
feedback uncertainty (though approx. a factor of 2 smaller) is from the
combination of water vapour and temperature lapse rate feedbacks [32]. The
other major climate forcings are albedo (snow, ice, land cover), aerosols and solar
irradiance, all of which have significant modelling uncertainties and therefore need
high-accuracy observations to be better constrained. All these forcings require
benchmarking for future studies.
(ii) Cloud feedback on climate
Clouds can amplify or counteract global warming; current climate models show
a range of amplification values from close to zero to up to 60 per cent [32]. The
global distribution and radiative properties of clouds are likely to alter in response
to global warming, which could have several different feedbacks on the surface
temperature. For instance, during day-time, reduced coverage of low-level cloud
would result in less reflection of solar radiation, and increased warming [30].
Thus, detecting a long-term trend against high natural decadal variability needs
high-accuracy observations of cloud properties.
Reducing the uncertainty associated with climate prediction by a factor of 2
would require observation stability of 0.5 per cent per decade for visible spectral
radiances and 0.3 per cent per decade for broadband SR radiances [33] (approx.
a factor of 10 better than is currently achieved). This thus needs high-accuracy
spectrally resolved observations with high stability over decadal time scales to
test the capability of different climate models in accounting for the various
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N. Fox et al.
cloud feedbacks (also dependent on cloud type). Spectral resolution of 10–
20 nm is needed for cloud-phase and cloud-type characterization, for observing
the transition zone between cloud and clear sky, and for seeing the effect of
aerosol on cloud formation. For reflected solar radiation, the differences between
spectral signatures in cloud fraction change and in cloud optical-depth change
are very similar. Therefore, sufficient spatial resolution (<500 m) is required to
allow cloud masking. A cloud fraction analysis would then allow determination
of cloud radiative forcing (CRF) (all-sky minus clear-sky), which is needed for
cloud feedback [34]. It would also help in identifying decadal change of all-sky
versus clear-sky properties, when an all-sky spectrum change alone might not be
sufficiently discriminating.
On a global mean basis, the changes are small but critical: a 1 s.d. of about
0.3 W m−2 K−1 for net and SR CRF. The recent IPCC AR4 report [1] predicts
about 0.2 K per decade change for the next few decades, or an order-of-magnitude
change of 0.06 W m−2 per decade for SR CRF. Given the global mean value of SR
CRF of 50 W m−2 , this is a relative uncertainty of approximately 0.12 per cent
(k = 1) or 0.25 per cent (k = 2).
The same conclusion can be derived by an alternative (order-of-magnitude)
approach. Expected anthropogenic radiative forcing of the climate system is
expected to be approximately 0.6 W m−2 per decade for the next few decades [30].
A 25 per cent cloud feedback would amplify or dampen this forcing [30] by
approximately 25 per cent, or roughly 0.15 W m−2 . This change would be 0.3
per cent of the average SR CRF of 50 W m−2 .
(iii) Optimizing uncertainty
Although accuracy requirements derived from expected climate signals are
useful, a much more rigorous approach was recently developed by the CLARREO
science team [7], which established a model to determine the optimum uncertainty
needed from a sensor, taking account of all observational aspects, i.e. instrument
drift, sampling, seasonal variation, etc., in order to detect a trend in a climate
parameter above a background of natural variability in a given time frame. Of
course, there are fundamental limits to the accuracy of measuring climate trends,
which can be set with reference to the perfect climate observing system following
the methodology of Leroy et al. [35].
Figure 4 presents the results of such an analysis for the CRF example using
the proposed operational characteristics of TRUTHS and CLARREO. The details
can be found in Wielicki et al. [7].
Figure 4 shows that there is a strong dependence of climate trend accuracy on
the TRUTHS calibration accuracy. An accuracy of 0.3 per cent (k = 2) shown as
the indicated blue line provides trend accuracy within 20 per cent of a perfect
observing system (black line), and time to detect trends within 15 per cent of
a perfect system. The upper horizontal red dashed line is the observed signal if
the feedback factor was 100 per cent, and the lower green dashed line is for the
signal if the CRF feedback effect was at a level of 50 per cent. Such feedbacks,
if they exist in the climate system, will amplify climate change, and it is critical
to determine from observations if they exist. It is clear from figure 4 that the
TRUTHS requirement of 0.3 per cent (k = 2) is optimal. Further improvements
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Accurate radiometry from space
4
trend accuracy (% CRF per decade, 95% confidence)
calibration accuracy (k = 2)
3
2
0%
perfect obs
0.15%
0.30% TRUTHS
0.60%
1.20%
1.80% CERES
2.40%
3.00%
3.60% MODIS/VIIRS
100% cloud feedback:
decadal change in CRF =
anthropogenic forcing of
0.6 W m–2 per decade =
1.2% SW CRF per decade
1 perfect obs
TRUTHS
95% confidence for 50%
cloud feedback; time to detect cloud
feedback trends is a strong function of calibration accuracy
0
10
20
30
40
length of observed trend (year)
50
Figure 4. The trend in cloud radiative forcing (CRF) with an accuracy in per cent per decade
(k = 2) in relation to the length of observation (abscissa) of different calibration accuracies currently
achieved (MODIS, CERES) and TRUTHS.
in accuracy are hindered by the limit of natural variability (the perfect observing
system, black line), and reduced accuracy rapidly increases the time for society
to detect moderate to large cloud feedbacks.
For an accuracy change from 0.3 to 1.2 per cent, the time to detect cloud
feedback increases by almost a factor of 2. Society would need to wait an
extra decade for accurate climate sensitivity information to facilitate difficult
policy decisions concerning the implementation of rigorous and potentially costly
mitigation and adaptation strategies. We conclude that a TRUTHS relative
accuracy of 0.3 per cent (k = 2) is required to observe decadal changes in
cloud feedback. Of course, a similar analysis can be carried out for other
climate-sensitive feedbacks.
(c) Establishing SI traceability for the Earth observing system
Although every effort is made to ensure that any pre-flight calibration and
characterization of a satellite sensor can be relied upon and is traceable to SI
units, when the instrument is operational in orbit, it is well known that for optical
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sensors this is rarely if ever the case. The high vibration of launch and sensitivity
to contamination of optical coatings and surfaces make some form of postlaunch calibration, or at best validation, an essential component of any mission.
Some sensors have on-board calibration systems to offer the prospect of direct
calibration, but often these are themselves subject to the same sorts of issues as
the instruments themselves, particularly in the solar-reflective spectral domain.
For the thermal IR, blackbody radiators, which for practical (mass/size) reasons
may not have quite as good emissivities as the standards used on the ground,
are probably reasonably good at providing some level of in-flight performance
monitoring. With effective design, these might be considered to offer a sufficient
level of ‘SI traceability’ for operational missions, as they are relatively insensitive
to most of the common degradation mechanisms.
However, sensors operating in the solar-reflective domain (<2500 nm) are a
very different issue. Here, various on-board approaches have been and are being
used, including the flight of relatively fragile ‘standard lamps’. More commonly,
diffusers illuminated by solar irradiance to establish a Lambertian radiance
ideally capable of filling the sensor field of view are employed. Knowledge of
the diffuser reflectance and solar irradiance allows full in-flight calibration. Even
in the absence of reliable values for these, if they can be relied upon to be
stable, then they can at least monitor and provide a correction for drift. In
practice, SSI, although reasonably stable in the short/medium term, is only
known to at best a few per cent in an absolute sense. Similarly, diffusers degrade
with time, particularly when exposed to sunlight. Effort is of course made to
limit exposure, in some cases, e.g. Medium Resolution Imaging Spectrometer
(MERIS) of ESA (http://envisat.esa.int/instruments/meris/), using multiple
diffusers. These methods have never claimed to be more accurate than a few
per cent. Thus, although the best of these are probably adequate for many
operational applications, none are really sufficient for the needs of long-term
studies such as climate.
All satellite operators use some alternative vicarious method (or often a
collection of methods) to attempt to establish some level of confidence in the
satellite observations and, if appropriate, on-board calibration systems. These
approaches are briefly reviewed below but none at present can come close
to the uncertainties needed for climate. However, if they could be validated,
and in some cases calibrated, by a sensor in orbit of sufficient accuracy,
spectral and spatial resolution, then the uncertainty of these methods could
be reduced. This would allow the establishment of an operational global
calibration system capable of upgrading the performance of existing sensors
(which usually have adequate sensitivity but not accuracy) and of facilitating
a ‘global climate observing system’ rigorously tied to SI units. The TRUTHS
satellite (http://www.npl.co.uk/truths) described in this paper and its sister
CLARREO (http://clarreo.larc.nasa.gov/) are designed for this very purpose.
(i) Vicarious calibration
Post-launch vicarious calibration relies on the use of remote targets that
are calibrated or characterized independently of the sensor under test, e.g.
Earth targets. Historically, the most commonly used approach has involved
specifically identified test sites. Many ‘test sites’ have been identified and
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Accurate radiometry from space
4041
used. Some are characterized by local survey teams, with a few of these now
permanently instrumented, providing automated information on their properties
to Web-based servers. These form the basis of a potential operational calibration
network called Landnet, which follows an earlier concept called GIANTS [36].
Others are too remote to visit directly (e.g. the Moon [37,38] and some North
African/Arabian deserts [39]) but instead use regular observations by satellites
or ground observing stations to build a database of knowledge of their properties.
A catalogue of all test sites has recently been established by the United States
Geological Survey (USGS) for CEOS and is accessible through the Cal/Val portal
(http://calvalportal.ceos.org/cvp/web/guest), operated by ESA for CEOS and
GEO. In the case of optical imaging sensors, such sites (targets) exist for both
land and ocean, and a subset has been formally endorsed by CEOS to serve as
international reference standards.
Vicarious calibration methods can be used not only to establish absolute
values on the radiometric performance of a sensor and relative ‘band-to-band’
performance but also to facilitate sensor-to-sensor bias evaluation, and they are
not limited to fixed sites; e.g. the use of Rayleigh scattering over the ocean [39]
is also a well-established method.
(ii) Sensor-to-sensor cross-calibration (reference inter-calibration)
The concept of reference inter-calibration in its broadest sense is not new,
nor specific to space. In fact, the principle is widely practised and underpins SI
traceability and the measurement system as a whole. When reviewed in detail,
there can be and are a number of approaches that can be followed to implement
it, often bringing slightly different information to aid in analysis, e.g. different
spectral characteristics, dynamic range, etc.
In all cases, the instrument under test is presented with a ‘signal’ from a
‘reference standard’, which is similar in characteristic to that which it normally
observes. However, a prerequisite is that the appropriate characteristic of the
‘reference standard’ is well characterized/quantified or defined (calibrated) by
some independent SI-traceable means, e.g. the test sites mentioned above. For
this to be useful, the property of the ‘reference standard’, as observed by the
instrument under test, must be sufficiently stable compared to when it was
originally calibrated, or at least any change quantified. In the ideal case, the
‘reference standard’ is calibrated at the same time as or close to (approx. ±10 min)
the time that it is observed by the instrument under test; this is generally called
simultaneous nadir observation. However, for most standard orbits, this does not
happen very often, and so pointing of the reference sensor may be necessary, and
ideally an orbit chosen that has frequent cross-over opportunities, i.e. a 90◦ polar
precessing orbit.
Of course, the calibration can only be as good as the sensor under test.
For example, the reference sensor, e.g. TRUTHS, can only define with high
accuracy the radiometric characteristics of the spectral radiation that it observes
and is then observable by the sensor under test. If the sensor under test
is not well characterized, has unknown or drifting spectral characteristics,
limited signal-to-noise ratio capabilities, poor stray light, etc., then TRUTHS
is unlikely to provide significant help, other than to provide known values
to constrain, test or monitor drift. However, if the sensor is relatively stable,
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as most now are, is well characterized pre-flight and only lacks knowledge of
radiometric gain and traceability, then TRUTHS will be able not only to validate
performance but also to improve the overall accuracy significantly, in some
cases bringing them up to the levels needed to make decadal climate quality
measurements.
3. A benchmark mission for climate: a metrology institute in space
(a) Introduction
Section 2 has presented an outline science case based on a few example topics
highlighting the urgent need to make high-accuracy SI-traceable ‘benchmark’
measurements of the Earth–Sun system. It has alluded to the prospect of satellite
missions capable of fulfilling that role, e.g. TRUTHS and CLARREO. This section
will introduce one of these missions, TRUTHS, and its unique in-flight calibration
strategy.
The overall concept of TRUTHS can be summarized as:
a mission to measure, SI-traceably and with an unprecedented accuracy, the interaction of
solar radiation with the Earth as a benchmark reference to enable the detection of decadal
climate change.
TRUTHS will also enhance the performance and traceability of other EO systems
to deliver additional societal benefits, and, in some cases, ‘climate quality’ data
products, from existing instruments.
TRUTHS will achieve this objective using two independent but complementary
methodologies:
— direct sampling of key climate-specific fingerprint signatures as baselines
from which future change can be detected;
— facilitating performance improvements in other observing systems,
satellites and others, to enable them to collect better ‘climate quality’
data through reference inter-calibrations.
These objectives drive the specification of the mission and, although there is a
wide range of potential observables, the overall mission driving characteristics—
instrument and platform—can be relatively easily defined. While demanding
high performance and, in some cases, adaptations, the requirements are not
pushing technology beyond the established and already proven state of the art
for space instrumentation. Thus, this paper will not dwell on the details of the
instrumentation, but will only seek to highlight any key characteristics to aid the
reader to understand the concepts.
The key to the mission’s success lies not in the complexity or novelty of its
observational instruments, nor in the platform, but in the innovative in-flight
calibration system and the effective transfer of state-of-the-art terrestrial primary
calibration and SI traceability methodologies into space. The TRUTHS mission
will fly the radiometric capability of a national measurement institute into space.
In this way, it will be able to act like a calibration or ‘standards laboratory
in space’—TRUTHS will be SI traceable by design.
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Accurate radiometry from space
Solar Spectral Irradiance Monitor
(SSIM)
optical fibre
transfer
Spectral Calibration
Monochromator
(SCM)
Polarizing Transfer
Radiometer (PTR)
rotates from the Sun to
the Earth axis
Cryogenic Solar
Absolute Radiometer
(CSAR)
Sun
high-sensitivity
cavity
total solar
irradiance cavity
Earth imager
solar diffuser plate
(deployed)
solar diffuser plate
(part deployed)
Earth
Figure 5. Schematic of the TRUTHS satellite payload and its operational configuration. The
satellite has two viewing directions, towards the Sun or the Earth, shown here nominally
perpendicular to each other.
The key challenge to this mission and thus one where we will concentrate
our efforts in terms of explanation is the ability to achieve SI traceability at
uncertainty levels a factor of 10 below what has been achieved to date. We will
seek to show how this step change is not in practice as radical as it might initially
appear, but rather the result of simply taking the calibration chain directly
into orbit. In fact, when the methodology that will be described here was first
introduced into national standards laboratories, some 25 years ago, a similar step
change of more than a factor of 10 was also achieved. It is worth noting that
this goal to achieve high SI-traceable accuracy is also being pursued in the USA
by NASA for the complementary mission CLARREO, although the technical
approach they are pursuing to achieve this is very different. In the CLARREO
case, the IR traceability aspects of the mission mimic the traceability routes
of national standards laboratories, with the onboard reference standard being a
gallium melting point blackbody. The SR route differs significantly and relies on
some key measurements from other missions. Ideally, both missions and all their
components would fly simultaneously, allowing the establishment of a climate
and calibration constellation and some inter-calibrations at very high accuracy.
However, a minimal partnership would see the combination of IR elements from
CLARREO and SR from TRUTHS.
TRUTHS is implemented through a small agile satellite platform with two
perpendicular observation axes—Solar and Earth viewing (figures 5 and 6).
The majority of observation time is allocated to global nadir spectrally and
spatially resolved measurements of Earth-reflected solar radiation for decadal
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N. Fox et al.
0.75 m
CSAR
PTR–
calibration axis
SCM
SSIM
Earth imager
solar diffuser
CSAR–cooler
PTR–Earth view
Figure 6. Schematic of the satellite payload configured on the satellite bus. (Online version in
colour.)
climate change benchmarking. A nominal 5–10 min per day is allocated to solar
observations (total and spectrally resolved). In addition, the agile platform
enables:
— near-simultaneous angularly co-aligned observations for sensor reference
calibrations;
— multi-angular measurements of specific targets for reference intercalibration of sensors (surface and Moon); and
— multi-angular measurements of specific targets to support ‘process studies’.
The very high radiometric accuracy required by TRUTHS means that it seeks
to be largely self-reliant in most of its key measurands and thus includes the
instrumentation required to provide the information to retrieve the atmospheric
parameters (e.g. aerosols, water vapour) required to account for radiation
propagation losses in the atmosphere.
(b) Requirements
Table 1 provides a summary of the driving requirements for a benchmark
decadal climate mission. For reference calibration, the spatial requirement is
driven by the sensors targeted for calibration. Most climate-focused sensors have
relatively large spatial footprints, as the objective is generally global-averaged
measurements rather than local, while, increasingly, many operational satellites
have relatively high spatial resolution of a few tens of metres or less. This suggests
that the emphasis should be on high spatial resolution provided it does not
compromise the other measurements.
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Accurate radiometry from space
Table 1. Characteristics of the key measurands needed to meet the needs of decadal climate studies.
Note that the Earth/Moon radiance also represents the requirements needed to provide reference
calibration to other EO systems.
key parameters
spectral
range
(mm)
spectral
resolution
(nm)
spatial
resolution
(m)
estimated
accuracy
2s (%)
TSI
SSI
Earth/Moon radiance
0.2–30
0.2–2.5
0.32–2.45
total
0.5–1
4–10
n.a.
n.a.
<500
0.02
0.2
0.3
(c) Scientific payload
Three separate instruments on-board TRUTHS deliver the above science
objectives. In some cases, more details are provided later, but to provide an
overview of the mission concept, they will be briefly introduced here.
(i) TSI (0.2–30 mm)
Measurement is provided by an instrument called CSAR. In essence, this
instrument has a similar operational principle to those already used to measure
TSI in space, e.g. PMOD VI on-board VIRGO, TIM on SORCE, etc., and as
described in §2a(iii). The CSAR compares the heating effect of optical power
with that of electrical power. However, CSAR, through cooling to cryogenic
temperatures of approximately 20 K, reduces uncertainty by more than 10 times
over the typical ambient instrument. It is also the same operating principle as
that used at the NMIs for establishing primary radiometric scales.
(ii) SSI (0.2–2.5 mm)
This is measured on TRUTHS by the Solar Spectral Irradiance Monitor
(SSIM). In simple terms, this instrument consists of a spectrally dispersing
element to enable solar radiation to be dispersed onto a detector, e.g. a linear
array detector. In our baseline concept, the SSIM consists of two gratings and
two array detectors to enable simultaneous collection of spectrally resolved solar
irradiance and operates in a passive staring mode, removing any movements.
The exact design of this instrument can take many forms, including a spectrally
scanned system, but all aspects can be delivered by heritage components and
concepts and contain no significant risk. There are many examples of such
instruments used in space and on the ground for similar spectral irradiancebased measurements.
(iii) Earth spectral radiance (0.32–2.45 mm)
This is measured on TRUTHS by an imaging spectrometer baselined to be
an upgrade of the ESA aircraft imager APEX (http://www.apex-esa.org/), but
again could be based on many other existing designs and instruments. Our
instrument uses prisms to disperse the radiation and provides the opportunity
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N. Fox et al.
for full continuous spectral sampling of a scene. This Earth imager (EI) samples
the Earth, in our case, at 40 m GIFOV (geometric instantaneous field of view) and
with a swath of 40 km, driven by the availability of existing detector technologies.
In normal operational mode, it views the Earth at nadir but can be rotated by
the spacecraft to make observations at multiple angles. It can provide in-flight
switchable spectrally resolved radiances at bandwidths from approximately 1 nm,
but normally these will be integrated to around 10 nm. The EI is designed to
have a low sensitivity to polarization but contains a means to check and quantify
this in orbit. The number of spectral bands and spatial resolution transmitted
to the Earth is changeable in flight to maximize the trade-off between ‘useful
information’ and data transfer rates. For example, high spectral and spatial
information is useful over land, but less so over the ocean or cloud, where coarser
spatial resolutions are more than adequate; and similarly, other than in a few
specific spectral bands, high spectral resolution is also unnecessary. However,
even when optimizing these observational criteria, we anticipate a data collection
of some 4500 Gbits d−1 .
(d) Satellite platform
The instruments described above form the key science payload and are mounted
on a small agile pointable satellite platform in a 90◦ precessing polar orbit
(see schematic configuration in figure 6). The payload is configured so that the
entrance ports of the solar instruments, CSAR and SSIM, are on one axis and the
EI is perpendicular to them. Operationally, the satellite will alter its orientation
so that the solar instruments have approximately 5 min observing time per day.
For the remainder, the satellite will point the EI to the Earth, largely at nadir
but also occasionally (per orbit) off-nadir to co-locate with another sensor (to
provide a calibration) or to make angular measurements on specific targets.
The total payload mass for a fully redundant system is approximately 160 kg
and the peak power requirement is less than 200 W.
(e) The ‘in-flight calibration system’
This is the key technological innovation within TRUTHS and is designed to
enable direct traceability to an SI primary standard in orbit at unprecedented
uncertainties. This calibration system again consists of three instruments,
although one is common to the science payload. The heart of the calibration
system is the primary standard. In TRUTHS, this is the CSAR outlined above
and described in detail in §4b(ii), and the key operational principle is the same as
for TSI measurements. However, when performing as the primary standard, we
need in this case to measure the power of monochromatic radiation and not of
TSI. The spectrally dispersed radiation is significantly lower in power than direct
sunlight and so for these measurements a cavity with higher (approx. 100×)
sensitivity has been incorporated into the CSAR and serves as the primary SI
standard, able to measure the power in a monochromatic beam of radiation with
an uncertainty of less than 0.1 per cent.
Once the power of the monochromatic beam of radiation is determined, it can
be used as a known input to provide calibrations and traceability to other optical
instruments. For example, it could be used as an input to SSIM instead of the
Sun, allowing an absolute calibration of the instrument at any or each wavelength.
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In practice, this is carried out on TRUTHS through an intermediate instrument
called a polarizing transfer radiometer (PTR; see §4f ), which in turn also provides
the calibration to the EI (see §3c(iii)).
The source of the spectrally dispersed solar radiation on TRUTHS is the
spectral calibration monochromator (SCM; see §4c). This instrument disperses
incident solar radiation using (in our baseline) a diffraction grating. The SCM
includes an optical fibre delivery system as a single integrated instrument. In use,
the CSAR and other instruments measure the output power of the optical fibre
bundle that is translated between them.
The final instrument in the TRUTHS calibration system is the PTR. This
instrument is relatively simple in nature and provides the means to act as a
short-term secondary standard to minimize measurement time with the CSAR
and also to provide calibrations of the EI. The PTR is a transfer detector including
both broadband and spectrally filtered detectors. Monochromatic radiation from
the SCM can enter the PTR and provide spectral calibration of non-filtered
photodiodes traceable to CSAR.
In a secondary step, the output of the SCM can be measured by these detectors
in the PTR before being used to calibrate the SSIM. In a similar way, the filtered
detectors of the PTR can be calibrated across their spectral bandpass using
radiation from the SCM. The PTR is mounted on a small simple rotation device,
which can rotate the PTR so that it views towards the Earth viewing axis and
measures the radiance in spectral bands of a reference source—a solar diffuser
and/or the Moon or Earth reference site. The source is simultaneously viewed by
the EI to provide full SI traceability to that instrument.
In TRUTHS, the PTR has been configured to serve a secondary purpose, as
it will also be able to measure s and p polarizations separately. In this way, the
PTR can also provide information on the state of the atmosphere, particularly
aerosol optical depth, for atmospheric correction. TRUTHS has two PTRs for
redundancy.
The above calibration system mimics that used terrestrially in NMIs, with the
exception that, instead of the SCM as a source of tunable radiation, on the ground,
tunable laser sources are generally used. In addition, it is more convenient to use a
high-temperature blackbody as an artificial source of continuum radiation rather
than the Sun. The traceability chain is described in more detail in §4.
4. Radiometric calibration of an optical sensor
(a) Traceability chain
A typical radiometric traceability chain from an NMI can be described
schematically by the left-hand panel in figure 7.
— The primary standard, the cryogenic radiometer, establishes a scale
of spectral responsivity, through the calibration of a photodiode. The
cryogenic radiometer first measures the radiant power in a monochromatic
beam; in this case a fixed-wavelength laser, e.g. argon ion, can be used.
Uncertainties of less than 0.005 per cent can be achieved in this way [15].
— The now calibrated monochromatic beam is then used to illuminate
a photodiode and thus determine its spectral response. This process
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4048
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fundamental constants (SI)
total solar
irradiance
terrestrial traceability
cryogenic radiometer
Cryogenic Solar Absolute
Radiometer (CSAR) (TSI cavity)
Sun
laser
cal. interval
~100 nm
CSAR HS cavity
Solar Calibration Monochromator
cal. interval ~100 nm
photodiode
(spectral responsivity)
reference photodiode
laser
cal. interval
~0.1nm
Solar Calibration Monochromator
Filter Radiometer
cal. interval ~1 nm
Solar Spectral
Irradiance Monitor
Filter Radiometer
radiance via 10
spectral channels
radiance temperature
solar/lunar spectral
irradiance
ultra-high-temperature
blackbody (3500 K)
solar diffuser plate
radiance continuum
radiance continuum
Earth imager
spectroradiometer
(multi-band filter
radiometer)
other EO
instruments
hyperspectral
Earth/lunar radiance
spectral
radiance/irradiance
calibrations
TRUTHS traceability
Figure 7. Calibration traceability chain for TRUTHS (right) contrasted with that of the typical
terrestrial traceability of an NMI (left).
is repeated at sufficient wavelength intervals (usually 0.05–0.1 mm) to
allow interpolation of the slowly varying spectral response of the
photodiode.
— These photodiodes can then be used to calibrate spectrally selective
detectors (filter radiometers, FRs) using a similar monochromatic beam of
radiation but making measurements at much finer spectral intervals using
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Accurate radiometry from space
4049
a tunable laser, e.g. a titanium sapphire. The FR can have narrow or broad
spectral response functions, e.g. 0.001 mm for a monochromator-based
instrument or 0.01 mm for one based on coloured glasses [40].
— The calibrated FRs are then used to measure directly the spectral
irradiance or radiance of conventional polychromatic sources such as lamps
[41–43]. Often, the Planckian radiation of an intermediate, ultra-hightemperature (3500 K) blackbody (UHTBB) is used to interpolate finer
spectral intervals for irradiance and radiance measurements, its radiant
temperature having been previously determined using a group of FRs
across the spectral region of interest.
The monochromatic source used for this traceability chain is typically a laser.
Lasers provide high radiant power and monochromaticity of easily measured and
potentially tunable wavelength, and are optically well defined and controllable.
However, they are not very portable and require significant power for operation
and thus are not suitable for space flight.
This traceability chain is quite simple and can allow spectral radiance
measurements to be made with uncertainties less than 0.05 per cent. In
principle, this traceability chain and its resultant uncertainty can be used to
calibrate an instrument bound for space. In practice, this is rarely carried out
using this primary stage of the traceability chain and thus to this level of
uncertainty, because of additional challenges that have to be faced, e.g. the
physical size of the sensor, level of ‘cleanliness’ and need for vacuum operation.
However, the readiness to accept this increased uncertainty is underpinned by
the acknowledgement that any laboratory calibration uncertainty is unlikely
to be maintained in space and thus achieving an uncertainty of a few per
cent pre-launch, while still very challenging, is usually set as the target. This
uncertainty level allows additional steps and thus uncertainty to be added
to the traceability chain to accommodate the challenges described above. For
example, large-aperture integrating sphere sources can be built to fill the aperture
of the sensor under test but considerable effort is required to make them
spatially uniform; windows can be used on vacuum tanks but these need to
have a known transmittance and are a source of inter-reflections and stray
light.
It should however be noted that the drive for improved pre-flight accuracy is
leading to new efforts at NMIs to reduce the length of the traceability chain and
adapt the primary methodologies so that they can be more easily transported
to the calibration facilities: vacuum tanks and clean rooms, located at sensor
manufacturers (http://www.emceoc.org).
However, of greater potential interest is the realization that this complete
traceability chain can actually be replicated directly on-board a satellite in space,
allowing similar uncertainties to be achieved, tied robustly to SI. This space-based
traceability chain is described in the right-hand column of figure 7 and is the heart
of the TRUTHS satellite.
The major difference between the two traceability chains is that in TRUTHS
the monochromatic radiation is not from a laser but instead is spectrally dispersed
solar radiation from a monochromator, the SCM. Lasers are too large and power
hungry for space applications, at least where multiple wavelengths are required.
Only short-term stability (<1 min) of the radiation from the SCM is required. Any
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N. Fox et al.
long-term degradation of the SCM optics is calibrated out each time it is used,
as the output beam power from the optical fibre delivery system is referenced to
the on-board primary standard cryogenic radiometer, the CSAR.
Similarly, the spectral radiance response of the Earth viewing imager (EI) is
calibrated in orbit through the measurement of solar irradiance reflected from
a solar-illuminated diffuser plate instead of a high-temperature blackbody. This
procedure is in common use, e.g. by MERIS (http://envisat.esa.int/instruments/
meris/). However, in contrast to other EO missions, the spectral radiance of this
system is measured directly in orbit using a group of FRs contained within the
PTR calibrated using the SCM. This practice removes errors due to drifts in
spectral shape and absolute level caused by ageing or contamination.
In TRUTHS, the complete spectral response of the FRs (gain and shape)
can be routinely measured in flight using radiation from the SCM traceable to
the CSAR. The PTR can be rotated between the calibration plane and the EI
by means of a simple rotation device (figure 5). The resultant uncertainty in
this process is dominated by the signal-to-noise ratio caused by the relatively
low power emitted through the SCM. As the CSAR itself has an uncertainty
of less than 0.01 per cent, the overall uncertainty is likely to be less than 0.1
per cent in radiance, more than an order of magnitude better than any other
EO mission. Although this may seem optimistic to the relatively conservative
space instrumentation community, this step change reduction in uncertainty is
similar in magnitude to that obtained when NMIs started to introduce cryogenic
radiometers into their terrestrial calibration chains nearly 30 years ago [15–17].
The in-flight calibration procedure described above does not of course
reduce the need for normal detailed pre-flight characterization of the sensor
characteristics, e.g. stray light, out of band, linearity, etc. However, in most
modern EO optical instrumentation, the dominant uncertainty in the quoted
error budget is ‘radiometric calibration’ and its maintenance over mission life. It is
this source of uncertainty that the TRUTHS calibration methodology addresses,
through the regular recalibration, ‘in flight’, of its instrumentation against an
essentially electrical, as opposed to optical, standard.
Some of the critical elements of this traceability chain will be described
below, highlighting and contrasting differences between the terrestrial and space
equivalent versions where appropriate.
(b) Primary radiometric standard: cryogenic radiometer
(i) Terrestrial primary standard
Most NMIs have adopted the cryogenic radiometer as the primary standard
for all optical radiation measurement quantities [16,17,44,45]. The cryogenic
radiometer is an extension of the concepts of the electrical substitution radiometer
(ESR), summarized in §2a(iii) [14,16], which were independently developed by
the metrologists Ångström [46] and Kurlbaum [47]. Cooling the technology to
cryogenic temperatures (<30 K) reduces uncertainty levels to less than 0.01 per
cent (95% confidence) as first shown by NPL in the 1980s [14] due primarily to
better equivalence between electrical and optical power and the ability to build a
large cavity to absorb optical radiation while maintaining a reasonable sensitivity.
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Accurate radiometry from space
15 cm
Two-Axis
Sun
Sensor
(TASS)
radiometer
head
flexible link
(T = 15 K)
flexible link
(T = 120 K)
cooler
(cold head)
cooler
(compressor)
cooler
(drive
electronics)
measurement
electronics
Figure 8. Representation of the CSAR. The radiometer head sits on the twin cooler cold platform
via a flexible cooling link. Note that the orientation and location of the radiometer head with
respect to the cold platform is fully open. (Online version in colour.)
Cryogenic radiometers are now the primary standard of choice for NMIs.
Different designs and operators have been compared showing equivalence less
than 0.02 per cent, generally limited by the transfer standards used in the
comparisons [44,48,49]. The use of a mechanical cooling engine instead of the
need for liquid cryogens made possible the practical possibility of flying such an
instrument into space [18,19]. It subsequently resulted in a terrestrial version,
which has formed the basis of the NPL primary radiometric scales for the last
15 years and has been commercially exploited for use in other NMIs around the
world [50].
(ii) Space-based primary standard—CSAR
The space-based radiometer follows exactly the same principles as that
for the ground. As both are already required to operate in vacuum, the
principle differences relate to adaptation for vibration, redundancy and the
specific operational characteristics required for the space application, e.g.
different power levels and irradiance. Its configuration for space flight is
shown in figure 8, where the radiometer head is attached to the cooling
system by a flexible connector to aid configuration on the spacecraft
platform. This cooling system is shown here consisting of two Astrium 10 K
coolers (http://www.astrium.eads.net/en/equipment/ 10-k-stirling-cycle-cooler.)
for redundancy, but only one is needed and used during operation.
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N. Fox et al.
thermal
shields
high-sensitivity
cavity
mounting
surface
detector stage
total solar
thermal
irradiance shields and
cavity
light trap
aperture
wheel
shutter wheel
Figure 9. CAD-based schematic of CSAR. (Online version in colour.)
TRUTHS will have a cryogenic radiometer (CSAR) with two different types
of cavity shown schematically in figure 9. The high-power cavities will measure
TSI directly. As the TSI cavities are optimized to measure power levels close to
30 mW, they do not have sufficient dynamic range to measure the relatively lowintensity monochromatic radiation from the SCM. This is done instead using
high-sensitivity cavities (optimized for 10 mW to 1 mW), which serve as the
primary references and provide traceability to SI for all the other instruments
on TRUTHS: SSIM, EI and PTR.
The cavity absorptances are approximately 0.99998 and spectrally flat,
allowing measurements in any spectral band. The only additional source of
error in the space environment, which is different from those on Earth, is
potential degradation of the cavity. But with such high absorptance, considerable
degradation would be required before any change would be observed and have
impact on the overall uncertainty, particularly for applications where it is
used as a primary standard for other instruments. CSAR will minimize any
residual sensitivity to this degradation by providing four primary cavities for TSI
measurements operated in an exposure time-limited sequence. It will have two
high-sensitivity cavities to ensure redundancy. A small (approx. 0.8 mm) aperture
will be used to allow direct comparison between the two types of cavity in orbit.
This aperture, positioned in front of each type of cavity in turn, will bring the
detected solar power down to a level within the dynamic range of both cavities.
The absolute area of this small aperture does not need to be known as it will be
common and only relative measurements are required.
For traceability of irradiance (units: W m−2 ), a defined detector area is also
required. The TRUTHS CSAR will have five reference apertures, any one of
which can be used with any of the cavities. The aperture areas will be calibrated
traceably to the metre on the ground, and then cross-compared in the space
environment (in an exposure time-limited sequence), using either direct solar or
monochromatic radiation from on-board the spacecraft. The uncertainty planned
for TRUTHS (for TSI) is less than 0.02 per cent (k = 2) and as a primary standard
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Accurate radiometry from space
Table 2. Estimated uncertainty budget of CSAR.
source of uncertainty
spectral–VIS
uncertainty
(k = 2) (%)
spectral–UV and
IR uncertainty
(k = 2) (%)
total solar irradiance
uncertainty
(k = 2) (%)
measurement of electrical power
area of defining aperture
cavity absorptance
diffraction correction
scattered light
random noise
0.010
0.010
0.010
n.a.
0.006
0.010
0.010
0.010
0.010
n.a.
0.006
0.020
0.010
0.008
0.004
0.020
0.004
0.002
total
0.03
0.04
0.024
less than 0.2 per cent (although we anticipate significantly better; table 2), a
factor of 10 less stringent than that already proven on the ground, for TSI
[51,52,53] and for spectral responsivity [16,17].
The full performance characteristics of CSAR, which is the heart of the
TRUTHS concept, is currently being tested on the ground for terrestrial
measurements of TSI. A fully working terrestrial CSAR has been manufactured
and used to make comparisons against the WSG [54]—the group of (ambienttemperature) electrical substitution radiometers that maintain the WRR [13].
The terrestrial CSAR was designed to fully meet the requirements of TRUTHS
and can largely be considered an engineering model of the space application
[54]. Only the TSI cavities are required for the WRR comparison, but the
instrument has been built to include also the high-sensitivity (SI traceability)
cavities as well. Further, the instrument has been built to space standards
(e.g. with space-compatible materials, with a design able to withstand launch
vibrations and accelerations). The instrument design specification is also based
around the relatively limited cooling power available from space refrigerators,
although terrestrially it is being used with a non-space cooler and so effort has
been made to degrade the effective performance of this cooler so that it can truly
replicate space operational conditions. On the ground it also has to operate in a
vacuum enclosure.
(c) Source of monochromatic radiation on TRUTHS
Monochromatic radiation is provided on TRUTHS by the SCM. This
instrument disperses solar radiation using (in our baseline concept) a grating.
We have chosen to use three monochromators, each optimized for different
spectral regions, stacked together with a common drive shaft. Spectrally dispersed
radiation from each grating (monochromator) is then coupled into an optical
fibre bundle, which, through arrangement of the fibres, converts radiation from
a rectangular slit into a circle that can then be imaged into a near-collimated
beam using a lens. This fibre can be translated along an optical axis without
significant change to its bending radius and thus throughput. The SCM together
with the optical fibre delivery system should be considered as a single integrated
instrument. The output of the optical fibre bundle is measured by the CSAR and
used to calibrate the other instruments.
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transmitted power (mW)
N. Fox et al.
40
30
20
10
0
1
2
wavelength (µm)
3
Figure 10. Spectral radiant power transmitted through SCM and fibre (through SCM in 0.5 nm
bandwidth). (Online version in colour.)
In the short time scale between measurement of the SCM by CSAR and its
use with another instrument (<60 s) there is no significant change in the output
of the Sun, so the power will be constant. However, as the mission progresses,
the throughput of the SCM, including the optical fibre, is likely to deteriorate,
but this degradation will be calibrated out every time it is used. The anticipated
signal of the SCM can be seen in figure 10.
(d) Transfer radiometers in standards laboratories
In NMIs, the cryogenic radiometer is used to calibrate the spectral responsivity
of a solid-state transfer detector, e.g. a silicon photodiode. Solid-state transfer
devices have a faster response than a cryogenic radiometer and can therefore
be used over a wider spectral range more quickly. Also, it is easy to add an
aperture (or apertures) to a transfer detector and therefore convert from power
responsivity to irradiance or radiance responsivity and they are highly portable.
On TRUTHS, the PTR will be used for the same reasons—a faster method of
achieving full spectral coverage, to convert from power to irradiance and radiance
responsivity, and to transfer from the solar viewing axis to the Earth viewing axis
of the satellite.
An ideal transfer standard should:
— be spectrally predictable (so that calibrations at a few wavelengths can
provide full spectral coverage),
— be stable between calibration and use,
— possess good linearity and uniformity, and
— be insensitive to geometrical changes (e.g. the addition of an aperture
should not change the power responsivity).
The most commonly used transfer standard (for the important spectral region
300–900 nm) is the silicon trap detector [55,56]. Over this spectral region, it was
shown [55,56] that the internal quantum efficiency of certain types of photodiode
is near unity and therefore the intrinsic responsivity is directly proportional
to wavelength. The reduction from unity for the external quantum efficiency
is almost entirely due to specular reflectance. Therefore, by arranging silicon
photodiodes in a ‘trap’ configuration (where the light specularly reflected from
one photodiode reaches the next) and combining the output currents, a device
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Accurate radiometry from space
4055
with predictable spectral responsivity directly proportional to the wavelength can
be obtained. The spectral responsivity of trap detectors in the spectrally linear
region can be predicted without calibration to within 0.05 per cent, and this
performance (across the full spectral range) can be improved to 0.01 per cent by
calibration against a cryogenic radiometer at a handful of wavelengths [55].
In the IR region, photodiodes have not performed well in a trap
configuration [57] (radiometric uncertainties approx. 0.3%), primarily due to poor
(low) shunt resistance, small size and spatial non-uniformity of their internal
quantum efficiency. Therefore, alternative detectors have been developed using
integrating spheres [58–60].
With a silicon trap detector, only a very small number of calibration
wavelengths are required, as the relative spectral responsivity is predictable. With
sphere detectors, more calibration wavelengths are needed to account for the
potential spectral reflectance changes of the sphere. However, with a smoothly
varying sphere reflectance and smoothly varying spectral responsivity of the
detector (as in the case with the TRUTHS PTR), interpolations can still be
performed between relatively widely spaced spectral calibrations.
(e) Filter radiometers in standards laboratories
Cryogenic radiometers are, by requirement and design, ‘black’ (they absorb
radiation in a spectrally unselective manner). Solid-state transfer detectors, while
not ‘black’, also respond over a wide spectral region. In order to obtain spectral
information, it is necessary to introduce wavelength selectivity. By combining
photodiodes with spectrally selective filters (and apertures: one, for irradiance,
or two, for radiance), a device known as an FR is created. FRs are key instruments
at any NMI in the measurement of spectral radiance or irradiance. FRs can
have broadband filters (for example, to mimic the human eye response curve—a
photometer) or narrowband filters. The latter are already in common usage as
the basis for many space imagers.
A set of FRs with different central wavelengths and appropriately narrow
bandwidths (approx. 10–20 nm) can be used to measure the spectral radiance or
irradiance of a smoothly varying source of optical radiation directly. This is used
to realize the primary spectral irradiance scale of several NMIs [41–43], where,
for example, the irradiance of a tungsten lamp is determined at the wavelengths
of the interference filters and then interpolated in the intermediate region using
an appropriate smoothly varying model of the lamp’s spectral irradiance.
Other NMIs, including NPL, Physikalisch-Technische Bundesanstalt (PTB)
and the National Institute of Standards and Technology (NIST), use FRs to
determine the thermodynamic temperature of a blackbody as the basis of their
(more accurate) spectral irradiance scales [61]. Because the blackbody’s spectral
radiance can be entirely described by its temperature (through Planck’s law), a
single FR reading is sufficient to determine the entire spectrum. In the early 1990s,
NPL pioneered the absolute radiometric measurement of blackbody temperature
(filter radiometry) with measurements of the transition temperature of fixedpoint blackbodies (the freezing temperature of metals at around 1000◦ C) and
measured the radiance of these blackbodies with uncertainties of 0.06 per cent
(k = 2) [62]. Today, the radiometry and thermometry communities are using the
same techniques to calibrate novel high-temperature fixed points as references for
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N. Fox et al.
trap
detector
apertures
filter
radiometer
filter
radiometer
filter
radiometer
trap
detector
integrating
sphere
translation stage
fibre-optic
cable
laser
stabilizer
fibre
coupler
ultrasonic
bath
tunable laser source
Figure 11. Schematic of NPL’s FR calibration facility. (Online version in colour.)
high temperature measurement (approx. 3000◦ C). Radiometric uncertainties of
0.1 per cent (95% confidence) in the measurement of blackbody radiance are now
relatively straightforward [63,64].
FRs can be calibrated in power mode (under-filled), in irradiance mode (with
a single aperture over-filled) or in radiance mode (imaging within a large uniform
source). In all cases this is done with a monochromatic source. For broadband
FRs, such a source can be obtained using a lamp-illuminated monochromator. For
higher stability, stronger signal and better wavelength definition, a laser-based
source can be used (figure 11). Such a source can be obtained by illuminating
a small integrating sphere with a tunable laser, fed through a vibrating fibre to
remove speckle [40,65]. Traceability to SI comes via the trap detector, which is
itself calibrated for spectral power responsivity against the cryogenic radiometer.
An aperture is added to the trap to convert this from power responsivity to
irradiance responsivity.
Pre-flight calibration of any instrument, including those intended for space,
can also be carried out using this approach.
(f ) Transfer radiometers on TRUTHS
The TRUTHS PTR (figure 12) combines the functions of the transfer
radiometer and FR used at NMIs to transfer traceability from a primary standard
to a measurement instrument. In essence, we describe here a PTR that consists
of an integrating sphere to ensure uniform and spatially insensitive collection
of optical radiation from a range of sources, and a small group of solid-state
detectors, some spectrally filtered, to defined bandpasses that can view the
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Accurate radiometry from space
FoV defining
apertures
Glan-Thompson
polarizer
photodiodes
integrating
spheres
Figure 12. Conceptual design of PTR. (Online version in colour.)
radiance of the internal sphere walls. The PTR (figure 12) is mounted on a
simple rotation device, which can rotate the PTR so that it views towards the
Sun or Earth viewing axis, thus providing traceability from the CSAR to the EI
(figure 5).
When viewing a small (point) source, the instrument acts as an irradiance
mode sphere-based transfer detector. The silicon (200–1100 nm) and InGaAs
(1000–2500 nm) detectors provide full spectral coverage as intermediate reference
detectors to allow detailed spectral interpolation between more widely spaced
calibration points made with the CSAR. The instrument (entirely under-filled in
power responsivity mode and with the internal aperture over-filled in irradiance
responsivity mode) provides the fast, spectrally smooth, portable detector needed
to transfer traceability from the CSAR to the other instruments.
The PTR (figure 12) consists of a housing of two apertures—a polarizing beam
splitter and two spheres (each illuminated by a different polarization state). The
detectors are mounted on a thermo-electrically cooled housing block that sits
on the sphere and views the sphere wall, well away from the input port or the
first reflection. The Si and InGaAs detectors have smoothly varying spectral
responsivities and the reflectance of the sphere (and transmittance of the beam
splitter) is also smoothly varying spectrally. This means that the power (underfilled) responsivity of the instrument can be calibrated at a few wavelengths
against the cryogenic radiometer using the SCM. For calibration transfer, the
instrument can also be calibrated in irradiance mode by over-filling the second
aperture. These solid-state reference detectors are then used to transfer the
calibration to the SSIM through a similar process, but in this case between the
PTR and the SSIM.
The PTR is also an FR. In addition to the unfiltered solid-state detectors,
a number of spectrally filtered detectors are also mounted in the temperaturecontrolled block. In this case, the PTR can effectively use the SCM to
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N. Fox et al.
self-calibrate between the reference detectors and FRs, as both see the same
radiation. The SCM only needs to spectrally tune over the bandwidth of each FR
and its response compared with that of the reference photodiode.
The PTR can then rotate between the solar axis (its self-calibration mode)
and the Earth viewing axis; see figure 5, where it can view sunlight diffusely
reflected from a diffuser plate, which can be deployed in front of the EI. The
PTR effectively measures the spectral radiance of the diffuser, which is then
subsequently used as a ‘standard source’ to calibrate the EI.
This Lambertian diffuser, typically made from Spectralon, will be calibrated
for full spectral reflectance during the pre-launch calibration. As Spectralon has
high and nearly spectrally flat reflectance properties, and any degradation will
not exhibit sharp spectral features, it is not necessary to measure the radiance
with high spectral resolution. Therefore, measurements in a few spectral bands
will be sufficient.
The PTR can also view the Earth or Moon simultaneously with the EI
as alternate transfer sources, for both redundancy and integrity checks. The
polarized nature of the instrument and choice of the spectral filters allows the
PTR also to be deployed in an off-nadir view, pointing (approximately 50◦ ) to
the fore of the imager during normal observation mode. This provides essential
additional information to aid in the determination of aerosol optical depth and
atmospheric correction.
The mission baseline has two identical PTRs (each on a separate rotation
arm) partly for redundancy and also to provide additional spatial information on
the diffuser for the characterization of the EI. The SSIM can also view targets
such as the Moon and can be used to provide another internal cross-check on
calibration.
Figure 13 shows a block diagram of the traceability chain for TRUTHS
in terms of its measurement of Earth spectral radiance. Note that in this
paper we have not described all the steps in any great detail, e.g. wavelength
calibration.
5. Summary
Climate change is the most critical issue facing mankind today. The enormous
cost implications of policy decisions based on forecasted impacts resulting from
the predictions of a warming Earth demand that the science community finds
and delivers the necessary information with the highest possible confidence in
the shortest possible time. The challenge to the metrology community is equally
severe.
The IPCC [1] concludes that the mix of natural variability and anthropogenic
effects on decadal time scales is far from fully understood or measured, requiring
significant improvements in accuracy. Unequivocal attribution and quantification
of subtle fingerprint indicators from this noisy background are fundamental to
our ability to predict climate reliably and use appropriate mitigation/adaptation
strategies. The uncertainty in climate prediction lies in the complexity of the
models, our inadequate understanding of the Earth system and its feedback
mechanisms, and the relatively poor quality of available data against which to
test predictions on the necessary decadal time scales.
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4059
Accurate radiometry from space
radiance from Earth imager
W m–2 sr–1 nm–1
0.3%
radiance
calibration
0.3%
solar spectral
irradiance
0.2%
diffuser
spectral
reflectance
0.2%
in-orbit
calibration by
SSIM
pre-flight
calibration
full spectral
0.2%
0.2%
wavelength
accuracy
0.01%
radiance
geometry m2 sr
from PTR
apertures
<0.1%
in-orbit
calibration at 13
spectral
bandwidths
0.3%
pre-flight
calibration
in-orbit
check
internal
aperture
absorption lines
in
Sun/atmosphere
pre-flight
calibration
<0.06%
PTR FRs
in-orbit
calibration
Figure 13. Traceability chain for Earth spectral radiance, showing directly SI-traceable steps in
deep shade and expected uncertainties (at the k = 2, or 95% confidence level). (Online version in
colour.)
Establishing rigorous SI traceability to the key measurands underpinning
the ECVs, with sufficient accuracy, is a central pillar to achieving this
goal. The satellite community has over the years developed a number of
strategies to support this objective, but as yet all fall dramatically short of
the required accuracy, forcing the adoption of high-risk philosophies seeking to
monitor ‘change’ through normalization of overlapping datasets. While pre-flight
calibrations can be made traceable, the harsh environment of space following the
shock of launch means that few, if any, of the radiometric ground calibrations can
be relied upon in space. Efforts to recover some of the information can be carried
out using a variety of post-launch methods, but none have sufficient accuracy to
meet the needs of climate.
This paper has described the only real solution available to the EO community,
i.e. to establish high-accuracy SI traceability in flight, on-board the spacecraft.
One mission, TRUTHS, which is designed to achieve this for the solar-reflective
domain, has been described in some detail. Its sister CLARREO complements
this in the solar-reflective domain and extends the capability into the IR spectral
region.
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N. Fox et al.
TRUTHS would become ‘a standards laboratory in space’ providing directly
traceable measurements of unprecedented accuracy (factor of 10) in SI units, in
orbit, through the deployment of a primary radiometric standard and associated
calibration methodology. This unprecedented accuracy will establish benchmark
measurements for the detection of decadal change in the most sensitive (but
least understood) climate radiative forcings, responses and feedbacks in the solarreflective domain.
With spectrometer resolution sensors of unprecedented accuracy, TRUTHS
can observe climate-relevant processes related to the atmosphere, the oceans and
the land surface. TRUTHS’ observations will test and advance the development
of climate models, allowing more accurate climate change hindcasting and
forecasting. In addition, TRUTHS’ measurements of ground-based reference sites
(and the Moon) will allow retrospective improvements to the calibration of
existing satellites and their data.
TRUTHS/CLARREO will provide an in-orbit standard for reference intercalibration for other EO satellite instruments (with benefits for both science
applications and operational services). The TRUTHS and CLARREO missions
are planned to fly in an orbit that allows frequent ‘cross-overs’ with other
in-flight sensors. Together with their own pointing capabilities, and ability to
match spectral and spatial resolution, the high calibration accuracy and SI
traceability will transfer to other sensors through simultaneous observations of a
target.
SI-traceable observations at climate change accuracy from different
instruments can then be objectively linked to a common baseline time series
even if no sensor-to-sensor overlaps are available. Had an in-orbit calibration
observatory similar to TRUTHS/CLARREO already flown, this benefit would
exist now. Delaying the installation of such a system reduces the value of the
climate records and creates the constant risk of data records being lost completely
should an overlap be lost. The full value of TRUTHS/CLARREO will be gained
in the future, when a series of small platform in-orbit calibration instruments
anchor EO time series, providing the highest possible confidence in any observed
trends and at relatively low cost.
The realization of SI-traceable measurements in orbit is achievable now
and time critical. This ‘grand challenge’ project between the metrology and
Earth/climate science community must become a priority if society is to reap
the commercial benefits of EO, but more fundamentally, without it, policy-makers
will be acting blindly in their efforts to ensure long-term sustainability and growth
within an ever-changing climate.
The authors would like to acknowledge the support and contributions of the following individuals
(alphabetical by host institute) and organizations to the TRUTHS mission concept, which is in
part summarized by this paper: T. Quinn FRS (BIPM), G. Myhre (CICERO, Norway), P. Henry
(CNES, France), R. Bantges, R. Brindley, J. Haigh and J. Russell (Imperial College London,
UK), M. Verstraate and J.-L. Widlowski (JRC-Ispra, EC), J.-P. Muller (MSSL, UK), A. Shaw
(NCEO, UK), X. Briottet (ONERA, France), S. Groom (Plymouth Marine Laboratory, UK), S.
Mackin (SSTL, UK), R. Saunders (UK Meteorological Office), P. Teillet (University of Lethbridge,
Canada), R. Allen and K. Shine FRS (University of Reading, UK), M. Schaepman (University
of Zurich, Switzerland), G. Stensaas and T. Stone (USGS, USA), EADS Astrium UK, OIP
Belgium, Serco, Italy, SSTL UK and STFC (RAL) UK. In addition, Nigel Fox would like to
thank the National Measurement Office of the BIS Department of the UK for financial support to
this work.
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