U. Hugentobler

The accurate localization of gamma-ray bursts (GRBs) remains a crucial task. Historically, improved localizations have led to the discovery of afterglow emission and the realization of their cosmological distribution via redshift measurements; however, a more recent requirement comes with the potential of studying the kilonovae of neutron star mergers. Gravitational wave detectors are expected to provide locations to not better than 10 square degrees over the next decade. With their increasing horizon for merger detections the intensity of the gamma-ray and kilonova emission also drops, making their identification in large error boxes a challenge. Thus, a localization via the gamma-ray emission seems to be the best chance to mitigate this problem. Here we propose to equip some of the second-generation Galileo satellites with dedicated GRB detectors. This saves costs for launches and satellites for a dedicated GRB network, the large orbital radius is beneficial for triangulation, and...

In the framework of the ESA GSTP study “Accurate Orbit Determination of Space Debris with Laser Tracking / Tasking” both two-color and multi-static laser tracking data was acquired. We validate these observations and assess orbits computed on their basis. In this context, we deliver insight into the technology’s accuracy and precision by comparing observed versus computed measurements from reference orbits and by assessing their high-frequency components with regard to object geometries. Moreover, based on post-fit and cross-validation residuals of our orbit products we validate the overall processing procedure which comprises data filtering, estimating force model coefficients from Two-LineElements (TLEs), as well as the actual orbit determination process. Eventually, further comparisons with external orbit data support our statements about the validity and quality of our data and orbit products, respectively.

ABSTRACT At the altitude of the CPS satellites the most important non-gravitational perturbation is caused by the solar radiation pressure acting on the satellite body and its solar panels. The development of high-fidelity radiation pressure models may be motivated by the following observation: The GPS satellites are orbiting in a 2:1-commensurability with the Earth's rotation which causes resonance. The expected sensitivity to specific coefficients of the geopotential is, however, significantly reduced by strong correlations of these parameters with radiation pressure parameters. Sophisticated radiation pressure models rely on a precise knowledge of the satellite's attitude which does not only affects the location of the antenna phase center or the phase windup of the signal carrier but, through radiation pressure, also the orbital dynamics. PRN 23, whose attitudinal behaviour was modified early in 2002 is an interesting case. Due to this change an impressive improvement in the orbit quality could be achieved.

The time transfer techniques used to generate TAI are currently the TWSTFT (TW), GPS C/A, and GPS P3. About 19% of all TAI laboratories possess P3, 12% possess TW backed up with P3, i.e., in total only 31% of the TAI laboratories transfer 100% of the primary frequency standards (PFS) including all the Cs fountains and 80% of the total weighted clocks for TAI. GPS carrier phase (CP) data are co-products of P3 receivers and they are never used in TAI. An effective strategy to improve TAI is to strengthen these TW+P3 links by using the already available CP information. We first review the classical TAI time transfer techniques and solutions of different analysis strategies of the CP data, namely the IGS [1], the AIUB [2], and the NRCan PPP [3]. To evaluate all the different time transfer techniques, we carried out an extensive comparison between the above techniques, over different periods from hours to 4 months and over different distances on inner- resp. inter-continental baselines b...

The altimetry satellite JASON-1 carries several independent instruments for precise orbit determination such as a DORIS receiver, a GPS receiver, and an SLR reflector. DORIS and GPS are both based on microwave phase measurements and not fundamentally different. A new analysis technique for processing DORIS observations has been developed which follows a GPS-like approach. This new method is used to

ABSTRACT QZS-1, the first satellite of the Japanese Quasi Zenith Satellite System (QZSS) was launched in September 2010. Transmission of the standard codes started in December 2010 and the satellite was declared healthy in June 2011. Five stations of the COoperative Network for GIOVE Observation (CONGO) were upgraded to provide QZSS tracking capability. These five stations provide the basis for the precise orbit determination (POD) of the QZS-1 spacecraft. The stability and consistency of different orbital arc lengths is analyzed based on orbit fit residuals, day boundary discontinuities, and Satellite Laser Ranging residuals. As QZS-1 simultaneously transmits navigation signals on three frequencies in the L1, L2, and L5 band, different ionosphere-free linear combinations can be formed. The differences of the orbits computed from these different observables (ionosphere-free linear combination of L1/L2 and L1/L5) as well as the stability of the differential code biases estimated within the POD are studied. Finally, results of the attitude determination based on the navigation signal transmission from two different antennas onboard QZS-1 are presented.

Troposphere zenith delays estimated from observations of the Global Positioning System (GPS) provide valuable information about the neutral atmosphere. On the other hand, troposphere long time series of operational GPS analyses are degraded by model changes, in particular as regards the tropospheric mapping function, hydrostatic a priori delays, the elevation cut-off angle and the reference frame used for datum definition.

The two GRACE satellites provide a unique platform to study precise orbit determination strategies for constellations of low Earth orbiters (LEOs) using GPS and inter-satellite K-band data. We apply pseudo-stochastic orbit modeling techniques to the two GRACE satellites to derive normal equation contributions from the K-band measurements and from orbit positions previously established for both satellites with GPS carrier phase measurements. The three contributions are combined on the normal equation level using appropriate weighting. Special attention is paid to the numerical stability of the procedure due to the very different accuracies of the GPS carrier phase and the ultra-precise K-band measurements. We use GRACE data for the year 2003 to study the mutual benefits for precise orbit determination of a LEO constellation based on GPS and K-band data. We discuss the differences to GPS-only orbit solutions and show that precisions of a few tens of micrometers in the relative orbit p...

Precise and high precision ionosphere models are important for modern satellite navigation and positioning systems. In most cases, the ionosphere models are based on pure mathematical approaches, e.g. by applying spherical harmonic expansions for the vertical total electron content. In order to achieve a deeper understanding of the complex phenomena within the ionosphere, physical conditions have to be considered and introduced. The physics-motivated Chapman function is very efficient for describing the vertical structure of the electron density. Introducing the Chapman function and a plasmasphere layer, the vertical distribution of the electron density can be described by five parameters altogether, namely (1) the F2 peak electron density (NmF2), (2) the peak height (hmF2), (3) the topside scale height (HF2), (4) the plasmasphere basic density (NP) and (5) the scale height (HP). In our approach, each of these parameters is decomposed into an initial part, derived from a given ionos...

We propose to deploy small (2-3 kg) Lunar geodesy packages on the Moon's surface, consisting of an optical Laser receiver, a small retroreflector, as well as a radio beacon. The optical receiver will maintain Earth pointing through the Lunar libration cycles and record arrival times of Laser shots from Earth. Judging from the photon budget for a 50 mJ pulse

ABSTRACT The motion and the rigidity of the Nubian plate provides a critical constraint to the geodynamics of the surrounding plates. Unfortunately, the sparse distribution of geodetic continuous GNSS stations across the plate does not provide a high precision solution for an Eulerian pole and to test statistically for the rigidity. The presence of 3 separate cratons, Rift Valleys, and old deformation belts along the cratons' sutures, indicate that in geological time the plates have not been completely rigid. The amount of current deformation is very difficult to derive from the GPS observations due to the sparse distribution of receivers within the rigid plate. Using IGS sites the deformation has been constrained to be smaller than a few mm/yr. Here, we present the velocity field for 42 stations of the continuous GPS network TrigNet, a network covering South Africa with an average distance of 200 km. We present the velocity field for the period 2002-2008, the strain rates between individual TrigNet sites, and the relative Eulerian pole and corresponding residuals assuming rigid motion of the network. The distribution of these stations on the stable part of the Kalahari craton, allows computing a pole of rotation that can be compared with the rest of the stations within the Nubian plate. Preliminary results show that the entire network behaves as a rigid block with negligible average residual. Exceptions to this rigid motion are some of the coastal stations, mining areas around Johannesburg and the Northeast portion of the TrigNet network. The relative motion of the TrigNet network with respect to the African plate is investigated using the Eulerian pole that minimizes the residual for the IGS stations on the Nubian plate and shows that the average residuals are well within the errors indicating that the Nubian plate behaves as a rigid block within our resolution. On the other hand, the analysis of the strain within the TrigNet network and the non-random distribution of the azimuth of the residual with respect to the motion described by a TrigNet only Eulerian pole, does not exclude a possible counter clock wise rotation of the craton with respect to the Nubian plate and an influence of the Nubia/Somalia plate boundary for the Northeastern region of South Africa that at this stage still cannot be distinguished from the noise.

ABSTRACT The F2 layer of the ionosphere plays an essential role in radio communication and positioning applications as well as in space weather research. In particular, the characteristics of the F2 layer are defined by the F2 peak density , the F2 peak height as well as the scale height . The International Reference Ionosphere (IRI) is the internationally recognized and recommended standard for predicting these critical parameters. However, IRI provides median monthly values of these parameters based on the International Radio Consultative Committee (CCIR) or the International Union of Radio Science (URSI) models, which were developed from data of the worldwide network of ionosondes during the years 1954 to 1958. These models provide monthly averages, and therefore, they are required to be updated with up-to-date measurements to get more accurate predictions. In this contribution, we provide a procedure to improve the parameters and from modern space geodetic measurements, which can serve to update IRI. Specifically, we model these key parameters spatio-temporally by tensor product of B-spline expansions, and estimate the model coefficients using two types of GNSS data, namely, ground-based two-frequency GPS observations of total electron content and electron density profiles retrieved from ionospheric GPS radio occultation measurements acquired by the FORMOSAT-3/COSMIC mission. In this manner, the solution of the model parameters benefits from different sensitivities as well as from different spatio-temporal resolutions of the two observation techniques. The model is applied exemplarily over a South American region on three selected days under high solar activity (1 July 2012), moderate solar activity (16 July 2011) and low solar activity (16 July 2008), respectively. A comparison of our results with ionosonde data, a comparison of vertical total electron content (VTEC) values and a cross validation show the strengths of our approach and the potential to update the IRI model.

ABSTRACT GPS position time series provide an outstanding tool to measure crustal motion and deformation. The availability of longer time series and the enhanced data quality allow more and more to analyse time dependent effects in continental deforming zones. These analyses may provide insights to seismic cycle processes and allow to drawing conclusions on the rheology of the crust and the underlying asthenosphere. However, the time series are subject to many different kind of (time correlated) noise, whose various sources like clock and orbit errors, ionospheric and tropospheric effects can hardly be quantified properly. The effect of time correlated noise on the time series can be similar to the one caused by transients posing the problem of separating the signal from the noise. We developed an algorithm based on the Allan Variance that is capable of classifying time correlated noise and used it to calculate the covariance of the rate along with the spectral indices for many different azimuth directions. Unlike noise, transient signals usually occur in some preferred direction. Based on this assumption, we are able to identify sites in North-western America and Costa Rica that are affected by transients and to estimate the quality of models accounting for transients.

The satellites of the Global Navigation Satellite Systems (GNSS) are orbiting the Earth according to the laws of the celestial mechanics subject to gravitational and non-gravitational forces. As a consequence, the satellites are sensitive to the instantaneous center of the mass of the Earth. The coordinates of the (ground) tracking stations are referring to the center of figure as the conventional origin of the reference frame, which is supposed to be the long-term mean location of the center of mass. The difference between the center of mass and the center of figure is the instantaneous geocenter. Current GNSS-based determinations for the X and Y coordinates of the instantaneous geocenter (in ITRF) seem to be competitive with those from the primary geocenter supplying technique (SLR). For the Z component, however, strong correlations with parameters from orbit modeling are present. Due to this circumstance, the evaluation of Z component geocenter results may be used for assessment ...

ABSTRACT The development of techniques for the comparison of distant clocks and for the distribution of stable and accurate time scales has important applications in metrology and fundamental physics studies. Additionally, the rapid progress of frequency standards in the optical domain is presently demanding additional efforts for improving the performances of existing time and frequency transfer links. Present clock comparison systems in the microwave domain are based on GPS and TWSTFT (Two-Way Satellite Time and Frequency Transfer). ELT (European Laser Timing) is an optical link presently under study in the frame of the ESA mission ldquoAtomic Clock Ensemble in Spacerdquo. The on-board hardware consists of a corner cube retro-reflector (CCR), a single-photon avalanche diode (SPAD), and an event timer board connected to the ACES time scale. Light pulses fired towards ACES by a laser ranging station will be detected by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse towards the ground station providing precise ranging information. This paper will present the ELT scientific objectives, the recent studies performed on the ELT hardware, and the dedicated test campaign carried out at the Wettzell laser ranging station to demonstrate the experiment feasibility. Recent test results will be also discussed.

As a prototype for the satellites of the future European Global Navigation Satellite System (GNSS) Galileo, the European Space Agency (ESA) launched two satellites (GIOVE-A and GIOVE-B) as part of the Galileo in Orbit Validation Element (GIOVE). To gain experience with the signals transmitted by these satellites and to estimate satellite orbit and clock parameters, a global network of GIOVE-capable

Satellite based geodetic techniques - above all GPS - provide an outstanding tool to measure crustal motions. They are widely used to derive geodetic velocity models that are applied in geodynamics to determine rotations of tectonic blocks, to localize active geological features, and to estimate rheological properties of the crust and the underlying asthenosphere. However, it is not a trivial task to derive GPS velocities and their uncertainties from positioning time series. In general time series are assumed to be represented by linear models (sometimes offsets, annual, and semi-annual signals are included) and noise. It has been shown that error models accounting only for white noise tend to underestimate the uncertainties of rates derived from long time series and that different colored noise components (flicker noise, random walk, etc.) need to be considered. However, a thorough error analysis including power spectra analyses and maximum likelihood estimates is computationally e...

ABSTRACT The first two Galileo In-Orbit Validation satellites were launched in October 2011 and started continuous signal transmission on all frequencies in early 2012. Both satellites are equipped with two different types of clocks, namely rubidium clocks and hydrogen masers. Based on two test periods, the quality of the Galileo orbit determination based on Global Navigation Satellite System (GNSS) and Satellite Laser Ranging (SLR) observations is assessed. The estimated satellite clock parameters are used as quality indicator for the orbits: A bump at orbital periods in the Allan deviation indicates systematic errors in the GNSS-only orbit determination. These errors almost vanish if SLR observations are considered in addition. As the internal consistency is degraded by the combination, the offset of the SLR reflector is shifted by +5 cm, resulting in an improved orbit consistency as well as accuracy. Another approach to reduce the systematic errors of the GNSS-only orbit determination employs constraints for the clock estimates with respect to a linear model. In general, one decimeter orbit accuracy could be achieved.

... improvement at CODE R. Dach, C. Urschl, M. Ploner rolf.dach@aiub.unibe.ch Astronomical Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern S. Schaer Federal Office of Topography (swisstopo), Seftigenstrasse 264, CH-3084 Wabern U. Hugentobler ...

• Since the beginning of May 2003, the CODE AC has been computing a rapid orbit product for both the GPS and the GLONASS satellite constellation.• GPS and GLONASS orbits are generated at the same time in a rigorous GNSS analysis, ensuring best possible ...

Optical detection of space debris is a technical challenge from the point of view of the observations as well as of the data processing. The most promising technique for ground based optical observations consists of using CCD detectors in combination with specially tuned digital image processing techniques.We present observation scenarios and image processing algorithms allowing to recognize the faint trailed