Bau Geo Umwelt

Geodesy and Remote Sensing, Germany (email : [email protected]) GRACE – OBP Comparisons GRACE and OBP monthly variations expressed in equivalent water height are compared for the period 2003-2008. The GRACE observables are brought to the same reference frame as the OBP (CF) by applying a correction for the geocenter motion [5] . The similarity criterions are the mean correlations and absolute amplitude differences, both weighted epoch-wise. A criterion for selecting reliable OBP stations for the comparison is the SNR. serves to select the stations with a strong geophysical signal that can be detected by GRACE. A common set of stations for each solution with SNR > 1 was selected for a more rigorous comparison. GRACE monthly variations expressed in vertical displacements are compared with selected GPS time series reprocessed (CODE) and brought to the reference frame of GRACE (CM) by removing the frame translations of (CF) and (CM). The similarity criterions are the mean cor...

Since GPS week 1052, 5 March 2000, the IGS is producing a new combined orbit called the IGS Ultra rapid product, IGU. The combined IGS Ultra rapid products are being made available twice every day, at 3:00 and 15:00 UTC, with a delay of 3 hours after the end of the included data interval, and are based on solutions from up to seven different IGS Analysis Centers. The main reason for the generation of the Ultra rapid products are the requirements, in both timeliness and accuracy, for near-real-time atmospheric monitoring, e.g., weather predictions. Each ultra rapid orbit file covers 48 hours. The first 24 hours of the orbit are based on actual GPS observations (real orbit), the second 24 hours are extrapolated (predicted orbit). Like the IGS Predicted (IGP) orbits, the Ultra rapid orbits are available for real-time usage. However, the quality of the Ultra rapid orbits should be significantly better because the average age of the predictions is reduced from 36 hours (IGP) to 9 hours (IGU). When the quality of the IGU products reaches a satisfactory level the IGU products will replace the IGP products.

Rigidity and continuity of the Nubia plate is a fundamental assumption for the kinematic description, the dynamic implications of its interaction with surrounding plates and ultimately an important constraint to the geodynamics processes involved in continental lithospheric rupture. Geophysical, neotectonic and geodynamics considerations suggest the possibility that the Nubia plate is not completely rigid but could be undergoing internal deformation due to the southward propagation of the East African Rift. Here, we utilize the South African TrigNet geodetic network to evaluate the amount of internal deformation within the South African region and the possibility of motion between South Africa and the rest of the African continent. Our results show that the South African region behaves rigidly, with deformation of the order of 1 nanostrain yr −1 or less. The analysis shows some higher strain rates in the eastern region, and the presence of spatially correlated residuals in the Cape Town region and the region east of Johannesburg. Although not statistically significant, the spatial coherence of those residuals could indicate tectonic activity. A comparison of the Euler vector for the South African region with previously published Euler poles for the Nubia plate as well as the analysis of the residuals of Nubia sites with respect to a 'rigid' TrigNet are compatible with clockwise rotation of the South African block with respect to the African continent, consistent with a propagation of the East Africa Rift along the Okavango region.

Gravity missions are equipped with onboard Global Positioning System (GPS) receivers for precise orbit determination (POD) and for the extraction of the long wavelength part of the Earth's gravity field. As positions of low Earth orbiters (LEOs) may be determined from GPS measurements at each observation epoch by geometric means only, it is attractive to derive such kinematic positions in a first step and to use them in a second step as pseudo-observations for gravity field determination. The drawback of not directly using the original GPS measurements is, however, that kinematic positions are correlated due to the ambiguities in the GPS carrier phase observations, which in principle requires covariance information be taken into account. We use GRACE data to show that dynamic or reduced-dynamic orbit parameters are not optimally reconstructed from kinematic positions when only taking epoch-wise covariance information into account, but that essentially the same orbit quality can be achieved as when directly using the GPS measurements, if correlations in time are taken into account over sufficiently long intervals. For orbit reconstruction covariances have to be considered up to one revolution period to avoid ambiguity-induced variations of kinematic positions being erroneously interpreted as orbital variations. For gravity field recovery the advantage is, however, not very pronounced.

Apart from the gradiometer as the core instrument, the first ESA Earth Explorer Core Mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) will carry a 12-channel GPS receiver dedicated for precise orbit determination (POD) of the satellite. The EGG-C (European GOCE Gravity-Consortium), led by the Technical University in Munich, is building the GOCE HPF (High-level Processing Facility) dedicated to the Level 1b to Level 2 data processing. One of the tasks of this facility is the computation of the Precise Science Orbit (PSO) for GOCE. The PSO includes a reduced-dynamic and a kinematic orbit solution. The baseline for the PSO is a zero-difference procedure using GPS satellite orbits, clocks, and Earth Rotation Parameters (ERPs) from CODE (Center for Orbit Determination in Europe), one of the IGS (International GNSS Service) Analysis Centers. The scheme for reduced-dynamic and kinematic orbit determination is based on experiences gained from CHAMP and GRACE POD and is realized in one processing flow. Particular emphasis is put on maximum consistency in the analysis of day boundary overlapping orbital arcs, as well as on the higher data sampling rate with respect to CHAMP and GRACE and on differences originating from different GPS antenna configurations. We focus on the description of the procedure used for the two different orbit determinations and on the validation of the procedure using real data from the two GRACE satellites as well as simulated GOCE data.

Ground based observations of orbital debris are predominantly performed by either radar or optical techniques. It has been demonstrated that radar is best suited for routine surveys of low Earth orbits (LEO). Optical observations, on the other hand, are better than radar for objects at high altitudes, particularly in the geostationary ring and the geostationary transfer regions. The new Zimmerwald 1 m telescope has been designed as a multipurpose instrument with special emphasis on debris observation capability (the telescope is used for satellite laser ranging during daytime). The detector system consists of a 2k x 2k four section CCD with fast readout electronics. Due to detector size and short exposure times of a few seconds the debris observation system produces large data volumes (more than 1 MRyte/sec). Therefore efficient algorithms for automated detection of faint moving objects on CCD frames were developed and integrated into an on-line processing scheme.

The satellites of geostationary navigation overlay systems such as EGNOS (European Geostationary Navigation Overlay System) are equipped with single-frequency microwave transponders. The tracking data contain a GPS-like signal corresponding to the GPS C/A-code (Clear Access code) in the GPS L 1-band of the electromagnetic spectrum. This signal is tracked by (some of the) commercially available GPS receivers and may be used for orbit determination and for the estimation of EGNOS clock corrections. GPS and EGNOS observations, acquired by eight combined GPS/EGNOS receivers in a time interval of five days in spring 2004, were used (a) to precisely position the GPS/EGNOS tracking sites (using the conventional GPS observables) and (b) to determine the EGNOS orbits and clock corrections using the EGNOS C/A-code observable. The resulting orbit determination scheme proved to be robust and accurate-in particular in view of the fact that only L 1 C/A-code was analyzed. The quality of the procedure, which might be easily transformed into a routine EGNOS orbit determination scheme, is discussed in our article. The EGNOS satellites can also be observed with optical telescopes (situated in the appropriate geographical longitude sector). The observed astrometric places may be used to determine EGNOS orbits, as well. In spring 2004 an observation campaign using the 1-m telescope in Zimmerwald (near Bern in Switzerland) was organized to acquire observations for two EGNOS satellites. Based on these optical observations orbits were determined for one of the EGNOS spacecrafts. The quality of this complementary and independent orbit determination procedure is discussed, as well. Accurate EGNOS orbits may thus be derived with two independent methods. For the current study, however, no observations of the two types could be made available and orbits resulting from the two methods could not be compared directly. This step, as well as a combined processing of the two observation types, is planned for the future.

Reduced-dynamic orbit determination for low Earth orbiting satellites is usually performed to obtain trajectories of highest accuracy without making use of additional accelerometer data. We use efficient stochastic orbit modeling techniques based on least squares estimation to compare estimated piecewise constant accelerations of the CHAMP satellite with measurements from the STAR accelerometer to have an alternative orbit quality assessment. Stimulated by such comparisons, simulated CHAMP data are used to discuss the mutual benefits of a statistically correct combination of GPS phase and accelerometer measurements with accelerometer data treated as additional observations.

Satellite Laser Ranging (SLR) observations allow for a completely independent validation of orbits derived using microwave measurements as provided by the International GPS Service (IGS). The orbit validation is based on the difference between the observed range (SLR measurements) and the computed range (assuming that the satellite positions are know from the IGS). The resulting range residuals are primarily an indicator for the radial accuracy of the microwave orbits. This validation method is well-known and several validation results have been published in the last 10 years. The IGS analysis center CODE (Center for Orbit Determination in Europe) provides on a routine base daily range residuals for all GNSS satellites observed by both, SLR and microwave techniques. We present results of our recent range residual analysis for two GPS satellites (PRN G05, G06) and three GLONASS satellites (PRN R03, R22, R24). Microwave orbits provided by IGS and CODE are used as well as SLR normal point observations of 13 globally distributed sites. The resulting range residual time series of about 3.3 years, starting in January 2001, for GPS satellites and of about 10 months, starting in June 2003, for GLONASS satellites are analyzed. The comparison of the range residuals with previous analysis results shows the impact of several improvements, i.e., the increasing quality of the microwave orbit as well as the use of new laser retroreflector arrays on GLONASS satellites launched after 1995. The validation of the CODE orbits shows a standard deviation of 2.7 cm for the GPS satellites and of 4.9 cm for the GLONASS satellites. A bias of about À5.8 cm for GPS satellites and of À2.3 cm for GLONASS satellites between the SLR measurements and the microwave orbits was determined. The cause of these biases is not yet fully understood. Pass-specific systematics in the range residuals were found, but it was not possible to assign them to station-or satellite-specific error sources. Deficiencies in microwave orbit modeling might probably cause pass-specific patterns.

Optical observations in astronomy have been revolutionized by the use of solid state CCD detectors. Their enhanced sensitivity combined with digital image processing opened new prospects for automated sky surveys. Such techniques may be used for space debris surveys. Ground based optical surveys of the geostationary ring and the geostationary transfer orbit region outperform Radar observations in terms of minimum detectable object size, even when using telescopes of moderate size. On the other hand, automated optical surveys-given the large data volume produced by the sensors-ask for a high degree of on-line processing. Furthermore carefully planned observation scenarios are mandatory. We discuss different detection techniques and survey types. Optical surveys may yield a wide variety of results. In the simplest case a correlation with catalogued objects on the basis of observed positions is performed. It is also possible to aim for precise orbits, visual magnitudes, albedo, geometrical size, attitude motion, etc. The techniques will be illustrated by results from three surveys conducted with the 1 m Zimmerwald Laser and Astrometry Telescope (ZIMLAT) in the geostationary orbit region.

SLR tracking data allow for a completely independent validation of GNSS orbits that are derived from microwave data. SLR validation results show mean range residuals of several centimeters for both, GPS and GLONASS satellites, as well as significant seasonal ...

We provide suggestions for the approved COSMIC-2 satellite mission regarding the field of view (FOV) and the clock stability of its future GNSS receiver based on numerical analyses using COSMIC GPS data. While the GRACE GPS receiver is mounted on the zenith direction, the precise orbit determination (POD) antennas of COSMIC are not. The COSMIC antenna design results in a narrow FOV and a reduction in the number of GPS observations. To strengthen the GPS geometry, GPS data from two POD antennas of COSMIC are used to estimate its orbits. The phase residuals of COSMIC are at the centimeter level, compared to the millimeter level of GRACE. The receiver clock corrections of COSMIC and GRACE are at the microsecond and nanosecond levels, respectively. The clock spectra of COSMIC at the frequencies of 0-0.005 Hz contain significant powers, indicating potential systematic errors in its clock corrections. The clock stability, expressed by the Allan deviation, of COSMIC ranges from 10-9 to 10-11 over 1 to 10 4 s, compared to 10-12 to 10-14 for GRACE. Compared to USO-based clock of GRACE, the clock of COSMIC is degraded in its stability and is linked to the reduction of GPS data quality. Lessons for improvement of COSMIC-2 over COSMIC in FOV and receiver clock stability are given.

Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations, where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses. Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations. We use GPS data of the aforementioned LEOs of the year 2007