Peter Steigenberger

Initial assessment of the COMPASS/BeiDou-2 regional navigation satellite system

An initial characterization and performance assessment of the COMPASS/BeiDou-2 regional navigation system is presented. Code and carrier phase measurements on up to three frequencies have been collected in March 2012 with a small regional network of monitoring stations. The signal and measurement quality are analyzed and compared with the Japanese Quasi Zenith Satellite System. A high level of stability is demonstrated for the inter-frequency carrier phase biases, which will facilitate the application of triple-frequency undifferenced ambiguity resolution techniques in future precise point positioning applications. The performance of the onboard Rubidium frequency standards is evaluated in comparison to ground-based hydrogen masers and shown to be well competitive with other GNSS satellite clocks. Precise orbit and clock solutions obtained in post-processing are used to study the presently achievable point positioning accuracy in COMPASS/BeiDou-2-only navigation. Finally, the benefit of triple-frequency measurements and extra-wide-lane ambiguity resolution is illustrated for relative positioning on a short baseline.

Imaging Geodesy—Toward Centimeter-Level Ranging Accuracy With TerraSAR-X

In this paper, we report on experiments to measure large-scale Earth surface displacements, such as those caused by solid Earth tides, with centimeter-level accuracy using TerraSAR-X radar images. With two totally different approaches, corner reflectors and image correlation techniques, we show the clear interrelation between the radar range measurements and the projection of the solid Earth tide motion vector onto the radar line of sight. Pixel location accuracies of up to 2.6-cm standard deviation can be achieved after a single calibration. We further demonstrate that solid Earth tides and tropospheric water vapor variations are the largest sources of ranging error if not compensated for. Alternatively, tropospheric water vapor can be estimated with centimeter accuracy using our proposed technique of synthetic aperture radar (SAR) image correlation and solid Earth motion compensation by the existing models. We also consider ionospheric delays which improve the results marginally in the X-band. Our results show the best ranging accuracies so far reported for spaceborne radar amplitude images and make TerraSAR-X-together with our simple compensation methodology-suitable for the imaging of centimeter-level Earth displacements. Absolute measurements of volcanoes or glaciers are possible without the use of ground equipment and without the use of SAR interferometry, thus avoiding the associated problems of phase ambiguity, phase unwrapping, and reference points.

Validation of Precipitable Water Vapor within the NCEP/DOE Reanalysis Using Global GPS Observations from One Decade

Abstract In contrast to previous studies validating numerical weather prediction (NWP) models using observations from the global positioning system (GPS), this paper focuses on the validation of seasonal and interannual variations in the water vapor. The main advantage of the performed validation is the independence of the GPS water vapor estimates compared to studies using water vapor datasets from radiosondes or satellite microwave radiometers that are already assimilated into the NWP models. Tropospheric parameters from a GPS reanalysis carried out in a common project of the Technical Universities in Munich and Dresden were converted into precipitable water (PW) using surface pressure observations from the WMO and mean atmospheric temperature data from ECMWF. PW time series were generated for 141 globally distributed GPS sites covering the time period from the beginning of 1994 to the end of 2004. The GPS-derived PW time series were carefully examined for their homogeneity. The validation of the NWP mo...

Signal, orbit and attitude analysis of Japan's first QZSS satellite Michibiki

Results are presented for Michibiki, the first satellite of Japan’s Quasi-Zenith Satellite System. Measurements for the analysis have been collected with five GNSS tracking stations in the service area of QZSS, which track five of the six signals transmitted by the satellite. The analysis discusses the carrier-to-noise density ratio as measured by the receiver for the different signals. Pseudorange noise and multipath are evaluated with dual-frequency and triple-frequency combinations. QZSS uses two separate antennas for signal transmission, which allows the determination of the yaw orientation of the spacecraft. Yaw angle estimation results for an attitude mode switch from yaw-steering to orbit-normal orientation are presented. Estimates of differential code biases between QZSS and GPS observations are shown in the analysis of the orbit determination results for Michibiki. The estimated orbits are compared with the broadcast ephemerides, and their accuracy is assessed with overlap comparisons.

Gravity Model TUM-2Sp Based on the Energy Balance Approach and Kinematic CHAMP Orbits

We have used one year of CHAMP data for deriving a gravity field model based on the energy balance approach. In order to avoid the use of any a priori gravity information, purely kinematic orbits have been computed from GPS measurements only. Subsequently velocities have been derived from these kinematic positions by two different methods, namely smoothing splines and Newton- Gregory interpolation. Using the principle of energy conservation, the satellites positions and velocities are transformed into gravitational potential. CHAMP onboard micro-accelerometry is used to correct for surface forces. For spherical harmonic analysis the so-called direct approach has been implemented using the full normal equation matrix. The model, called TUM2Sp, was found to be a more accurate gravity field than EIGEN-2 model.

Impact of Albedo Radiation on GPS Satellites

GPS satellite orbits available from the International GNSS Service (IGS) show a peculiar pattern in the SLR residuals at the few centimeter level that is related to radiation pressure mismodeling. Part of the mismodeling may be attributed to neglecting the solar radiation reflected and reemitted from the Earth, the albedo radiation, as most IGS analysis centers do not yet take into account this radiation pressure component. In this study the relative importance of different albedo model constituents is analyzed. The impact of nine albedo models with increasing complexity is investigated using 1 year of global GPS data from the IGS tracking network. The most important model components are the solar panels of the satellites while different Earth radiation models have a minor impact on orbits at GPS altitudes. Albedo radiation has the potential to remove part of the anomalous SLR residual pattern observed by Urschl et al. (Calibrating GNSS orbits with SLR tracking data. Proceedings of the 15th International Workshop on Laser Ranging, 2008) in a Sun-fixed reference frame.

Effects of Different Antenna Phase Center Models on GPS-Derived Reference Frames

In GPS week 1400, the International GNSS Service (IGS) switched from a relative antenna phase center model (APCM) for receiver antennas only to an absolute model including receiver and satellite antenna corrections. At the same time the International Terrestrial Reference Frame 2005 (ITRF2005) was adopted. These changes had a significant influence on the terrestrial reference frame (TRF). In order to study the influence of different APCMs on GPS-derived TRFs, four TRF solutions have been computed from 11 years of homogeneously reprocessed GPS data. The processing strategy for the four solutions is completely identical except for the APCM applied. The following models have been used: (1) the relative model IGS01 used by the IGS till GPS week 1400, (2) the new absolute IGS model IGS05 including radome calibrations, (3) IGS05 ignoring the radome calibrations for the receiver antennas, and (4) IGS05 including azimuth-dependent satellite antenna phase center variations (PCVs). Station coordinates and velocities have been estimated simultaneously with daily pole coordinates. Consistent time series of station coordinates have been generated using the corresponding reference frames for datum definition. This paper compares the station coordinates and velocities as well as the station coordinate time series arising from the four different reference frames.

High resolution geodetic earth observation with TerraSAR-X: Correction schemes and validation

Previous studies have shown the unprecedented absolute pixel localization accuracy of the German SAR (Synthetic Aperture Radar) satellites TerraSAR-X and TanDEM-X. Now, by thoroughly correcting all signal path delays and geodynamic effects like tides, loadings and plate movements, range accuracies of about 1 centimeter are demonstrated to be attainable. While Global Navigation Satellite System (GNSS) data provide local correction values for the atmospheric delays, correction values for the geodynamic effects are based on the IERS (International Earth Rotation and Reference Systems Service) conventions. Our recent measurements are based on a corner reflector with very precisely known ground position which we installed at Wettzell, Germany, close to the local GNSS reference stations. Further comparable high precision test sites in the world are in progress and shall prove the worldwide reproducibility of the achieved results.

A Priori Gradients in the Analysis of Space Geodetic Observations

We introduce a static a priori gradient model (APG) based on a spherical harmonic expansion up to degree and order nine to describe the azimuthal asymmetry of tropospheric delays. APG is determined from climatology data of the European Centre for Medium-Range Weather Forecasts (ECMWF), and the refined model can be used in the analysis of observations from Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry (VLBI). Comparisons reveal that gradients estimated in GNSS analysis are mostly smaller than those provided by APG. This difference is also confirmed by station and source coordinate changes if APG is used in GNSS and VLBI analysis.

GPS-Specific Local Effects at the Geodetic Observatory Wettzell

Global Navigation Satellite Systems (GNSS) are important contributors to the realization of the International Terrestrial Reference System (ITRS). For the combination of different space geodetic techniques, terrestrial measurements between the corresponding reference points are necessary. Discrepancies between these so-called local ties on the one hand and the coordinate differences derived from space techniques on the other hand are a major limitation for the realization of the ITRS nowadays. In the past, these discrepancies have often been attributed to inaccurate terrestrial measurements. This paper shows that a major part of the differences can be explained by systematic GNSS-specific errors, if a global data analysis is simulated. One of the most important error sources for GNSS are interactions of the antenna with its immediate vicinity, primarily multipath.