Yoaz E. Bar-Sever

Estimating horizontal gradients of tropospheric path delay with a single GPS receiver

We present evidence that modeling troposphere delay gradients in precise Global Positioning System (GPS) geodesy improves the accuracy and precision of the estimated quantities, and that the estimated gradients resemble real atmospheric moisture gradients observed with a water vapor radiometer (WVR). Using a low elevation angle cutoff, combined with a model of the atmospheric delay gradient as a random walk process leads to 19.5% and 15% average improvement in radial and horizontal site position repeatabilities, respectively, relative to a current state-of-the-art estimation strategy that does not model horizontal gradients and imposes high elevation angle cutoff. The agreement between estimated values of zenith wet delay from collocated GPS receivers and WVRs was improved by at least 25%. Merely lowering the elevation angle cutoff improves the repeatability of the radial component of the sites position vector but tends to degrade the repeatability of the horizontal components of the position vector if troposphere gradients are not properly modeled. The estimates of wet delay gradients from a collocated GPS receiver and a WVR at Onsala, Sweden, seem to be correlated over timescales as short as 15 min. The agreement in azimuth between the GPS-based and the WVR-based gradients was at the 10° level, for significant gradients. The GPS was found to under-estimate the magnitude of the gradients by about 60% relative to the WVR-based gradients. The ability to sense atmospheric moisture gradients from a single GPS receiver increases the useful information content from networks of GPS receivers by providing additional spatial information for weather forecasting applications.

GPS precise tracking of TOPEX/POSEIDON: Results and implications

A reduced dynamic filtering strategy that exploits the unique geometric strength of the Global Positioning System(GPS) to minimize the effects of force model errors has yielded orbit solutions for TOPEX/POSEIDON which appear accurate to better than 3 cm (1 σ) in the radial component. Reduction of force model error also reduces the geographic correlation of the orbit error. With a traditional dynamic approach, GPS yields radial orbit accuracies of 4–5 cm, comparable to the accuracy delivered by satellite laser ranging and the Doppler orbitography and radio positioning integrated by satellite (DORIS) tracking system. A portion of the dynamic orbit error is in the Joint Gravity Model-2 (JGM-2); GPS data from TOPEX/POSEIDON can readily reveal that error and have been used to improve the gravity model.

A new model for GPS yaw attitude

Abstractmodeling of the GPS satellite yaw attitude is a key element in high-precision geophysical applications. This fact is illustrated here as a new model for the GPS satellite yaw attitude is introduced. The model constitutes a significant improvement over the previously available model in terms of efficiency, flexibility and portability. The model is described in detail and implementation issues, including the proper estimation strategy, are addressed. The performance of the new model is analyzed and an error budget is presented. Finally, the implementation of the yaw bias on the GPS satellites is reviewed from its inception until it reached a steady state in November, 1995.

First assessment of GPS‐based reduced dynamic orbit determination on TOPEX/Poseidon

The reduced dynamic GPS tracking technique has been applied for the first time as part of the GPS experiment on TOPEX/Poseidon. This technique employs local geometric position corrections to reduce orbit errors caused by the mismodeling of satellite forces. Results for a 29-day interval in early 1993 are evaluated through postfit residuals and formal errors, comparison with GPS and laser/DORIS dynamic solutions, comparisons on 6-hr overlaps of adjacent 30-hr data arcs, altimetry closure and crossover analysis. Reduced dynamic orbits yield slightly better crossover agreement than other techniques and appear to be accurate in altitude to about 3 cm RMS.

One-Centimeter Orbit Determination for Jason-1: New GPS-Based Strategies

The U.S./French Jason-1 satellite is carrying a state-of-the-art GPS receiver to support precise orbit determination (POD) requirements. The performance of the Jason-1 “BlackJack” GPS receiver was strongly reflected in early POD results from the mission, enabling radial accuracies of 1–2 cm soon after the satellites 2001 launch. We have made further advances in the GPS-based POD for Jason-1, most notably in describing the phase center variations of the on-board GPS antenna. We have also adopted new geopotential models from the Gravity Recovery and Climate Experiment (GRACE). The new strategies have enabled us to better exploit the unique contributions of the BlackJack GPS tracking data in the POD process. Results of both internal and external (e.g., laser ranging) comparisons indicate that orbit accuracies of 1 cm (radial RMS) are being achieved for Jason-1 using GPS data alone.

Monitoring the TOPEX Microwave Radiometer with GPS: Stability of columnar water vapor measurements

Using data from terrestrial global positioning system (GPS) receivers, we describe an anomalous drift in columnar water vapor measurements from the TOPEX microwave radiometer (TMR). Based on their long-occupation histories and proximity to open-water TOPEX/POSEIDON (T/P) ground tracks, we identified four geodetic-quality GPS stations that are suitable for calibrating the TMR. We constructed time series at each site based on the differences of instantaneous vertical wet tropospheric path delay (PD wet ) derived independently from GPS and TMR data at T/P overflight times. The ensemble results span 1992-1997 and suggest that the TMR measurements of PD wet are gradually drifting lower. Our nominal estimate of the drift is - 1.2 ± 0.4 mm yr -1 (one standard error). Accounting for this would increase the estimated rate of change in global mean sea level from T/P by the same amount.

Topex/Jason combined GPS/DORIS orbit determination in the tandem phase

Abstract In December 2001, the Jason-1 satellite was launched to extend the long-term success of the TOPEX/POSEIDON (T/P) oceanographic mission. The goals for the Jason-1 mission represent both a significant challenge and a rare opportunity for precise orbit determination (POD) analysts. Like its predecessor, Jason-1 carries three types of POD systems: a GPS receiver, a DORIS receiver and a laser retro-reflector. In view of the 1-cm goal for radial orbit accuracy, several major improvements have been made to the POD systems: 1) the GPS “BlackJack” TurboRogue Space Receiver (TRSR) tracks up to 12 GPS spacecraft using advanced codeless tracking techniques; 2) a newly developed DORIS receiver can track two ground beacons simultaneously with lower noise. In addition, the satellite itself features more straightforward attitude behavior, and a symmetric shape, simplifying the orbit determination models compared to T/P. On the other hand, the area-to-mass ratio for Jason-1 is larger, implying larger potential surface-force errors. This paper presents Jason-1 POD results obtained at JPL using the GIPSY-OASIS II (GOA) software package. Results from standard tests (orbit overlaps, laser control points) suggest that 1 to 2 cm radial orbit precision is already being achieved using the JPL reduced-dynamic estimation approach. We also report new DORIS POD strategies that make full profit of the additional number of common DORIS observations due to the T/P·Jason-1 tandem mode of orbit as well the additional dual-channel capability of the upgraded JASON receiver (allowing simultaneous tracking of two ground stations). New information on the satellites time scale is availed through this new estimation strategy. Results show that a significant improvement to DORIS-based orbits could be gained using this strategy. Building on these results, we have extended the GIPSY/OASIS 11 software capability to more fully exploit the combined benefit of both GPS and DORIS measurements from T/P and Jason-1 in their preliminary tandem mode. POD test results are used to demonstrate the accuracy of these orbits and to compare results in different cases: DORIS-alone, and GPS and DORIS together in both single- and multi-satellite modes. On the other, we have demonstrated and explained an anomalous behavior of the on-board oscillator when crossing the South Atlantic Anomaly region. Finally, plans for future software enhancements, processing strategies and modeling improvements are presented.

Probing Europa's Hidden Ocean From Tidal Effects on Orbital Dynamics

Recent observations of Europa suggest that the Jovian satellite may have a liquid ocean underneath its icy surface. Geophysical models indicate that the tidal Love number k2 has a strong dependence on the presence or absence of an ocean. The k2 dependence on the ice shell thickness is also significant. Measurements of the static and tidal gravity fields through their dynamic effects on the trajectory of a low Europan orbiter can be essential in the detection of an ocean and inference of other internal structures. Covariance analyses have been carried out to assess accuracies using simulated Doppler tracking data. With 15 days of tracking from 2 Earth stations, the uncertainties for k2, mantle libration amplitude and the epoch radial position of the spacecraft are expected to be 0.0004, 2.8 arcsec and 5.7 m, respectively. These tight constraints will strongly contribute to ocean detection and ice thickness determination when combined with altimeter measurements.

Atmospheric Media Calibration for the Deep Space Network

Two tropospheric calibration systems have been developed at the Jet Propulsion Laboratory (JPL) using different technologies to achieve different levels of accuracy, timeliness, and range of coverage for support of interplanetary NASA flight operations. The first part of this paper describes an automated GPS-based system that calibrates the zenith tropospheric delays. These calibrations cover all times and can be mapped to any line of sight using elevation mapping functions. Thus they can serve any spacecraft with no prior scheduling or special equipment deployment. Centimeter-level accuracy is provided with 1-h latency and better than 1-cm accuracy after 12 h, limited primarily by rapid fluctuations of the atmospheric water vapor. The second part describes a more accurate line-of-sight media calibration system that is primarily based on a narrow beam, gain-stabilized advanced water vapor radiometer developed at JPL. We discuss experiments that show that the wet troposphere in short baseline interferometry can be calibrated such that the Allan standard deviation of phase residuals, a unitless measure of the average fractional frequency deviation, is better than 2times10-15 on time scales of 2000 to approximately 10 000 s.

WVR-GPS comparison measurements and calibration of the 20-32 GHz tropospheric water vapor absorption model

Collocated measurements of opacity (from water vapor radiometer brightness temperatures) and wet path delay (from ground-based tracking of global positioning satellites) are used to constrain the model of atmospheric water vapor absorption in the 20-32 GHz band. A differential approach is presented in which the slope of opacity-versus-wet delay data is used as the absorption model constraint. This technique minimizes the effects of radiometric calibration errors and oxygen model uncertainties in the derivation of a best-fit vapor absorption model. A total of approximately five months of data was obtained from two experiment sites. At the Cloud and Radiation Testbed (CART) site near Lamont, Oklahoma, three independent water vapor radiometers (WVRs) provided near-continuous opacity measurements over the interval July-September 1998. At the NASA/Goldstone tracking station in the California desert two WVRs; obtained opacity data over the September-October 1997 interval. At both sites a Global Positioning Satellite (GPS) receiver and surface barometer obtained the data required for deriving the zenith wet delays over the same time frames. Measured values of the opacity-versus-wet delay slope parameter were obtained at four WVR frequencies (20.7, 22.2, 23.8, and 31.4 GHz) and compared with predictions of four candidate absorption models referenced in the literature. With one exception, all three models provide agreement within 5% of the opacity-versus-wet delay slope measurements at all WVR frequencies at both sites. One model provides agreement for all channels at both sites to the 2-3% level. This absorption model accuracy level represents a significant improvement over that attainable using radiosondes.