Bruce J. Haines

The Jason-1 Mission Special Issue: Jason-1 Calibration/Validation

On December 7, 2001, the Jason-1 satellite was successfully launched by a Boeing Delta II rocket from the Vandenberg site in California, USA. Its main mission was to maintain the high accuracy altimeter measurements, provided since 1992 by TOPEX/Poseidon (T/P), ensuring continuity in observing and monitoring the ocean for intraseasonal to interannual changes, mean sea level, tides, and so forth. Despite four times less mass and power, the Jason-1 system has been designed to have the same performances as T/P, measuring sea surface topography at the centimeter level. This new Centre National dEtudes Spatiales/National Aeronautics and Space Administration (CNES/NASA) mission also provides near real-time data for sea state and ocean forecast. The first 10 months of the Jason mission were dedicated to the verification of the system performance and cross-calibration with T/P measurements. A complete CALVAL plan was conducted by the Science and Project Teams of the mission based on in situ and regional experiments, global statistical approaches, and multisatellite comparisons, taking advantage of the T/P-Jason overlap during the first months of the mission. CALVAL and first science results showed that the Jason-1 performances were compliant with prelaunch specifications. This was a needed preamble before starting the routine phase of the mission in July 2003 with generation and distribution of validated geophysical data records to the whole user community.

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.

Improved elevation-change measurement of the southern Greenland ice sheet from satellite radar altimetry

A new analysis of Seasat and Geosat satellite radar altimeter measurements over the Greenland ice sheet was performed to determine surface elevation change. The new analysis includes twice as many measurements and has 50% greater spatial coverage than the authors previous study. In addition, a precise global ocean reference network created from four years of Topex/Poseidon altimeter data is used to obtain improved estimates of altimeter orbit errors and measurement system biases. The results show that the average elevation change of the southern Greenland ice sheet above 2000 m from 1978 to 1988 is not significantly different than zero. This contradicts earlier and even more recent studies that reported positive ice sheet growth rates and suggested increased precipitation due to a warmer polar climate.

Recent improvements in GEOSAT altimeter data

Altimetric sea level measurements made by the U.S. Navys geodetic satellite (Geosat) during its exact repeat mission (1986–1989) have been successfully applied to the study of a variety of oceanic phenomena [Douglas and Cheney, 1990], and comparisons with in situ measurements have shown that monthly mean altimetric sea level is accurate to approximately 5 cm rms [Miller and Cheney, 1990; Taietai, 1989]. Nevertheless, the original release of geophysical data records (GDRs) produced by the National Oceanic and Atmospheric Administration/National Ocean Service [Cheney et al., 1987] showed significant uncertainties in two important fields: the tropospheric water vapor correction and the satellite orbit. Data from other satellites have been used to derive improved water-vapor fields specifically for Geosat, while satellite ephemerides that are more precise by an order of magnitude have resulted from the application of advanced gravity models and the incorporation of new tracking data. To make these data readily accessible to the scientific community, new versions of the Geosat GDRs have been produced and are being distributed on compact disc/read only memory, or CD-ROM [Cheney et al., 1991a]. In this report, we document the increased sea level accuracy attainable in the tropical oceans using these new T2 GDRs.

Leveling the Sea Surface Using a GPS-Catamaran Special Issue: Jason-1 Calibration/Validation

In the framework of the TOPEX/Poseidon and Jason-1 CNES-NASA missions, two probative experiments have been conducted at the Corsica absolute calibration site in order to determine the local marine geoid slope under the ascending TOPEX/Poseidon and Jason-1 ground track (No. 85). An improved determination of the geoid slope was needed to better extrapolate the offshore (open-ocean) altimetric data to on-shore tide-gauge locations. This in turn improves the overall precision of the calibration process. The first experiment, in 1998, used GPS buoys. Because the time required to cover the extended area with GPS buoys was thought to be prohibitive, we decided to build a catamaran with two GPS systems onboard. Tracked by a boat at a constant speed, this innovative system permitted us to cover an area of about 20 km long and 5.4 km wide centered on the satellites ground track. Results from an experiment in 1999 show very good consistency between GPS receivers: filtered sea-surface height differences have a mean bias of −0.2 cm and a standard deviation of 1.2 cm. No systematic error or distortions have been observed and crossover differences have a mean value of 0.2 cm with a standard deviation of 2.7 cm. Comparisons with tide gauges data show a bias of 1.9 cm with a standard deviation of less than 0.5 cm. However, this bias, attributable in large part to the effect of the catamaran speed on the waterline, does not affect the geoid slope determination which is used in the altimeter calibration process. The GPS-deduced geoid slope was then incorporated in the altimeter calibration process, yielding a significant improvement (from 4.9 to 3.3 cm RMS) in the agreement of altimeter bias determinations from repeated overflight measurements.

The Effects of GPS Carrier Phase Ambiguity Resolution on Jason-1

We have used GPS carrier phase integer ambiguity resolution to investigate improvements in the orbit determination for the Jason-1 satellite altimeter mission. The technique has been implemented in the GIPSY orbit determination software developed by JPL. The radial accuracy of the Jason-1 orbits is already near 1 cm, and thus it is difficult to detect the improvements gained when the carrier phase ambiguities are resolved. Nevertheless, each of the metrics we use to evaluate the orbit accuracy (orbit overlaps, orbit comparisons, satellite laser ranging residuals, altimeter crossover residuals, orbit centering) show modest improvement when the ambiguities are resolved. We conservatively estimate the improvement in the radial orbit accuracy is at the 10–20% level.

Calibrating the Jason-1 measurement system

We present calibration results from Jason-1 (2002-) and TOPEX/Poseidon (1992-) overflights of dedicated verification sites on the Mediterranean island of Corsica and on a California offshore oil platform (Harvest). Harvest served for a decade (1992-2002) as a calibration site for the TOPEX/Poseidon (T/P) mission, and is serving in a similar capacity for Jason-1. Initiated in 1996, the Corsica experiment features a fiducial reference station near Aspretto, and a primary sub-satellite tide-gauge deployment site 40 km south at Cape Senetosa. Both Corsica and Harvest feature carefully designed collocations of space-geodetic and tide-gauge systems to support the absolute calibration of the altimetric sea-surface height (SSH). By incorporating improved estimates of the Jason-1 sea-state bias and columnar atmospheric wet path delay, we observe a bias of about 12 cm.