Pascal Willis

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.

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.

Precision Orbit Determination Standards for the Jason Series of Altimeter Missions

The Jason-1 altimeter satellite and its follow-on mission Jason-2/OSTM were launched in December 2001 and June 2008, respectively, to provide the scientific community with a high-accuracy continuous record of observations of the ocean surface topography. Both missions carry on board three state-of-the-art tracking systems (DORIS, GPS, SLR) to meet the requirement of better-than-1.5 cm radial accuracy for the operational orbit included in the geophysical data record (GDR) product. This article outlines the common set of models and processing techniques applied to both Jason reprocessed and operational orbits included in version C of the GDR, referred to as GDR-C standards for precision orbit determination (POD), and describes the systematic components of the radial error budget that are of most interest for the altimeter data analysts. The nonsystematic component of the error budget, quantified by intercomparison of orbits using similar models or with reduced dependency on the dynamic models, is generally at or below 7 mm RMS (root-mean-square). In particular, the average daily RMS of the radial difference between the JPL and CNES reduced-dynamic orbits on Jason-2 is below 6 mm. Concerning the dynamic models employed, the principal contributors to residual systematic differences appear to be the time varying gravity and solar radiation pressure, resulting in geographically correlated periodic signals that have amplitudes at the few-mm level. Concerning the drifts of the orbits along the North/South direction, all solutions agree to better than the 1 mm/year level.

Current status of the doris pilot experiment and the future international doris service

Abstract The aim of the DORIS Pilot Experiment is to assess the need and feasibility of an International DORIS Service. A Call for Proposals was broadcasted in September 1999 to prompt qualified organizations to submit proposals for components of the future IDS. DORIS Days were held in Toulouse (May 2–3, 2000). This second version of these Doris days was in particular devoted to a review of the start-up of the Doris Pilot Experiment. This paper recalls the objectives of the future IDS, points out its components and structure, and gives information on the current and future activities.

IGEX: International GLONASS experiment — Scientific objectives and preparation

The GLONASS system is rapidly becoming of great interest for navigation, timing and geodetic applications, usually in combination with the GPS system. The goal of this paper is to describe the first world-wide campaign of observations of GLONASS satellites, organized with a large international participation. This first campaign, called IGEX98 will be organized in fall 1998. In this paper, we will focus on the scientific objectives and present the on-going preparation for this campaign. We will address several important scientific issues that could be solved in the short term by such a campaign. A large number of organizations have already answered positively to the IGEX98 International Call for Participation.

Parameter sensitivity of TOPIX orbit and derived mean sea level to DORIS stations coordinates

Abstract When determining precise orbits from altimetric satellites, the choice of the terrestrial reference frame is a key issue but still an open scientific topic. The terrestrial reference frame realization choice in the precise orbit determination processing will usually lead to systematic effects coming from the adoption of translations, scale factor and rotations but also others effects more difficult to assess, coming from a possible erroneous choice of a station coordinate. The goal of this paper is to try to characterize the effect of an erroneous individual station coordinate in vertical and horizontal components on the satellite orbit determination (characterization of the error, order of magnitude, consequences assessment of the on derived oceanographic products). Simulations have been realized at Institut Geographique National using the GIPSY/OASIS software from actual TOPEX DORIS data. These studies show that the stations whose latitude are close to the orbit inclination have the largest effect on the orbit error. These simulations will also help us to better estimate the required accuracy of a new tracking station. Additionally the study has been extended to try to predict the future uncertainty in the mean sea level determination in the 2000–2010 period derived for the present accuracy of the ITRF97 coordinates and velocities. Present uncertainties in the ITRF97 realization will create systematic errors in the TOPEX derived mean sea level at the 1 mm level in 2010.

Isostatic stability of the East Antarctic station Dumont d’Urville from long-term geodetic observations and geophysical models

Geodetic measurements of the vertical crustal displacement collocated with absolute gravity changes provide a discriminatory measurement of present-day glacial changes, versus more deeply seated rock motions caused by glacial isostatic adjustment (GIA). At the East Antarctic station of Dumont d’Urville, we compare the displacements derived from continuous DORIS (1993.0– 2006.0) and Global Positioning System (GPS) (1999.0–2005.7) data, and observed changes in absolute gravity (2000–2006), with the predicted vertical displacement and change in gravity from GIA modelling. The geodetic results have mutual self-consistency, suggest station stability and provide upper bounds on both GIA and secular ice mass changes. The GIA models tend to predict amplitudes of rock motion larger than those observed, and we conclude that this part of Antarctica is probably experiencing a slight gain in ice mass, in contrast to West Antarctica.

DPOD2008: A DORIS-Oriented Terrestrial Reference Frame for Precise Orbit Determination

While accuracy of tracking station coordinates is of key importance for Precise Orbit Determination (POD) for altimeter satellites, reliability and operationality are also of great concern. In particular, while recent ITRF realizations should be the most accurate at the time of their computation, they cannot be directly used by the POD groups for operational consideration for several reasons such as new stations appearing in the network or new discontinuities affecting station coordinates. For POD purposes, we computed a new DORIS terrestrial frame called DPOD2008 derived from ITRF2008 (as previously done by DPOD2005 with regards to ITRF2005). In a first step, we will present the method used to validate the past ITRF2008 using more recent DORIS data and to derive new station positions and velocities, when needed. In particular, discontinuities in DORIS station positions and/or velocities are discussed. To derive new DORIS station coordinates, we used recent DORIS weekly time series of coordinates, recent GPS relevant time series at co-located sites and also dedicated GPS campaigns performed by IGN when installing new DORIS beacons. DPOD2008 also contains additional metadata that are useful when processing DORIS data, for example, periods during which DORIS data should not be used or at least for which data should be downweighted. In several cases, a physical explanation can be found for such temporary antenna instability. We then demonstrate improvements seen when using different reference frames, such as the original ITRF2008 solution, for precise orbit determination of altimeter satellites TOPEX/Poseidon and Jason-2 over selected periods spanning 1993–2013.

A high‐quality, homogenized, global, long‐term (1993–2008) DORIS precipitable water data set for climate monitoring and model verification

For the first time a high-quality, consistent, global, long-term data set of zenith tropospheric delay (ZTD) and precipitable water (PW) is produced from Doppler orbitography radiopositioning integrated by satellite (DORIS) measurements at 81 sites. The data set was screened using a two-level procedure. First, postprocessing information is used to apply range checks and outlier checks to ZTD and formal error estimates. Second, outliers are detected by comparing DORIS ZTD with European Centre for Medium-Range Weather Forecasts reanalysis (ERA-Interim) data. These procedures reject 3% and 1% of the data, respectively. A linear drift is evidenced in the screened DORIS ZTD data compared to ERA-Interim and Global Positioning System (GPS) data, which potentially results from biases introduced by the progressive replacement of Alcatel antennas with Starec antennas. The DORIS PW is homogenized by applying a bias correction computed form comparison with ERA-Interim data each time station equipment is changed. The homogenized DORIS data are in excellent agreement with GPS data (correlation of 0.98 and standard deviation of differences of 1.5 kg m−2) and with ERA-Interim and satellite PW data (correlation > 0.95 and standard deviation of differences < 2.7 kg m−2). The agreement with radiosonde data is less good. Preliminary results of water vapor trends and variability are shown for 31 sites with more than 10 years of data. Good consistency is found between DORIS PW trends and ERA-Interim trends, which demonstrates the high potential of the DORIS PW data set for climate monitoring and model verification. The final DORIS PW data set is freely available in the supporting information.