Michiel Otten

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.

Routine operational and high-precision orbit determination of envisat

Abstract ESAs Earth observation satellite Envisat was successfully launched on 1 March 2002 by an Ariane-5 launcher, and ESOC immediately took over the task of determining and predicting the orbit using S-band tracking data, and optimising the manoeuvre sequence to bring the spacecraft into an orbit accurately phased with ERS-2. On-board, Envisat carries, among others, a radar altimeter, a DORIS instrument and a laser retro-reflector array (SLR). Data from these instruments are being used at ESOC for high-precision orbit determination, for verification of the routine orbit determination and for cross-comparison with orbits computed on-board by the DORIS navigator and with those delivered with the Envisat products. This paper presents the first consolidated results obtained for Envisat routine and high-precision orbit determination. All orbit determination and control activities were performed with the software package NAPEOS, which was developed in-house.

The International DORIS Service (IDS): Recent Developments in Preparation for ITRF2013

The International DORIS Service (IDS) was created in 2003 under the umbrella of the International Association of Geodesy (IAG) to foster scientific research related to the French DORIS tracking system and to deliver scientific products, mostly related to the International Earth rotation and Reference systems Service (IERS). We first present some general background related to the DORIS system (current and planned satellites, current tracking network and expected evolution) and to the general IDS organization (from Data Centers, Analysis Centers and Combination Center). Then, we discuss some of the steps recently taken to prepare the IDS submission to ITRF2013 (combined weekly time series based on individual solutions from several Analysis Centers). In particular, recent results obtained from the Analysis Centers and the Combination Center show that improvements can still be made when updating physical models of some DORIS satellites, such as Envisat, Cryosat-2 or Jason-2. The DORIS contribution to ITRF2013 should also benefit from the larger number of ground observations collected by the last generation of DGXX receivers (first instrument being onboard Jason-2 satellite). In particular for polar motion, sub-milliarcsecond accuracy seems now to be achievable. Weekly station positioning internal consistency also seems to be improved with a larger DORIS constellation.