SSALTO (Coutin-Faye, 2001) produces on a routine basis the ephemerides of the satellites and the network station coordinates. The so-called MOE or Medium Orbit Ephemerides is calculated within 48 h for all the satellites with the ZOOM software developed by the CNES DORIS Orbitography Service (SOD). The arc length is 30 hours. The DORIS data are first pre-processed to eliminate spurious data. The measurement model then includes the ionospheric correction based on the dual frequency correction model, the tropospheric correction based on the CNET (Berrada-Baby et al., 1987) model and uses three meteorological parameters measured in-situ and transmitted in the beacon data stream to the orbiting satellite and solid earth tides. The processing also includes corrections at the instrument level such as the satellite mass center - antenna phase center offset. The orbit model uses the following Earth gravity field models: GRIM5-S1 (Biancale et al., 2000) for the Spot series satellite and Eigen-CG03C (Förste et al., 2005) for Envisat. Luni-solar gravitational force is also included. The DTM94 air drag model is applied for Spot satellites (Berger et al., 1998), MSIS for Jason1 and Envisat1, and a drag coefficient is estimated every 6 hours. The solar pressure radiation is also calculated and a 30-hour coefficient is adjusted. Nonconservative forces, including atmospheric drag and solar radiation pressure are modelled using a macromodel and appropriate attitude model for each satellite (Spots and Envisat)1. Hill's empirical accelerations are also adjusted per arc (Cretaux, 1994). The station coordinates and velocities are fixed to ITRF2000 for the permanent beacons in the DORIS network (Altamimi, 2002). A clock bias containing both the onboard and beacon ultra stable oscillator contribution and a zenital tropospheric delay are also estimated every satellite pass over a beacon. The MOE orbits are calculated within 48 hours and their 3D precision is estimated to be 4-5 cm by comparison to POE (Precise Orbit Ephemerides). The accuracy may be degraded during periods of intense solar and magnetic activity (Barotto and Berthias, 1996). The ephemeris are then used to derive the DORIS antenna coordinates2. The point positioning processing in SSALTO is realised considering each station independently and the orbit satellite as fixed (Valette et al, 2000). The DORIS measurement model includes the corrections of the troposphere and the ionosphere, the clocks bias, the antenna offsets.
The model for the estimated parameter includes the cartesian beacon coordinates and two parameters per satellite pass: one is a zenital troposphere delay offset and another is a satellite-beacon clock offset. The arc length of the observations and the selection of the satellites are adapted to the objective. A two day arc is used for daily monitoring of the whole DORIS network. A one week arc is used for IDS deliveries of sinex and per station time series of coordinates (Noll et al, 2005, Tavernier et al, 2005).
1 The macromodel of the Spot and Envisat satellites used at the SOD are described in ftp://ftp.ids-doris.org/pub/ids/satellites/macromodels/.
2 The geometry of the antenna and the exact position of the DORIS reference point is shown in ftp://ftp.ids-doris.org/pub/ids/stations/antennas.pdf file (see the Starec antenna type).
References
Altamimi Z., P. Sillard and C. Boucher, 2002. ITRF2000: a new release of the International Terrestrial Reference Frame for earth science applications, J. Geophys. Res., 107 (B10), 2002.
Barotto, B; Berthias, JP. (1996). First results of reduced dynamics with DORIS on TOPEX/Poseidon and SPOT. J. of Guidance Control and Dynamics, 19 (6): 1296-1302.
Berrada-Baby H., P. Golé et J. Lavergnat (1987), Effets de la troposphère sur les mesures de distances Terre-Satellite. Application au projet DORIS. Note technique CRPE/158.
Biancale, R., Balmino, G., Lemoine, J.-M., Marty, J.-C., Moynot, B., Barlier, F., Exertier, P., Laurain, O., Reigber, Ch., Bode, A., Gruber, Th., König, R., Massmann, F.-H., Raimondo, J.C., Schmidt, R., Zhu, S.Y., (2000), A New Global Earth's Gravity Field Model from Satellite Orbit Perturbations: GRIM5-S1. Geophysical Research Letters 27, 3611-3614.
Coutin-Faye S. (2000), SSALTO: a new ground segment for a new generation of altimetry satellites. AVISO newsletter 7.
Cretaux J.F., F. Nouel, C. Valorge, P. Janniere (1994), Introduction of empirical parameters deduced from the Hill's equation for satellite orbit determination, Manuscripta Geodaetica (19): 135-156.
Förste C., F. Flechtner, R. Schmidt, U. Meyer, R. Stubenvoll, F. Barthelmes, R. König, K.H. Neumayer, M. Rothacher, Ch. Reigber, R. Biancale, S. Bruinsma, J.-M. Lemoine, J.C. Raimondo (2005), A New High Resolution Global Gravity Field Model Derived From Combination of GRACE and CHAMP Mission and Altimetry/Gravimetry Surface Gravity Data. EGU General Assembly 2005, Vienna, Austria.
Guier W., H. and R.R. Newton (1965), The Earth's gravity field as deduced from the Doppler tracking of five satellites, J. Geophys. Res., (70), 4613-4626.
Noll C, Soudarin L (submitted) On-line resources supporting the data, products, and infrastructure of the International DORIS Service. J Geod Same issue.
Tavernier, G; Fagard, H; Feissel-Vernier, M; Lemoine, F; Noll, C ; Ries, J ; Soudarin, L ; Willis P. (2005). The International DORIS Service (IDS), ADVANCES IN SPACE RESEARCH 36(3):333-341.
Tavernier G., H. Fagard, M. Feissel-Vernier, K. Le Bail, F. Lemoine, C. Noll, R. Noomen, J.C. Ries, L. Soudarin, J.J. Valette, P. Willis (submitted), The International DORIS service : genesis and early achievements. J Geod Same issue.
Valette J.J., M.N. Loaec, B. Nhun Fat (2000), DORIS accurate location service: from request to delivery and the applications, DORIS Days meeting.