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ERS-2

The ERS-1 and ERS-2 operational orbit determination is performed by the Flight Dynamics Division at the European Space Operations Centre (ESOC) in Darmstadt. The purpose of this orbit determination is, among others, to provide the ERS-1 and ERS-2 ground segment with the latest orbit determination and prediction, for satellite acquisition (at the ground stations), mission planning and fast delivery data processing purposes. The operational orbit determination uses S-band tracking and fast delivery altimeter height data. The S-band data comprises range and range-rate measurements from the Multi-Purpose Tracking System MPTS installed at the Kiruna station. An automatic software sequence checks the arrival of data in the ESOC computer after each planned pass, and sends warning messages to the Spacecraft Controller’s console if anomalies are detected. Once per day, the full sequence runs to process tracking data from the last three days, and to update the orbit file, including a prediction for the next nine days. The central day of the three day moving window provides the final orbit. As a result, the operational orbit is available to users with a delay of only one day.

In parallel with this operation, precise orbit determination is performed using more complete models and all the data available: quick look laser, S-band range and doppler, and corrected fast delivery altimetry from Kiruna (16-sec normal points). The precise orbit determination is also performed automatically outside normal working hours, including the retrieval and preprocessing of tracking data and the generation of residual and orbit comparison plots. Solutions are being generated in 4-day arcs, with a delay of typically one week necessary to collect most of the laser tracking.

A comparison between the operational and precise orbits allows an estimate of the operational orbit determination accuracy. This is currently estimated at 2-4 m along-track, 1-2 m cross track and about 50 cm radially.

The quality of the orbit prediction provided by the operational orbit solution is monitored constantly. This is done by comparing the predicted orbit for each day with the final orbit determined afterwards. The statistical information of the 1-day, 3-day and 6-day prediction errors are presented in tabular form.

Information about the latest manoeuvres of ERS-1 and ERS-2 is provided in tabular form. The success of maintaininng the 1-km deadband of the ground tracks of both satellites is shown graphically.

The quality of the individual tracking types is summarised in terms of the amount of data available and the rms residuals from the precise solution. Individual statistics are plotted for the laser data (including a detailed breakdown per laser station), for the altimeter data and for the MPTS S-band tracking data.

A comparison is made between the ESOC precise and D-PAF preliminary orbits, showing a radial consistency between both solutions at the level of 10 cm. Another indicator of the radial accuracy is the internal consistency of the orbit determination method, which is monitored by computing overlapping 1-day arcs and comparing them with the 4-day solution.

The models used in the operational and precise orbit determinations can be summarised as follows:

  • Reference frame
    • mean equator and equinox of J2000.0
    • Station coordinates computed from a Topex/Poseidon and Lageos multi-arc solution
  • Dynamics
    • JGM-3 (36,36) gravity model (operational), JGM-3 (70,70) (precise)
    • MSIS density model (Hedin 1983); detailed CD modelling; 1 daily (operational) or sub-daily (precise) drag scale factor estimation
    • luni-solar gravity
    • frequency-dependent solid-earth tides, Wahr model
    • detailed ocean tide model, augmented Schwiderski (precise)
    • direct solar radiation pressure model (operational), taking into account spacecraft geometry (precise)
    • albedo, infrared perturbations (precise)
    • modelling of manoeuvre accelerations, estimation of corrective factors
    • one cycle per revolution accelerations
  • Measurement processing
    • Hopfield tropospheric correction (S-band), Murray-Marini (laser)
    • Rawer-Bent ionospheric correction (S-band)
    • spacecraft transponder delay, and ground calibrations
    • centre of mass correction (precise)
  • Altimeter data processing
    • fast delivery dry tropospheric corrections
    • Rawer-Bent ionospheric correction (precise)
    • Sea State Bias: FD correction plus an additional percentage of the SWH (precise), currently 0.0%
    • ESOC wet tropospheric correction model (precise)
    • ERS-1/ESOC Preliminary Mean Sea Surface, 0.3 deg resolution. Reference ellipsoid a = 6378.1367 Km , f = 1 / 298.257
    • Solid tide correction, including permanent tides (precise)
    • NSWC ocean tide models, including ocean loading (precise)
    • ESOC Dynamic Ocean Topography model to degree and order 20: previous month’s solution (precise)

In parallel to the precise orbit determination the PRARE orbit determination for ERS-2 is computed (since october 1999). The PRARE orbit determination is also performed automatically outside normal working hours, including the retrieval and preprocessing of tracking data and the generation of residual and orbit comparison plots. Solutions are being generated in 4-day arcs, with a delay of typically one week necessary to collect the PRARE tracking (currently PRARE quick look data revision 4 from the PRARE data server in GeoForschungsZentrum Potsdam (GFZ)).

The results of the PRARE orbit determinations are generated in terms of global rms of the orbit determination residuals and number of passes per processed arc; this gives an oveview of the quality of PRARE as tracking system.

A comparison of the PRARE orbit determination is made with respect to the ESOC precise and the D-PAF preliminary orbit determinations showing a radial consistency between solutions at the level of 10 cm

The models used in the PRARE orbit determination can be summarised as follows:

  • Reference frame
    • mean equator and equinox of J2000.0
    • PRARE station coordinates computed from ERS-2 multi-arc solution constrained to the ITRF-93 within 0.1 mas
  • Dynamics
    • JGM-3 (70,70)
    • MSIS density model (Hedin 1983); sub-daily drag scale factor estimation
    • luni-solar gravity
    • frequency-dependent solid-earth tides, Wahr model
    • detailed ocean tide model, augmented Schwiderski
    • direct solar radiation pressure model (operational), taking into account spacecraft geometry
    • albedo, infrared perturbations
    • modelling of manoeuvre accelerations, estimation of corrective factors
    • one cycle per revolution accelerations
  • Measurement processing
    • tropospheric, ionospheric, centre of mass, antenna phase, station mechanical centre and external corrections as delivered with the quick look data.
    • Range and time biases estimated per station per arc
    • Atmospheric correction factor estimated per station per pass

Further to orbit determination other complementary off-line activities such as Sea Level Analysis are carried out with ERS data.