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2.8.5 Orbit interpolation

This section is intended to provide a description of the EnviSat orbit CFI functions (PPF_ORBIT libraries) in the NRT RA2/MWR processing chain.

The only orbit routines to be used within the processing are the orbit propagator (po_ppforb) and the orbit interpolator (po_interpol), that allow the calculation of the orbit and the satellite ground track related parameters (i.e. latitude, longitude, altitude and altitude rate).

In the Level 2 OFL chain, different interpolation routines are used, with input DORIS preliminary orbit files (for the production of IGDR/IMAR products) and DORIS precise orbit files (for the production of GDR/SGDR products).

2.8.5.1 po_ppforb

This routine is currently used for the NRT (IPF) processing since no restituted orbit file is available within 3 hours after data take.

In this case, only one state vector from a predicted orbit state vector file is used for the calculation of the orbital parameters for all records in the input product, by propagating the state vector.

A proper sequence of orbit propagator calls consists of one initialisation call and a number of propagation calls (one per record).

The predicted orbit files are:

  • DORIS Navigator Level 0 file (product id= DOR_NAV_0P)
  • Predicted orbit file from FOS (product id= AUX_FPO_AX)

Until now, only the predicted FOS file has been used for the propagation of the OSV in the RA2/MWR NRT processing.

The use of a predicted OSV file, instead of a restituted one, seriously impacts the quality of the NRT orbital parameters, where errors of several meters in the orbital altitude can be observed.

This is why an interpolation strategy is being implemented for the NRT processing, by using the DORIS Level 0 Navigator files.

This strategy implies an update of the orbit routines, so they can accept a new orbit file for the interpolation, and an upgrade in the NRT official processor.

When this scheme will be implemented, the NRT orbital errors will be reduced to some centimeters.

The info on the orbit file used for the selection of the OSV to be propagated, appears in the product header: field 25 in MPH (vector source).

2.8.5.2 po_interpol

This routine, based on the interpolation of state vectors from restituted OSV files, is not currently used in the NRT RA2/MWR processing, due to the specific time constraints (i.e. the restituted orbit files are not available within ∼ 3 hours after data take to be used for the NRT processing).

The restituted orbit files are the following:

  • DORIS precise orbit file (product id=DOR_VOR_AX)
  • DORIS preliminary orbit file (product id=DOR_POR_AX)
  • FOS restituted orbit file (product id=AUX_FRO_AX)

In order to avoid the huge inaccuracies in the NRT orbital parameters (see previous paragraph) a new strategy will become operative soon, based on the use, in NRT, of the full-rate DORIS Navigator file for the orbit state vectors interpolation.

A proper sequence of orbit interpolator calls consists of one initialisation call and a number of interpolation calls (one per record), as in the case of the propagator routine.

The info on the specifc orbit file used for the interpolation appears in the L2 product SPH.


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry