Minimize POD Products and Requirements

The Precise Orbital products and auxiliary data generated by the Copernicus POD (Precise Orbit Determination) Service for the Sentinel-1 mission can be divided up into three categories depending on the associated timeliness.

  1. Near Real-Time (NRT) products are generated based on immediate GPS L0 availability with the near real-time GPS orbits and clocks provided by EGP.
  2. Near Real Time predicted (PRE) products are computed a certain time ahead of an astronomical event, particularly the ascending node crossing.
  3. Non Time Critical (NTC) products are computed after a number of days to allow for all highly accurate inputs to be available, such as GPS orbits and clocks from IGS.

Details on the official POD products are available in the File Format Specification File Format Specification document. A brief description of the POD products is included hereafter.

The NRT orbital product (AUX_RESORB) generation is associated to the availability of a new GPS L0 file, which typically have a coverage of several orbits with significant overlaps. It has the coverage of the latest ascending node crossing (ANX) contained in the GPS L0 that triggered its generation plus 2 orbits backwards. Thus, the official POD NRT products for Sentinel-1 typically show overlaps of 1 or 2 orbits, depending on the coverage of the GPS L0 file.

The PRE orbital product (AUX_PREORB) generation is associated to the availability of a new GPS L0 file as the NRT case. It has the coverage of the latest ANX contained in the GPS L0 file that triggered its generation plus 4 orbits to the future. In this way, the official POD PRE products for Sentinel-1 typically show overlaps of 3 or 4 orbits, depending on the coverage of the GPS L0 file.

The NTC orbital product (AUX_POEORB) is generated with a timeliness of 20 days and covers 26 hours, from 1 hour before the start of day N-20 until 1 hour after the end of day N-20 (being N the current day). Therefore, there is an overlap of 2 hours between consecutive NTC orbit files.

The NTC restituted attitude product (AUX_RESATT) has the same timeliness and coverage as the NTC orbital product.

Each of these products has different timeliness and accuracy requirements, which are summarised in the following table:

Copernicus POD Sentinel-1 Orbital Files Requirements
Category Latency Orbit Accuracy
NRT Predicted (AUX_PREORB) 30 minutes 1 m (2D RMS 1-sigma)
NRT (AUX_RESORB) 180 minutes 10 cm (2D RMS 1-sigma)
NTC (AUX_POEORB) 20 days 5 cm (3D RMS 1-sigma)

The timeliness and performance results of the precise orbital products are provided in the Quality Control section.

Additionally, the GNSS L1B RINEX files for Sentinel-1 are delivered on a daily basis to the PDGS with a latency of 7 days. These files contain the decoded GPS observations (pseudorange and carrier phase) following the International GNSS Service (IGS) RINEX ICD. The Sentinel-1 GNSS L1B RINEX data products are made available to the user community through the ESA Copernicus Open Access Hub.

Moreover, performing POD requires making use of the satellite attitude Quaternions files and validating the results obtained against an external reference solution. The user community can also find the Quaternions files on the ESA Copernicus Open Access Hub. The precise orbit files are not available yet, but they will be soon. Thus, a proper validation against an external solution could be done.

In the meantime, a preliminary Test Data Set (TDS) has been made available to the user community to perform a proper POD. The files for this TDS have been computed by the Copernicus POD Service for August 2018. A release note describing the TDS and where to find the inputs required to do POD is included in the TDS itself.