Minimize POD Products and Requirements

The Precise Orbital products and auxiliary data generated by the Copernicus POD (Precise Orbit Determination) Service for the Sentinel-3 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. Short Time Critical (STC) products make use of the STC products delivered by the EGP with a timeliness of 1 day.
  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, clocks and satellite biases from CODE (the NTC products are currently ambiguity fixed).

For Sentinel-3, the platform data files contain the platform off-nadir angle derived from the on-board computed quaternions and the nadir projection of the SRAL antenna and the satellite centre of gravity.

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

The NRT orbital product (SR___ROE_AX) and Restituted platform data file (SR_2_NRPPAX) generation is associated to the availability of a new GPS L0 file, which typically have a coverage of one orbit. This product is generated by the S3 POD IPF running directly at the PDGS (see CPOD Service section for further information). They have the coverage of the latest ascending node crossing (ANX) contained in the GPS L0 that triggered its generation plus 1 orbits backwards.

The STC orbital product (AUX_MOEORB) and Preliminary platform data file (AUX_PRLPTF) are generated with a timeliness of 1.5 days and cover 26 hours, 1 hour before the start of the previous day and 1 hour after the end of it. Therefore, there is an overlap of 2 hours between consecutive files.

The NTC orbital product (AUX_POEORB) and Precise platform data file (AUX_PRCPTF) is generated with a timeliness of 25 days and covers 26 hours, from 1 hour before the start of day N-25 until 1 hour after the end of day N-25 (N being the current day). Therefore, there is an overlap of 2 hours between consecutive NTC orbit files.

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

Table 1: Copernicus POD Sentinel-3 Orbital Files Requirements
Requirements of Orbital Products
Category Latency Orbit Accuracy
NRT – S3 POD IPF
(SR___ROE_AX)
30 minutes 10 cm radial RMS 1-sigma (target of 8 cm)
STC
(AUX_MOEORB)
1.5 days 4 cm radial RMS 1-sigma (target of 3 cm)
NTC
(AUX_POEORB)
28 days 3 cm radial RMS 1-sigma (target of 2 cm)
Table 2: Copernicus POD Sentinel-3 Platform Data Files Requirements
Requirements of Platform Data Files
Category Latency Pointing Accuracy (w.r.t. theoretical attitude law)
NRT – S3 POD IPF
(SR_2_NRPPAX)
30 minutes 0.09 deg in pitch and roll (3-sigma) 0.1 deg in yaw (3-sigma)
STC
(AUX_PRLPTF)
1.5 days 0.09 deg in pitch and roll (3-sigma) 0.1 deg in yaw (3-sigma)
NTC (AUX_PRCPTF) 28 days 0.09 deg in pitch and roll (3-sigma) 0.1 deg in yaw (3-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-3 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-3 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. In addition, different types of precise orbit files (i.e., NRT, STC and NTC) are available on the ESA Copernicus Open Access Hub. Thus, a proper validation against an external solution could be done.

If necessary, 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.