Impact of high solar activity on Sentinel-1A AUX_PREORB POD files

19 July 2023

Worse than usual orbit accuracies have been obtained in the Sentinel-1A POD predicted orbit file (AUX_PREORB) for the past months due to the high solar activity. The S-1 SAR Mission Performance Cluster (SAR MPC) has confirmed that the current levels of accuracy of the predicted orbit files are still good enough to support the SAR focusing. Restituted orbit files (AUX_RESORB) show no routine degradation, except for a few cases in which strong solar geomagnetic storms took place – these were reported as dedicated S1 Quality Disclaimers (ie Precise orbit files (AUX_POEORB) are in principle not affected by the higher solar activity. 

In order to quantify the degradation in the predicted orbit files, the AUX_PREORBs are routinely compared against the combined solution, which is a weighted average of non-time critical precise orbit products generated by the members of the Copernicus POD Quality Working Group (CPOD QWG), and is taken as reference to assess the accuracy requirements. The comparisons since the AUX_PREORB are generated are shown below, together with the F10.7 solar flux index, where the strong correlation between the increasing solar activity and the reported degradation can be observed. We attach also the 1-sigma per year (the value below which 68% of the comparisons are) of the AUX_PREORB and the S-1A GPS on-board navigation solution (which was used before the AUX_PREORB to support SAR focusing). This illustrates that predicted orbits are still much better than the navigation solution, even considering the recent worsening.

For context, the solar activity varies with a 11-year cycle. When there is high solar activity, a combination of effects makes the orbit computation more challenging: on the one hand, GNSS observations on-board become noisier due to the effects of the solar flux on the receiver; on the other hand, and more relevant for Precise Orbit Determination (POD), the atmosphere becomes more difficult to model because it changes its state more rapidly. This is especially cumbersome in predicted and near-real time products because predicted solar coefficients are used which then may differ greatly from the observed ones. The impact is especially remarkable when there are strong solar geomagnetic storms, of which there have a few occurrences in the past few months (e.g. 23/03/2023 and 23/04/2023).

At the Copernicus POD Service we are taking action to mitigate the effects of the high solar activity in the two mentioned fronts:

  • The CPOD QWG has issued a recommendation to modify the tracking settings of the GPS receivers on-board the Copernicus Sentinel satellites to reduce the impact of the high solar flux on the observations.
  • The CPOD Service is carrying out analysis to find a more robust predicted orbit parametrization which allows to model better the atmosphere, and is also assessing the possibility to update more frequently the solar coefficients.

Any future update or noteworthy degradation will be properly communicated through this channel, and via dedicated Quality Disclaimers if it impacts any AUX_RESORB or AUX_POEORB products. In addition, further information and any status update will be reported at the FRINGE ’23 (11-15 September 2023, University of Leeds).

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