SAR Yaw Weekly Reports - Description
Attitude pointing is derived from wave data (imagette) using the Doppler frequency at near range and its variation rate from near range to far range. The process is not straightforward due to the nature of wave data and the limitation of the wave mode processor. In particular the Doppler frequencies estimated by the SAR wave processor and annotated in the fast delivery UWA products, are wrapped in the base band (+/- PRF/2) and they have to be unwrapped in order to obtain the true frequencies values. In nominal 3 gyro operations the Doppler frequencies stay in the baseband for almost the entire orbit and therefore there is no more that 1 PRF error between the wrapped Doppler and the corrected unwrapped Doppler. For the new AOCS configuration (gyro-less) the error can be up to 10 PRF and it is therefore critical to unwrap the Doppler. A simple unwrapping algorithm is applied in "near real time" as soon as UWA data are received in PCS. The algorithm assumes as reference baseband the one when the data become available and unwraps the Doppler frequency taking this baseband as reference. Discontinuities are expected for large data gaps (e.g. due to SAR in image mode or AMI instrument switch-off).
After the unwrapping the yaw angle is computed along the orbit and yaw information (mainly the mean yaw and standard deviation per orbit) are sent to ESOC to re-compute and uplink on satellite new AOCS parameters.
Yaw error angle is also estimated by analysing the Scatterometer raw data. In particular from the Doppler frequency shift of the received echo is possible to retrieve the yaw error angle. That processing is not performed in "near real time" because it needs raw Scatterometer data. Those data are sent from Kiruna ground station to PCS via a satellite link and the fast delivery constraints is not always complied.
One advantage to use Scatterometer data is that the Doppler frequency shift of the echo can be estimated without ambiguity. That Doppler cannot be compared directly with the SAR wave Doppler due to different geometry during the acquisition but it is possible to compare the retrieved yaw angle.
For that reason Scatterometer data are used to assess the quality of the yaw information retrieved by wave processing.
AMI Active Microwave Instrument
AOCS Attitude On-board Control System
PCS Product Control Service
PRF Pulse Repetition Frequency
SAR Synthetic Aperture Radar
UWA Fast Delivery product: AMI wave