ERS-2 AMI Low bit rate data (fast
delivery wave spectrum from SAR wave mode and raw Scatterometer data) are
systematically used in PCS to derive satellite attitude information.
Untitled
Weekly Results from 110620
to 110626
Figure 1 shows the evolution of the mean yaw (over one orbit) and its standard
deviation for the period covered in this report.
During the YCM operations, a mean yaw per orbit within [-2.0, +2.0] degrees is expected.
The following table resume the yaw statistics for this period:
Yaw statistics
Prod
Mean
Std. Dev.
Min.
Max.
HEY
-0.42175837
0.89887170
-1.7200
1.3621
click on image for enlargement
Figure 1 Mean Yaw per orbit
Yaw statistics over KS anx+1350\
Prod
Mean
Std. Dev.
Min.
Max.
HEY
-0.20492791
0.77288850
-1.7200
1.3621
click on image for enlargement
Figure 2 Mean Yaw over anx [1350, 1500] secs
Long term results
click on image for enlargement
Figure 3 Mean Yaw per orbit
High Level Processing 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.
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