4 Final Report - Emits - ESA

4 Final
Report
4.4 Line of Sight (LoS) Stabilisation Concepts
4.4.1 LoS stabilisation main issues: microvibrations and post-integration
The LoS pointing stability requirements are very stringent for the aspects of the mission involving high
resolution imaging. This is particularly the case during PAN imaging for disaster monitoring, since the
20 m resolution over Europe corresponds to 0.28 µrad nadir resolution, and to a lower extent when
acquiring full resolution VNIR images (40 m resolution over Europe, 0.56 µrad nadir resolution) for
disaster or fire monitoring. Image quality is then highly sensitive to LoS motion over the short
integration time (up to ~100 msec) required to meet the moderate SNR requirements.
On the contrary, the Marine applications, with a resolution relaxed to 80 m over Europe (i.e. 1.1 µrad
nadir), are less sensitive to pointing stability over the image integration time. Nevertheless, since
several images need to be post-integrated to reach high SNR requirements, image quality is more
sensitive to pointing drifts over the total image acquisition time (several sec).
LoS Stabilisation requirements derived from instrument design are summarised in the following table:
RPE: Relative Pointing Error
(stability over the integration time)
RME: Relative Measurement Error
(over image acquisition time)
PDE: Pointing Drift Error
(drift over integration time)
0.15-0.2 µrad peak-to-peak for high frequency jitter (>10 Hz)
0.1 µrad over 5 s max. acquisition time
Marine applications: 10 µrad/s
Fire/Disaster monitoring: 5 µrad/s
Such specifications can not be met without proper management of the LoS pointing stability issue.
Depending on the type of disturbances that challenge the LoS stability requirements, and in particular
depending on the frequency band affected, different solutions might be proposed for Geo-Oculus :
• High frequency perturbations, with period lower than typical integration time (100 msec), i.e.
frequency > 10 Hz, require disturbance reduction techniques. Such disturbances are mainly due to
microvibrations generated by moving parts (e.g. reaction wheels and cryocoolers).
• Medium frequency disturbances (with period in the range of a few sec, corresponding to the time
to acquire an image based on accumulation of several shots) require image processing
techniques to enable post-integration. Such disturbances are mainly due to solar array flexible
mode excitation after slew manoeuvres.
• Low frequency disturbances are handled by the AOCS for those observed by the attitude sensors
and by ground-based processing (so-called INR, Image Navigation & Registration) for LoS
pointing errors due to orbit errors and thermo-elastic distortions between LoS and AOCS
reference.
Therefore, the LoS stabilisation issues to be addressed are:
• Microvibrations: level reduction through careful selection of actuators and identification of
potential other disturbances.
• Post-integration: Evaluation of the technique to be used to estimate the shift between one
image and another one before adding them.
Doc. No: GOC-ASG-RP-002 Page 4-43
Issue: 2
Date: 13.05.2009 Astrium GmbH