4 Final Report - Emits - ESA

4 Final
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If the LoS motion during the summation of the successive images is large (typically > 1 pixel), the
motion must be compensated for by shifting the pixels. The simplest correction is the so-called
"nearest pixel motion compensation", where the shift is limited to an integer number of pixels, simply
achieved by a shift in memory and an accumulation. The average MTF loss at the Nyquist frequency is
0.64 on the accumulated image, i.e. equivalent to a shift of one pixel over the whole accumulation. To
reduce this significant degradation of the MTF, refined offset correction methods based on pixel
interpolation are possible, but not retained for Geo-Oculus for their low maturity and the required large
on-board computation and storage capabilities.
The first step is however to measure the LoS motion between two integration phases, with an
accuracy significantly better than half a pixel (0.2 pixel i.e. ~0.1 µrad).
4.4.3.2 LoS drift measurement
The LoS motion information can be extracted from gyroscope measurements, provided they are
mounted close to the focal plane. The following figure shows the LoS drift estimation error for two high
accuracy gyros (Pleiades Astrix 200 FOG and SIRU HRG): Over the maximum image acquisition time
(5 sec), the error is 0.3-0.5 µrad, well above the 0.1 µrad requirement. Gyros are therefore not
adequate for LoS drift estimation.
Gyro drift (µrad)
0,5
0,4
0,3
0,2
0,1
0,0
0 1 2 3 4 5
Time (secs)
Figure 4.4-3: LoS drift estimation accuracy using gyroscopes
ASTRIX200
In the case of GEO-observation with a staring instrument, the motion information can also be
extracted from the image itself, which removes the need for additional motion sensor. The principle is
to correlate in real-time on board the spacecraft the incoming image with the accumulated image, so
as to determine the relative image to be corrected. Either the full image or vignettes of interest are
used. In the first case, the processing load is high, but the algorithm is simple (correlation over a small
moving window) and repetitive, which is well adapted to FPGA or ASIC implementation. In the latter
case, the system shall identify vignettes of interest within the first image using an algorithm detecting
areas with contrasted variations. Then the correlation is performed between the selected vignettes
extracted from the accumulated & current image.
Such techniques are actively investigated at Astrium, primarily for on-ground processing to improve
image quality without relying on pre-defined landmarks. The resulting accuracy of image correlation is
in the order of 10% to 20% of a pixel, that is to say below 0.03 to 0.06 µrad, well with in the 0.1 µrad
requirement. Image correlation is therefore the selected approach for LoS drift measurement.
Doc. No: GOC-ASG-RP-002 Page 4-47
Issue: 2
Date: 13.05.2009 Astrium GmbH
SIRU