International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
  
1.2 HRS' Geometry 
HRS is fully integrated on board SPOTS and ancillary data 
provided with each image of an HRS stereopair has got same 
characteristics as SPOTS High Geometric Resolution principal 
mission ancillary data. 
All data necessary for a physical geometric modelling of an 
image are given in a DIMAP format file named 
METADATA.DIM : 
- dating is made through two models : correspondence 
between board time (BT) and universal time (UT) is given 
by Doris ultra stable oscillator and correspondence 
between image line and board time is given by onboard 
oscillator. Image centre line time, line period and each 
ephemeris and attitude sample time are deduced from these 
models and given in universal time ; 
- satellite position and velocity is given every 10 s by 
DORIS DIODE on board orbitography determination. The 
best orbit arc is fit on DIODE data thanks to TRIODE 
software and result in ephemeris points given every 30 s. ; 
- Satellite yaw, pitch and roll orientation angles are 
computed out of quaternions delivered by the on board 
stellar location unit (ULS) and given every 125 ms (8 Hz) ; 
- pair of viewing angles psiX, psiY are calibrated on ground 
.. and given for each detector of the CCD array. 
Details about how to proceed with the geometric modelling of 
an image out of its METADATA.DIM are given in [Spot 
Image 2002]. 
With those, HRS location performance is expected to be better 
than 50 m rms (specification). Before launch expectations even 
showed that a 25 m rms performance could be reached. 
This performance level, much better than for the previous 
SPOT satellites, is mainly due to the onboard stellar location 
unit. This system uses both star tracker and gyroscopes data to 
compute absolute orientation of the satellite. The result is a 
good low frequency accuracy thanks to the star tracker, and a 
good high frequency accuracy thanks to the gyroscopes. 
1.3 Geometric calibration of HRS performed during the 
quality image commissioning phase 
The image quality commissioning phase was divided into two 
parts. The first two months were dedicated to first calibrations 
and characterisation in order to ensure that SPOTS was 
fulfilling its specifications and ready for commercial 
exploitation. During the next 4 months, image quality 
commissioning went on, with a fine characterisation, and 
improvement of the first calibration processed. 
HRS' geometric calibration during this phase has consisted in : 
- measuring orientation bias of the two cameras on SPOTS 
platform thanks to ground control points (outer 
orientation) ; 
- calibration of the optic distortion and relative orientation 
of the two cameras by comparison of HRS images with a 
precise aerial reference (inner orientation). 
This process has resulted in updates of the list of viewing 
directions psiX, psiY given for each detector (See [Bouillon, 
2002], [Breton, 2002] and [Gachet, 2004] for more details about 
geometric calibrations and performance measurements carried 
out during the commissioning phase). 
At this point, the goal for HRS was to meet the location 
specification of 50 m rms and it was attained. Still, HRS 
challenge is to provide a worldwide database of Digital Terrain 
Models and Orthoimages, as automatic as possible, called 
Reference3D. This database is produced from HRS acquisition 
with no use of ground control points and a horizontal location 
specified as 16 m for 90% of the points (see [Airault, 2003] and 
[Bernard, 2004]). 
So, at the end of the commissioning phase, some steps remained 
to get location results in accordance with pre flight expectations 
and it was also necessary to check that Reference3D 
specifications were held. 
2. PRINCIPLES FOR OPTIMISATION OF HRS 
». LOCATION PERFORMANCE 
After the end of the commissioning phase, some possible 
improvements of HRS location performance were already 
identified. In this section, we will describe method and means 
used to complete the work performed during the commissioning 
phase in order to get the best possible location performance for 
HRS. 
2.1 Adopted working method 
A working group involving French Space Agency (CNES), 
French Mapping Agency (IGN), SPOTS satellite integrator 
(ASTRIUM) and star tracker developer (SODERN) has been 
created. The method held back was to carry on an analysis of 
every potential error source affecting HRS' location 
performance at three levels of interpretation : 
- elementary sources of errors, such as attitude restitution's 
accuracy, structure deformations or geometric calibration 
remainders ; 
- System location performance : in this context, each image, 
even of an HRS stereopair, is considered independently and 
the location performance is computed separately for each 
camera ; 
- location performance obtained with the added value of tie 
points between the two images of an HRS stereopair 
(stereoscopic location performance) and of several HRS 
stereopairs considered in a bundle block adjustment process 
(with only tie points between the different strips, no ground 
control points are used). 
Each level of interpretation allows the detection of different 
phenomena likely to have some impact on the location 
performance. Each phenomenon has been analysed to 
understand its source and exact impact. Potential improvements 
have been examined, but only in terms of on board modification 
of existing threshold or parameter, or of ground algorithms. 
Finally, improvements considered as necessary have been 
carried out and validated, as well as monitoring activities 
necessary to uphold the performance.