SPOT 5 Builds a Global Multi-Purpose 3D Database: Reference3D
2. HRS SENSOR ON-BOARD SPOT 5
The HRS instrument is made of two telescopes allowing a 20° fore view and a 20° aft view over
a 120-km swath, respectively (Figure 1). This concept of along-track stereo view enables both
panchromatic pair images to be obtained within an interval of a 90 seconds for each pixel line in
a single pass (Figure 2). This is a major advantage which ensures maximum correlation between
the two stereopairs images since the time interval is the chief obstacle when generating a DEM
(due to the changes of landscape). The resulting B/H ratio is slightly higher than 0.8. Each
telescope has a 580 mm focal length and a focal plane made of 12000 detectors, size 6.5µm. The
instantaneous field of view is 10 m but the sampling rate is 5 m along the track allowing higher
altimetric precision of the DEM (Digital Elevation Models) to be obtained (in-flight calibration
assessment was 4m rms). As fore and aft image acquisition operations are exclusive, the length
of a continuous stereo segment cannot exceed 600 km. However several segments can be
processed together for mass production.
Figure 1 : HRS camera
Figure 2 : The acquisition process of HRS stereopairs
The accurate on-board determination of the satellite position (1m rms) and the absolute dating
are supplied by DORIS. Both gyroscopes and a star tracker provide the attitude of the satellite.
This information is sent with the HRG and HRS instruments telemetry for further precise
geometric processing on ground : 35m location accuracy for HRG and 25m for HRS without
any ground control point. The SPOT 5 satellite is steered around the yaw axis so as to balance
the earth rotation and both optimize the overlap between the stereo images taken by HRS and
ensure that the shift between the 2 images taken in very high resolution (VHR) mode is exactly
0.5 pixels across the track (Gleyzes, 2003).
