# THE SPOT SATELLITES

#### Image acquisition

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### OBLIQUE VIEWING : STEREOSCOPY AND ACCESSIBILITY

#### Stereoscopy

SPOT's oblique viewing capacity makes it possible to produce stereopairs by combining two images of the same area acquired on different dates and at different angles, due to the parallax thus created.
A Base/Height (B/H) ratio of 1 can be obtained for a viewing angle of 24 deg. to the East and to the West. For a stereopair comprising a vertical view and one acquired at 27deg., a B/H of 0.5 is obtained.

Oblique viewing offers two key advantages

#### Use of stereopairs

Stereopairs are mainly used for stereoplotting, topographic mapping, and automatic stereocorrelation, from which Digital Elevation Models (DEMs) can be directly derived without the need for maps.

#### Repeat viewing capability

SPOT's oblique viewing capacity allows it to image any area within a 900 kilometer swath. Oblique viewing can be used to increase the viewing frequency for a given point during a given cycle. The frequency varies with latitude: at the equator, a given area can be imaged 7 times during the same 26-day orbital cycle. At latitude 45 deg., a given area can be imaged 11 times during the orbital cycle, i.e 157 times yearly and an average of 2.4 days, with an interval ranging from a maximum of 4 days to a minimum of 1 day.

Typical image acquisition opportunities in real-time mode for the Toulouse station

### ONBOARD DATA STORAGE AND TRANSMISSION TO GROUND

SPOT satellites can transmit image data to the ground in two ways, depending on whether or not the spacecraft is within range of a receiving station. As the satellite proceeds along its orbit, four situations arise concerning imagery acquisition and image data transmission to ground.

1. The satellite is within range of a Direct Receiving Station (DRS), so imagery can be down-linked in real-time provided both satellite and DRS are suitably programmed.

2. The satellite is not within range of a SPOT DRS. Programmed acquisitions are executed and the image data stored on the onboard recorders.

3. The satellite is within range of a main receiving station (Kiruna or Toulouse). It can thus be programmed either to down-link image data in real-time or play back the onboard recorders and transmit image data recorded earlier during the same orbital revolution.

4. The rest of the time, the satellite is on standby ready to acquire imagery in accordance with uplinked commands.