Direct receiving stations
Archive and Preprocessing Centre

Direct receiving stations

Worldwide, over 20 receiving stations access the SPOT system on a daily basis. They receive and process imagery gathered by the operational SPOT satellites (currently SPOT 1 and SPOT 2).

This map shows the stations and their "acquisition circles". A station can receive imagery whenever a satellite is within its acquisition circle.

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Archive and preprocessing centre

In preparation for SPOT 4's entry into service, CNES set up the archive and preprocessing centre (CAP) on Spot Image premises.
The CAP, produced by MS&I and operated by Spot Image, was commissioned in August 1995. Every day it processes image data from the operational SPOT satellites. Designed from the outset to handle data flows from three spacecraft at once, the CAP is ready to process the first images returned by SPOT 4.

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The CAP in the SPOT acquisition chain

The image telemetry received by receiving stations cannot be converted directly into usable imagery. It is the CAP's job to make the imagery accessible to users.
This involves two tasks : inventorying and production.


cap_loc.jpg (45755 octets)Inventorying involves determining the area covered by the image telemetry, then framing the 60-km-wide data strips acquired by an HRV or HRVIR instrument into scenes. Each scene is then assessed for cloud-cover, snow and radiometric quality.

The resulting data and "quick-looks" are then loaded into the Dali catalogue containing reference data on all scenes acquired by SPOT satellites. The raw data are stored in the SPOT archive.

The SPOT archive

cap_hddt.jpg (10951 octets)The SPOT archive contains all the scenes acquired by the successive SPOT satellites since SPOT 1 was launched in February 1986.

Until 1995, image telemetry downloaded during passes over the Aussaguel station near Toulouse were stored on high-density digital tapes (HDDTs) holding 7 Gbytes each. Today, the CAP's physical archive contains over 15 000 HDDTs corresponding to 15 000 SPOT passes.

cap_sony.jpg (13284 octets)Since 1995, image telemetry has been stored on type-D1 cassettes. These employ a helical recording technique similar to that used by VCR recorders and offer about ten times the capacity of HDDTs. Today, the CAP's physical archive contains 1500 type-D1 cassettes.

The HDDTs and D1 cassettes in the CAP's physical archive represent over 20 000 SPOT passes, or over 50 Terabytes of image data (1 Tbyte = 1 million megabytes).

CNES is responsible for managing and conserving the physical archives. After teaming up with the IGN (Institut Géographique National), CNES launched a programme to store the entire archive first on optical tape, then on digital cartridges.

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Preprocessing involves converting the image data (recorded on HDDTs and other media) into standard SPOT products. More specifically, it involves applying radiometric and geometric corrections to raw image data. Several levels of correction have been defined. Preprocessed imagery can be delivered on computer-compatible tape (CCT), CD-Rom, or photographic media (film, etc.).

The CAP performs so-called "system preprocessing" to levels 1A and 1B. These two levels of preprocessing are designed to compensate for the instrument transfer function (for radiometric corrections) and SPOT geometric imaging conditions and the satellite's orbital motion (for geometric corrections).

Level "1A"

Level-1A preprocessing leaves the data in raw form, save for radiometric corrections. Level-1A data are intended primarily for users requiring image data that have undergone only minimal preprocessing.


Level-1A products are supplied with the following auxiliary data:

spectral band(s),
Grid Reference System (GRS) designators for scene centre,
time and date of acquisition,
preprocessing level,
angle of incidence and scene orientation,
geographic reference marks for corners of scene,
distance between scene centre and nearest GRS node,
Sun azimuth and elevation.

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Level "1B"

Level-1B is the basic preprocessing level including both radiometric and geometric corrections. These corrections compensate for effects due to the viewing conditions and satellite/Earth system.

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page updated on the 04-09-29