SPOT


Satellite Pour l'Observation de la Terra (SPOT) was designed by the CNES (Centre National d'Etudes Spatiales), France, and developed with the participation of Sweden and Belgium.

SPOT 1 was launched on 22 February 1986, and withdrawn from active service on 31 December 1990. SPOT 2 was launched on 22 January 1990 and is still operational. SPOT 3 was launched on 26 September 1993. An incident occured on SPOT 3 on November 14, 1997. After 3 years in orbit the satellite has stopped fonctioning. After that incident, SPOT 1 was recalled into active service again from January 1997. The next satellite, SPOT 4, was launched on 24 March 1998. Engineering work for SPOT 5 has began so that the satellite can be launched late in 2001 to ensure service continuity.

SPOT data are visible and near-infrared radiance data obtained from High Resolution Visible (HRV) sensors carried on the SPOT satellites. For SPOT 1,2 and 3, each satellite carries two HRVs with the capability of scanning in either a multispectral mode or a panchromatic mode. The multispectral mode captures data in three spectral bands: .50-.59, .61-.68 and .79-.89 micrometers. The three bands are co-registered and have a ground resolution of 20 meters. For SPOT 4, the multispectral mode is improved to provide 4 spectral bands. The 4th band is the 1.58 - 1.75 micrometer SWIR band. The panchromatic mode of SPOT 1,2 and 3 images data in the spectral range of .51-.73 micrometers at a 10 meter ground resolution. For SPOT 4, the panchromatic mode images data in teh spectral range of .61-.68 micrometers at a 10 meter ground resolution.

The viewing angle of each HRV sensor can be adjusted to collect data up to 27 degrees right or left of satellite nadir. This cross-track pointing capability allows the same point on the earth to be viewed from several different orbits and enables the acquisition of stereoscopic imagery.

Extent of Coverage

The SPOT system provides global coverage between North 87 degrees latitude and South 87 degrees latitude. Each nominal scene covers a 60 by 60 km (37 by 37 square mile) area.

Acquisition

The SPOT satellites operate from a sun-synchronous, near polar orbit 832 km (517 mi) above the earth. The satellites are inclined 98.7 degrees, cross the equator at 10:30 AM local time and have an orbital cycle of 26 days. The ground imaging swath is 60 km (37 mi) per HRV sensor. With both HRVs scanning in the twin vertical viewing mode the cross-track swath is 117 km (73 mi). Each HRV sensor has the capability to scan 27 degrees off-nadir (earth curvature effects produce a 31 degree angle) allowing for repeat coverage of an area up to every three or four days depending upon the latitude of the area. The SPOT network consists of 18 worldwide ground receiving stations that acquire SPOT data in a real-time mode. Additionally, the stations at Toulouse, France and Kiruna, Sweden can download data acquisitions during night passes that were recorded on board the satellites.

Processing Steps

Data acquisition begins when a station's antenna has locked onto the SPOT payload telemetry signal after automatically tracking the satellite from the time it rises above the horizon. The incoming signals received are demodulated, synchronized (8 GHz carrier filtered away) and then recorded on two high density data tapes (HDDTs) operating in full-redundancy. Only one of the two tapes is considered to be a master at any given time, the other serving as a back up should the first develop an error.

The HDDT contains all the image data transmitted by the satellite and is the system archive. It is not, however, directly usable by researchers.

Archiving and inventory operations consist of playing back the raw data from the HDDTs, locating the image data, dividing the strip imaged by each HRV sensor into scenes and then creating a directory for the HDDT with this information in it.

Following those steps, the SPOT catalog is automatically updated. Each archived scene is defined by the following characteristics:

When a user requests a particular scene, the archived data undergoes preprocessing. Five standardized preprocessing levels are available to the user:

The raw data are decompressed, corrected and recorded onto magnetic tapes usable by researchers. These tapes are known as Computer Compatible Tapes (CCTs). The tapes are needed to produce full resolution photographic film as well as being end products themselves.

Data Organization

SPOT digital tapes are unlabeled with the number of volumes being determined by the imaging mode, viewing angle and level of processing. SPOT scenes vary from approximately 27 megabytes to approximately 100 megabytes of data depending on the level of processing and geometric corrections performed. A panchromatic scene consists of a single band image and the 3-band multispectral scene is organized as band interleaved by line (BIL).

Image data are recorded such that the first data pixel corresponds to the extreme Northwest corner of the scene; succeeding pixels corresponding to those immediately East of the first pixel, then line by line from North to South. All image data regardless of spectral imaging mode or level of processing are recorded as right-justified 8-bit pixels.

For additional information on data organization, select one of the topics listed below.

Products and Services

CRISP's ground station is capable of generating the following standard products:

in SPOT CAP format,
and GEOSPOT format (SPOTVIEW®).

Standard SPOT products consist of digital 9-track tapes at 6250 bpi, 8mm Exabyte and CD-ROM products.

Appendix

Grid Reference System (GRS)

The SPOT Grid Reference System (GRS) is used to identify the geographic location of SPOT images. The grid is made up of nodes located at the intersection of columns (K) and rows (J).

When data strips are split into scenes, the SPOT GRS links each scene with two K,J designators representing a node on the GRS. Once the K,J designators have been identified for a new scene, they are added to the list of the scene's main characteristics file.

The Grid Reference System indicates the nominal location of scenes that can be acquired in the twin vertical viewing configuration for any region in the world minus the polar zones. In the case of oblique viewing, the scene centers do not normally coincide with the GRS nodes (defined by the surveying conditions of twin vertical viewing). SPOT scenes acquired in oblique viewing mode are identified by the K,J designators of the node closest to the scene center.

The GRS divides the Earth into five zones forming a symmetrical pattern on either side of the Equator. This division is dictated by the satellite's orbital characteristics and more specifically by the convergence of the ground tracks at high latitudes:

In the north, intermediate and south zones, the K columns are arranged parallel to the satellite ground tracks while the J rows are latitude lines.

The pattern of nodes within the three zones is defined in terms of satellite viewing conditions corresponding to the twin vertical viewing configuration. It indicates the nominal location of the centers of scenes yielded by this viewing configuration. Oblique viewing will place the scene center always on a J row but the center may not coincide in longitude with a GRS node.

In the two polar zones the GRS node pattern is independent of satellite orbital and viewing characteristics. The pattern is obtained by hexagonal dissection using quasi-equilateral triangles where each side represents approximately 26 km (16 mi).

The K columns are derived directly from the SPOT reference tracks. Each track number N corresponds to two K columns:

K = 2N-1 associated with HRV-1 and located West of track N (odd number)
K = 2N associated with HRV-2 and located East of track N (even number)

The distance between these two columns (i.e. between K=2N-1 and K=2N) is constant at about 58 km (36 mi) and is a direct result of the twin vertical configuration. Since the GRS nodes are located on either side of the reference tracks, the scene centers obtained in vertical viewing do not coincide with the GRS nodes. This constitutes an important difference from the Landsat Worldwide Reference System (WRS), where the nodes are located on the tracks and not on either side.

The J rows correspond to latitude lines (i.e. all GRS nodes at the same latitude share the same J designator). The interval between the rows has been calculated to ensure that endlap occurs between two successive scenes. The scenes of a given data strip are segmented in such a way that the centers are located on two adjacent rows, J and J+1.