SPOT

Since 1986 France has operated the highly regarded SPOT (Satellite Pour l'Observation de la Terre) high resolution imaging satellite system. Developed and funded by ONES, SPOT is managed on a commercial basis by SPOT Image with multi-national shareholders, which hope to be free of all government subsidies by the late 1990's (Reference 533). Experience gained by the French through the SPOT program is being applied to a series of space endeavors including ESA's Polar Platform and French national security satellites.

SPOT satellites orbit the Earth at an altitude near 825 km and an inclination of 98.7 degrees to produce a 26-day repeating groundtrack pattern with sun-synchronous conditions. SPOT 3 joined SPOT 1 (February, 1986) and SPOT 2 (January, 1990) after its launch on 26 September 1993. All three spacecraft are in essentially co-planar orbits.

Each successive SPOT spacecraft has been approximately 40 kg heavier than its predecessor, culminating in SPOT 3's 1,907 kg launch mass. The spacecraft bus is 2 m by 2 m by 3.5 m with a hydrazine-controlled 3-axis stabilization system. A 15.6-m-long solar array produces nearly 1.4 kW of electrical power at start of life.

The heart of SPOT is a pair of high resolution visible COD scanners with both multi-spectral (0.50-0.59 µm, 0.61-0.68 µm, and 0.790 89 µm) and panchromatic (0.51-0.73 µm) features. The former mode returns images with a ground resolution of 20 m, while the latter is capable of 10 m resolution. The swath of each scanner is 60 km, but the pair can be operated simultaneously to produce a 117 km swath with a small (3 km) overlap region. Off-nadir viewing is also possible with the aid of a tilting mirror extending the swath to 80 km for each scanner at an angle of 27 degrees from nadir. Images can be transmitted in realtime directly to a world-wide network of ground stations or may be stored on board the spacecraft for later downlinking via two Odetics tape recorders, which were improved for SPOT 3. SPOT 3 also carried the US Polar Ozone and Aerosol Measurement (POAM) experiment (References 534-539).

Developed by a team led by Matra Marconi and Aerospatiale, SPOT 1 was initially retired at the end of 1990, nearly two years past its design life of three years. However, the spacecraft was recalled to service during March-October, 1992, and April-July, 1993, to help meet imaging demands during the principal Northern Hemisphere growing season, although both its tape recorders had failed. The vehicle was placed in standby mode again after SPOT 3 was declared operational. SPOT 2, also with limited tape recording capability, serves as the prime backup to SPOT 3 as new uses of the SPOT system continue to be found (References 540-547).

Whereas the 2,500 kg SPOT 4, scheduled for launch in 1997, will possess several significant improvements over its predecessors (in particular, greater power availability, longer life, and additional mid-IR band of 1.58-1.75 µm), SPOT 5 (~2000) and SPOT 6 (~2005) will represent the second generation SPOT spacecraft. The imaging instruments will be modified to permit 5-m resolution or better in each of two bands (panchromatic and near-IR) while the multi-spectral capability will be improved to 10 m resolution. The mass of the spacecraft will likely be at least 1,000 kg more than SPOT 4. Greater use of stereo imaging will also be possible (References 527, 548-555).

Studies are also underway for a SPOT variant hosting a synthetic aperture radar not unlike ERS (Reference 556). Another French proposal named BEST would carry even more sophisticated radar and lidar sensors for environmental studies (Reference 557). Meanwhile, FrenchEarth observations are operational on the multinational Topex/Poseidon (DORIS [Doppler Orbitography and Radio-positioning Integrated Satellite] and radar altimeter) and UARS (Wind Imaging Interferometer). In 1994 the French SCARAB radiometer was flown on a Russian Meteor 3 spacecraft, in 1995 the French Alissalidar will be on board the Russian Priroda module for the Mir space station, and in 1996 the French POLDER instrument will be carried by the Japanese ADEOS spacecraft.