The Spot 4 launch has breathed new life into the French Earth observation satellite programme

Andrzej Jeziorski/KOUROU

Just another 2t of junk in a relentlessly growing orbital scrapheap, Spot 3 still zips from pole to pole, awaiting its end as a fiery skid mark across the upper atmosphere. In October 1996, a failed gyroscope caused it to tumble out of control. Its solar array no longer aligned with the sun, the low-Earth orbit observation satellite's batteries drained, leaving it a multi-million dollar, deaf, dumb and blind Spot.

Spot 3 had lived out its official three-year design lifetime. Nevertheless, its end came earlier than its operator, the French National Space Studies Centre (CNES), had anticipated. As an emergency measure, the agency reactivated the 11-year-old Spot 1, which was still serviceable despite the failure of its on-board data recording system. Spot 2, launched in January 1990, remained similarly serviceable, although its recorder had failed. The two veterans have since continued to provide data to Spot Image - a commercial venture, 35.3%-owned by CNES, which markets and sells the satellite images - filling the gap until the launch of Spot 4, now completed by Arianespace using an Ariane 4 on 23 March (Flight International, 1-7 April).

Described by the programme's deputy director Michel Arnaud as "the first of the second generation" of Earth observation satellites, Spot 4 offers many new capabilities.

At 2,750kg, it is 50% heavier than Spots 1 and 2. Its solar arrays generate 2,100W - 90% more power than that generated by the older satellites and it has a design life of five years - two years longer than its predecessors.

In place of the earlier Spots' high resolution in the visible (HRV) cameras, Spot 4 carries two high resolution in the visible and infrared (HRVIR) instruments, offering 10m resolution in the monospectral mode, and 20m in the multispectral mode across four wavelength bands: B1 (0.5-0.59 microns); red, or B2 (0.61-0.68 microns), near-IR, or B3 (0.79-0.89 microns); and the new short-wave IR (SWIR) band (1.58-1.75 microns). The SWIR band is useful for vegetation monitoring and crop forecasting.

The resolution offered by the HRVIR cameras is the same as that on the earlier Spots, and has up to now been limited by the French authorities, which have refused to allow the commercial use of images with resolution better than 10m. Spot Image, which now claims 60% of the orbital image market, will soon find itself competing against US companies such as OrbImage offering resolution as high as 1m.

Such images will be expensive, however, and will only offer a swath of (in OrbImage's case) some 8 x 8m, while Spot 4 covers 60 x 60km. Arnaud adds that CNES was granted permission early last year to provide better resolution to commercial customers:the next generation Spot 5 satellite, due to become operational in 2002, will be capable of 2.5m resolution.

Another major new feature of Spot 4 is the Aerospatiale-built Vegetation instrument. Vegetation is 50% funded by the European Union (EU), with the rest of the money coming from the Belgian, French, Italian and Swedish governments, and has been developed to monitor crops and investigate environmental change.

The 160kg instrument offers images with 1km resolution across a 2,200km swath in the B0 (blue, 0.43-0.47 microns), B2, B3 and SWIR bands. The EU plans to use the imagery to support its agricultural, environmental and research policies, and the data will be used particularly by the Institute of Applications of Remote Sensing, at the Common Research Centre in Italy. The USForeign Agricultural Service is also understood to be interested in Vegetation data, which will be marketed commercially by Spot Image.

The failure of the magnetic tape recorders on Spots 1 and 2 have meant that they can only transmit data to Earth in "real time" when within a 2,500km radius of the receiving stations at Toulouse, France, and Kiruna, Sweden. The satellites pass at least once daily and are in range for about 13min. Without the recorders, which taped 22min of data (66 gigabytes) on the older satellites, images which could have been collected out of range of the ground stations are lost.

Spot 4 also has a magnetic tape recorder, with 40min (120 gigabytes) capacity, and this is backed up by an additional 10 gigabytes of solid state memory. According to Arnaud, Spot 5 will offer 190 gigabytes of solid state recording capacity, but there is an alternative to this system of data collection and transmission - to relay collected data to Earth "live" via a system of three geostationary satellites.

On board Spot 4 is one part of a semiconductor laser inter-satellite link experiment which should prove the workability of such a system. Data will be transmitted from Spot 4 by laser to an optical terminal aboard the Artemis communications satellite, due for a 1999 launch, which will then relay the signal to Earth.

Arnaud hopes that Spot Image sales will cover the operational cost of the Fr3.4 billion ($560 million) satellite, but believes it will be decades before the market could cover development costs as well. Still, the market is growing, and with work well under way on Spot 5 and plans already being drafted for Spot 6, Spot Image plans to remain a front runner.

Source: Flight International