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Efficient Design and Analysis of Airborne Radomes
The French Space Agency's (the CNES) Spot 5 imaging satellite was successfully placed in an 832 km, sun-synchronous, circular polar orbit by an Ariane 42P rocket on 3 May 2002. Developed in parallel with the Helios II military observation satellite, the 3,000 kg Spot 5 device is described as being designed to offer the best compromise between image resolution and swath width as well as a considerable increase in performance when compared with its immediate predecessor, the Spot 4 vehicle. Spot 5's payload includes a high resolution stereoscopic (HRS) and two high resolution geometric (HRG) instruments, a star sensor, a 90 Gbit solid-state memory, a vegetation instrument (VI) and a Doris station-keeping unit.
Looking at some of these elements in more detail, the satellite's HRS instrument incorporates fore and aft telescopes and can generate digital elevation models (DEM) with an accuracy of 10 m. Here, Spot 5's manufacturer notes that such precision DEMs have a ready market for use in mapping, database, telecommunications, air traffic control and geo-information applications. The two HRG instruments are described as providing multispectral imagery in the B1, B2, B3 and mid infra-red bands and as offering resolutions of 2.5 m, 5 m and 10 m in their super, panchromatic and multispectral modes, respectively. Each HRG operates along a 60 m swath for off-track imaging of ±420 km (±27°). The satellite's star sensor facilitates precise imaging positioning to within 50 m while its solid-state memory is noted as being able to process five image acquisition channels simultaneously and to image telemetry throughput boosted to 2 x 50 Mbps to increase the vehicle's operational capacity. Spot 5's VI operates in the B0m, B2 and mid IR bands, and offers resolution and swath values of 1000 m and 2250 km, respectively. The Doris station-keeping unit is designed to maximise orbital accuracy.
Designed for an in orbit life of 5 years, the successful launch of the 2.4 kW Spot 5 vehicle has ensured that the current Spot satellite constellation has the capacity to image any point on the earth's surface within 24 hours. Equally, the bus design used will form the basis of the European Space Agency/European Meteorological Satellite Organisation's next-generation Metop weather satellite.
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