Spot 4

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General

Designation	25260 / 98017A
Launch date	24 Mar 1998
Country of origin	France
Mission	Remote sensing
Perigee/Apogee	832 km
Inclination	98.7°
Period	101 min
Orbital cycle	3.7, 26 days
Launch vehicle	Ariane V107

The total cost of the satellite over 6 years (contruction, launch, exploitation) is approx. $500 million

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External resources

http://spot4.cnes.fr/

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External resources

sat-index articles

Technical data

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Specifications

Prime contractor	Matra Marconi Space
Platform	Spot Mk 2
Mass at launch	2755 kg
Dry mass	2600 kg
Payload mass	1390 kg
Dimension	2.5 x 2.5 x 5.35 m
Solar array	4 x 8 m
Stabilization	3-axis
DC power	EOL: 2200 W
Design lifetime	5 years

Telemetry: 1704 MHz
Telemetry: 2218 MHz (4096 kbps)
Telemetry: 8253 MHz
Command: 2042.408 MHz (2 kbps)
Command: 5690 MHz

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2 HRVIR (High Resolution Visible IR) detectors

Wavebands	0.50-0.59 µm (green) 0.61-0.68 µm (red) 0.79-0.89 µm (near IR) 1.53-1.75 µm (mid-IR)
Swath width	60 km (each HRV)
Resolution	Monospectral: 10 m Multispectral: 20 m

CCD with 6000 points. Has the same on-board recorder as Helios 1A provided by Enertec (2 x 120 Gbits). HRVs are steerable to either side of the ground track (+/- 27°) to provide re-visits and stereoscopic images

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Vegetation

Wavebands	0.43-0.47 µm (blue) 0.61-0.68 µm (red) 0.78-0.89 µm (near IR) 1.58-1.75 µm (mid-IR)
Swath width	2000 km
Resolution	1.15 to 1.7 km

Vegetation monitoring Instrument. Onboard storage: 2.25 Gbit

The PASTEL system (PAssager Spot de TElécommunication Laser) use the SILEX (Semiconductor Intersatellite Link Experiment) payload to downlink via Artemis at 50 mpbs. The link is operated at optical frequencies at a wavelength of 800 to 860 nanometers. The light source is a solid state laser diode operated typically at 60 mW optical power. The applied modulation scheme is direct intensity modulation. A photo diode is used as optical data detector.

The difficulty involves aligning and stabilizing two terminals which are on average 38 500 km away from each other, with a pointing error of less than 2 micro-radians. In other words, the beam from PASTEL (which at that distance, would have a diameter of 200 m) would have to be kept centred on OPALE with a tolerance of +/- 38m. This is equivalent to to hitting an orange with a rifle shot from 51 km!

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