The Vegetation instrument The Vegetation cameras Simulations of Vegetation imagery

  VEGETATION is an independent instrument which complements the main SPOT 4 payload, HRVIR (High Resolution Visible Infrared).

It is a complete and autonomous system made up of the following subsystems :

• an imaging instrument consisting of 4 independent cameras,
• an image processing module for synchronizing transmission rates and formatting,
• an onboard management system for activating and monitoring equipment,
• a recorder storing up to 97 minutes of imagery,
• X-band telemetry for transmitting recorded data,
• L-band telemetry for direct transmission of imagery,
• a control unit and calibration lamp as well as heaters for thermal control.

These subassemblies are integrated into a single package which is mounted at the base of the HRVIR instruments, with a volume of 1 x 1.1 x 0.7 metres and a mass of 152 kg.

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The Vegetation cameras

The instrument design uses 4 cameras, one for each spectral band, with each one covering the whole field of view.

These spectral bands are the same as those for the SPOT 4 HRVIR instruments.

However, unlike the HRVIRs, the VEGETATION cameras cover a very wide field of view of 101° producing a swath width of 2 250 km. The nominal resolution for optimizing the instrument mission is defined by pixels of 1.165 x 1.165 km instead of the ten metres for the HRVIRs.

The instrument gathers light reflected by the Earth's surface and transforms it into discrete pixel values.

The radiation emitted by the area under observation passes through a set of thirteen lenses. A wide- angle or spherical-parabolic lens at the entrance of the instrument transforms the incoming light into constant illumation in the focal plane, where the sensors are placed.

The sensors for the visible channels are CCD silicon detectors very similar to those used in digital cameras.
The shortwave infrared array consists of light-sensitive Gallium Arsenide, InGaAs detectors

Each camera has :

• an array of 1 728 light-sensitive detectors (elementary detectors), identical to those used by SPOT 4 in the visible (the B0, B2 and B3 spectral bands), each measuring 13 x 13   mm (micrometres),
• and also a line of 10 modules of 300 detectors each for the shortwave infrared, with each elementary detector measuring 20 x 30 mm).

Following analogue equalization, the four spectral channels are grouped together and multiplexed by an analogue-to-digital converter (ADC) using 10 bit encoding.

The signals are sent to the video electronics package for digitizing. They are then transmitted to the image processing device which sends them either to the onboard recorder or to the telemetry channel.

Telemetry can be sent at a ' global mode' level at 8 153 MHz known as the X-band,   at a rate of 3.4 Megabits per second (Mbps). Over certain regions it can also be transmitted in L-band (at 1 704 MHz) at a rate of 0.51 Mbps.

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Simulations of Vegetation imagery

	This animated illustration shows the swath covered by the Vegetation instrument. The scenes are acquired during the day over exposed land and viewing stops when the satellite crosses the oceans. In this example, the viewing begins over Greenland, then starts again over Newfoundland before covering South America.
If you are patient enough, click on the image to get an enlarged animated picture. (be warned, this is a 1.17 Mbyte file)

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page updated on the 00-06-06