Continuity and innovation


SPOT 4 is now in the final development stage and is scheduled to be launched at the beginning of 1998.

SPOT 4 offers a number of improvements over its predecessors.

The design lifetime will be extended from three to five years, thus guaranteeing continuity of SPOT services beyond the year 2000. SPOT 4 will consist of a new-generation satellite bus and service module accommodating twice the payload of the SPOT 3 bus. This increased capacity has been used to add an 'equipment bay' housing both the payload telemetry unit and the onboard recorders. The equipment bay can also accommodate passenger payloads, such as the Vegetation instrument.

Improvements to the payload include:

Addition of a new multispectral-mode band in the mid-infrared (1.58-1.75 µm).

  • Onboard registration of all spectral bands. This has been achieved by replacing the panchromatic band (0.51-0.73 µm) by band B2 (0.61-0.68 µm) operating in both a 10-m and 20-m resolution mode.

  • Improved knowledge of ground reflectances, acquired by SPOT 1 and -2, will be used to introduce electronic sensor gains matching according to landscape type and season, thus ensuring greater dynamic range. In addition, the imaging instruments will no longer be susceptible to glare or affected by the polarization of the incident light.

  • The two HRV-IR imaging instruments will be programmable for independent image acquisition, increasing significantly the total number of imaging opportunities. In particular, it will be possible to change the viewing direction of one instrument without affecting the quality of the images acquired at the same time by the other instrument.

  • The recording capacity of each of the two onboard recorders will be increased from 22 to 40 minutes. In addition, an 8.5-Gbit solid-state memory will be added to increase the overall reliability of onboard recording and extend the design life.

  • Payload telemetry transmission to direct receiving stations will be made secure to protect the commercial confidentiality of transmitted data.

  • The prime SPOT 4 mission will be supplemented by those of the following passenger payloads

  • Pastec, a technology demonstration passenger, to study the orbital environment.

  • Doris (precision satellite-based orbit determination and radiopositioning system) onboard package, identical to those carried by SPOT 2 and -3. This system, successfully demonstrated by the Doris/SPOT 2 mission, permits the calculation of the spacecraft's position in space (after ground data processing) to within 10 cm. It is also used to locate ground beacons with the same accuracy. For the SPOT 4 mission, additional software will be tested (Doris/Diode experiment) to determine the spacecraft's position in real time, to within a few tens of metres, using data generated by the Doris package. This position data will, in turn, be included in the auxiliary data transmitted with the payload (image) telemetry.

  • Radar transponder, for calibrating ground radars for satellite tracking (at the CSG Guiana Space Centre and elsewhere).

  • Pastel, or SPOT laser communications passenger, a component of the European Space Agency's Silex experiment. Silex (semiconductor intersatellite link experiment) is a satellite-to-satellite laser communications system using solid-state laser transmitters and receivers. Pastel will transmit image telemetry over a laser link at a high bit rate, via the Artemis geostationary relay satellite scheduled to be launched in 1996.

  • Vegetation instrument, a very-wide-angle (2 000-km-wide swath) earth observation instrument offering a spatial resolution of around 1 km and high radiometric resolution. It uses the same spectral bands as the HRV-IR instruments (B2, B3 and mid-IR) plus an additional band known as B0 (0.43-0.47 µm) for oceanographic applications. The Vegetation instrument is being developed as a European cooperative project including the EC.

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