ENVISAT will be the largest free flying satellite built in Europe. Ten instruments, accommodated on the Polar Platform, compose its payload. The most demanding instrument in terms of mass, volume and power resources is the ASAR with an antenna of some 700 kg but 6 other large instruments also share the satellite resources with mass ranging between 100 kg and more than 300 kg.

ENVISAT's configuration concept provides a large, modular construction, with sufficient earth facing mounting surface for payload instruments and an anti-sun face, free of occultation by satellite subsystem equipment. In-flight, the spacecraft longitudinal axis (the X-axis) is normal to the orbit plane, the Y-axis is closely aligned to the velocity vector and the Z-axis is earth pointing.

The satellite comprises two major assemblies:

• The Service Module (SM) which provides the main satellite support functions
• The Payload Module (PLM) on which the payload instruments are mounted

(Photo: ESA)
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The Payload Module itself is composed of three major subassemblies:

• The instruments either externally mounted or partly accommodated in the Polar Platform
• The Payload Equipment Bay (PEB) which accommodates payload support subsystems and some instrument electronics
• The Payload Carrier (PLC) which provides the main structural support for externally mounted instruments and antennas.

This split provides the basis for a convenient physical and functional separation between the Service Module subsystems and equipment which is needed for every mission and those in the Payload Module which are mission specific and therefore dedicated only to the needs of the particular payload complement being flown. In addition, the interface, decoupling at module level, facilitates parallel development and integration of Service and Payload Modules and allows for an efficient satellite AIT programme where only a minimum of system level activity is needed for final verification.