Description of Mission

More detailed figure of TOPEX/Poseidon

Satellite Bus (Multimission Modular Spacecraft or Multimission Platform)

The satellite bus was taken from the Multimission Modular Spacecraft (MMS), which has been proven on previous MMS-based missions: the Solar Maximum Mission and Landsat 4 and 5. The MMS consists of four primary modules and two sub-modules. The primary modules include the Propulsion Module, the Power Module, the Altitude Control Module, and the Command and Data Handling Module (CDHM). The sub-modules include the Earth Sensor Assembly Module and the Signal Conditioning and Control Unit.

Propulsion Module

The Propulsion Module, which is attached to the stern of the Multimission Modular Spacecraft, consists of four identical Rocket Engine Modules, three main hydrazine tanks, one auxiliary tank, and an aluminum support structure. The Propulsion Module provides the ways for the spacecraft to make some adjustments to its position ("Orbit Maintenance Maneuvers") and its orientation ("Attitude Control") by firing the thrusters.

Power Module

The Power Module holds three rechargeable batteries, electronic components to keep electrical power within safe limits, and the Electrical Power Subsystem electronics to distribute electrical power to all subsystems and sensors on the spacecraft and also to control charging and regulating the batteries.

Attitude Control Module

The Attitude Control Module, which is attached to the zenith side of the MMS, contains most of the electronic hardware used by the Attitude Control System (ACS) to control the orientation of the satellite in space. They include two optical sensors called advanced star trackers (sticking out of the starboard wall of the Attitude Control Module), the digital fine sun sensor, four reaction wheels and magnetic torquer bars. These functions are controlled by the on-board computer in a normal mode and by the electronics in the ACS in some safe-hold modes.

Command and Data Handling Module

The Command and Data Handling Module, which is attached to the port side of the MMS, contains on-board computer, the tape recorders, the transponders, and other electronic equipment responsible for telemetry, data, and computational support for all of the spacecraft's subsystems and sensors.

Earth Sensor Assembly Module

The Earth Sensor Assembly Module (ESAM), which is visible from both the nadir and stern sides of the MMS, has two solid-state infrared sensors which automatically scan in a circular pattern to locate the Earth. The sensors detects the infrared signature of carbon dioxide in the Earth's limb and the ESAM reports the location of the Earth to the ACS

Signal Conditioning and Control Unit

The Signal Conditioning and Control Unit (SC&C), which is attached to the nadir side of the MMS, contains heater control and pyrotechnics activation circuitry. The heater control regulates the temperature inside the spacecraft so that all the equipment can function properly. The pyrotechnics activation circuitry is for pyrotechnic devices which are used to fasten components on the spacecraft that are to be released later in a mission. Pyrotechnic bolts, for instance, were used to fasten the solar panels and were blown apart by SC&C after launch so that the solar panels could unfold and expand automatically.

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This page is created by
Masaharu Suzuki
The University of Texas at Austin


Last Modified: Wed Feb 11, 1999