Description of Mission

Rocket Engine Modules

Each of the four rocket engine modules contains one large thruster which produces 22 N (4.9 lb) of thrust and three smaller thrusters, each of which produces 1 N (0.22 lb) of thrust.

Digital Fine Sun Sensor

The spacecraft has a digital fine sun sensor mounted on the port side of the Attitude Control Module. What it does is that like the star trackers, it helps the on-board computer to determine the orientation of the spacecraft. The sensor locates the sun and send out that information to the on-board computer, which uses it for orbit and orientation adjustments.

Star Trackers

The spacecraft has two (advanced) star trackers mounted on the starboard side of the ACM. What they do is that like the digital fine sun sensor, they help the on-board computer to determine the orientation of the spacecraft. They are fixed solid-state cameras and are pointed at different areas of the space. They detect and track stars, whose information is sent out to the on-board computer, which compares it with that from the catalogue of 327 of the brightest stars stored in the computer and determines the orientation.

Reaction Wheels

The spacecraft has the primary attitude control system called reaction wheels inside the Attitude Control Module. The wheels are electromagnetic devices that adjust the orientation of the spacecraft by using the momentum of spinning wheels. There are four of them, three of which are assigned to adjust the orientation along three major axes (X, Y, and Z) of the spacecraft and the other is set at an angle to all the three axes so that it provides an auxiliary force to the other three wheels.

Magnetic Torquer Bars

Magnetic torquer bars are electromagnets that is used to turn the spacecraft by pushing against the magnetic field of the Earth.

On-board Computer

The on-board computer stored in the Command and Data Handling Module is the heart of TOPEX/Poseidon. It is a NASA Standard Spacecraft Computer integrated from the previous MMS-based missions: the Solar Maximum Mission, Landsat 4, and Landsat 5. The computer is responsible for most of the essential part of the operations. It receives, stores, and executes commands from the ground stations, and it handles telemetry for downlink. It performs attitude control computations and it controls attitude in a normal mode. In addition, the computer controls the scientific instruments, the high gain antenna, the solar array, the tape recorders, launch vehicle separation detection, and ephemeris computation. Monitoring power and the health and status of the spacecraft is also a very important role of the on-board computer. One of the fascinating features of the on-board computer is the back-up system. When a system fails, the computer launches the software that tries to locate the fault and correct it if possible. As far as the attitude control system is concerned, the computer can isolate or bypass attitude sensors and switch the spacecraft into a safe-hold mode so that the problem can be dealt with from the ground.

Tape Recorders

Three NASA Standard Tape Recorders are housed in the Command and Data Handling Module. They are all identical and each is capable of storing 500 Mbytes (about 8 hours or about 4 revolutions) of collected science data and engineering telemetry at a rate of 16 Kbytes/sec. The recorded data is played back later 3 times each day and sent to the ground through the high-gain antenna and the TDRSS. Also, as a backup, the data can be sent to the Deep Space Network on the ground. Since it is very critical to keep those recorders functioning properly, they are rotated in service to prevent any particular one from being worn out too much.

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