Description of Mission

More detailed figure of TOPEX/Poseidon

Instrument Module

The Instrument Module (IM), which is a large aluminum box attached to the MMS, consists of all the scientific sensors and systems that are required for data collection, communication, and solar power use. These instruments require about 1000 watts during normal operation; as a result, a lot of heat is generated. In addition, the sun has a great effect on the temperature of the spacecraft. Since many of the instruments inside function best at a certain range of temperature, the IM has thermal control louvers on the both port and starboard sides of its walls to control the internal temperature by opening and closing them, thereby removing the heat into the space.

DORIS Antenna

The DORIS Antenna is part of a CNES's radio tracking system of the satellite called the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), which is one of the primary satellite tracking systems for the TOPEX/Poseidon mission. The DORIS antenna is connected to the DORIS dual Doppler tracking system receiver inside the IM. About every ten seconds, the DORIS receives microwave signals from the ground based beacons, fifty of which are located around the world, for satellite tracking, gravity field measurements, and ionospheric correction for the CNES single-frequency solid-state radar altimeter. It measures the Doppler frequency shift in the signals as the Earth and the satellite move with respect to each other. This data along with that from the Laser Raging System provides very accurate satellite tracking data for precision orbit determination. The DORIS has an accuracy of less than 0.3 mm/s radial velocity.

Laser Retroreflector Array

The Laser Retroreflector Array (LRA) is a NASA's laser tacking system of the satellite, which is one of the primary satellite tracking systems for the TOPEX/Poseidon mission. It is a set of 192 quartz corner-cubes mounted in two concentric rings around the altimeter antenna on the nadir side of the IM. There are about 30 ground-based laser tracking stations located around the world. One of the stations reflects laser beams off the LRA and measures the time it takes the beams to come back, from which the orbital radial position of the satellite can be calculated with an accuracy of 3 cm or less to provide satellite tracking data for precision orbit determination. The tracking data is also used to calibrate and verify altimeter measurements (calibration of radar altimeter bias).

Global Positioning System Antenna

The Global Positioning System (GPS) Antenna , which is mounted on the zenith side of the IM, is connected to the GPS Demonstration Receiver (GPSDR) inside the IM. The GPSDR was a NASA's new experimental technique for accurate and continuous tracking of the spacecraft and TOPEX/POSEIDON was the first mission to use it for satellite tracking. The GPSDR receives signals through the GPS Antenna from the U.S. Defense Department's Global Positioning System constellation satellites orbiting at 20200 km (12552 miles) above the Earth and determines the position of the satellite in its orbit with an accuracy of 3 cm or less to provide satellite tracking data for precision orbit determination.

Altimeter Antenna

The altimeter antenna is used by the NASA dual-frequency radar altimeter and the CNES single-frequency solid-state radar altimeter stored inside the IM. The antenna is a 1.5 m diameter parabolic dish and is attached to the nadir side of the IM so that it points toward the center of the Earth in order to take altimeter measurements. Since the two altimeters have to share the same antenna, only either one can be used at a time.

The NASA altimeter, which is the primary instrument aboard the spacecraft, is used to measure the height of the satellite above the sea surface and to calculate wind speed, wave height, and ionospheric correction. The altimeter operates at two separate frequencies at 13.6 GHz (Ku-band) and 5.3 GHz (C-band), which allow the correction for the pulse delay in the ionosphere (ionospheric correction). The altimeter has an accuracy of 3.4 cm (1.3 in.) for altitude and 0.5 m (20 in.) for wave height.

The CNES altimeter, which is a new experimental technology with low power and low weight, is used to measure and calculate the same things as the dual-frequency altimeter does except for the ionosphere correction. It has an expected accuracy of 2.5 cm (1 in.) for altitude.

The altimeters send out pulses to the sea surface and measures the time that it takes the pulses to come back to the satellite, which provides the distance between the satellite and the sea surface by multiplying the time by the speed of light. Also, the altimeters measure the characteristics (amplitude, shape, etc.) of the backscattered echo of the pulses to determine wind speed and wave height. As the sea surface wind increases, the sea surface gets rougher, leading to a decrease in the amplitude of the echo, which can be used as a measure of the wind speed. The shape of the echo depends strongly on the surface variations. For example, the echo is a mirror image of the incident pulse for a perfect smooth surface. As the surface gets rougher, the echo drops off slowly and its leading edge rises gradually. This rising time depends on the root mean square (rms) height of the waves, which can be used as a measure of the waves rms.

High Gain Antenna

The high gain antenna is a 1.2 m (1.3 m) parabolic dish antenna mounted on a 2.6 meter boom on the zenith side of the IM. It is part of the primary communication link between the ground and the satellite called NASA's Tracking and Data Relay Satellite System (TDRSS). The antenna is used to communicate between the spacecraft and the TDRSS satellites which relays the signal to the ground stations. Two gimbals joints, perpendicular to each other, are mounted between the end of the dish and the boom, which allows the antenna to point in any direction above or to the side of the spacecraft and communicate with any one of the TDRS satellites which happens to be in the satellite's zenith hemisphere. The antenna was not operable until it was deployed 22 hours after launch, during which the nadir omni antenna and the zenith omni antenna were used for all the communications.

Nadir Omni Antenna

The nadir omni antenna, which is attached on the nadir side of the IM, is a low-gain omnidirectional antenna. Unlike a high gain antenna, a low gain antenna has a low efficiency because the radio signal is transmitted to all directions and the power that reaches a receiver is reduced very much. Because of that characteristic, the nadir omni antenna is used to communicate directly with NASA's Deep Space Network (DSN) in the event of an emergency, such as malfunction of the high gain antenna . In fact, it was used to communicate with the DSN during launch though the reason was not because the high-gain antenna malfunctioned but because it was not deployed until 22 hours after launch

Zenith Omni Antenna

The zenith omni antenna, which is attached on the zenith side of the IM, is a low-gain omnidirectional antenna like the nadir omni antenna. The difference is that it is used not only in the event of an emergency, but also for some command, telemetry, and tracking support during normal operations. Another difference is that until the high gain antenna was deployed 22 hours after launch, the zenith omni antenna was used as part of the primary communication link between the spacecraft and the TDRSS satellites. (The nadir omni antenna was used to communicate with the ground stations.)

Microwave Radiometer

The microwave radiometer is a three-frequency (18, 21, and 37 GHz) sensor that provides estimates of water vapor content in the troposphere along the path of the signal The estimates are used to correct the altimeter measurements because the water vapor slows down the signal that travels between the altimeter and sea surface, making the travel distance that the altimeters read seem longer than it really is.

The radiometer uses the 21 GHz primary channel to make water vapor measurement and the 18 GHz and 37 GHz channels to remove the effects of wind speed and cloud cover on the water vapor measurement. There is a backup receiver for the measurement channel.

Thermal Louvers

The thermal louvers, which are located on both port and starboard sides of the Instrument Module, are to control the internal temperature by removing a large amount of heat generated by the instruments inside the spacecraft through these louvers (windows) into space.

Solar Array

The solar array, which is mounted on the port side of the IM, is a large solar paddle (8.89 m long and 3.30 m wide) that provides power up to 3385 watts for instruments and systems on the spacecraft. During launch, the array was folded along the instrument module. After deployment, during the Assessment Phase of the mission, it expanded to the dimensions above. The array is continuously pointed at the sun by the on-board computer during normal operations so that it can get as good reception of sunlight as possible. When the spacecraft is in the shadow of the Earth and can not receive sunlight, which is known as "occultation," power is supplied by three batteries which have been charged during the normal operations. The batteries are NASA Standard 50 amp-hour Rechargeable Nickel-Cadmium Batteries which installed inside the Power Module.

Course Sun Sensors

The spacecraft has several course sun sensors mounted on the edges of the solar array. They measure the angle between the sun and the solar array and report it to the Attitude Control System (ACS) so that the array can be pointed toward the sun at the desired angle. However, this operation by the ACS is a back up system because it is the on-board computer that points the array.

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

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Last Modified: Wed Feb 11, 1999
CSR/TSGC Team Web