Description of Mission

Communication Link

Communication is one of the most vital and critical functions in order to accomplish the TOPEX/Poseidon mission. There are a lot of information that have to be passed between the spacecraft and the ground. For instance, command data has to be delivered from the ground stations to the spacecraft. The engineers on the ground need to know various things about the satellite, such as its position, the status of hardware, instruments, and spacecraft subsystems. All these things have to be received from the spacecraft to the ground stations by a communication link. Most importantly, collected altimeter measurements have to be delivered to scientists and personnel on the ground. After all, you couldn't obtain the most precious collected scientific data without the communication link even if everything else went perfectly.

All communication is established on this TOPEX/Poseidon mission through radio transmission called telecommunications. The major communication link on the mission is between the ground stations and the spacecraft. The process of communication from the spacecraft to the ground is called downlink while the process of communication from the ground to the spacecraft is called uplink. Either way, information is digitized and transmitted over several microwave frequency bands. The signals in both links are called telemetry.

Mission operations are conducted from the Project Operations Control Center (POCC) at JPL in Pasadena, California, with which the satellite communicates via the TDRSS, DSN, and NASCOM. Between the spacecraft and the ground comes what is called the TDRSS, NASA's Tracking and Data Relay Satellite System, which is a constellation of six satellites and a ground station that relay telemetry between the spacecraft and the ground. The TDRS satellites were launched from the space shuttles into geosynchronous orbits 35880 km (22300 miles) above the Earth between 1983 and 1995. TOPEX/Poseidon was the first JPL mission to use the TDRSS for communication.

For downlink, during normal operations, telemetry transmitted from the high-gain antenna aboard the spacecraft is relayed via the TDRSS satellites, one or more of which are almost always in sight of the spacecraft, and received by the ground station at White Sands, New Mexico. Then, the telemetry is relayed via another satellite called Domestic Communications Satellite (DOMSAT) to the NASA Goddard Space Flight Center (GSFC) at Greenbelt, Maryland, which does some processing and recording and relays the processed data to the Project Operations Control Center (POCC) at JPL in Pasadena, California. For uplink, such as command information from the ground stations, telemetry goes the other way around.

Compared with the direct communication link with spacecraft using the Deep Space Network (DSN) which is NASA's traditional ground station network, the TDRSS link is superior in many respects. First of all, the TDRSS link provides real-time communications for the satellite for over 20 hours a day while no more than 3-1/2 of real-time communication out of 24 hours is possible with the DSN. Second, the TDRS is capable of providing tracking and communications for 32 satellites at the maximum simultaneously while the DSN can handle no more than 2 satellites at a time. Since there are other highly prioritized missions that preempt the use of the TDRSS satellites, the more satellites they can handle, the less chance that the TOPEX/Poseidon is cut in by the others. In case the TDRSS is not available either because it is out of sight or because it is preempted, the TOPEX/Poseidon stores the data on one of three tape recorders, waits for the right moment to come later, and play it to transmit the data. Finally, the TDRS provides faster command and telemetry transmission rates which is about 300 million data bits per second. Obviously, this helps to send a vast amount of data in short time. Also, it is advantageous in conjunction with the availability of the TDRSS.

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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