Mission: possible

TOPEX/Poseidon is a joint mission of the National Aeronautics and Space Administration (NASA) and the French space agency, Centre National d'Etudes Spatiales (CNES). In 1979, Jet Propulsion Laboratory (JPL) of NASA started to plan a mission called Ocean Topography Experiment (TOPEX) that would utilize a satellite equipped with an altimeter to measure the surface of the global ocean. Meanwhile, the CNES was designing an oceanographic mission called Poseidon, named for the Greek god of the sea. In order to achieve common scientific goals, the two space agencies decided to join together and design a single mission called TOPEX/Poseidon that would utilize a satellite altimeter to help them to understand how the ocean interacts with our planet the Earth.

TOPEX/Poseidon builds on the outcomes of previous missions GEOS-3 (Geodynamics Experimental Ocean Satellite, 1975), Seasat (Sea Satellite,1978), and Geosat (Geodetic Satellite, 1985), which proved validity and precision of a satellite altimeter in measuring the sea surface height from space over time. A challenge of TOPEX/Poseidon was to improve an accuracy in measurement using a state-of-the-art dual-frequency radar altimeter and three independent precision orbit determination systems. Due to tremendous efforts by every crew and staff involved in the mission, however, it has resulted in a great success with less money than it would have cost if the two teams had worked separately.

The following table shows how some of the responsibilities for the mission are shared.

Launch vehicle and launch CNES
Design and development of the spacecraft The Fairchild Space Systems Division (FSSD) under contract to JPL of NASA
Development of two altimeters and Precision orbit determination The Goddard Space Flight Center (GSFC) of NASA and CNES
Operation of the spacecraft JPL
Communication with the spacecraft NASA
Scientific research and analysis The Science Working Team (SWT)
U.S. portion of the project JPL on behalf of NASA
France portion of the project CNES
Overall NASA and CNES

The satellite orbits the Earth at altitude of 1336 km (830 miles) with an inclination angle of 66 degrees and a period of 112 minutes, carrying two altimeters aboard, one built by NASA and another built by CNES. The satellite takes measurements of height of the ocean at the same points every 10 days using the two altimeters. TOPEX/Poseidon distributes the data to 38 science investigators from 9 countries: USA , France, Japan, Australia, The United Kingdom, Germany, Norway, South Africa, and the Netherlands. Then, they conduct scientific investigations on various subjects in oceanography and marine geophysics to search for the scientific truth in conjunction with two other world climate research programs: the World Ocean Circulation Experiment (WOCE) and the Tropical Ocean and Global Atmosphere (TOGA) Program.

Mission Phases

As far as a timetable is concerned, the TOPEX/Poseidon mission can be divided into four major phases. They are: launch, assessment, initial verification, and observation. (See the diagram above.)

The launch phase starts from launch preparation at Kourou (French Guiana ) and ends with separation of the satellite from the third stage of the launch vehicle and placement into an injection orbit.

The assessment phase starts after the launch phase and ends when the satellite is in the operational orbit. In this phase, the satellite and its systems are initialized and prepared for routine operations: the antenna and the solar array are deployed, attitude references are established, sensor performance is assessed, and the satellite is maneuvered into the operational orbit. This phase takes up about 48 days after launch.

The initial verification phase starts after the assessment phase. In this phase, performance of two altimeters and precision orbit determination (POD) are verified. Also, the algorithms used to produce the Geophysical Data Records (GDR) are optimized. The orbit is designed to fly over two verification sites: the NASA site located on Oil Platform Hermosa off Pt. Conception in California and the CNES site at Lampione Rock near Lampedusa Island in the Mediterranean Sea. This phase ends when the desired GDR algorithms are obtained and it (takes up 5 months) continues till 6 months after launch.

The observational phase starts after the initial verification phase. In this phase, the mission goals are achieved, that is, scientific data is collected, recorded, and analyzed. This phase continues till three years after launch and it could be extended for another three years or more if the status of the satellite and resources are allowed.

The satellite arrived at European Space Agency's Guiana Space Center located in Kourou (click for the detailed map), French Guiana on June 23, 1992. It was reassembled and checked for any damage due to shipping over the next three days, and then it was set on the launch vehicle, an Ariane 42P rocket. Although the launch was originally scheduled for July 21, 1992, there was a delay due to last-minute modifications to a Eutelsat 2 F4 communication satellite that was scheduled to be launched ahead of TOPEX/Poseidon. Finally, on August 10, 1992, TOPEX/Poseidon was successfully launched into Earth's orbit by the Ariane 42P launch vehicle.

As soon as TOPEX/Poseidon was placed in its initial (injection) orbit 19 minutes 57 seconds after liftoff, the solar array and two antennas for the Tracking and Data Relay Satellite System (TDRSS) and the Global Positioning System (GPS) were deployed. Among the on-board sensors to be checked, special attention was paid to the ones that were to check for the Earth horizon and Sun position, such as a digital fine sun sensor, course sun sensors, and sensors in the Earth Sensor Assembly Module because they would help the Attitude Control System to make sure that the altimeter antenna would be pointing straight down to the ocean surface. Over the next few days, the sensors were initialized, a communication link was established through the TDRSS, and a sequence of maneuvers was refined to place the satellite into the operational orbit, which repeats itself every 10 days and covers 90 % of the ocean. According to Rod Zieger, TOPEX mission manager, I quote, "We could have given them orbits with repeat cycles anywhere between 3 and 40 days, but the oceanographic community chose 10 as a compromise between its principal investigators around the world. Each investigator will now receive 36 data sets per year for the area of ocean he is studying."

In order to place the satellite into the operational orbit, which is a nearly circular orbit with a period of 112 minutes and an inclination angle of 66 degrees at an altitude of 1336 km (830 miles), six maneuvers were performed. On August 20, the first of six maneuver was made to transfer the satellite from the elliptical orbit, which the satellite was originally placed into at launch, into a nearly circular orbit by firing the four large thrusters to accelerate. A week later, the second maneuver was made to change an inclination angle of the orbit to 66 degrees. On September 2, two in-plane maneuvers were made by firing the large thrusters; as a result, the satellite was brought closer to the desired orbit which would fly over the two verification sites. On September 15, another in-plane maneuver was made, which was followed by the trim maneuver on September 21 by firing small thrusters to bring the satellite into the target orbit. The satellite was placed into the operational orbit three days ahead of schedule, and finally on September 23, TOPEX/Poseidon started its first of 10 day period cycles of a long way to go. By 43 days after launch, TOPEX/Poseidon was in the operational orbit and all the scientific instruments were activated.

Back to Main Page

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


Last Modified: Wed Feb 11, 1999