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Ocean Surface Topography from Space
Ocean Surface Topography Mission/Jason-2 Spacecraft to Payload Integration and Testing
November 01, 2007

Just because instruments designed to fly on a satellite do well when tested individually on the ground doesn't mean the whole satellite will function properly once it's flying in space. To make sure everything on a satellite works well together, spacecraft undergo an important phase called integration and testing, or I&T, before launch.

PROTEUS platform
Figure 1: PROTEUS platform
Image credit: Alcatel Space/JL. Bazile, 1999
PROTEUS platform flight configuration for Corot mission.
Figure 1b: PROTEUS platform flight configuration for Corot mission
Image credit: CNES/Alcatel
That's what's happening right now in Cannes, France, for the Ocean Surface Topography Mission (OSTM) on the Jason-2 satellite. If all goes as planned the mission will be ready for launch next summer. OSTM/Jason-2 will expand to two decades the record of sea surface height begun by TOPEX/Poseidon and continued by Jason-1. These measurements are critical for understanding the ocean's role in climate.

Integration and testing begins after all the spacecraft's components, including its scientific instruments, have been tested individually. A January feature described this process for the Jason-2's Advanced Microwave Radiometer (AMR). The AMR is one of three instruments supplied by NASA for the mission. In addition to five primary instruments, Jason-2's payload also includes three "passenger" instruments, increasing its science capability.

The instruments are being integrated onto the spacecraft bus at Thales Alenia Space in Cannes, France. The bus is the framework, or platform, onto which all of the payload instruments are connected. It was provided by the French space agency CNES, one of the mission's four partners. A team of engineers from CNES, NASA, and Thales are conducting the testing.

Jason-2's bus is based on Thales' PROTEUS low-earth-orbit platform (Figure 1). PROTEUS is a French acronym that translates to "reconfigurable platform for observing, telecommunications, and scientific uses." First used for Jason-1, this bus has been used for several other satellites including the NASA/CNES Calipso mission. The Jason-2 platform includes service and payload modules designed and adapted specifically for the mission.

JPL's Advanced Microwave Radiometer is being integrated on the PROTEUS Platform Payload Instrument Module.
Figure 2: JPL's Advanced Microwave Radiometer is being integrated on the PROTEUS Platform Payload Instrument Module.
Image credit: NASA/JPL
Jason-2 payload integration began in December 2006. It's a complicated process involving both mechanical and electrical integration of the instruments and spacecraft subsystems. For Dr. Mohamed Abid, OSTM Project System Engineer, the most challenging part of I&T is the planning required. "Experience definitely weighs in here," he says.

"During mechanical integration you secure, the instrument in its allocated area on the payload instrument module (PIM)," says Abid. (See Figure 2)

Next is electrical integration. "Electrical integration involves safely connecting the instrument to its power source and data cables, and then verifying that it performs per specifications," says Abid. "We perform an instrument aliveness test to verify that the instrument is receiving power. It's like checking for the instrument's heart beat."

Then, engineers run an instrument health check to be sure the instrument can use the power to do what it's designed to do. "If the AMR were a person, a successful health check test would show us that this person is able to walk and talk without any problems,"
Abid says. The next step is to find out how well it can perform. "The health check test lets us know that our instrument can walk. We conduct a performance verification test to see if it can run a marathon and win," says Abid.

The fully integrated OSTM/Jason-2 spacecraft is ready for further testing.
Figure 3: The fully integrated OSTM/Jason-2 spacecraft is ready for further testing.
Image credit NASA/JPL
After all the instruments have been successfully integrated and tested, the spacecraft engineers perform aliveness, health check, and performance verification tests on the entire payload to make sure that the instruments work together as specified. (Figure 3)

At different times during system level tests, the entire spacecraft, including its payload, is shocked, shaken, frozen and cooked. What would engineers do if something were to go wrong?

"In the unlikely event that there is a mishap during testing, for example, if the hardware cracks or shows a change in performance, all activities stop, the environment is stabilized and secured, and all stakeholders notified in a timely manner," says Abid. "Typically, an investigation team would provide recommendations on how to proceed thereafter." After understanding the reason for a failure, a waiver might be granted or corrections made and tests repeated.

So far, Jason-2 and all its instruments have proven to be shipshape.

The entire integration and test phase, including operations simulations, is scheduled to be complete by March 2008. Then, the satellite will be transported from France to the launch facility at Vandenberg Air Force Base in California for integration onto a United Launch Alliance Delta II rocket in April 2008. The target launch date is June 15, 2008.

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