TES NEWS, Volume 5, Number 2, June 1996

TES Meets Mars Global Surveyor

It's "Bolted On!"

by Greg Mehall

TES Systems Engineer, Arizona State University

TES In Denver.

The past four months have been very exciting, as well as hectic, for the Thermal Emission Spectrometer (TES) project. Since my last TES News update, we've worked around the clock to complete the final testing on the instrument so that it could be installed on the Mars Global Surveyor (MGS) spacecraft. The original delivery date to the spacecraft contractor, Lockheed Martin Astronautics (LMA) in Denver, Colorado, was April 1, 1996. However, as we entered into the final phase of instrument testing at the beginning of March, we discovered several serious problems with the TES. As a result, we missed our planned delivery date. Since then, we have worked frantically to meet an ever-shrinking schedule. I'll get into the problems in the next few paragraphs-- however, I am happy to report that we were finally able to solve the problems and we delivered the TES to LMA on May 28, 1996. MGS is scheduled to launch Nov. 6, 1996.

Shaken, not Stirred.

In my last update, the TES was entering the final phases of the instrument testing and characterization. At the end of February, the TES was subjected to vibration testing. This test simulates the vibrations that the TES will experience during launch on the Delta II rocket. TES was attached to a vibration table at Hughes Santa Barbara Remote Sensing (SBRS, in Goleta, California) and was vibrated at various levels. These levels exceed the launch vibration, in order to ensure that the TES won't break during launch. It was very nerve-racking to watch the TES being shaken on the table!! It's amazing that such a sophisticated optical and electronic instrument is able to survive such a violent event! After the testing was completed, TES's performance and optical alignment were checked for any signs of degradation. We were all relieved when the TES passed all of these tests with no problems.

Pump it Down.

After vibration testing, the TES was ready for the thermal/vacuum chamber by the end of February. This was the most important phase of instrument testing and had to be completed successfully prior to delivery. The TES instrument was placed in the vacuum chamber on February 29, 1996, and was to be tested for three weeks. This test helps us understand how well the TES will perform under the various conditions it will experience during the mission in space. The data we get during this test allows us to "calibrate" the instrument so that the scientists who use the TES data from Mars can interpret it accurately. The chamber was "pumped down" to simulate the vacuum of space over a temperature range of -20 C to +40 C (-4 F to 104 F).

Image, below: TES engineer Greg Mehall (front) working at Hughes Santa Barbara Remote Sensing, Goleta, Calif., during Thermal-Vac testing. The thermal vacuum chamber is behind Greg. Photo by P. Christensen, March 1996.

First Glitches.

At the start of the test we warmed the chamber to +40 C for two days in order to "bake out" all of the contaminants (like water, carbon dioxide) that might be in the chamber and in the TES. Although the performance of the TES dropped slightly at this elevated temperature, it still was within the acceptable range. Next, we started lowering the temperature of the chamber and the TES towards -20 C. As the temperature dropped, we noticed that the infrared spectra, which are the primary data from the TES, began to degrade. The colder we went, the worse it became. We could not figure out why the instrument was performing so poorly at these cold temperatures. The bad luck continued when we decided to warm the instrument back up to room temperature and "break vacuum". As we started warming the TES, the signals on the other two detector arrays began to behave strangely. They continued to behave this way until the TES approached room temperature. In addition, during this cold-to-warm transition, the primary neon lamp failed and the TES switched over to its backup. This was alarming since the neon lamp is a critical element of the TES spectrometer. When the TES finally warmed up, we turned it off and opened the thermal-vac chamber.

Testing Hypotheses.

It was frustrating. We assembled a team of experts to try and determine what might be happening. We spent the next week analyzing the ~500 Mbytes of data that we collected in an attempt to develop some hypotheses. We also started working on adding additional test connectors to the instrument to help analyze the problem the next time we would cool the instrument down. On the second try, we tested out several of our hypotheses but were unable to determine the exact cause of our problem. However, we were able to eliminate most of these hypotheses, thereby narrowing it to a couple of possibilities. We determined that it had to be a temperature-dependent misalignment of one or more of the optical elements in the TES interferometer. The interferometer (also known as the spectrometer) is the subsystem of the TES that generates the signals (interferograms) that will be used to measure the spectra of the rocks and minerals on Mars. After we removed the TES from the chamber the second time, we took it down the hall-- back to the optics lab-- and opened up the TES aft optics cover.

Image, below: Neon Lamp (like a light bulb) from the TES instrument.

Beamsplitter "Glue".

We were able to measure the positions of the various optical elements in the lab over a smaller temperature range. We determined that the degradation that we had been seeing in the chamber was due to a very slight misalignment of the Cesium Iodide beamsplitter over the range of temperatures. The beamsplitter is the primary optical element in the interferometer (see TES News, Feb. 1996). We were able to determine that it was moving about 5 millionth's of a meter over the ~60 C (140 F) temperature range. Amazingly, this was enough to seriously degrade the performance of the TES at the cold temperatures. We finally discovered that the movement of the beamsplitter was caused by the epoxy ("glue") that we used to hold the beamsplitter in place!

Once we determined the cause, we spent the next month and a half redesigning, refabricating and installing a new beamsplitter. We also fixed the problem with the neon lamps, and we modified the two detector arrays that were behaving strangely (they were "self-oscillating"). These modifications were completed by the end of April and the instrument was "buttoned up" and prepared for a second vibration test.

More Tests, Fixes.

After successful completion of the second vibration test, we prepared for our third try at the thermal/vacuum test. We started the third round on May 5, 1996. Five days into this test we noticed that two of the three temperature sensors mounted on the TES internal calibration surface were not reading out the correct temperature. Without these sensors it would be very difficult to accurately calibrate the data collected by the TES at Mars. We decided to "break vacuum" one last time to repair these sensors. They were replaced within three days. After completing the repairs we started the fourth and final round of the thermal/vacuum testing on May 14, 1996. The testing lasted twelve days and the TES finally performed as expected over the entire temperature range. We were quite relieved! We were able to collect all of the data we needed to be able to calibrate the observations that we will eventually get from Mars.

Image, below: Hughes aircraft getting ready to ferry the TES from Santa Barbara, California, to Denver, Colorado. Photo by P. Christensen, May 28, 1996.

Flight to Denver.

After the successful completion of thermal/vacuum testing, the chamber was opened and the TES was removed. The "Pre-shipment Bench Acceptance Test" was performed to get a benchmark of the instrument's performance prior to shipment. After this test was completed, the TES and its test equipment were packed and prepared for shipment to LMA in Denver. On May 28, 1996, five members of the TES team carried the TES instrument aboard the Hughes Aircraft corporate jet and flew with it to Denver. The TES team was met at the airport by LMA personnel who took the TES to the Mars Global Surveyor assembly facility. The following day, the test equipment arrived by truck from Goleta, California, and the "Post-shipment Bench Acceptance Test" was performed on the TES in order to verify it made the trip safely and without damage. The instrument passed this test and was officially transferred to the responsibility of the Jet Propulsion Laboratory and Lockheed Martin for integration with the rest of the MGS spacecraft.

TES Meets MGS.

The TES was installed on the spacecraft "nadir deck" on May 31, 1996. The nadir deck is the panel that holds the MGS scientific instruments and is the side of the spacecraft that will face the planet when it orbits Mars. The TES was the final instrument to be installed on the spacecraft. The Mars Orbiter Laser Altimeter (MOLA) was installed two days earlier, and the other instruments had been there for several months already.

During the TES installation, the TES was hoisted into its position on the nadir deck and then bolted in-place. Next, the electrical interface between TES and the spacecraft was verified by the "Initial Power-On Test" (IPTO). This was an extremely important test, because any mistakes in the electrical connections could severely damage the spacecraft and/or the TES. When the spacecraft finally applied power to the TES, the instrument's computer initialized itself and the TES pointing mirror began to turn just as it was supposed to! Shortly after that, our test equipment began to receive the data from the TES! This was a major milestone for us: the TES was now an integrated part of the MGS spacecraft!

Image, below: TES Principal Investigator, Phil Christensen, with the new MGS Thermal Emission Spectrometer, May 1996.

TES-ting the Spacecraft.

The next day, the TES "Functional Electrical Test" (FET) was performed. The purpose of this test was to verify that we could successfully run our instrument using the new spacecraft commanding interface. We re-ran the TES "Bench Acceptance Test" and the instrument functioned properly. On June 3-5, 1996, we participated in the "Spacecraft Functional Test," which was the first test in which all of the spacecraft sub-systems and payloads were installed and operating. The purpose of this test was to verify that the spacecraft subsystems and the science instruments did not degrade each others' data by interacting with each other. This test also simulated the mapping mode we will be in when we start systematically observing Mars in early 1998. The test was only partially successful, because several problems were encountered that need to be addressed. This test will be fine-tuned and run again before the spacecraft gets shipped to Florida.

The Weeks to Come.

The final preparations for the spacecraft thermal/vacuum testing are being performed during the first two weeks of June. Thermal blankets (the gold covering that looks like foil) are being put on the spacecraft, and temperature sensors are being placed on the spacecraft and throughout the vacuum chamber. On June 16, 1996, the entire spacecraft will be placed in the 30 m (100 ft) tall by 15 m (50 ft) diameter chamber. The chamber will then be "pumped down," and the entire spacecraft will be tested over various temperatures and configurations, just as we did with the TES in May.

The thermal/vacuum testing is scheduled for the last two weeks of June. The TES will be turned on during the first half of the test when the spacecraft is in the mapping simulation mode. This will be the best opportunity for us to verify the performance of the TES with all elements of the spacecraft operating prior to launch. After spacecraft thermal/vacuum testing is completed, the spacecraft will be removed from the chamber and the final alignments and preparations for shipment will be performed.

To Florida.

The MGS team is aiming for an August 16, 1996, delivery of the spacecraft to Cape Canaveral, Florida. The spacecraft will be loaded aboard a military transport plane and flown to the Air Force base at Cape Canaveral. Once there, it will be tested some more, then placed on top the Delta II launch vehicle. I will update you on final stages of the spacecraft integration and testing as they are completed over the next few months in the next edition of the TES News. Until then... To Mars!!

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