SBRS engineer Steve Silverman (right) explains the MGS TES (inside plastic box, foreground) to K-12 educators from Arizona and California on January 13, 1996. In November, the TES shown here will be leaving for Mars. SBRS Photo 96-1-103(14). For full image view, click on above image icon.
by Greg Mehall, TES Systems Engineer, ASU
Many exciting events have occurred with the Mars Global Surveyor Thermal Emission Spectrometer (TES) project since our last TES News (December 1995). Most importantly, the instrument is now completely assembled and has begun to be tested to make it ready for the 10 & 1/2-month journey to Mars. Mars Global Surveyor (MGS) is set to launch from Florida on or around November 5, 1996. It will reach Mars in September 1997. TES and the five other MGS instruments will scrutinize the planet through at least January 2000.
Mid-December '95: Electronics Tested in Space-like Conditions.
The thermal/vacuum testing of the TES flight electronics module was completed just prior to the December holiday season at Hughes Aircraft's Space and Communications Division in El Segundo, California. The module was tested for two weeks in a chamber that simulated the conditions that MGS and TES will encounter in space. The chamber was "pumped down" to simulate the vacuum of space and the electronics module was tested at a range of temperatures from -20 C to +75 C (-4 F to +167 F). The good news-- the electronics performed exactly as expected over this temperature range. The success of these tests was a major milestone toward the completion of the TES electronics' module.
Changes at Hughes.
After the thermal/vacuum testing was complete, the electronics module was taken back up the coast to Hughes Santa Barbara Remote Sensing, SBRS (formerly known as Hughes Santa Barbara Research Center, SBRC) in Goleta, California, to be integrated with the rest of the TES instrument. The SBRS name change occurred at the first of the new year and is a part of Hughes Aircraft's recent reorganization and restructuring effort. These changes at Hughes are taking place in order to make the company more competitive in the shrinking defense/aerospace market. The changes at Hughes will not affect together has remained the same.
Late-December '95: TES Comes Together.
The SBRS engineers finished the final assembly of TES during the last week of December. This involved mating the electronics module with the main optical and mechanical assemblies. Engineers connected and routed all of the final wires needed for TES's operation. The last step in assembly was to enclose the aft optics plate with its "flight cover". The aft oftics plate contains all of the science components of TES-- particularly the spectrometer's detectors, mirrors, plus the Michelson interferometers' optics and motors. The interferometer (also known as the "spectrometer") is the part of TES that will be used to measure the spectra of rocks and minerals on Mars.
January '96: A Tale of Two, then Three, Beamsplitters.
One of the most important parts of the interferometer is the Cesium Iodide beamsplitter. The beamspitter helps the instrument to detect many different wavelengths of infrared radiation ("light"). The beamsplitter that we installed in the TES performs very well, but we found that it behaves differently than the one that was in our first TES that flew on Mars Observer, MO (remember that Mars Observer was lost in space in 1993). The computer software designed to work with the MO TES beamsplitter was also going to be used for the Mars Global Surveyor TES... but we had to modify the software to compensate for the differences we observed in the new beamsplitter. Computer programming experts working on the TES project, Nuno Bandiera (formerly of SBRC) and Don Anderson (of ASU) developed the original TES software. These two gentlemen spent a lot of time in January writing "patches" for the flight software to account for the differences between the MO and MGS TES beamsplitters. There was still a possibility that they would not be able to modify the software sufficiently to work with the new beamsplitter, so SBRS also went ahead and ordered yet another one and began to test it. If we find that we need to replace the beamsplitter, it will mean a major change in our schedule because TES is now "buttoned up." Opening the instrument to trade beamsplitters would take several days, perhaps weeks.
Drawing of Mars Global Surveyor Thermal Emission Spectrometer.
January-February '96: Let the Tests Begin.
Now that TES is essentially "all put together," the SBRS engineers have begun to test the instrument. The testing and calibration is the last major hurdle before TES is ready to ship to Lockheed-Martin in Denver. The testing will help us understand how well the instrument will perform under various conditions it might encounter in space. We will calibrate the instrument so that the scientists who use TES data will be able to interpret the observations correctly.
The first system test that we did is called the "Electromagnetic Compatibility / Electromagnetic Interference" (EMC/EMI) test. This was performed for two weeks in January at a facility in Fullerton, California. The EMC/EMI test characterizes how susceptible the TES is to various forms of electromagnetic energy it might encounter during the mission, plus how much the instrument itself generates and gives off to its environment. This is important in part because electronics on the MGS spacecraft and from other MGS instruments could give off electromagnetic energy that might affect TES-- perhaps causing interference in the data TES gathers. A good analogy occurs when you run your vacuum cleaner at home, and it causes "noise" or "snow" to appear on your TV screen. To do the EMC/EMI test, TES was placed in a shielded room so that the background electromagnetic energy that is around us all the time did not reach the instrument. Then, various antennas and signal amplifiers were used to transmit (toward the TES) and receive (from the TES) the frequencies that might occur during the MGS mission. These tests are complete and now SBRS engineers are looking at the data-- so far, the results look good.
February-April '96: Delivery Will Follow Tests.
On February 6 we met with engineers from the Jet Propulsion Laboratory (JPL) and Lockheed-Martin (the company building MGS), to determine that we are ready to perform the final environmental tests on TES. The meeting allowed the MGS engineers to determine that our plan for final testing of our instrument will meet all of NASA's requirements. After this review, the plan is to subject TES to vibration testing. This test simulates the shaking that TES will experience during launch on the Delta II rocket in November. Can TES withstand the violent shaking? We will soon find out by putting TES on a vibration table that can be adjusted to shake at different speeds that simulate the rocket launch. After the test is complete, TES will be powered up and checked to make sure it still works as well as it did before it was shaken. This procedure made us pretty nervous when we did it back in 1991 for the Mars Observer TES, but the instrument came through without any problems. Spacecraft instruments are built to be hearty and durable!
The most important test lies ahead: the thermal/vacuum test. In December, we only put the electronics module in a thermal/vacuum chamber. Later this month, we will do it for the whole instrument. TES will go into the chamber, which simulates the vacuum and temperatures of outer space, for about three weeks. We are still not sure if we will be doing this test at SBRS in Goleta, or if we will have to box up the instrument and drive it down to the Hughes Aircraft facility in El Segundo (near Los Angeles). The data we collect from TES during this test, known as thermal-vac, will allow us to calibrate and characterize our instrument. Thermal-vac tests will run 24-hours a day. All of the TES science team members (from across the U.S.) and some of the ASU graduate students working on TES will travel to California to assist the SBRS engineers during this phase (some of them will be working all-night shifts!).
Once thermal-vac is completed, TES will be packed up and flown by airplane to Denver, Colorado, to be placed onboard the MGS spacecraft at Lockheed-Martin. We are on target for an April 1st delivery. The next few months will be very exciting. I will update you on the final stages of TES integration and testing in the next edition of TES News. To Mars!