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Ocean Surface Topography from Space
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All Shook Up - Key OSTM/Jason-2 Components Pass Important Tests
January 01, 2007
Line drawing of Jason-2 instrument package showing advanced microwave radiometer on top.
Figure 1: Line drawing of Jason-2 instrument package showing advanced microwave radiometer on top.
Image credit: NASA/JPL-Caltech
There was a "whole lotta shakin' goin' on" as the reflector structure assembly, a major component of Jason-2's advanced microwave radiometer, underwent shake, acoustic and thermal vacuum tests in the Jet Propulsion Laboratory's Environmental Test Laboratory. The radiometer is an important instrument that will fly as part of the Ocean Surface Topography Mission on the Jason-2 satellite (see OSTM/Jason-2 Advanced Microwave Radiometer). The test results for the instrument were all positive, and the antenna reflector design passed all requirements.

The Environmental Test Laboratory, affectionately known as the "shake and bake" lab, includes "shake tables," which move large objects rapidly in horizontal and vertical directions, exposing them to the forces they will experience during launch. Recently, the lab's vertical table held the spare flight model of the antenna for the reflector structure assembly, one of the radiometer's two subsystems. Attached to the antenna was a test model of the electronics structure assembly, the radiometer's other subsystem. The test model weighs exactly the same as the flight instrument.

Although the shake test was only for the antenna with its lightweight graphite composite support structure, engineers included the electronics structure assembly model so they could observe how the antenna will behave when fully integrated into the reflector structure assembly.

All spacecraft instruments undergo rigorous testing to ensure they can survive the tremendous forces they'll be subjected to during launch. After all, if an instrument is unable to withstand the shakes, rattles, and rolls of a rocket launch, it will never get the chance to be proven in space. According to Alex Nicolson, the contracts technical manager for the Ocean Surface Topography Mission, most of the force generated during launch is from shaking and from the acoustic noise generated by the launch rockets.

The reflector structure assembly with attached test model of the electronics structure assembly is positioned on the vertical shake table.
Figure 2: The reflector structure assembly with attached test model of the electronics structure assembly is positioned on the vertical shake table.
Image credit: NASA/JPL-Caltech
A photograph of the actual advanced microwave radiometer reflector structure assembly.
Figure 3: The actual advanced microwave radiometer reflector structure assembly.

Image credit: NASA/JPL-Caltech

The ~23 kg (50.2 lb) reflector structure assembly antenna is held up by extremely strong graphite legs, which support the entire structure. (Figure 4) Each leg can carry a load of more than 5 tons.

Antenna graphite support legs and the electronics structure assembly test model.
Figure 4: Antenna graphite support legs and the electronics structure assembly test model.
Image credit: NASA/JPL-Caltech
Backside view of the advanced microwave radiometer antenna showing wires that connect to shake table.
Figure 5: Backside view of the advanced microwave radiometer antenna showing wires that connect to shake table.
Image credit: NASA/JPL-Caltech

For the recent shake tests, the antenna was shaken vertically, in what engineers call a "3/4-g swept sine" test. In this process, the shake table moves, or sweeps, through a series of frequencies from 5 to 140 Hz while a constant acceleration of 3/4 g is applied. At the lower frequencies, you can actually see and hear the structure shaking. But at some point, the structure begins to shake so fast that, to the human eye, all motion appears to have stopped. However, wires from the shake table to the antenna structure (Figure 5) record the antenna's motion and display the results on a computer screen.

In addition to the swept sine test, the antenna underwent two other shake tests. In a "sine burst" test, the antenna was shaken at only one frequency (30 Hz), over an amplitude range that has the shape of a teardrop. During the "random" test, the antenna was shaken through random frequencies from 20 to 2000 Hz. Each of the three tests was performed along each of the instrument's three directional axes for a total of nine tests per instrument component. Testing at the various frequencies helps engineers better understand the instrument's behavior in launch conditions.

AMR team members prepare instrument for thermal vacuum tests.
Figure 6: AMR team members prepare instrument for thermal vacuum tests.
Image credit: NASA/JPL-Caltech
The reflector structure assembly antenna in thermal vacuum chamber.
Figure 7: The reflector structure assembly antenna in thermal vacuum chamber.
Image credit: NASA/JPL-Caltech

After the shake tests, the antenna had to endure the thermal vacuum chamber (Figure 6), the baking component of "shake and bake". The antenna was baked at 80° C (175° F) in a complete vacuum for 96 hours to remove any gaseous materials embedded in the solid structure (Figure 7). When solid structures release gases when exposed to heat or vacuum, the process is known as outgassing. If a spacecraft instrument isn't thoroughly "cleaned" before launch, the vacuum of space could cause the instrument to outgas, making for some potentially serious problems.

The reflector structure assembly antenna in the acoustic chamber.
Figure 8: The reflector structure assembly antenna in the acoustic chamber.
Image credit: NASA/JPL-Caltech
After the thermal vacuum and the shake tests, the antenna was subjected to more testing in the acoustics chamber. Here the antenna was suspended from the ceiling of a room with 0.60-meter (2-foot) thick concrete walls (Figure 8). Two, large acoustic speakers, each measuring 1.52 meters x 1.52 meters (5 feet x 5 feet), are imbedded in one wall, and microphones and video cameras are placed at strategic locations around the room. When everything is set, the door is closed and operators (from outside the room, of course) subject the component to 5 to 10 minutes of synthesized launch sounds while engineers monitor the antenna's behavior. The acoustic tests can expose the instrument to sound levels up to 144 decibels. For humans, sound levels greater than 80 dB are potentially dangerous, says Nicolson. The threshold of pain is 130 dB, the sound of a military jet take-off registers at 140 dB, and at 160 dB, human eardrums are instantly perforated.

Following trials in the acoustic chamber, the antenna was integrated on the reflector structure assembly for another series of environmental tests. When all the tests were complete, engineers deemed that nothing moved significantly or changed shape as a result of the tests. The results were good news. The instrument components appear to be flight worthy and flight ready.

Jason-2 is on schedule for a June 2008 launch from Vandenberg Air Force Base in California. Following in the tracks of TOPEX/Poseidon and Jason-1, Jason-2 will extend the time series of ocean surface topography data to two decades. This additional information will allow scientists to improve our understanding of ocean circulation and heat transport leading to better climate models and predictions.

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