NPP Runs the Gauntlet of Environmental Testing
The NPP satellite sits surrounded by 144 rock concert speakers. They're stacked in a circle 16 feet high in a testing room at Ball Aerospace in Boulder, Colorado.
As engineers set up for the environmental test, Pink Floyd's song "Money" plays gently in the background. The music stops. The room clears. Then the sound engineer wearing earplugs and headphones in the control room next door flips a switch.
Slowly, the noise of thousands of pounds of exploding rocket fuel builds louder and louder until it blasts the satellite at a deafening 143.6 decibels -- loud enough to cause serious damage and pain to unprotected ears. "I was outside the building when they did the full level acoustics," says Glenn Iona, NPP Chief Engineer at NASA Goddard Space Flight Center, Greenbelt, Md. "and I could feel the ground shaking."
The acoustic test is one of a gauntlet of environmental tests a satellite must pass to prove that it can survive launch and life in space. For Large Class Observatory mission NPP, this process took years to plan, 15 months to execute and was fraught with as many engineering challenges as building the satellite itself.
The NPOESS Preparatory Project (NPP) is the prototype for the next generation of Earth-observing satellites that will monitor daily weather and long-term ozone levels and climate change.
NPP's five instruments will continue data collection now done by an aging fleet of satellites. NASA's oldest Earth Observing System (EOS) satellites are more than 10 years old, with instrument designs and technology dating back to the early 1990s. NPP is the bridge between the original EOS missions and the Joint Polar Satellite System (JPSS). JPSS, previously called the National Polar-orbiting Operational Environmental Satellite System (NPOESS), will be developed by NASA for the National Oceanic and Atmospheric Administration (NOAA).
Testing to evaluate whether a satellite is ready for space occurs at several levels. Some individual parts and each individual instrument from the satellite go through three types of testing: dynamic, electromagnetic compatibility, and thermal vacuum.
Then the parts are integrated onto the main satellite bus, a wedge-shaped block the size of a four-door sedan. The bus has propulsion systems, a flight computer, a data processing computer, data storage and a solar panel wing that powers it all. Engineers then put the spacecraft and instruments through their paces to get a performance baseline before the whole satellite is run through the suite of environmental tests again.
The challenge, according to Goddard's Glenn Iona, who oversaw environmental testing for NPP, which took place in 2010 and 2011, is testing the satellite while taking into account all the different instruments' requirements and restrictions: Will the electromagnetic field generated by one instrument’s electronics interfere with the instrument sitting next to it? Will the jitter caused by the spacecraft or other instruments affect the sensitive Cross-track Infrared Sounder (CrIS)?
Iona says they weren’t sure about the shaking, so just in case, they designed a way to isolate CrIS’s platform from vibrations using frangi-bolts that will break in a controlled manner when heated on command, allowing the instrument to "float" on shock absorbers.
Keeping out Dust Bunnies
Engineers also must figure out how to run the tests without damaging or contaminating the instruments.
Most of the tests happen in specially ventilated clean rooms -- no dust allowed. Engineers work in white coveralls, called bunny suits, which prevent contamination from clothes, skin and hair. But even those precautions weren’t enough for the super-sensitive Ozone Mapping and Profiler Suite (OMPS). It detects solid particles and molecular gases in the atmosphere, and is sensitive to contamination from the tiniest amount of dust. During testing, frequent inspections and a plastic bag protected it. While that worked for OMPS, Iona says that solution wouldn’t work on other instruments.
"CrIS has paint you can’t touch," he says. The specialized paint reflects the sun's heat because part of the instrument’s design is to have a stable operating temperature. Anything touching it may fleck the paint away. Iona says the challenge was, "How do you keep it clean from contamination if you can't put a bag over it?" The answer: special hard covers or, during dynamics testing, a tented drape that avoided the paint.
In the dynamics testing room, the whole satellite wears protective bagging and sits on a giant shaker table where it's rattled up and down and side-to-side to simulate its rocket ride. In another chamber, testers bombard the satellite with the types of electromagnetic radiation it will encounter in space -- and then test for how much radiation it emits that might affect neighboring satellites.
The ‘Iron Maiden’
But the most complicated and challenging test is thermal vacuum (TVAC) where the satellite goes through four cycles of extreme cold to extreme heat in a vacuum chamber. The test simulates the temperature changes NPP will encounter on the day and night sides of the Earth, as well as worst case scenarios of whether the instruments can come back to life in case of a shut down that exposes them to even colder temperatures.
According to Scott Compton, Integration and Test Manager at Ball Aerospace, Boulder, Colorado, preparing for the thermal vacuum test took a year and a half and involved building a scaffold that engineers fitted to the satellite like a dress. It was an engineering "project within a project," says Compton.
Called the "Iron Maiden" after the medieval torture device, the scaffold held heaters and coolers less than an inch away from each instrument to meet their individual hot and cold temperature requirements. Liquid nitrogen was used to cool OMPS, the Advanced Technology Microwave Sounder (ATMS) and the Clouds and Earth’s Radiant Energy System (CERES) while CrIS and the Visible Infrared Imaging Radiometer Suite (VIIRS) were subjected to even colder liquid helium, to reach temperatures ranging from 30-120 Kelvins. VIIRS and CrIS complicated matters because they are both designed to be thermally stable -- they resisted cooling down and heating up.
During the test, the temperature changes were carefully monitored because too quick of a change would damage the instruments. Coordinating the many heaters and coolers "was a ballet for the thermal engineers," says Compton, who adds that NPP's thermal vacuum test was the most complex he's been involved with.
Seventy five people, from the testing team to each instrument's engineering and data analysis teams, camped out on site for the 24-hour testing that lasted 49 days in March and April, 2011. And the scientists who will be using NPP's data were on standby across the country to evaluate the instruments' performance.
Air Hockey, Anyone?
Last, but perhaps most important, the testing team unfolded NPP's three solar panels. Looking like a set of blackboards on wheels, the team simulated weightlessness by using what acts like the world’s largest air hockey table. Hoses attached to temporary support legs for the solar panels pushed air underneath hockey pucks on the feet. This created a localized 30 thousandths of an inch air cushion. With reduced friction, the pucks then slid across a slick dance floor made of polymer roofing material, and the three panels locked themselves into place perfectly.
After resolving the 107 test anomalies they found during months of vibration, noise, electromagnetic radiation and controlled swings in temperature, the satellite's onboard computers and instruments passed their final performance tests. NPP is ready for space. Glenn Iona's says his favorite part of the process was that, even with all the complications and problems that cropped up, the environmental testing team passed every stage on or ahead of schedule. "Seeing the environmental testing come together and leading the team, TVAC in particular, was really satisfying on so many levels," he says.
NPP is scheduled to launch into orbit from Vandenberg Air Force Base in southern California on October 25, 2011.
NASA's Goddard Space Flight Center, Greenbelt, Md.