SUMMERLAND KEY, Fla. — Now that the U.S. military’s experimental Coriolis satellite is in orbit and operating well, some engineers are breathing sighs of relief while others are insisting they had confidence all along in the models they used to predict the performance of the $220 million spacecraft.
The U.S. Air Force launched Coriolis in January from Vandenberg Air Force Base, Calif. The spacecraft carries two radically different sensors, one paid for largely by the U.S. Navy and the other by the Air Force. The designs of these sensors were difficult to accommodate on a single satellite, program officials acknowledged.
The main payload is a $70 million spinning microwave reflector built by the Naval Research Laboratory in Washington. That sensor, dubbed WindSat, will gather microwave emissions from Earth, amplify them a trillion times and use the data to determine the wind direction at the ocean’s surface. U.S. sailors would rely on the wind information to target munitions and steer clear of wind-born biological- or chemical-weapon agents. The Air Force, NASA and the U.S. Commerce Department contributed about $20 million to Windsat as a test of a similar sensor that could fly aboard a new series of weather satellites being developed by the agencies.
The second Coriolis instrument consists of three cameras that will photograph streams of hot plasma belched Earthward by the sun. When these particles interact with Earth’s magnetic field, they are accelerated into storms of electrical current that can disrupt spacecraft electronics, scramble computer data bits and block communications. The Air Force hired the University of Birmingham in England to build the Solar Mass Ejection Imager, which is designed to provide up to three days of warning of geomagnetic storms to military satellite operators.
As late as July and August 2002, just four months before the scheduled launch, engineers from the Navy, Air Force and satellite prime contractor Spectrum Astro were still debating the compatibility of the two instruments. Spectrum Astro of Gilbert, Ariz., provided the Coriolis satellite platform and was in charge of integrating and testing the payloads.
Ground tests and predictive models had raised fears among Air Force engineers that the WindSat radiometer, which spins at 31.6 revolutions per minute, might cause enough wobble to blur pictures taken by the solar imager, which is affixed to the stable part of the spacecraft, said Air Force Capt. Earl Burnette, who oversaw development of Coriolis.
The spacecraft’s total wobble, or jitter, was predicted to be within the acceptable range, but by an uncomfortably slim margin, Burnette said. Other potential issues, such as the degree of electromagnetic interference given off by the Spectrum Astro satellite frame, had been resolved. The spacecraft’s electronics were not expected to interfere with the highly sensitive WindSat radiometer.
Coriolis had passed a series of tests inside a thermal vacuum chamber at the Naval Research Laboratory, but engineers had to remove the spinning microwave dish for it to fit inside the chamber. Program officials briefly considered shipping the entire spacecraft to a facility in California with a larger thermal vacuum chamber to settle the jitter debate, but ultimately decided there was not enough time or money to add that test, Burnette said. Program officials instead asked the federally-funded Aerospace Corp. in El Segundo, Calif., to review their jitter predictions.
Engineers at the Naval Research Laboratory had spun up WindSat in nonvacuum conditions and then subtracted out the aerodynamic forces the dish-shaped microwave reflector generated as it moved through the air. The jitter number calculated by the Aerospace Corp. "stayed right around" that predicted by the WindSat team, Burnette said.
Even so, Air Force officials breathed a sigh of relief in late January after technicians sent commands to Coriolis to begin spinning the WindSat antenna. Subsequent analysis showed that the jitter in orbit was much less than the worst case scenario predicted in ground tests, program officials said. Engineers are now testing the accuracy of Coriolis’ instruments and refining software to process the data.
"The problem with ground testing is it’s never the same as what you encounter in space," said Janet Johnston, the Air Force program manager in charge of the Solar Mass Ejection Imager at Hanscom Air Force Base, Mass. "The jitter has turned out to be a non-issue," she said.
Spectrum Astro officials said the performance of Coriolis should give confidence to engineers who are planning to use similar Spectrum Astro satellite frames, known as buses in the industry.
The Coriolis satellite frame resembles the one built by Spectrum Astro for NASA’s Deep Space 1 mission, said John Sutila, the company’s Coriolis program manager. Similar frames will be used on NASA’s Swift gamma-ray burst astronomy spacecraft and the Gamma Ray Large Area Space telescope, he noted.
Spectrum Astro also is a partner with NASA’s Jet Propulsion Laboratory on a proposed remote sensing spacecraft called Hydros, which would measure Earth’s soil moisture from space. Hydros currently is listed as a backup mission under NASA’s Earth System Science Pathfinder program. It would be built if one of the primary program missions encounters insurmountable technical, cost or schedule problems.
Spectrum Astro officials also envision new markets for scientific missions requiring electromagnetically quiet satellite frames and spinning payloads. "Having worked through these issues on Coriolis, we have learned how to deal with large, yaw-spinning spacecraft," Sutila said.
