Military Space Quarterly | U.S. Air Force Squeezing Extra Life from GPS Satellites
WASHINGTON — The U.S. Air Force’s intensifying scramble to do more with less in difficult fiscal circumstances has produced a brewing success story on the GPS program, where engineers are implementing a plan to extend the life of up to 60 percent of the satellites in the positioning, navigation and timing constellation.
The plan involves a new charging method that reduces the rate of satellite battery degradation, thereby extending the satellites’ lives. The scheme is being successfully tested on orbit, and Air Force officials say that if all continues to go well, the project could add a combined 20 years to the life of the GPS 2R- and 2R(M)-series satellites, which together comprise the backbone of the constellation.
The life extension is expected to save the Air Force tens of millions of dollars, and that carries a premium as the combined pressures of declining defense spending and sequestration put the squeeze on all Air Force programs, space being no exception.
Gen. William Shelton, commander of Air Force Space Command, cited the effort during a Senate hearing in April, and credited Capt. Jacob Hempen, who serves with the Air Force’s 2nd Space Operations Squadron, with making it happen.
An electrical engineer by training, Hempen’s introduction to military space came on a different program. While serving in Afghanistan as a flight test engineer, Hempen, who comes from a military family and graduated from the Air Force Academy, was tasked with fixing a satellite connectivity problem that was hampering critical unmanned aerial vehicle operations.
The stakes were high: Because of the problem, certain military activities were facing a suspension of a month or more.
Hempen had virtually no experience in the space arena — he had never collaborated with the Joint Space Operations Center, which coordinates command and control of U.S. military satellites and never worked with the Air Force space community.
His superiors gave him two to three days.
The fix worked. Operations continued.
In an interview, Hempen said the experience taught him the importance of military space. It also landed him in a position with the Air Force’s space program.
It was there that Hempen was presented with a new challenge: implementing a plan to extend the lives of the existing GPS satellites. The Air Force is always looks for ways to stretch the lives of its operational satellites, but implementing solutions across large constellations presents obvious challenges.
“The key is identifying a large group of satellites where a single improvement solution can be applied,” Hempen said. “In this case, 60 percent of on-orbit GPS satellites have a common bus with common components that wear out, one of which is batteries. By extending the use of these batteries, constellation sustainment can be improved across GPS.”
Managing battery life for satellites requires a tricky calculus.
Satellites charge when their solar arrays are directly exposed to the sun’s rays. During this time, the arrays simultaneously charge the batteries and power the space vehicle. While the satellite is on the dark side of the orbit, the vehicle runs on batteries.
Unlike a cell phone or electric toothbrush that reaches a full charge over a predetermined amount of time, satellite batteries can be charged at different rates. The energy can be pushed into the battery at a high rate quickly, or it can be stored more gradually at a slower pace, according to industry sources.
Air Force officials discovered that when some of the GPS satellite batteries are powered over a certain rate threshold, they can overheat, accelerating their natural rate of decay.
And since batteries in space today are virtually impossible to replace, longer-lasting batteries represent one of the keys to longer-lasting satellites.
The current GPS constellation consists of 31 operational satellites of different generations made by two different manufacturers. The Air Force focused its effort on the GPS 2R and 2R( M )spacecraft built by Denver-based. The first of these satellites was successfully launched in 1997 and 19 — representing 60 percent of the constellation — are operational, according to GPS.gov, a government-managed GPS information website.
Hempen credited Warren Hwang, who works in the energy technology department at Aerospace Corp., with coming up with the idea of lowering the charging rate of the GPS 2R and 2R (M) batteries. Based in El Segundo, Calif., Aerospace is a federally funded not-for-profit research center that provides engineering advice on Air Force space programs.
According to Hempen, Hwang suggested that the battery charging rates could be lowered to a level that enabled the satellites to perform reasonably well while minimizing the rate of degradation. Hwang declined to be interviewed for this article.
“My role was to lead the project from theory to start of execution,” Hempen said. This entailed devising and conducting a series of tests, first in laboratory-based simulations and later on the satellites themselves.
The group needed to act quickly: The more time spent developing and implementing the test plan, the more degradation incurred by the satellite batteries, reducing the return on investment of resources.
The test program, which is ongoing, has since been taken over by the Air Force GPS operators and is running smoothly, Hempen said. “We knew our approach was sound,” he said.
If the project is successfully implemented, GPS experts at the Air Force and Aerospace Corp. estimate that it will help the service wring an additional 20 years of operation across 2R and 2R(M) satellites. Given that each satellite was designed to operate for eight years, that is the equivalent of more than two additional spacecraft that likely would cost in the hundreds of millions of dollars to build and launch.
Perhaps equally important is the possibility that the same life-extending technique can be used on other Air Force satellite programs, according to Hempen. “Both the new battery method and GPS methodology used to determine how to improve constellation sustainment can be applied to other satellite constellations,” he said.