NASA Goddard Team Sets High-Flying Record with Use of GPS

The red ellipses show the MMS orbit paths during the first and second phases of the mission. Each spacecraft uses GPS signals — which come from satellites situated along the green circle shown surrounding Earth — from the far side of Earth to track its position. (Credit: NASA/MMS)

Editor’s Note: See more about the MMS mission in our August issue. The article is also posted here.

News courtesy of NASA

After years of hard work building a spacecraft, a mission team anxiously awaits after a launch — will the instruments they’ve crafted all work as well as planned? This is all the more true when flying new hardware, such as the onboard navigation tool on the four spacecraft that make up the Magnetospheric Multiscale, or MMS, mission, which launched on March 12. This navigation system had never before flown on a spacecraft with an orbit traveling so far from Earth — but if it worked, it would provide the precision navigation needed for MMS.

And the results are now in: Not only has the MMS Navigator system exceeded all of the team’s expectations, it has set the record for the highest GPS use in space.

• At the highest point of the MMS orbit, at more than 43,500 mile above the surface of the earth, Navigator set a record for the highest-ever reception of signals and onboard navigation solutions by an operational GPS receiver in space.
• At the lowest point of the MMS orbit, Navigator set a record as the fastest operational GPS receiver in space, at velocities over 22,000 miles per hour.

A precise tracking system is crucial for MMS, which requires extremely sensitive position and orbit calculations. The four spacecraft must fly in a tight pyramid formation to gather science data as they move through Earth’s magnetic environment. The formation is required to obtain three-dimensional observations of a phenomenon called magnetic reconnection that occurs when magnetic fields from the sun connect and disconnect with magnetic fields of Earth, which can allow energy and solar material to funnel into near-Earth space. With its instrument booms deployed, each spacecraft is the size of a baseball field — while flying as close as six miles from each other.

Artist’s concept of the MMS (courtesy of NASA).

“Demonstration airplanes like the U.S. Navy’s Blue Angels fly in closer formations, but those planes are also much, much smaller and the pilots are always controlling the movements,” said Brent Robertson, deputy project manager for MMS at NASA Goddard Space Flight Center in Greenbelt, Md. “We have four giant spacecraft each with its own unique orbit that we make maneuvers on about every two weeks. It’s quite challenging to control this formation.”

Tracking spacecraft can be done by radar stations from the ground, but it’s much more expensive and takes longer than an inflight system. However, using GPS as is typically done on Earth by such things as cars, boats and smartphones isn’t nearly as simple for something like MMS. For one thing, the bulk of its highly-elliptical orbit occurs above where the GPS transmitters orbit. So MMS must have specialized, extremely sensitive receivers to capture GPS signals transmitted from the far side of Earth. In addition the MMS spacecraft spin; each one makes three revolutions per minute.

“Spinning adds a whole new dimension to trying to figure out where you are,” said Ken McCaughey, MMS GPS Navigator Product Development Lead at Goddard. “We have four GPS antennas on each spacecraft. As the spacecraft rotates we have an algorithm running that allows us to hand off from one antenna to the next without losing the signal.”

In the first month after launch, the MMS team began turning on and testing each instrument and deploying booms and antennas. During this time, the team compared the Navigator system with ground tracking systems and found it to be even more accurate than expected. At the farthest point in its orbit, some 43,500 miles away from Earth, Navigator can determine the position of each spacecraft with an uncertainty of better than 50 feet.

What’s more, the receivers on MMS have turned out to be strong enough that they consistently track transmissions from eight to 12 GPS satellites — excellent performance when compared to pre-flight predictions that there might be frequent drop outs during each orbit.

Even if the receiver were to lose all GPS signals for part of the orbit, Navigator is specifically designed to handle such dropouts. By gathering as many observations as possible, integrated software called GEONS — for Goddard Enhanced Onboard Navigation System — can still compute the orbit by incorporating additional information including drag force, gravity, and solar radiation pressure.

This system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 95,000 miles from Earth.

“It’s going to be very interesting to see how far out MMS can still receive signals,” said Robertson. “But Navigator has already far exceeded expectations. I think there’s a good chance we’ll end up being able to use GPS and save us some of the expense of using ground observations.”

While Navigator technology and GPS receivers were previously flown for testing and to help navigate a low-earth-orbit mission, this is the first time that the complete Navigator package has been used to actively navigate a high-altitude mission. Now that the team knows it works so well, Navigator can be used for other missions that travel in similar high orbits.

— By Karen Fox, NASA Goddard Space Flight Center