Receivers Reach New Heights

The four MMS spacecraft host the highest ever operational GPS receivers in space. (artist’s rendition, credit: NASA)

Editor’s Note: See additional coverage of the MMS mission here.

September Marks Start of Magnetosphere Mission, but Navigators Already Perform

A NASA mission to explore magnetic reconnection also made GPS history this spring. The Magnetospheric Multiscale (MMS) mission, led by NASA’s Goddard Space Flight Center in Greenbelt, Md., is flying four identically equipped spacecraft in a tight formation to take measurements 100 times faster than any previous space mission.

Each of the four spinning MMS spacecraft — roughly the size of a ballpark once eight booms deploy — is equipped with 25 sensors and other components provided by more than 40 partner institutions in the U.S., Europe and Japan. One key component is a GPS receiver dubbed Navigator.

Magnetic reconnection is a fundamental, yet poorly understood process.While reconnection occurs throughout the universe when magnetic field lines within plasma connect and disconnect, it can impact our technological society, since it drives virtually all space weather events that can disrupt low-Earth-orbiting spacecraft and lead to GPS, communications and power blackouts on Earth.

During the mission’s first phase, which begins in September, the spacecraft will travel through reconnection sites on the sun-side of Earth, where the orbit extends out toward the sun to around 47,500 miles. One year later, ground controllers will move the spacecraft to Earth’s night-side or magnetotail where the magnetic fields also reconnect — an orbit that extends away from Earth to almost 99,000 miles, nearly halfway to the moon.

However, science operations can’t begin before the four move into a highly elliptical orbit and assume their pyramid-shape formation that places the spinning spacecraft just 6.2 miles apart. It required a breakthrough to accomplish such an exacting formation, and the Goddard-developed Navigator GPS provided the solution.

Begun in the early 2000s as an enabling technology for MMS-type missions, the Navigator receiver and associated algorithms can quickly acquire and track GPS radiowaves even in weak-signal areas well above GPS’s 30-plus-satellite constellation positioned about 12,550 miles above Earth. In addition to continuously tracking weak signals, the Navigator also must operate as the spacecraft spin at three revolutions per minute. As a result, each MMS satellite is equipped with two Navigator receivers (primary and redundant), with four antennas placed around the perimeter of each, assuring continuous contact with the tracked GPS satellites

“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. “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.”

Robust Receivers. To the satisfaction of the technology’s architect, Goddard technologist Luke Winternitz, the receivers have proven very robust. Shortly after the GPS receivers were powered on after the launch, Navigator became, at more than 43,500 miles above Earth’s surface, the highest ever operational GPS receiver in space. “We’re tracking up to 12 GPS satellites at maximum altitude and track on average about nine,” Winternitz said. “We’re really excited about their performance so far.”

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 — Goddard Enhanced Onboard Navigation System — can still compute the orbit by incorporating additional information including drag force, gravity, and solar radiation pressure.

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)

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 Mission Deputy Project Manager Brent Robertson. “But Navigator has already far exceeded expectations.”

Almost all activities associated with operating the mission depend on where the satellites will be positioned a few days hence. That includes everything from determining the best time to downlink telemetry and scientific data to calculating when ground controllers would command the firing of the satellites’ onboard thrusters, which move and help maintain their orbital formation — an exercise that will happen at least once every couple weeks.

“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,” Robertson said.

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.

The four MMS observatories are processed for launch in a clean room at the Astrotech Space Operations facility in Titusville, Fla. The MMS mission launched March 12, 2015. (Credit: Ben Smegelsky/NASA)

Navigator Highlights

• 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.
• 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.