April 5, 2019: NASA’s MMS probes, which use GPS signals to orbit Earth in tight formation, have just shattered the record for long-distance GPS navigation. The four probes recently located themselves 116,300 miles above Earth’s surface, surprising experts who once thought that GPS could function no higher than about 22,000 miles.
“When we began the mission, we had no idea high-altitude GPS would be such a robust capability,” says Trevor Williams, the MMS flight dynamics lead at NASA’s Goddard Space Flight Center.
MMS, short for “Magnetospheric Multiscale,” is a constellation of 4 spacecraft launched in 2015. They are on a mission to study magnetic explosions in our planet’s magnetosphere. High above Earth where the magnetic field is buffeted by solar wind, magnetic lines of force criss-cross, reconnect and—Bang! Magnetic energy is unleashed, with charged-particles flying off near the speed of light. The process is called “magnetic reconnection, and it serves as a power source for geomagnetic storms.
To study the inner physics of reconnection, the MMS probes must fly in precise formation, as close as 10 km apart, so that they can sample particles and fields inside the tight reconnection zone. With the aid of GPS, the fleet maintain formation with an accuracy of only 100 meters, which is crucial to their measurements.
GPS satellites are not designed to assist spacecraft. They focus their radio energy on Earth where we use the signals for terrestrial navigation. So how do the MMS probes do it? The answer is “side lobes.” This diagram shows a simplified but typical GPS antenna pattern:
All GPS antennas allow a little bit of radio energy to leak out in unwanted directions through side lobes. Receivers on the MMS probes tap into the leaked signal and use it to locate themselves. The first time MMS attempted navigation at the extremes of its orbit, the satellites had as many as 12 GPS fixes, each requiring signals from four GPS satellites. Not bad for a “leak.”
This type of navigation could reach all the way to the Moon. NASA analysts have run simulations suggesting that all six international GPS-like constellations (collectively known as GNSS) when working together could guide spacecraft in lunar orbit 238,000 miles from Earth. NASA is even considering adding GPS navigation to its Gateway outpost, a proposed space station for the Moon.