A U.S. Air Force airman plugs coordinates into a Defense Advanced GPS Receiver, or DAGR. ( Credit: U.S. Defense Department)

This article originally appeared in the Nov. 20, 2017 issue of SpaceNews magazine.

The often-heard narrative about the military’s global positioning system is that it is a victim of its own success. When the subject of GPS satellites comes up, Pentagon officials tout the constellation as one the military’s greatest technological feats, but one that has become a strategic target for enemies of the United States.

The military has moved to upgrade GPS spacecraft, control systems on the ground and user terminals with new security features. Modern satellites transmit high-power signals that can resist jamming and interference.

Fears are growing, however, as Russia’s military and other potential adversaries step up their electronic warfare game and come up with fresh techniques to deny U.S. forces access to positioning, navigation and timing data. The GPS service combines precision navigation and timing, or PNT, with map data and other information.

When speaking about GPS, Air Force Heather Wilson puts it this way: “We built a glass house before the enemy had stones. Now they have stones.”

Congressional defense committees have been studying this issue for years and worry that the Pentagon is not doing enough to provide “assured” PNT capabilities. The 2018 National Defense Authorization Act approved by the House last week gives the Departments of Defense, Transportation and Homeland Security $10 million to continue ongoing efforts to develop a GPS backup system. This follows a “requirements and alternatives analysis study” that Congress mandated by last year’s NDAA.

The defense secretary and the commander of U.S. Northern Command are being asked to brief the House Armed Services Committee by Dec. 15 on the “risks associated with disruptions to the Global Positioning System that could affect defense of the homeland and other defense activities in the United States.”

Lawmakers want a detailed account of what the Pentagon is doing about “PNT reliability and redundancy” and a study of potential alternative sources of PNT that could be available in the event of a major disruption to GPS.

Another way to strengthen PNT is to give the military access to other systems operated by U.S. allies. The NDAA requires the Pentagon to look into the benefits and risks of having military GPS receivers incorporate Europe’s Galileo and Japan’s QZSS satellite signals alongside GPS.

The Pentagon’s largest investment in secure PNT is in the future GPS 3 constellation. It has spent about $9 billion so far to develop new satellites, ground control system and terminals. The first one is scheduled to launch in 2018. The satellites, ground control segments and receiver units are all required to fully implement the more advanced and secure PNT technology.

GPS 3 increases signal strength and resistance to jamming, and provides new military and civilian signals known as M-Code and L1C, along with the three existing civilian signals. L1C will be compatible with the European Galileo constellation. The biggest headache in this program has been the OCX command and control system, which is years behind schedule. The latest projection is that it will be up and running by 2021, meaning that initial GPS 3 satellites in orbit will have to use older signals until OCX is ready.

The U.S. military will test new GPS M-Code receivers on the Air Force B-2 bomber, Navy DDG-51 destroyer (above) and Army Stryker fighting vehicles. (Credit: U.S. Navy)
The U.S. military will test new GPS M-Code receivers on the Air Force B-2 bomber, Navy DDG-51 destroyer (above) and Army Stryker fighting vehicles. (Credit: U.S. Navy)

New Receivers

The new M-Code receivers will be tested on the Air Force B-2 bomber, Navy DDG 51 destroyer, Army Stryker armored fighting vehicle, and joint light tactical vehicles from 2019 until 2021.

“The big news about assured PNT is that our customer base has started to understand that GPS has vulnerabilities that give them some pause,” said Mark Johnson, business development manager at Rockwell Collins, the military’s largest supplier of GPS receivers.

The receivers the military now uses — and that eventually will be replaced by the M-Code version — are called SAASM, short for Selective Availability Anti-Spoofing Module.

The Air Force is developing an “early user” M-Code receiver to fill the gap until the OCX ground control system is completed. The early user system works with eight GPS 2R-M and 12 GPS 2F satellites currently on orbit, as well as future GPS 3 satellites.

Rockwell Collins recently delivered the last of a 770 M-Code receiver order to the Air Force. Rockwell Collins, Raytheon and L3 Technologies have been under contract since 2012 to produce M-Code receivers.

“It’s important to remember that M-Code does not equal ‘high assurance,’” Johnson told SpaceNews. The M-Code “enhances assurance” but the only way to guarantee PNT is to add other backup sensors or use techniques that do not require GPS.

“High assurance PNT is bigger than the M-Code program,” he said. The military wants alternative technologies that will provide PNT without GPS. There are many, including inertial, visual, celestial and odometer sensors, map matching and pseudolite systems (ground based transmitters that look like a GPS satellite).

“Pound for pound, GPS is the best thing there is going,” he said. “It’s cheap, lightweight, low power, extremely precise, it’s a wonderful technology. But it’s vulnerable,” Johnson noted. “You can’t duplicate all of that. But you can get good enough PNT capability” by other means.

Johnson said Rockwell already is working on receivers that support Galileo signals. It signed an agreement with British manufacturer QinetiQ to develop “multi-constellation” global navigation satellite system (GNSS) receivers for military and government users.

GNSS is a global position, navigation and time determination system enabled by satellite constellations, aircraft receivers and system integrity monitoring sensors. Space constellations that support GNSS include GPS, Galileo and Russia’s Global Navigation Satellite System (Glonass).

Rockwell Collins plans to bring a combined GPS Galileo receiver into the market in the near future, said Johnson. The company has not worked with Japan’s QZSS. “It’s our understanding that DoD has not taken an official position on those signals.”

If the DoD wanted to bring QZSS into U.S. receivers, the industry would figure out how to do it, Johnson said. It’s already done in consumer devices. “Your smartphone has multiple GNSS receive capabilities” and get signals from GPS, Glonass and Galileo.

Security is an issue for DoD. “You want to know who is sourcing your signal. If it’s one of your enemies that has control over the signal, you might be reluctant to use it,” he said. ”In the commercial world, there’s an assumption of ‘no foul play’ by providers.”

Sandra Erwin writes about military space programs, policy, technology and the industry that supports this sector. She has covered the military, the Pentagon, Congress and the defense industry for nearly two decades as editor of NDIA’s National Defense...