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Space weapons meant to target U.S. satellites are a growing concern for the U.S. military. Especially worrisome are electronic jamming devices designed to interfere with GPS signals.

That threat is “real today and concerning,” the chief of space operations of the U.S. Space Force Gen. John Raymond told the House Appropriations Committee’s defense subcommittee during a hearing in May.

Raymond pointed at China and Russia as the primary actors pursuing technologies aimed at “robust jamming of GPS and communications satellites.”

A constellation of 31 satellites (24 plus spares) orbiting the Earth at an altitude of 20,000 kilometers, GPS is operated by the U.S. Space Force like a global utility. GPS satellites in six orbital planes circle the Earth twice per day broadcasting positioning, navigation and timing (PNT) signals that are critical to the daily functioning of the civilian economy and essential to every facet of military operations.

The first satellite, called Navstar 1, was launched in 1978. The full constellation became operational in 1995.

The military’s huge dependence on PNT data makes GPS an attractive target for adversaries, said the RAND Corp. in a recently published report.

The new generation of GPS satellites provides a stronger signal known as M-code for military users U.S. Marine Corps combat engineers use utilize GPS devices during a July 6 exercise aboard the amphibious assault ship USS America in the Phillipine Sea. Credit: U.S. Marine Corps Photo by Cpl. Karis Mattingly

In addition to denying GPS through electronic jamming attacks, foreign militaries could also target U.S. military GPS users with falsified PNT data, a technique known as spoofing. A GPS outage could wreak havoc across all military activities involving aircraft, ships, munitions, land vehicles and ground troops. “In an active military conflict, even brief denials and spoofing of PNT might make a difference if well timed with other operations,” said the report.

In response to these vulnerabilities, the Space Force is hardening what it calls the “GPS enterprise” that includes three segments: satellites in space, the ground control system and user equipment. “We are looking at how we will continue to evolve this architecture into one that is more robust and resilient than it is today,” said Col. Ryan Colburn, director of the spectrum warfare division at the Space and Missile Systems Center.

DoD uses a number of PNT technologies to complement GPS or to serve as backup when GPS is degraded or unavailable.

Some military platforms use onboard sensors to track their position and keep time without the use of an external signal. Other PNT technologies use external sources of information other than GPS to determine the position of a platform. Military combat aircraft use GPS paired with inertial navigation systems so if GPS goes out the pilot can still complete the mission. Inertial sensors and clocks allow a platform to identify its position and keep track of time without an external signal like GPS. Other technologies rely on celestial and magnetic navigation to determine position. There’s also a growing number of satellites in low Earth orbit that transmit PNT information.

None of these alternative PNT technologies, however, has reduced the U.S. military’s dependence on GPS, said the Government Accountability Office in a May report.

“Alternative PNT does not appear to be a particularly high priority for DoD,” said Karen Howard, GAO’s director of science, technology assessment and analytics.

Howard’s office investigated the issue at the request of the Senate Armed Services Committee.

“We interviewed a number of people at DoD and outside experts,” she said. “From everything we’ve heard, developing and integrating alternative PNT technologies isn’t highly prioritized,” she added. “They are doing research, they’re looking into it, but the department is heavily reliant on GPS.”

Jon Ludwigson, GAO’s director of contracting and national security acquisitions, said Congress for years has questioned DoD’s strategy of keeping GPS as the centerpiece of its PNT architecture. Because of the success of GPS, “the alternatives really haven’t had to step forward,” he said. Meanwhile, “there’s a rising call of concern that if we lost GPS, what would that mean for our operational plans?”

“There are known vulnerabilities and military officials talk about the need for alternatives,” Howard said. But when it comes down to funding programs, by default they choose GPS.

“Even if they don’t need that level of precise PNT, they’re asking for GPS because that’s what they’re used to,” Howard added. “A ship in the middle of the ocean doesn’t need precision within a few meters of its position but they’re still defaulting to that. It’s their comfort zone.”

Concerns about military and civilian infrastructure dependence on GPS led Congress to mandate a study in the 2017 National Defense Authorization Act. It directed federal agencies to assess and identify PNT technologies to back up and complement GPS for national security and critical infrastructure.

The study, conducted by the RAND Corp. and delivered to Congress in April 2020, listed an array of technologies that can help to augment global navigation satellite systems like GPS, but concluded that few can replace them.

No single system is a perfect backup for GPS, said the report. Some systems such as Europe’s Galileo are similar to GPS and could be a suitable substitute if GPS were spoofed or disrupted by a cyberattack. There are also ground-based signals used for timing and positioning, including signals from public and private Wi-Fi transmitters and cellphone towers. Despite greater availability of alternatives, “all research shows that dependence on GPS and other GNSS [Global Navigation Satellite Systems] continues to grow,” noted RAND.


A SpaceX Falcon 9 rocket on June 17 launched the fifth of 10 planned GPS 3 satellites, the newest generation of GPS now being deployed to replace aging satellites that have been in service since the 1990s.

GPS 3 satellites are built by Lockheed Martin under a 2008 contract from the U.S. Air Force. L3Harris supplies the navigation payload. Compared to earlier generations, GPS 3 signals are more accurate and better protected against jamming, said Col. Edward Byrne, senior materiel leader at the Medium Earth Orbit Space Systems Division at the Space and Missile Systems Center (SMC).

The GPS 3 constellation broadcasts a new L1C civil signal, which is compatible with international GNSS like Europe’s Galileo.

DoD is investing $4.1 billion in the GPS 3 space segment, including production, launch and operating costs of the 10 satellites, an SMC spokesman said. Five GPS 3 satellites have been launched since 2018. The remaining five are projected to go to space from 2022 through 2024.

To provide additional security for military users, GPS 3 satellites broadcast a stronger signal called M-code. Short for military code, M-code transmits at much higher power and is more jam-resistant than civilian signals. The Air Force launched the first GPS satellite capable of broadcasting the M-code signal in 2005.

A minimum of 24 M-code enabled GPS satellites are needed to provide global coverage of the stronger signal, Byrne said. The launch of the fifth GPS 3 last month was significant, he noted, because it was the 24th M-code capable satellite.

Despite greater availability of M-code, most U.S. military forces still can’t take advantage of the more secure signal because they don’t have compatible user equipment, noted Ludwigson, the director of contracting at GAO.

GAO for years has called out the Pentagon for not producing enough M-code-capable receiver equipment and for taking too long to install these receivers across all weapon systems.

DoD is working to accelerate the fielding of advanced receivers, the Pentagon said in its 2022 budget proposal which seeks $434 million for M-code-compatible GPS receiver cards for Army, Navy, Air Force and Marine Corps platforms.

Ludwigson said DoD is making some progress fielding M-code equipment but cautioned that this technology is not a silver bullet. “It’s a stronger signal, but I don’t think anyone expected M-code was a one-stop shop or complete solution.” DoD still needs backup options, he added. “The focus should be on resilience, not on a specific technology.”


The Air Force in 2018 affirmed its commitment to GPS when it awarded Lockheed Martin a $7.2 billion contract with options to produce up to 22 GPS 3 Follow-on satellites, or GPS 3F. The Space Force already has ordered the first four.

The first GPS 3F is projected to launch in 2026. The satellites will have enhancements such as the ability to broadcast stronger military signals regionally to ensure U.S. forces in combat zones can get PNT data. The new satellites also will carry a new payload for search-and-rescue services, and laser retro-reflector arrays for better on-orbit position determination.

DoD in its proposed budget for fiscal year 2022 is seeking nearly $1.8 billion for the GPS program. About $1.1 billion is for research, development and testing of the GPS ground system and user receivers, and for the integration of the remaining five GPS 3 satellites that are in storage or in the final stages of production. An additional $688 million is for the procurement of two GPS 3F satellites.

“Adversaries have long recognized our dependence on GPS and have proliferated technologies to degrade, deny and spoof GPS signals for civil and military users,” DoD said in its budget request. “We are pursuing modernization efforts across the entire GPS architecture to include upgrades in space, ground and user segments.”

The ground system has been a major hurdle in the modernization of GPS. For the GPS 3 constellation, the Air Force selected Raytheon to develop a new ground control system designed to be more secure against cyber-attacks and compatible with the stronger M-code signals. Estimated to cost $6.7 billion, the Next Generation GPS Operational Control System (OCX) is considered one of DoD’s most troubled programs, hampered by delays and more than $2 billion in cost growth since the program started in 2012.

The Air Force and Raytheon restructured the program in 2018 and early versions of the ground control software have been delivered for testing. Contributing to the delays is a requirement that Raytheon remove IBM computer hardware from the OCX system by April 2022. This is for security reasons following the sale of IBM’s computer server product line to Chinese-owned Lenovo. Raytheon will replace the IBM computers with Hewlett Packard Enterprise hardware.

The Space Force meanwhile is relying on a ground system developed by Lockheed Martin to control and operate the GPS constellation as a bridge until OCX is ready. The interim ground system allows the Space Force to implement the M-code signal. Raytheon is under contract to complete by 2025 a new upgrade to make the system compatible with GPS 3F satellites.

A major criticism of the GPS program has been a lack of integration and coordination across the space, ground and user equipment segments. Both GAO and the Pentagon’s director of operational test and evaluation have pointed this out in numerous reports and have called for better synchronization among the three segments of the GPS enterprise.


With these challenges in mind, the Air Force in 2018 embarked on a research project to demonstrate a new PNT satellite with advanced technologies that could transition to the next generation of GPS.

The project, run by the Air Force Research Laboratory, was named Navigation Technology Satellite-3, or NTS-3, as it is the third major space experiment by the U.S. military to advance positioning, navigation and timing technologies. The Naval Research Laboratory in the 1970s launched the NTS-1 and NTS-2 demonstration satellites that served as technological building blocks of the current GPS.

Northrop Grumman said July 1 it had delivered an ESPAStar-D spacecraft bus to L3Harris in support of the NTS-3 mission launching in 2022 to demonstrate advanced PNT technologies. Credit: Northrop Grumman

L3Harris is producing NTS-3 using a Northrop Grumman ESPAStar bus. The satellite is scheduled to launch in 2023.

Unlike the GPS program, NTS-3 will pursue a more integrated approach to the development of the space, ground and user equipment segments, said Col. Eric Felt, director of the Air Force Research Laboratory’s Space Vehicles Directorate.

“One of the lessons learned from GPS was just how important it is to have tight integration and coupling between the user equipment, the on-orbit equipment and the software that flies the satellites,” Felt said.

Another key feature of NTS-3 is that the satellite, the ground system and user equipment are fully digital and reprogrammable, said NTS-3 program manager Arlen Biersgreen.

“This gives you the ability to recover quickly if there is a denial of service,” said Biersgreen. “If there’s an attack, we can pivot, we can do different encryption, we can do frequency changes so we’re able to defeat jamming as it occurs.”

Another benefit of reprogrammable satellites is that changes can be made without having to buy new hardware and fly it to space “which is expensive and takes ages,” Biersgreen said.

NTS-3 also is expected to add resilience to the military’s PNT architecture by adding a higher layer in geosynchronous Earth orbit, he said. “With NTS-3 we are looking at where we can move away from just mid Earth orbit with GPS and start diversifying.”

Felt said AFRL scientists are working on a more advanced military PNT signal that would be stronger than M-code. “It’s a concept we call Q code, an even more advanced way to do PNT signals using quantum and other advanced communications technologies that can potentially be applied to navigation.”

The Air Force in 2019 designated NTS-3 as one of its “Vanguard” programs, a category assigned to projects that promise trailblazing capabilities. If the experiment is successful, the Space Force likely will transition technologies demonstrated by NTS-3 to the GPS 3F program, or it could decide to build more satellites like NTS-3 to augment the GPS constellation.

NTS-3 is all about adding resilience to the GPS system to ensure availability of PNT, said Biersgreen.

“PNT is sometimes considered to be synonymous with GPS because GPS is so awesome, so accurate and so consistently available,” he said. “The thinking is that if a military user needs PNT information, they should get it from GPS.”

This article originally appeared in the July 2021 issue of SpaceNews magazine.

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...