WASHINGTON — The Pentagon is spending tens of billions of dollars on a new constellation of more secure GPS satellites and a stronger anti-jam signal in anticipation of a future conflict where adversaries will try to disrupt access to this critical global navigation system.

But even as the military is counting on a modernized GPS to be more resilient to electronic attacks, it is eyeing alternative means to get vital positioning, navigation and timing data, just in the event that GPS is denied.

“People don’t realize how weak the signal is,” said Kaigham “Ken” Gabriel, president and CEO of Draper Laboratories, a non-profit research and development firm with U.S. military and NASA contracts.

Draper has worked for decades on alternative technologies to GPS but the Pentagon only in recent years has expressed interest in exploring them amid warnings that military satellites are being targeted by enemies. In a recent interview with SpaceNews, Gabriel — a former deputy director of the Defense Advanced Research Projects Agency — cautioned that there will never be a single “silver bullet” that can replace a system as capable at GPS, but if U.S. satellite signals ever were disrupted, the military needs back-up options.

“I think you’re going to see many approaches,” he said. “There will not be a single substitute. There will be a need for multiple solutions all operating at the same time.”

Gabriel has offered the military a Draper-developed celestial navigation technology also known as a star tracker. “We’re excited about this system that is designed to be in GPS denied environments,” he said. Dubbed “celestial object sighting system,” it measures the position of satellites relative to stars. “Angular measurements can turn into GPS level accuracy,” Gabriel said. “We have so much junk in orbit that we have to track anyway. We can turn junk into a feature. We can do GPS level mapping using only stars and satellites.”

Draper has made non-GPS navigation and timing a central part of its business. In addition to celestial tracking technologies, it has developed inertial navigation systems, precision time transfer using optics and chip-scale atomic clocks, and vision-based navigation that relies on cell phone cameras and signal processing algorithms.

The GPS 3 satellites made by Lockheed Martin for the U.S. Air Force will start going into orbit in the coming years. They will have three times better accuracy and up to eight times improved anti-jamming capabilities compared to the existing satellites. These are “important improvements,” said Gabriel. “You can make the system stronger but you still need alternatives.” He noted that the current GPS signal is about 10,000 times less intense than most cellphone signals.

Draper’s deep expertise in this area partly comes from its work as a prime contractor for the Navy’s ballistic missile program. The company for decades has refurbished the guidance system in every missile that goes to the fleet. “That guidance does not use GPS, it never has,” said Gabriel. “When the Polaris missile was designed in the 1960s there was no GPS. For some reason the guidance system was never swapped out because it was working fine.”

In hindsight, said Gabriel, “it was brilliant to not put it on GPS because it is robust the way it is.” The guidance uses an inertial measurement unit and celestial navigation. “We need better inertial sensors. We’re working on that.”

Communications links
Another navigation option in the absence of GPS recently was tested by the U.S. Navy. It is a shipboard system that uses communications signals, regardless of whether they are ship-based, ground-based or from satellites.

The technology, called “enhanced link navigation system,” was developed by CTSi and L3 Technologies. An ELNS prototype was built under an $8.7 million small business innovative research Navy contract.

“We were focused on a system for aerial navigation and landing performance,” CTSi vice president Glenn Colby told SpaceNews.

“ELNS leverages existing communications systems to provide navigation and landing functions,” he said. Now that the technology has been demonstrated, “we’re working with the Navy and the Air Force to transition ELNS to other applications.”

The Air Force could use this system for aerial refueling, for example. “The idea is to provide a navigation reference between a tanker and a receiver aircraft,” said Colby.

A system like ELNS can be combined with GPS for added capability, “but we focused on the performance of ELNS assuming GPS was not there at all.”

“So much has gone into the development of GPS. But there is a a lot of interest in providing alternatives,” Colby said. “We’ve seen a lot of activity in this space. A lot of research. But so far not a lot of scale.”

L3 Technologies provided an anti-jam waveform that would be needed to prevent enemies from detecting and disrupting aircraft communications and navigation.

“DoD is properly looking at all options,” Colby said. There is inertial navigation, satellite based navigation, ground based navigation. One of the benefits of the ELNS is that it can can installed in aircraft without adding new avionics or antennas. “That is a big deal,” he said. In military operations, 75 percent of the cost of deploying an aircraft comes from on-board equipment and integration.

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...