Evolution of military navigation is well underway

3Military navigation is undergoing several significant changes - including the introduction of international satellite constellations, a transition from Selective Availability Anti-Spoofing Module (SAASM) secure GPS to M-code, and an increasing interest in augmenting GPS with other technologies.

The combined launch of new international satellite constellations and arrival of modernized GPS, a.k.a. Military code or simply “M-code,” means changes are underway on several fronts for the U.S. military and its allies in terms of achieving secure positioning, navigation, and timing (PNT) based on the global navigation satellite system (GNSS).

The GNSS currently encompasses all of the equipment capable of receiving signals from multiple satellite systems – including the U.S. NAVSTAR GPS and Russia’s GLONASS, as well as Europe’s Galileo and China’s Beidou systems, which are in the process of being deployed now. France, Japan, and India also plan to send up regional geostationary satellites, which will become part of GNSS.

Many of the signals sent by the new constellations are extremely robust and contain more complex data than the original GPS system. Notably, signals from the new constellations don’t necessarily operate within the same frequency bands as signals in the past, so it’s necessary to ensure antennas are capable of receiving multiple satellite signals.

“As a global company serving the U.S. military and other militaries around the world, other countries’ constellations will affect us and our markets,” says Al Simon, marketing manager for Rockwell Collins (Cedar Rapids, Iowa; www.rockwellcollins.com). “Conversations with customers are evolving into discussions about which markets, constellations, and specific capabilities they’ll need now.”

Multi-constellation capability is a rapidly evolving requirement, particularly for international militaries, points out Nik Hartney, Honeywell Aerospace’s director of Defense Navigation Aircraft (Phoenix, Ariz.; www.aerospace.honeywell.com). “Now that other countries are investing in their own constellations, they want to integrate them into their aircraft and navigation systems.”

This entire space “is becoming very interesting, and one in which secure GPS or GNSS will be of value – both in terms of M-code or Galileo’s PRS, which is Europe’s encrypted equivalent capability,” Simon notes.

M-code

The U.S. military is currently transitioning to next-generation GPS M-code, a central element in the modernization of military GPS capabilities.

“Upgrading to modernized or M-code, the follow-on to the current Selective Availability Anti-Spoofing Module (SAASM) military GPS signal in space, is a major priority for the U.S. military right now,” says Hartney.

Honeywell is working to complete the first integration of an M-code receiver in its embedded GPS inertial navigation system (EGI) to prepare for the FY2017 mandate. During 2014, the industry made solid progress preparing for the M-code mandate. Rockwell Collins, for example, tested its “GB-GRAM-M” M-code GPS receiver – using live M-code signals – to navigate an AeroVironment RQ-11B Raven unmanned aerial system. And the U.S. Air Force also demonstrated its M-code signal in a jamming environment using a Raytheon MAGR2K receiver.

In a related effort, backed by a $2 million contract from the U.S. Air Force Research Laboratory and the GPS Directorate, Rockwell Collins is developing a secure software-defined radio (S-SDR) GNSS receiver capability. “This program will help develop the security architecture required for future receiver equipment approvals and certifications,” Simon says.

Augmented GPS for GPS-denied or degraded environments

For U.S. military operations within GPS-denied or degraded environments, relying solely on GPS for navigation isn’t an option. “Everyone is pursuing a capability set that’s robust enough to ensure navigation if GPS experiences a temporary outage, or, in a wild scenario, even a permanent outage,” Simon says. “Navigation augmentation needs are different for each customer – it depends on the platform involved.”

Inertial measurement units (IMUs) are emerging as one of the hottest topics of “augmented” conversations taking place now, because these sensors don’t rely on external signals and can’t be jammed. “The performance of IMUs has improved significantly and they’ve also become smaller and more affordable, so they’re being discussed as legitimate navigation aids for specific missions,” says Simon.

Honeywell makes high-accuracy embedded GPS/INS (EGI) navigation systems with inertial sensors for military aircraft, and Hartney explains that their high performance “is particularly important for GPS-denied environments – allowing the navigation system to ‘coast’ through GPS outages without significant degradation of accuracy.”

Combining EGI with beam-forming anti-jam systems is another area Honeywell is focusing on. “Tests show this combination is capable of maintaining GPS tracking through the most significant jamming environments – with little to no degradation in accuracy,” Hartney says (see Figure 1).

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Figure 1: Pictured is Honeywell’s EGI FALCN navigation system, launched this year. It is a smaller, lighter version of the company’s H-764, for defense aircraft.
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For scenarios in which GPS is truly denied, Honeywell is integrating other inertial aiding sources to provide GPS levels of accuracy. “These include a star tracker capable of geolocation, which provides full position updates to the inertial solution, Precision Terrain Aided Navigation for updating the inertial solution over known ground locations, visual aided navigation, and many more,” Hartney explains.

In terms of other alternative technologies for GPS-denied environments, while DARPA and the military labs have all been exploring this realm for many years, we’re finally on the front edge of seeing some of those technologies actually entering programs now.

The U.S. Army’s Assured PNT program is one of the main ground navigation efforts helping to drive these technologies forward, Simon points out. This program’s mission is to provide optimal and affordable PNT capabilities with designs, products, and solutions that promote decisive action in all Army operations. To this end, Rockwell Collins has developed a stamp-sized GPS receiver, the MicroGRAM, which can be embedded into systems ranging from tactical radios to laser range finders (see Figure 2).

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Figure 2: Pictured is Rockwell Collins’ small GPS receiver, the MicroGRAM.
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The Army is also actively pursuing pseudolite technology, which provides GPS-like service in electronically or physically challenging environments by broadcasting a signal similar to a GPS signal that can be used to supplement signals from GPS satellites. “Within a theater or battle region, pseudolites enable more GPS signal availability without requiring more satellites in the sky,” Simon explains. “We’re seeing strong interest in pseudolites, but also in chip-scale atomic clocks, MEMS IMUs, and star tracking.”

Threat environment and security

In a “threat environment” in which there’s a possibility of satellites or signals becoming unavailable, two very different philosophies are being discussed: Advocates who support not being completely reliant upon satellite-based navigation vs. satellite and space advocates whose position is that if a satellite goes down – even intentionally – the culprits couldn’t get away with taking out more than one. Conversations of this type are likely underway at the most senior levels of the Department of Defense and, in fact, policy guidance is expected to emerge soon.

“During the past six months we’ve asked for clarity on this issue because it’s a complex, confusing environment,” Simon notes. “M-code is still in development and the user equipment is still maturing, so we’ll likely be talking about this for several years.”

Military receivers, from a security standpoint, are either based on SAASM or M-code and use encrypted signals and processing and algorithms, so vulnerabilities to hacking aren’t a big concern. “The likelihood of a military receiver being hacked is minimal,” Simon explains. “But once we start migrating to other devices or multisensor devices – with open service GNSS or GPS combined with other navigation augmentation – the manner in which it’s protected becomes more of an issue. While the SAASM or M-code side of the receiver can be protected, the question becomes: Can the rest of the receiver be protected for users who have migrated to other sensors?”

This is a concern expressed by military users – even at the policy level. They need to be able to trust that receivers won’t be vulnerable to attacks, so efforts are underway to develop algorithms to ensure continued receiver security.

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