PNT Roundup: DARPA advances on many fronts to reduce reliance on GNSS

Micro-Technology for Positioning, Navigation, and Timing towards PNT everywhere and always; slide from a 2014 DARPA presentation to the Space-Based Positioning, Navigation and Timing National Advisory Board (Image: Robert Lutwak, DARPA Micro-Technology Office). Click to enlarge.

The U.S. Defense Advanced Research Projects Agency (DARPA) has initiatives underway with a dizzying number of technologies, all seeking to reduce reliance on GNSS in challenged environments. Using cold atom interferometry and other techniques to reduce the size, weight and power consumption (SWAP) as well as cost of inertial sensors, employing other signals of opportunity (SOI), chip-scale atomic clocks (CSAC), micro-electro-mechanical systems (MEMS) and more, the Micro-Technology Office (MTO) and the Adaptable Navigation Systems (ANS) projects press relentlessly forward to provide U.S. forces with PNT “everywhere and always.”

DARPA’s ANS initiative explores tools to enable use of the many sensors available to warfighters and first reponders. “Over the past two decades, the field of robotics has done a lot for extracting features out of imagery and tracking those features as the robot moves through a given environment,” said Lin Haas says, program manger at the Strategic Technology Office. “We’ve been building upon those capabilities and using the capabilities for the purposes of navigation.”

ANS seeks to provide GPS-quality PNT to military users regardless of the operational environment. It addresses three basic challenges through its Precision Inertial Navigation Systems (PINS) and All Source Positioning and Navigation (ASPN) efforts:

• better inertial measurement units (IMUs) that require fewer external position fixes;
• alternate sources to GPS for those external position fixes;
• new algorithms and architectures for rapidly reconfiguring a navigation system with new and non-traditional sensors for a particular mission.

PINS is developing an inertial measurement unit (IMU) that uses cold atom interferometry for high-precision navigation without dependence on external fixes for long periods of time. Atom interferometry involves measuring the relative acceleration and rotation of a cloud of atoms within a sensor case, with potentially far greater accuracy than today’s state-of-the-art IMUs.

A company called AOSense has applied cold-atom interferometry to IMUs and demonstrated sensors that support system drifts of 5 meters per hour, by using quantum physical properties to measure the relative acceleration and rotation of a cloud of laser-cooled atoms. The next challenge is shrinking the lasers to microsystem size, because the concept requires three lasers generating five beams to cool and move the atoms through interferometers to determine movement and rotation of the device.

Because even long-duration IMUs require an eventual position fix, the ASPN effort is developing sensors that use signals of opportunity — non-navigation signals from sources like television, radio and cell towers, and satellites, as well as natural phenomena, such as lightning.

“Our navigation systems tend to be finely tuned, and as a result they are fairly brittle in terms of accepting new sensors without a lot of hands-on time to make it work,” said Haas.

Flexible Combinations. Integrating and tuning these diverse sensors, maps and other components into a navigation system is expensive and slow, producing platform and mission-specific solutions. The ASPN effort is also developing new fusion algorithms and plug-and-play processing architectures for rapid integration and near-real-time reconfiguration or upgrading of sensors, IMU devices, maps and databases on a navigation system. With flexible combinations of existing and new navigation sensors, ASPN can produce improvements in accuracy, robustness and cost of navigation systems across a range of platforms, environments and missions.

PINS is working towards a final subsystem demonstration in fiscal year 2017. ASPN has completed multiple field demonstrations on air, land and sea platforms, with final demonstrations scheduled in fiscal 2017.

Chip-Scale Atomic Clocks. Meanwhile, last year DARPA launched the Atomic Clocks with Enhanced Stability project under the direction of Robert Lutwak (recipient of GPS World’s Leadership Award for Products in 2012). “If ACES is successful, virtually every Defense Department system will benefit,” Lutwak said.

ACES seeks to create palm-sized, battery-powered atomic clocks that perform up to 1,000 times better than the current generation, employing experts and techniques from atomic physics, optics, photonics, microfabrication and vacuum technology. “All of our modern communications, navigation and electronic warfare systems as well as our intelligence, surveillance and reconnaissance systems depend on accurate time-keeping,” Lutwak added.

Pseudolites. In other, non-DARPA initiatives around the Department of Defense, the Command and Control Directorate of the Army’ Communications-Electronics Research, Development and Engineering Center (CERDEC) is “very concerned about what happens when we lose GPS,” according to Paul Olson. CERDEC is developing vehicle-based, dismounted and anti-jam antenna pseudolite systems.

The pseudolites have completed feasibility testing and entered acquisition for transmitters, receivers and command-and-control. Rockwell International and L-3 are developing the transmitters. The effort seeks to use current military GPS receivers with software modified to accept pseudolite signals.

This article draws on interview quotes that appeared in Signal magazine of the Armed Forces Communications and Electronics Association.