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The military has become increasingly dependent on Global Positioning Systems (GPS) for accurate and precise positioning, navigation and timing (PNT) in a wide variety of operational environments — from tactical vehicles to GPS-guided missiles.

However, as military operations are increasingly carried out in areas where GPS is denied, unreliable or not accessible, military use of GPS has evolved from being a strategic advantage to a vulnerability. Many environments in which the military operates — such as buildings, urban canyons, under dense foliage, underwater and underground — have limited or no GPS access. Moreover, adversaries can easily block GPS access by jamming, spoofing and other GPS-denial threats.

As a result, alternative sources of PNT are required. The Army is exploring the potential of atomic clocks, platform distribution PNT and inertial navigation systems as alternative technologies under the Assured PNT program.

"Threats to military GPS have evolved and improved at a rapid pace — from a proliferation of small-scale commercial jamming devices that can readily be purchased on eBay to large-scale military anti-access/area-denial (A2/AD) capabilities," said MAJ Christopher Brown, assistant program manager Dismounted PNT within the Assured PNT program.

Currently, GPS is the predominant technology the Army relies upon for positioning, navigation and timing. The Army has integrated GPS receivers into most technology-based systems used in the battlefield, such as Stryker, Nett Warrior, Rifleman Radio, the M777 howitzer and many others. These systems depend on PNT to varying degrees for some aspect of their functionality. The challenge is developing a resilient PNT capability the Army can confidently rely on as technological threats continue to increase, which is the intent of Assured PNT, according to Kevin Coggins, the product director for PNT in the Program Executive Office for Intelligence, Electronic Warfare and Sensors.

"The goal of Assured PNT is not to be 'independent of GPS.' In the Army, we think of PNT as a capability and GPS as one of many material solutions that provide a capability," MAJ Brown said.

"Assured PNT is a robust capability that provides a level of assured access and trust — a level that must be defined, measurable, and testable. GPS will be one of the tools we use to achieve this, along with non-GPS augmentation," MAJ Brown said.

The Assured PNT approach doesn't just involve material solutions such as hardware and software, but also requires that the Army's architecture, training, testing and operational concepts evolve, Brown said.

The initiative stems from a series of joint analyses that outline the gaps of "access to" and "integrity of" PNT information, with the realization that the (GPS) alone is not sufficient for all situations. The Army has been designated the lead service to address the Assured PNT challenge.

A system of systems architecture for PNT

A meaningful milestone in the move toward a resilient PNT is the initiation of the Army PNT System of Systems Architecture (SoSA). The Army has spent billions of dollars over the past 12 years at war developing, procuring, integrating and sustaining GPS receivers without a guiding architecture. The PNT SoSA "is an open systems architecture that is flexible enough to accommodate additional capabilities without incurring expensive system or platform integration and certification costs — a framework that enables a pathway for future innovation," Coggins wrote in a recent paper.

The framework will include the PNT Hub, under development within the Assured PNT program, which "will enable the integration of innovative technologies such as the Chip Scale Atomic Clock, which harnesses the stable oscillations of the cesium atom to preserve precise time, even in the absence of GPS," according to Coggins. Engineers with innovative approaches to determine positioning and timing will have an affordable pathway to insert this technology into a PNT SoSA-compliant product.

The Assured PNT program and the Defense Advanced GPS Receiver (DAGR) Distributed Device (D3) program incorporate the PNT SoSA. The D3 is the functional replacement for the DAGR, replacing up to eight GPS devices on a platform and is upgradable to Military Code (M-code), a new signal that is designed to improve both the security and anti-jamming properties of military navigation using GPS. PNT SoSA is also designed to provide an affordable migration path to M-code.

DARPA research moves beyond GPS

Meanwhile, the Defense Advanced Research Projects Agency (DARPA) is exploring novel approaches to PNT such as penny-sized inertial sensors, pulsed lasers and tracked lightning strikes to provide precise location-based insights in GPS-denied areas. In fact, early investments by DARPA to miniaturize GPS technology has made GPS so ubiquitous today, integrated in technologies as wide-ranging as smartphones to precision munitions.

"Position, navigation, and timing are as essential as oxygen for our military operators," DARPA Director Arati Prabhakar said. "Now we are putting new physics, new devices, and new algorithms on the job so our people and our systems can break free of their reliance on GPS."

DARPA's current PNT portfolio includes five programs, focused wholly or in part on PNT-related technology:

Adaptable Navigation Systems (ANS) is developing new algorithms and architectures for rapid plug-and-play integration of PNT sensors across multiple platforms. DARPA's goal is to reduce development costs and shrink deployment time from months to days. ANS aims to create better inertial measurement devices by using cold-atom interferometry, which measure the relative acceleration and rotation of a cloud of atoms stored within a sensor. By leveraging quantum physical properties, researchers hope to create extremely accurate inertial measurement devices that can operate for long periods without needing external data to determine time and position.

Additionally, ANS explores the use of non-navigational electromagnetic signals — including commercial satellite, radio and television signals and even lightning strikes — to provide additional points of reference for PNT. In combination, these various sources are much more abundant and have stronger signals than GPS, and so could provide position information in both GPS-denied and GPS-degraded environments, according to DARPA.

ASPN is currently in Phase 2 of development going through subsystem field demonstrations on a variety of platforms. An end-to-end system demonstration of GPS-independent PNT is planned for FY15.

Microtechnology for Positioning, Navigation and Timing (Micro-PNT) applies extreme miniaturization technology made possible by DARPA-developed micro-electromechanical systems technology. Micro-PNT comprises a portfolio of diverse efforts collectively devoted to developing highly stable and precise chip-scale gyroscopes, clocks and complete integrated timing and inertial measurement devices. Currently, DARPA researchers have fabricated a prototype with three gyroscopes, three accelerometers and a highly accurate master clock on a chip that fits easily on the face of a penny.

Quantum-Assisted Sensing and Readout (QuASAR) intends to make the world's most accurate atomic clocks — which currently reside in laboratories — both robust and portable. QuASAR researchers have developed optical atomic clocks in laboratories with a timing error of less than one second in 5 billion years. Making clocks this precise portable could improve upon existing military systems such as GPS, and potentially enable entirely new radar, LIDAR and metrology applications.

The Program in Ultrafast Laser Science and Engineering (PULSE) applies the latest in pulsed laser technology to significantly improve the precision and size of atomic clocks and microwave sources, enabling more accurate time and frequency synchronization over large distances.

The Spatial, Temporal and Orientation Information in Contested Environments (STOIC) program aims to develop PNT systems that provide GPS-independent PNT with GPS-level timing in a contested environment. STOIC comprises three primary elements that, when integrated, have the potential to provide global PNT independent of GPS: long-range robust reference signals, ultra-stable tactical clocks and multifunctional systems that provide PNT information between mulitple users.

Additionally, DARPA recently announced a new program related to PNT called "Precise Robust Inertial Guidance for Munitions: Navigation-Grade Inertial Measurement Unit."

This PRIGM program addresses the challenge of providing precise PNT for low-cost, -size, -weight and -power consumption platforms, such as smart bombs and guided munitions, in GPS-denied environments.

Raytheon Missile Systems has worked with DARPA on various projects related to precision navigation and, as a result, always takes a keen interest in the research agency's projects, said Chris Sprinkle, senior program manager for the company's Tomahawk cruise missile.

The Tomahawk is a highly accurate, GPS-enabled precision weapon that has been used over 2,000 times in combat, and flight tested more than 500 times. Although the Tomahawk uses GPS, it is a weapon system that predates GPS, Sprinkle said. The Tomahawk Block III was the first to employ GPS to strike targets more precisely. The Block IV, released in 2004, is GPS-enabled, but also uses Raytheon's anti-jamming GPS receiver AGER IV. Raytheon's systems incorporate satellite navigation, laser guidance, high-definition radars, advanced seekers and other technologies.

"We still have maintained our ability to navigate in the absence or degradation of GPS. We take advantage of GPS," Sprinkle said. "But if it were gone, we have the ability to hit targets with extreme precision even without GPS."

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