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2014-09-01

WORLD BEYOND GPS

It is difficult to imagine the modern world without the Global Positioning System (GPS), which provides real-time positioning, navigation and timing (PNT) data for countless military and civilian uses. Thanks in part to early investments that Defense Advanced Research Projects Agency (DARPA) made to miniaturize GPS technology, GPS today is ubiquitous.

It’s in cars, boats, planes, trains, smartphones and wristwatches, and has enabled advances as wide-ranging as driverless cars, precision munitions, and automated supply chain management.

As revolutionary as GPS has been, however, it has its limitations. GPS signals cannot be received underground or underwater and can be significantly degraded or unavailable during solar storms. More worrying is that adversaries can jam signals. GPS continues to be vital, but its limitations in some environments could make it an Achilles’ heel if warfighters rely on it as their sole source of PNT information.

To address this problem, innovative technologies and approaches are explored that could eventually provide reliable, highly accurate PNT capabilities when GPS capabilities are degraded or unavailable.

“Position, navigation, and timing are as essential as oxygen for our military operators,” said DARPA director Arati Prabhakar. “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.”

Easily blocked

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

Adaptable Navigation Systems (ANS) is one. The military relies heavily on GPS for PNT, but GPS access is easily blocked by methods such as jamming. In addition, many environments in which our military operates like inside buildings, in urban canyons, under dense foliage, underwater, and underground have limited or no GPS access. To solve this challenge, ANS seeks to provide GPS-quality PNT to military users regardless of the operational environment.

ANS addresses three basic challenges through its Precision Inertial Navigation Systems (PINS) and ASPN efforts. They are: better inertial measurement units (IMUs) that require fewer external position fixes; alternate sources to GPS for those external position fixes; and new algorithms and architectures for rapidly reconfiguring a navigation system with new and non-traditional sensors for a particular mission.

Complementing the Micro-PNT program, which is developing chip-scale inertial sensors that are navigation grade or better, PINS is developing an 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.

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

Integration challenge

Integrating and tuning different sensors, maps and other components into a navigation system is expensive and slow, resulting in platform and mission-specific solutions. To address this integration challenge, 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.

By allowing flexible combinations of existing and new navigation sensors, ASPN seeks improvements in accuracy, robustness and cost of navigation systems across a wide range of platforms, environments and missions.

Both PINS and ASPN are currently in the second phase of development, and are working toward subsystem field demonstrations on a variety of platforms for next year, followed by an end-to-end system demonstration of GPS-independent PNT planned for FY15.

The goal of the Microtechnology for Positioning, Navigation, and Timing (Micro-PNT) is to develop technology for self-contained, chip-scale inertial navigation and precision guidance for munitions as well as mounted or dismounted soldiers. Size, weight, power, and cost (SWaP+C) are key concerns in the overall system design of compact navigation systems. Breakthroughs in microfabrication techniques may allow for the development of a single package containing all of the necessary devices (clocks, accelerometers, and gyroscopes) incorporated into a small (8 mm3) low-power (1 W) timing and inertial measurement unit.

On-chip calibration should enable periodic internal error correction to reduce drift and thereby enable more accurate devices. Trending away from ultra-low drift sensors towards self-calibrating devices will allow revolutionary breakthroughs in PNT technology.

In 2010, a coordinated effort focused on the development of microtechnology specifically addressing the challenges associated with miniaturization of high-precision clocks and inertial instruments was launched. The Micro-PNT program comprises othree thrust areas: clocks, inertial sensors, and microscale integration. Each area is made up of various efforts exploring new fabrication techniques, deep integration, and on-chip self-calibration, which go hand-in-hand with the development of “plug-and-test” architectures.

Operational scenarios

The developments consider a number of operational scenarios, ranging from dismounted-soldier navigation to navigation, guidance and control of Unmanned Air Vehicles (UAVs), Unmanned Underwater Vehicles (UUVs), and guided missiles. The micro-PNT initiatives seek to increase the dynamic range of inertial sensors, reduce long-term drift in clocks and inertial sensors, and to develop miniature chips providing position, orientation, and time information.

Typically, the performance of measurement devices is limited by deleterious effects such as thermal noise and vibration. Notable exceptions are atomic clocks, which operate very near their fundamental limits. Driving devices to their physical limits will open new application spaces critical to future DoD systems. Indeed, many defense-critical applications already require exceptionally precise time and frequency standards enabled only by atomic clocks. The GPS and the internet are two key examples.

Measurement systems based on atomic physics benefit from the exquisite properties of the atom. Among these are precise frequency transitions, the ability to initialize, control, and readout the atomic state and environmental isolation. In addition, atomic properties are absolute, and do not “drift” over time. In this sense, atoms are self-calibrated, making them ideal for precision sensing.

The Quantum-Assisted Sensing and Readout (QuASAR) program will build on established control and readout techniques from atomic physics to develop a suite of measurement tools that will be broadly applicable across disciplines, helping to address outstanding challenges in physics, materials and biological sciences.

QuASAR will push toward fundamental operating limits by developing atom and atom-like sensors that operate near the standard quantum limit (SQL), constructing hybrid quantum sensors that combine the optimal sensing and readout capabilities of disparate quantum systems and entangling multiple sensors/devices to operate below the SQL. These types of devices will find broad application, particularly in the areas of biological imaging, inertial navigation and robust global positioning systems.

Improved radiation sources

Defense applications, such as geo-location, navigation, communication, coherent imaging and radar, depend on the generation and transmission of stable, agile electromagnetic radiation. Improved radiation sources - for example, lower noise microwaves or higher flux x-rays - could enhance existing capabilities and enable entirely new technologies.

Program in Ultrafast Laser Science and Engineering (PULSE) seeks the technological means for such improved radiation sources. Through precise spectral engineering in the optical domain, more efficient and agile use may be made of the entire electromagnetic spectrum. By generating and engineering waves in the optical domain, where engineers already exercise exquisite stability and control, these waveforms may be down or up-converted to the desired wavelength.

PULSE will also aim to apply this technology to enable synchronization, metrology and communications applications spanning the electromagnetic spectrum, from radio frequencies to x-rays. By building on established ultrafast laser techniques, PULSE seeks to:

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 multiples users.

Seven devices, one chip, no GPS

DARPA researchers at the University of Michigan have made significant progress with a timing & inertial measurement unit (TIMU) that contains everything needed to aid navigation when GPS is temporarily unavailable. The single chip TIMU prototype contains a six axis IMU (three gyroscopes and three accelerometers) and integrates a highly-accurate master clock into a single miniature system, smaller than the size of a penny.

Three pieces of information are needed to navigate between known points ‘A’ and ‘B’ with precision: orientation, acceleration and time. This new chip integrates state-of-the-art devices that can measure all three simultaneously. This elegant design is accomplished through new fabrication processes in high-quality materials for multi-layered, packaged inertial sensors and a timing unit, all in a tiny 10 cubic millimeter package. Each of the six microfabricated layers of the TIMU is only 50 microns thick, approximately the thickness of a human hair. Each layer has a different function, akin to floors in a building.

“Both the structural layer of the sensors and the integrated package are made of silica,” said Andrei Shkel, DARPA program manager.

“The hardness and the high-performance material properties of silica make it the material of choice for integrating all of these devices into a miniature package. The resulting TIMU is small enough and should be robust enough for applications (when GPS is unavailable or limited for a short period of time) such as personnel tracking, handheld navigation, small diameter munitions and small airborne platforms.” In time, dependence on GPS may be as unimaginable as is the idea today of living without it.

Reference text, Photos: www.darpa.mil

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