The GPS system provides critical positioning capabilities to military, civil, and commercial users around the world. However, in many environments in which military operates (inside buildings, in urban canyons, under dense foliage, underwater, and underground) have limited or no GPS access. It can be significantly degraded or unavailable during solar storms.
GPS signals are also subject to electronic attacks such as jamming by adversaries. “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. Apart from jamming by adversaries, GPS signals are also subject GPS-spoofing attacks whereby a malicious entity generates a GPS-like signal designed to mislead GPS receivers.
To address this problem, DARPA is giving thrust to multiple programs that are exploring innovative technologies and approaches that could eventually provide reliable, highly accurate PNT capabilities when GPS capabilities are degraded or unavailable.
DARPA wants to build radio navigation based on VLF radio signals under its Spatial, Temporal and Orientation Information in Contested Environments (STOIC) program. The Defense Advanced Research Projects Agency (DARPA) recently announced the award of Phase II and III of the Spatial, Temporal and Orientation Information in Contested Environments (STOIC) Very Low Frequency (VLF) Positioning System to a team led by Leidos and supported by ENSCO.
The US Defense Advanced Research Projects Agency (DARPA) is planning to conduct demonstrations that centre on the possibility of performing position, navigation, and timing (PNT) in GPS-denied or degraded environments using very low frequency (VLF) signals . DARPA’s present approach is to monitor the ionosphere – between 90 and 500 km above the Earth – using VLF receivers and then attempts to track its movement in real-time. By doing so, the agency hopes to get a more precise location than in previous efforts using VLF signals.
VLF signals get trapped in the wave guide between the ionosphere and earth, so they just keep propagating; and VLF signals travel extremely far, Tremper said. “If you know where [the VLF signal] is being transmitted from you can detect [it at] a very long distance and then establish a range for yourself from where it came from,” he explained. Using VLF signals to do positioning is not a new concept, Tremper noted. A comparable method had been employed for the Omega navigation system, which supported PNT requirements before GPS was introduced.
Precision timing and synchronization is essential to DoD communications, navigation, reconnaissance, and electronic warfare systems. The requirements for timing precision and stability have grown increasingly demanding as systems have evolved towards higher data rates, increased spectrum congestion, and time-dependent encryption algorithms. This demand will continue to grow over the next decade, particularly due to emerging requirements for precision timing in GPS-denied environments and synchronization between system-of-systems components on distributed platforms, says DARPA. “The requirements for timing precision and stability have grown increasingly demanding as DoD systems have evolved towards distributed engagement and surveillance architectures,” says DARPA.
Militaries are taking two approaches one is integration of GPS with complementary technologies such as chip-scale atomic clocks and small inertial measurement units of the Micro-Electro Mechanical Systems (MEMS). Other approach is developing entirely new PNT technologies.
OMEGA was the first truly global-range radio navigation system, operated by the United States in cooperation with six partner nations. It enabled ships and aircraft to determine their position by receiving very low frequency (VLF) radio signals in the range 10 to 14 kHz, transmitted by a network of fixed terrestrial radio beacons, using a receiver unit. It became operational around 1971 and was shut down in 1997 in favour of the Global Positioning Satellite system.
In Omega they predicted where they thought the ionosphere would be and then used that to determine where their position is,” Tremper noted. “Under STOIC, we are actually monitoring the ionosphere using VLF receivers and then attempting to account for the movement in the ionosphere in real-time, and then use that so we can update models in real time and drive that position error further down,” he added.
Omega had deficiencies in accuracy that were on the order of 1–2 km because VLF signals are susceptible to interference by channels that are created when signals reflect continuously between the ionosphere and the Earth, Tremper said. DARPA has also demonstrated that accuracy could be improved with post-processing techniques. “What we are attempting to do is take advantage of that signal that is traveling a long way and range off of it,” Tremper said.
Spatial, Temporal and Orientation Information in Contested Environments (STOIC)
In 2014, DARPA released a Broad Agency Announcement for the STOIC program, inviting private companies to compete. STOIC aims to develop a backup positioning, navigation and timing (PNT) capability. The program is comprised of three technical areas that when integrated have the potential to provide global PNT, including long-range robust reference signals, ultra-stable tactical clocks, and Multi-function tactical data links systems that provide PNT information between cooperative users.
The STOIC program seeks 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.
In Phase I, TA1 focused on using very low frequency (VLF) radio frequency (RF) signals to provide robust ranging in support of earth-fixed positioning. DARPA has released new BAA is a follow-on to TA1 Phase I for developing the detailed design (Phase II) and real-time demonstration (Phase III) of a VLF positioning system (VPS).
Very low RF frequencies are desired for long range communications due to low path attenuation, the atmospheric waveguide properties, and the ability of low frequency magnetic fields to penetrate underground or underwater. Information bandwidth and link propagation characteristics must be included as primary design considerations. DARPA requests responses from individuals and organizations with experience and capabilities in VLF communications, modulation protocols and RF waveform design, RF propagation models for atmospheric, underground and underwater applications, etc
The general system architecture is partitioned into three segments analogous to how the GPS architecture is partitioned.
The transmission segment comprises new VLF transmit antennas and signal waveforms with improved resistance to jamming as well as the ability to carry navigation data messages. Optical clocks being developed under STOIC TA2 keep the VLF stations synchronized for extended periods of time without depending on GPS.
The control segment comprises multiple monitor stations that are networked to a central processing facility. The monitor stations form a wide area network that measure VLF signals from the transmission segment. The central processing facility uses the monitor station data to calibrate measurement models for current conditions and generate system messages that are transferred to users via the transmission segment.
The user segment comprises VLF receivers integrated with other navigation sensors (e.g., inertial navigation system, altimeter, etc.) on stationary and moving platforms. One-way range measurements to the VLF transmitters are derived from precise carrier phase measurements with ranging codes and other means to resolve carrier phase ambiguities.
During STOIC Phase I, ENSCO worked closely with the prime Leidos and other team members to design a new class of VLF transmitters to be used for global VLF. ENSCO’s primary role was the design of navigation signals to achieve DARPA’s positioning requirements, and at the same time optimize VLF transmitter performance.
“ENSCO has made significant investment in the development of RF based PNT technology,” said Boris Nejikovsky, ENSCO President. “ENSCO PNT expertise helped the team to adjust to evolving customer requirements and successfully complete Phase 1. We are looking forward to working with Leidos on Phases II and III of this exciting DARPA project.”
The Phase II award is a follow-on to Phase I to develop the detailed design; Phase III is a real-time demonstration of a VLF positioning system. ENSCO’s engineering task in Phase II is to further enhance and test adaptive interference mitigation algorithms in post-processing. In Phase III, ENSCO algorithms will be integrated into the DARPA navigation system for real-time demonstrations.
Rockwell Collins wins DARPA award under STOIC program
Rockwell Collins has been selected to develop technologies DARPA’s Spatial, Temporal and Orientation Information in Contested Environments (STOIC) program that aims to reduce warfighter dependence on GPS for modern military operations. Under the terms of the agreement, Rockwell Collins will develop innovative architectures and techniques to enable communication systems that will support time transfer and positioning between moving platforms independent of GPS, with no impact on primary communications functionality.
“The time-transfer and ranging capabilities we are developing seek to enable distributed platforms to cooperatively locate targets, employ jamming in a surgical fashion, and serve as a backup to GPS for relative navigation,” said John Borghese, vice president of the Rockwell Collins Advanced Technology Center.
Borghese added that the goal of the STOIC program is to develop positioning, navigation, and timing (PNT) systems that provide GPS-independent PNT, achieving timing that far surpasses GPS levels of performance. The program is comprised of three primary elements that, when integrated, have the potential to provide global PNT independent of GPS, including long-range robust reference signals, ultra-stable tactical clocks, and multifunctional systems that provide PNT information between cooperative users in contested environments.
For this third technical element, Rockwell Collins is tasked with developing multifunction communication system solutions that yield DARPA STOIC objective picosecond-accurate time transfer and enable GPS-levels of relative positioning accuracy in contested environments.
Rockwell Collins is developing and testing a number of multi-function communication radio systems to provide PNT information while maintaining the basic communication system functions.
1) Two-Way Time Transfer and Ranging (TWTR) with omni-directional tactical data links. L/S-band data link radios based on the fielded Quint Network Technology (QNT) radio are being used to demonstrate sub-nanosecond or better TWTR performance. Using an existing QNT-200 radio with modified firmware, we have demonstrated <1ns performance, with minimal communications network impact. Innovative signal synthesis and time of arrival (TOA) processing techniques enable this performance. A new QNT radio design is being pursued that would have wider band digital transmit and receive capabilities, enabling further improved TOA processing. The new radio design will also have improved ability to maintain calibration of RF front end delay and phase over frequency.
2) 3D relative positioning with directional communication links. A new Ku-band directional communication system, called COMPASS, is being developed to demonstrate full 3D relative positioning while providing high bandwidth, highly secure communications. The COMPASS uses an electronically scanned array (ESA) to make angle of arrival measurements (AoA) from other COMPASS units; coupled with TWTR measurements, this enables 3D relative positioning and orientation transfer. The ESA technology being developed has the promise of being affordable for application on attritable platforms.
3) VLF positioning system receiver. An existing Rockwell Collins strategic VLF communications receiver, the KGR-72, is being adapted to make precision carrier phase measurements from Navy Fixed Submarine Broadcast System (FSBS) stations. These carrier phase measurements, when corrected for Earth-Ionosphere Waveguide (EIW) propagation effects can be used for 2D absolute positioning and timing. Initial test results with the modified VLF receiver have shown the ability of the receiver to support STOIC further development and testing.
“Future applications of STOIC technology could include a variety of precision relative navigation operations, such as autonomous aerial refueling and cooperative navigation and collision avoidance within unmanned aerial vehicle swarms,” said Borghese. “It also could support precise time transfer for networking operations in contested environments.”
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.
DARPA Electromechanical Transmitters for Very Low Frequency RF (EMT‐VLF) RFI
DARPA is seeking information on the exploitation of electromagnetic‐mechanical coupling for use in creating radio frequency (RF) transmitters operating at low frequencies (0.3‐30 kHz, or ultra‐low frequency (ULF) and very low frequency (VLF) and below frequency bands). At these frequencies, free‐space electromagnetic (EM) field wavelengths are measured in tens of kilometers, resulting in very large transmitter structures when employing conventional antenna approaches.
Electrically‐small antennas are defined as having dimensions much smaller than the EM wavelength, with examples in the literature of antenna‐sizes as small as 1/10th of the EM wavelength. DARPA is seeking innovation to bring that size below 1/10,000 of the EM wavelength or by at least a factor of 103 smaller than the current state of the art (SOA). Such a tremendous reduction in size is impossible to achieve through traditional antenna design due to extremely low radiation efficiency and very unfavorable impedance matching conditions.
A potential path to a successful solution is offered by a mechanically‐driven antenna where coulomb charge is accelerated mechanically. The moving charge is equivalent to electric current and oscillatory acceleration results in EM emission. Applications of this concept include linear (oscillatory) as well as rotational motion of an electret material or poled ferroelectric. Rotating a ferromagnetic material with permanent magnetic polarization (permanent magnet) also results in coupling to the EM field. RF transmission is achieved by modulating the rotational speed of the permanently polarized or permanently magnetized material. Mechanically actuated electric or magnetic devices promise to produce transmitter antennas whose sizes are orders of magnitude smaller than the free‐space electromagnetic wavelength of operation and whose field extends far enough to make long‐distance communication possible.
To realize the above concept in a practical transmitter design, DARPA is seeking innovative information in the areas of materials, mechanical actuation, and overall transmitter architectures to address impedance matching, power handling, signal modulation, scalability, and other system level considerations
“Position, navigation, and timing are as essential as oxygen for our military operators,” said DARPA Director AratiPrabhakar. “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.”