The GPS system provides critical positioning capabilities to military, civil, and commercial users around the world. The Global Positioning System (GPS), is a global navigation satellite system (GNSS) that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. GPS has become the ubiquitous technology that provides real-time positioning, navigation and timing (PNT) data 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.
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.
However in many environments in which military operates including 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.
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.
Mission success can come down to mere millionths or billionths of a second and current military systems that rely on global positioning system (GPS) timing updates are inherently vulnerable. Though GPS is a revolutionary capability, it is unreliable underground or underwater and can be degraded or unavailable due to adversarial signal jamming.
To overcome these limitations, DARPA launched H6 program in May 2022 that seeks to develop ultra-small, low-power, fieldable clocks that can maintain their microsecond timing precision for one week over an operating range of -40 to 85 Celsius without GPS fixes.
“When clockmaker John Harrison developed his H1 through H5 marine chronometers to compete for British Parliament’s 1714 Longitude Act prize, determining longitude was the tactical mission challenge of the era,” said Jonathan Hoffman, DARPA program manager for H6 in the agency’s Microsystems Technology Office. “Today, GPS denial is the most significant PNT [positioning, navigation and timing] challenge. H6 is the spiritual successor to Harrison’s H5, and with it we aim to remove GPS-timing dependency while maintaining signal assurance, pervasive security, and high-bandwidth communications. H6 is the clock Harrison would build to solve today’s tactical mission challenge of GPS denial.”
Successful H6 proposers will solve the GPS denial challenge with technology that achieves this goal within low size, weight and power (SWaP) constraints.
DARPA is interested in any technology that could attain this H6 goal. Potential approaches could include, but are not limited to:
1. Compound mechanical clocks. Current mechanical clocks are a well understood technology with very high performance in short timescales (< 1 s) but on long timescales are limited by the large temperature dependence of the oscillation and by aging-induced drift. In mechanical oscillators, thermal changes alter both the spring constant (via the temperature coefficient of the elastic modulus) and the dimensions of the device (through the coefficient of thermal expansion). The resulting mechanical resonant frequency changes from temperature will exceed the H6
requirements. Even in the absence of temperature changes, a mechanical oscillator’s frequency will drift with time (aging).
The dominant mechanisms responsible for this drift are mass transfer and stress relaxation. Surface contaminants can adsorb/desorb from the oscillator, changing the mass and hence the frequency of the oscillator. Similarly, stresses (that either originate in the fabrication of the oscillator or its attachment to a substrate) slowly relax and alter the resonant frequency.
Recent advances in MEMS technology have focused on resonators fabricated with multiple materials. Such resonators can fully cancel the first-order temperature coefficient around a selectable set point while reducing higher order effects. Designs have also been proposed that reduce the impact of stress relaxations by localizing the stresses away from the active resonator components.
2. Sub-THz molecular clocks. Small atomic vapor-based clocks have also been intensely studied over the past twenty years. Thermal changes in these clocks induce pressure changes that shift the frequency of atomic transitions. This behavior, inherent to the atomic species and buffer gasses used, has limited the long-term performance of such clocks caused by external temperature changes. In addition, the frequency of atomic vapor-based clocks drifts due to vapor cell leakage, vapor cell internal chemistry, and redistribution of the alkali metal atoms on the vapor cell walls.
Recently, sub-THz molecular clocks have been demonstrated. These clocks interrogate molecular rovibrational states, not found in atoms, that have reduced temperature dependence compared to the electronic transitions of atomic vapor clocks. In addition, the low boiling point and reduced reactivity of the molecules used should result in negligible deposition of the molecules on the cell walls and less significant internal chemistry.
The H6 program will have one Technical Area (TA) divided into three phases.
In Phase 1 (Base) will focus on both clock dependence on temperature and SWaP reduction. The program will not have a universal Allan deviation target; instead, performers are encouraged to offer targets relevant to the progress they expect to achieve for their specific approach. Vibration induced frequency changes need not be addressed but will need to be measured for the purpose of devising a strategy to address them in later phases.
In Phase 2 (Option 1) will focus on clock aging. Clock operation should be demonstrated throughout the tactical temperature environment.
In Phase 3 (Option 2), performers are expected to demonstrate a fully integrated tactical-grade clock and the fabrication of the deliverable of five (5) clocks.