SATELLITE-based navigation enables war-fighters to operate more swiftly and precisely than ever before, and it has supplanted — in barely two decades—legacy systems for managing civil air traffic as well. The modern PNT—or positioning, navigation and timing—paradigm is based on the assumption that the space-based Global Positioning System (GPS) is accessible most of the time to provide position, velocity and timing information, thereby allowing every user to operate on the same reference system and timing standard.
However, this much reliance upon GPS availability creates a potential vulnerability in both military theaters and domestic airspace if GPS signals become degraded or denied. Threats continue to proliferate—some from hostile sources, others simply from the complex infrastructure we have created:
• Tactical jamming of GPS reception in the battle-space;
• Conduct of military operations where use of GPS is counter-indicated;
• Radio-frequency interference in the GPS bandwidth in domestic airspace;
• Potential jamming of airport or commercial aircraft systems by terrorists;
• Brief satellite transmission outages that affect regional air traffic management.
When operators or systems cannot access GPS signals, critical PNT information can be gathered only with self-contained, inertial-based instruments. Inertial navigation systems (INS) are generally viewed today as a less accurate solution to be used only as a fallback in GPS-denied circumstances. However, the engineers who are developing the navigation and air-traffic management systems of the future recognize the value of INS not merely as a backup but as a unifying element that enhances navigational precision and provides a hub around which other avionics systems can be clustered.
For that reason, we have continued to refine the performance of inertial systems while reducing their size, weight and unit cost. We are also making them more rugged to withstand high G-forces and battle shock. As unmanned aircraft systems (UAS) continue to command more of the airspace, both military and domestic, we continue to explore new concepts of UAS operation, which in turn drives our R&D of inertial systems in new directions.
Embedded GPS/inertial navigation (EGI) systems have proven themselves in both military and civil operations, exploiting the capabilities of both technologies in truly synergistic fashion. The GPS provides signals that calibrate the INS, while the INS updates the platform’s position and angle at a higher rate than the GPS can. For high-dynamic vehicles such as missiles and precision-guided munitions, INS fills in the gaps between GPS positions. In the event that the GPS receiver loses its signal, or the aircrew chooses to increase stealth by temporarily shutting down GPS reception, the INS can continue to compute the platform’s position and orientation.
INTEGRATING EGI TECHNOLOGY INTO OLDER PLATFORMS
As we face fiscal pressures on all fronts today, US defense appropriations have sustained frequent reviews. We have seen cutbacks in most sectors already, but also a reprogramming of spending across the board. This shift in priorities toward reducing cost of ownership over a program’s entire life cycle has refocused the market on modernizing older platforms, increasing their operating efficiency and otherwise extending their service life.
Such scrutiny is commonplace to anyone who operates in the commercial air transport world, where every business unit monitors its payback from every investment. At Honeywell, we have continued to move the market forward with new technology but have also adapted our defense products to this demand for modifications, upgrades and other service-life extensions. In other words, we are applying the cost-of-ownership logic embraced by our commercial carriers to our defense customers’ needs.
Consider, for example, the H-764 Advanced Configurable EGI (ACE). Based on the Tri-Service standard H-764G, of which there are 13,000-plus units in service on US Army, Navy and Air Force platforms, this advanced edition is designed specifically for ease and economy of retrofit into legacy platforms. We aligned its architecture with that of standard control display units to make integration affordable. We made it flexible so it can be “missionized” to the job at hand. We made it versatile so it can operate in GPS/INS, INS-only or GPS-only mode. We added an optional multimode receiver to make it interoperable with existing air traffic management systems and with precision approach/landing systems, both military and civil, that are just now coming on line.
EGI FOR AFFORDABLE SURGICAL STRIKE AT STANDOFF RANGE
Among the challenges they face in Afghanistan, US and allied forces must fly sorties against insurgents hiding out in mountainous terrain or in densely populated civilian areas, frequently in GPS-denied conditions. The concept of operations focuses on two priorities: maximize the war-fighter’s chances of surviving the mission and, at the same time, minimize the likelihood of civilian casualties.
With the Joint Direct Attack Munition (JDAM), the precision-guided munition most often used by US forces in recent conflicts, guidance is achieved using a relatively inexpensive GPS/INS packaged in a tail kit with control surfaces that can be strapped onto various “dumb” bombs. Target coordinates can be loaded into the JDAM before takeoff, manually altered by the aircrew in flight before weapon release or entered by a data link from onboard targeting equipment.
Once released from the aircraft, the JDAM requires no supervision, navigating autonomously to the designated target coordinates with or without a GPS signal, regardless of clouds or sandstorms that would confuse a laser or infrared seeker.
At its most accurate—with the highly integrated GPS and INS working in tandem—the JDAM provides a circular error probability (CEP) of 13 meters or less. If the JDAM’s GPS reception is jammed or compromised by terrain factors, the INS still provides a CEP of 30 meters for free-flight times of up to 100 seconds.
UNMANNED AIRCRAFT SYSTEMS IN GPS-DENIED SCENARIO
More than half the sorties currently being flown in Afghanistan by NATO forces are unmanned, and planning for unmanned air traffic management in the National Airspace System (NAS) has reached the final stages. Although some military UAS are taking on combat missions, most are currently used for intelligence, surveillance and reconnaissance (ISR) missions.
ISR calls for highly accurate readings of the gazing vehicle’s pitch, roll and heading—or attitude. This is especially true of high-altitude, long-endurance platforms that perform more strategic missions. A higher-altitude “look” provides more value, but only if documented with precise attitude readings. This is where the enhanced accuracy of highly integrated EGI systems comes into play, with the GPS and INS working together to augment the value platform’s data feed.
At Honeywell, we are also exploring possible GPS-denied solutions for future operation of unmanned aircraft in the national airspace. One promising approach: integrating an EGI system with DME, or distance-measuring equipment. An existing radio technology that uses transponders, DME is currently used by all airliners and many general-aviation platforms to measure distance by timing the propagation delay of VHF or UHF signals. A project currently underway will network DME installations regionally, providing additional redundancy in the absence of GPS signals.
TOWARD THE FUTURE:
SMALLER IS BETTER
The Defense Advanced Research Projects Agency (DARPA), the Air Force Research Lab (AFRL) and the Office of Naval Research (ONR) have all issued recent RFPs for further development of micro-sized inertial measurement units (the basic sensor of an INS) with various objectives in mind. The DARPA contract, awarded to Honeywell in August 2011, calls for a high-velocity, shockproof timing and inertial measurement unit (TIMU) of approximately 10 cubic millimeters. This would reduce the industry standard of 50,000 cubic millimeters by a mind-boggling factor of 5,000.
The ideal solution, according to DARPA, would contain a local clock and two triads of inertial sensors¬—three accelerometers for position and three gyroscopes for orientation—confined to a chip-sized package that requires no external data such as a GPS or other satellite-generated signal. It would operate on very low power and deliver highly accurate performance.
We can only speculate as to what uses DARPA envisions for TIMU. In any case, these requirements clearly portend not obsolescence for inertial navigation, but a next generation of IMUs that could drive the technology in directions we can’t begin to foresee.