Mirza Jatt
SENIOR MEMBER
No GPS? No Problem
Navigation via Global Positioning System is ubiquitous. Soldiers, vehicles and even artillery rounds are guided by satellite navigation, as long as the service is accessible.
But what happens when it isnt? GPS jamming has always been a concern and much effort has been expended in making the system jam-proof. GPS, however, may be unavailable for other reasons: Obstructions can block signals; satellites may be attacked; and a software glitch such as the one that brought down several thousand U.S. Air Force GPS units earlier this year is a possibility.
A market in GPS-alternative technologies is developing, and these may emerge as redundant safeguards.
One approach uses a series of radio beacons that act as an alternative to satellite-based transmitters. The Ranger from Ensco Inc., for example, employs beacons that transmit signals over a broad spectrum. Users access the system with handheld radios that calculate an individuals location based on the distance to each beacon.
The U.S. Defense Advanced Research Projects Agency (Darpa) is using the same principle on a larger scale for its Robust Surface Navigation (RSN) program. RSN will use any available signal augmented by additional radio beacons. New hardware will be required for RSN, in this case software-defined radios. Some elements of the program will transition to the U.S. Navy and Air Force in Fiscal 2011.
Another approach involves portable inertial measurement units (IMUs), miniature versions of the systems in aircraft and other platforms. One example is the matchbox-sized Dead Reckoning Module (DRM) from Honeywell. This has gyroscopes and accelerometers that calculate a users position from his cumulative movement. The technical challenge is to compensate for the uneven motion of walking, which varies with each individual. Specially developed algorithms address this issue.
A more advanced version in the offing is Darpas Microscale Ring Integrating Gyro program, which uses a vibrating structure in place of a spinning gyroscope to calculate location. Inertia keeps the gyro device vibrating in the same plane as it is rotated. Darpa plans to make its gyro small enough for integration on a chip-scale unit. If successful, this could replace GPS for ground users, and in munitions, drones and other applications.
IMUs tend to lose accuracy over time. Honeywells DRM is claimed to have an accuracy rate within 2% of distance traveled, which is adequate for short missions on foot. Every IMU needs to be recalibrated at intervals with a correct location verified by other means including the stars. Stellar or celestial navigation requires accurate timing, which is not a problem with modern electronics. Millisecond accuracy will pinpoint a location to within 5 meters (16 ft.). The limit is the precision with which stars can be located. The Navys Stella navigation system is reportedly accurate to 30 meters, while the NAS-26 system on the B-2 bomber is said to be so accurate that GPS is virtually redundant.
A recent Air Force Research Laboratory presentation on MQ-X Predator unmanned aerial vehicles (UAVs) describes a future craft equipped with star-tracking systems and IMUs as well as GPS, terrestrial radio navigation and terrain-matching capabilities.
Modern stellar navigation systems have evolved from the gimbaled telescope on the B-2, which swivels to locate stars. Size and weight are at a premium in air and space platforms, of course, and current units use solid-state devices with a wide field of view to locate several stars at once. They are tinythe Miniature Star Tracker from Comtech AeroAstro weighs less than 1 lb.
Adapting this technology for terrestrial use involves challenges, such as locating stars in daylight. Trex Enterprises developed a system for the Navy that tracks 6.3-magnitude stars at sea level in daytime. Such stars are barely visible to the eye at night.
A terrestrial user may wait days to get a stellar fix because of clouds. Predator UAVs or other aircraft with stellar-inertial navigation and radio beacons could act as GPS for users below, as part of a system such as the RSN. Even occasional passes by such aircraft would ensure that IMUs are kept updated and accurate.
Putting the hardware in place will take time and money. But in principle, the loss of GPS need not prevent soldiers and others from having precision navigation at their fingertips.
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