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No GPS? No Problem

Mirza Jatt

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Mar 26, 2010
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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 isn’t? 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 individual’s 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 user’s 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 Darpa’s 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. Honeywell’s 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 Navy’s 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 tiny—the 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.

No GPS? No Problem | AVIATION WEEK GPS? No Problem
 
Radio beacons have been used since WWII for Navigation,and before GPS was in vogue,Loran-C,Omega and Decca had been pretty accurate for Navigation.All those systems were decommissio9ned when Satnav and then GPS came...This forgotten technology can be tweaked and reinvented for present day use.

For an Inertial Navigation system to be accurate,an extensive survey of gravitational fluctuations over different types of terrain is required,and the data is stored as correction in Inertial Nav's memory..For example,mountains have more gravity than plain ground,due to concentration of matter in a smaller area.An Inertial system passing over mountains will incur error due to the extra gravity..All pilots are taught alternative methods of navigation,specially taking reference from landmarks such as Mountain ranges and cities. PAF's F-16s are fitted with Inertil navigation system..dont know about other planes...And dont know if PAF has done any Gravitation fluctuation survey of Pakistan or not.
 
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^^ Very good post. I remember using LORAN and OMEGA systems before GPS came on line. The problem with these older systems is the range issue. To get long range, over the horizon, requires HF signals, and there was a problem with Omega especially called "track shift" that could put you 60 miles off course.

VHF/UHF signals can create better accuracy, but are line of sight only. And for portable battlefield use, you obviously need to know EXACTLY where the units are sighted, meaning you really need... GPS to set them down and turn them on.

Inertial units are fascinating. Imagine a sealed, black box. You tell it where it is, then turn it on. Now, you move it, and it tracks its own location in 3D space. This is what was used on ICBM's for decades, and still is in use.

The very best modern ring-laser INS systems (we call them IRU's) do work well, but when you compare their position with a known GPS, they do drift, and it can be miles after many hours at 500 knots. In a commercial aircraft like a 737, we have dual GPS, dual IRU, and RNAV (Area Navigation) equipment that uses standard VOR's. All of these inputs combine to calculate the exact aircraft position. A bonus is that the IRU's are also used to generate the artificial horizon for instrument flight.
 
INS does take its time to power up and align itself.. 2 minutes for the F-16 i think.. but there is that scramble where it aligns on the go?? :S

If there is any INS I was fascinated by.. it was the SR-71's..
 
INS does take its time to power up and align itself.. 2 minutes for the F-16 i think.. but there is that scramble where it aligns on the go?? :S

If there is any INS I was fascinated by.. it was the SR-71's..

There was a method to "hot c-o-c-k" fighter INS by aligning it, then putting it into a sort of sleep mode. The next time the fighter is scrambled, you can go to Quick Align and it'll be ready in 1/4 the time. But it still felt like an eternity during a scramble.

The reason an IRU is so important in a fighter (beyond instrument flight) is that the IRU inputs are critical to correct radar operation. In the F-15 at least, we'd watch with dread if the INS drifted badly, because the radar operation began to die. No INS = no radar = day visual fighter. The introduction of the ring-laser jobs fixed that.

What was special about the SR-71 unit? I know they had INS, but didn't they also use a celestial navigation system?
 
There was a method to "hot c-o-c-k" fighter INS by aligning it, then putting it into a sort of sleep mode. The next time the fighter is scrambled, you can go to Quick Align and it'll be ready in 1/4 the time. But it still felt like an eternity during a scramble.

The reason an IRU is so important in a fighter (beyond instrument flight) is that the IRU inputs are critical to correct radar operation. In the F-15 at least, we'd watch with dread if the INS drifted badly, because the radar operation began to die. No INS = no radar = day visual fighter. The introduction of the ring-laser jobs fixed that.

What was special about the SR-71 unit? I know they had INS, but didn't they also use a celestial navigation system?

Yup..My bad.. the celestial navigation system..
 

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