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    AW&ST article on non-GPS nav systems
    From: Frank Reed
    Date: 2010 Nov 8, 21:08 -0800

    An article at Aviation Week & Space Technology's web site on alternatives to GPS:

    The basic idea is to use radio beacons that can provide an alternative to GPS signals in an emergency, specifically a military operation. So you fly some UAVs (drones) at 50,000 feet or so and they emit radio signals which receivers on the ground can use to get their positions. But how do the UAVs determine their own positions so that they can enable a high-accuracy fix at ground-level? Why by using celestial navigation, of course, and since they're up in the lower stratosphere, they can see the stars all the time... Something like that.

    Note below that they mention sensors which can see stars in daylight down to magnitude 6. Wow! I want one. They also describe an Inertial Measuring Unit the size of a matchbox. I would also like one of those, please. :)

    Here's the section from the article relevant to celestial systems:
    "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."


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