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  • Where's True North?
    Posted by Bill Sweetman 10:55 AM on Oct 14, 2011

    If you have spent much time around the close air support and "joint fires" community in the last few years, you know that there's been one problem that joint tactical air controllers (JTACs) have wanted solved. It's a very basic one.

    With GPS, the JTAC knows exactly where he or she is. Laser rangefinding, built into the basic JTAC tool -- night-vision or infrared binoculars, usually tripod-mounted -- gives an exact distance to the target. But one of the vital elements in geolocation falls short. Most systems rely on a digital magnetic compass for azimuth, and that is subject to normal changes in the local magnetic field.

    The result is uncertainty about the target's position that increases with range, and that may prevent a JTAC from authorizing fire from GPS-guided weapons if friendlies or noncombatants are close.

    Various solutions have been tried. Rockwell Collins offers a system with a second GPS receiver, on a cable. By using differential GPS, it can determine the bearing between the two receivers and calibrate the compass. But it requires someone to emplace the second receiver, which may be slow or hazardous.

    Vectronix, a Switzerland-based subsidiary of Sagem, unveiled a new solution to the problem at this week's Association of the US Army show. The Sterna family of systems comprises a north-finding module mounted on a tripod, with a top mount for any of the company's rangefinder systems. When the operator starts the system up, it goes through a two-minute process in which it rotates left, right and then back to its start position. Within two minutes it has locked on to true north. Specified accuracy is +/- 5mil but the company says it routinely achieves close to +/- 1 mil.

    blog post photo
    Sterna and display showing +/- 1mil accuracy

    Vectronix director, orientation systems, Sabina Danczul explains how it works. Inside the unit is a hemispheric resonator gyro, basically the same Sagem model that is used on the A2SM Hammer guided bomb, which compares readings each time it turns. But, I said, how does that help if the system doesn't change position? It does, Danczul points out: "The system accounts for the rotation of the earth." A few algorithms later, it calibrates the DMC.

    Very neat. Sterna was getting some good high-level attention at AUSA.

    Tags: ar99, ausa, vectronix

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