NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: accuracy of automatic celestial navigation
From: Paul Hirose
Date: 2002 Dec 9, 11:42 -0800
From: Paul Hirose
Date: 2002 Dec 9, 11:42 -0800
George Huxtable wrote: > > down to affordable levels. And on a rough sea-surface, the disturbing > accelerations are orders of magnitude greater than a high-altitude aircraft > has to contend with. I don't remember the price tag on the B-2 AINS, but it must be staggering. The thing is an optical, mechanical, and electronic marvel. We never opened it - if there was a malfunction it went back to the factory to be repaired in a "clean room". An INS can handle motion far more violent than a seagoing vessel will experience. For many years fighter and attack aircraft have carried INS. The older ones, at least, were not "strap-down" systems, but mounted the gyro and accelerometer platform on gimbals. But since the platform maintains a fixed orientation in space as the plane gyrates around it, inertia is your friend. The gimbal torque motors need only compensate for the friction in the gimbal bearings, and of course this is reduced to a very low level. > In the days before GPS, how could the instaneous position in flight be > independently determined sufficiently accurately, for testing purposes? Is > Loran accurate enough, and fast-responding enough? The aircraft will > presumably pass through 15 metres in less than 30 milliseconds, so it's a > demanding requirement. A tracking instrument on the ground called a cinetheodolite is the old-tech method. You often see these in documentaries about the space program. The operator manually follows the target as a motion picture camera records the scene. Azimuth and elevation readings are optically superimposed on each frame, along with the time. With simultaneous film from two or more of these instruments it's possible to reconstruct the target's flight path. The "kinetos" I saw at Edwards AFB were so big, they were motorized and the operator rode the instrument. http://www.serve.com/mahood/nellis/ttr/sln5.htm http://www.emitechnologies.com/optped.html http://www.acq.osd.mil/te/mrtfb/commercial/afftc/time.html Rodney Myrvaagnes wrote: > > I don't understand how the earth's lumpy gravitation could affect > inertia in a B2. I know it goes pretty fast, but it would have to get > close to the speed of light for relativistic effects to become > comparable to the drift of the ANS. Am I missing something? The 1984 Bowditch explains this better than I can: "Since an accelerometer cannot distinguish between a kinematic acceleration and a gravitational acceleration, any uncertainty in the gravitational environment manifests itself as a system error. in the case of marine inertial navigation in locally level coordinates, it is the horizontal components of gravity that cause significant errors. These are directly due to deflections of the vertical (art. X4), which are tilts of the actual (plumb bob) vertical vector relative to the presumed reference vertical."