NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: automatic celestial navigation
From: Frank Reed
Date: 2008 Jan 10, 12:57 -0500
From: Frank Reed
Date: 2008 Jan 10, 12:57 -0500
Greg R., you wrote: " Although I think the inertial part of it might be a little pragmatic on a sailboat... ;-) " The inertial part is the principal navigation system. Though the description as posted makes it sound like this system is doing something akin to common celestial navigation, that's really unlikely. Consider: to do standard celestial navigation, you measure altitudes relative to a horizon. At the altitude where the SR-71 would fly, there is no visible horizon, so that means an artificial horizon. Now there's been no great advance in all these decades in artificial horizons. There's simply no way to get a vertical that is not influenced by the motion of the airplane. You're limited to measuring angles between celestial objects or angles relative to components within the airplane's structure. So why would that navigation system need the stars if not for a standard celestial fix? The key here is that it's an "inertial" navigation system. This is a sophisticated form of dead reckoning. It uses accelerations (including rotations) to calculate the current position by integration: acceleration integrated over time yields net change in velocity, velocity integrated over time yields net change in position. But an inertial system drifts, and the biggest drift is in the orientation of the platform. If we can compare the platform with a known inertial frame of reference, we can largely eliminate that drift. The distant stars, of course, constitute an excellent measuring standard for an inertial frame of reference. So, more likely than not, the SR-71 system used the stars to re-set the inertial system. In effect, the stars in this system are being used AS A COMPASS! The primary navigation is inertial. The paragraph from the Wikipedia article on the SR-71 strikes me as reasonably accurate: "The ANS primary alignment was done on the ground and was time consuming, but brought the inertial components to a high degree of level and accuracy for the start of a mission. A "blue light" source star tracker, which could detect and find stars during day or night, would then continuously track stars selected from the system's digital computer ephemeris as the changing aircraft position would bring them into view. Originally equipped with data on 56 selected stars, the system would correct inertial orientation errors with celestial observations. The resulting leveling accuracies obtained limited accelerometer errors and/or position growth." Incidentally, there's a way to dispense with the stars for inertial navigation. An inertial frame of reference can also be accurately maintained using a ring laser. Those supposedly began to replace astro-compass systems in the 1980s (so they say!). -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---