A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: John Blake
Date: 2017 Aug 5, 11:44 -0700
In an aircraft, the sight reduction methodology is the same as on the surface in that you must start with an assumed position based on dead reckoning (DR) time at your closest latitude and a longitude that makes your local hour angle (LHA) come out as an even angle. Coriolis Effect is calculated from a table in the Air Almanac and is applied by off-setting the assumed position. Usually, three stars are shot at four-minute intervals so that each star's LHA changes by a whole degree. An airborne bubble sextant has a two-minute averager to account for the wallowing of the airplane (as I best recall, a B52 wallows on about a 40-second cycle). Thus, each shot is started at one minute prior to the shot calculation time so that the average time of the shot coincides with what you calculated. Even the best of pilots cannot fly the plane truly straight and level for two minutes and auto-pilot is not as good as a good pilot. Once the shot average is found, you follow the same HoMoTo rules that you would do on the ocean or dry land. I flew in the E model and we had a Doppler radar which gave accurate groundspeed and groundtrack, so our calculations for distance and direction could be accurate during the 10 minutes whille we were making our shots. The greatest errors that were made during celestial navigation were in the reduction of the data, i.e., retrieving the GHA of Aries from the almanac, calculating ,the proper LHAs and treating HoMoTo's correctly. In other words, the same errors that are made on the surface. Properly taking into account of the fact that you are moving along at 7-10 miles per minute also causes errors, but to a much-lesser extent. In a B52, the navigator does the computation and plotting, the electronics warfare officer (EWO) does the shooting based on the navigator-provided stars' azimuth/altitude/countdown and the pilots make certain that the plane is flying as smoothly as possible during the shooting. It's a real crew coordination effort when done properly.