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Re: Question about Pub 249 combined Star and Planet sight reduction
From: Rod Deyo
Date: 2000 Nov 20, 8:36 PM
From: Rod Deyo
Date: 2000 Nov 20, 8:36 PM
My thanks to Richard B. Emerson, Dan Allen, and Paul Hirose for their responses. A few further comments: 1) Perhaps the issue is moot for small craft when the measured Hs values are uncertain, but as you get away from the epoch used for Pub 249 Vol 1 calculations, the nutation and procession corrections are as large as 4 nm . I hate to just give away that amount of error on what otherwise might be consistent sights. 2) The question I raised was how to properly weight multiple LOPs when some of them are intended to be corrected as a single group. Just eyeballing the answer is tough for me if there are five LOPs, three which need to be moved a few miles in a particular direction (for example, several Pub. 249 Vol 1 star sights corrected for nutation and procession) and then weighted against two that are not shifted (several planet or other star sights not done using Pub 249 Vol 1, but using the Almanac and 249 Vols 2-3 or Pub 229). Ideally, you then pick the point as your final fix that minimizes the square of the distance from each of the corrected LOPs (this is an unbiased estimate, since no line or set of lines is favored over the others). 3) At the moment, I've convinced myself that it's probably best to either a) assume it makes no real difference and do not correct (this is probably good enough if the correction is 1 nm or less) or b) correct each AP by the nutation and procession, plot the resulting LOPs, and then eyeball each of the LOPs together to get the final fix. Rod Deyo