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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
SA errors---averaged over a reasonable time?
From: Mal Misuraca
Date: 1997 Dec 15, 9:25 AM
From: Mal Misuraca
Date: 1997 Dec 15, 9:25 AM
Do we know that SA errors are not randomly distributed over a reasonably short period of time? It would be interesting to know if anyone can be certain of that. I have watched the progress of the SA watch program averaging, as it deposits one dot on the screen for each of the fixes it takes from the GPS and permits these accumulations to be measured against (1) the position you believe you occupy and (2) the position represented by the mean of the fixes. My impression---and I certainly intent to go back and look again---is that the GPS fixes are not random in the sense that they jump from one side of the putative position to another, but that they wander in a line back and forth across and around a point. Thus, you could argue that you must let them wander quite a long time before being sure you are getting the mean well established. On the other hand, after several hours, it seemed clear to me where the locus was, and I based that on seeing where all those fixes had been deposited. In maybe an hour, it seemed to me that the truth was beginning to emerge---there was a point that seemed to be the locus of all the fixes within that time. After that, it was fine tuning, not a significant shift in the mean, no matter how many hours the GPS was left on. This suggests that in as little as an hour, maybe less if I had looked more carefully, there was in the plot the ability to say pretty much where it was all going to come out. Maybe that's the "slowly" that the last two or three emails on the subject mean to suggest. This is for someone like me plenty fast enough to be useful---as in trying to correct some of the Baja charts, for example, in which it is not unusual to find that latitude is good, and longitude is off. This arises, I believe, from the fact that the charts have not been resurveyed since Admiralty days. Thus, I have tried to tie down one or two important spots in each anchorage, figuring that when I went back, they'd be all I needed to get started. Mal Misuraca Passage East Sausalito =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@ronin.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=