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Position from crossing two circles : was [NAV-L] Reality check
From: George Huxtable
Date: 2006 Jun 7, 12:10 +0100
From: George Huxtable
Date: 2006 Jun 7, 12:10 +0100
In the thread "Reality check",Lu asked the following perceptive and interesting question, which then got sidetracked into how easy it was to estimate a rough position without instruments.- | If I've got two Ho's (and obviously know what bodies were observed, what | time the observations were taken, etc, etc), is there a direct solution | to obtaining a position that does NOT require a DR, calculating two Hcs | and Zns, and crossing the resulting LOPs? | | As someone who works in the computer field and has at least a | rudimentary knowledge of numerical analysis, I can easily see that this | could be set up as an iterative solution (ie, guess at a position, | calculate what the observed body altitudes would be, compare to actuals, | use differences to get a direction to move the assumed L/Lo, repeat...). | This is, in fact, not much different from the way your friendly GPS | calculates its position. | | But there's a part of me that says some expert in spherical trig came up | with a way to cross two spherical triangles centuries if not millennia | ago... Yes, there is such a method, a remarkably simple one, that involves no dead-reckoning or assumed position. To start with, it's easiest to think about it geometrically, with a model, which is what Elizabethan mariners would do, using a globe of the Earth, which they would carry on board. Let's assume that you have measured the altitudes of two stars at pretty nearly the same moment, or the Sun at two different times from an anchored position, so that there is negligible travel of the vessel between the two observations. At the moment of its observation, body1 is known from the almanac to be at a position of dec1, GHA1, when its altitude was measured as alt1. It was then directly above a Geographical Position (GP1) on the Earth's surface, where lat1 = dec1, long1 = GHA1 ( measuring longitudes Westwards, just like GHAs). You can mark that GP1 on the surface of the globe. The vessel must be somewhere on a circle of radius (90 - alt1) degrees, which is the "locus" of all possible positions of a vessel making that observation. You can take a pair of compasses, open them to a spacing of (90 - alt1), measured off the latitude scale of the globe, stick the spike into the marked GP1, and draw a circle. The vessel is somewhere on that circle, but where on it is as yet unknown. It may be anywhere. Similarly, when body2 was observed to be at alt2, its almanac position put it at GP2, (dec2, GHA2). So mark a position for the second body, at GP2, and draw a circle round that, at radius (90 - alt2), and the vessel must also be somewhere unknown on that circle also. In which case, the vessel must lie at the intersection of the two circles. In general, any two circles which intersect must have two such intersections, not just one, and there's no telling which is the one the vessel must be at, from the given information. So some rough knowledge of the vessel's position is required, in order to eliminate the false solution. As long as the two bodies were chosen to have well-differing azimuths, according to normal navigational practice, the two solutions will be thousands of miles apart, and the false solution becomes obvious by commonsense, or from a rough knowledge of those azimuths. Other than for making that distinction, however, there's no requirement for any sort of DR or assumed position, no calculation of intercepts and azimuths; it couldn't be much simpler. Having rejected the false solution, the vessel's lat and long are read off the globe at the remaining crossing-point, and the job's done. The snag is, of course, that working from a model globe is inherently imprecise, when looking for a resulting position to within a mile (or few) on a globe that's only a few inches in radius. But there's nothing imprecise about the principle involved. So the question arises, can the same two solutions be found mathematically, without using the model globe as an analogue calculator? Yes they can. The algebra for doing the job is somewhat complex, so that it would be hard work to do the job in "longhand", but presents no challenge to a computer or programmable calculator. I have written a program in bastard-Basic which runs on my 1980s Casio programmable calculator (FX 730P or FX 795P), and if anyone is interested would be happy to send it or post it up. It would be simple to adapt it to another machine. It takes the 6 quantities, dec, GHA, and altitude for each of two bodies, and returns two possible positions in terms of lat and long, for the user to choose the appropriate one. It does not require a DR or AP, and provides an exact result without going through an iteration process. It's not original, in that versions of the method have been described previously beforehand. For example, in an article by George Bennett in the journal "Navigation" (which is, I think, the American one) Issue no. 4, vol 26, winter 1979/80, titled " General conventions and solutions- their use in celestial navigation", and to the book "Practical navigation with your calculator", by Gerry Keys, (Stanford maritime, 1984), section 11.12. The method has also been described in "The K-Z position solution for the double sight", in European Journal of Navigation, vol.1 no, 3, December 2003, pages 43-49, but that article was bedevilled by printing errors that render it more-or-less unintelligible, which were corrected in a later issue. Not to mention several serious errors and misunderstandings by the author, which have never been acknowldged or corrected in that journal. The method can be extended to simultaneous or almost-simultaneous sights of more than two bodies, by taking all possible pairs of bodies; for example, 3 pairs with 3 bodies. Each pair then results in a calculated position, and for 3 bodies each such position is the intersection at a corner of a "cocked-hat". The method of intersecting circles becomes much more difficult (impossible, perhaps?) when there's a need to account for the travel of the vessel in the time interval between the two observations, the "sun-run-sun" case. But that's another story. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.