Hello Tom and others I have just called my friend having regular GPS which he is using on his yacht (and dreams when I find enough time to teach him how to use sextant and do "real" navigation). His GPS piece gives him accuracy of about 15 m. This is at least what he told me at 10 PM tonight. The best I can do with my sextant in ideal conditions is 0.1 NM (Nautical Mile) which amounts to about 185.2 m.
And the time issue too. GPS position is instantaneous. Single line of position using sextant may take about 30 minutes total. You have to prepare for sightings, take the precious thing out of its box, prepare stop watch or check correction of your electronic watch, go to the deck, do sightings (best if a series of 3 more), note them down, secure the sextant back to its box and then work on calculations. You start with averaging sightings, then calculations. When using classic logarithmic tables of trigonoemtric functions (1 page for 1 degree) calculations may take 10 minutes, plotting on the chart 1 minute (depending on sea conditions). Using famous Sight Reduction Tables for Marine Navigation shortens calculations to about 2 minutes but plotting then is a bit more complex and may take 2 minutes. Specialized navigation calculator is faster than the famous Tables but regular scientific calculator is not, unless programmable and programmed properly. And all this is for just one line. You need at least two lines for the position, better if you have 3 or more but do not demand too much, better may be enemy of good sometimes. Taking altitudes of bodies when passing the meridian reduces calculations to simple arithmetic but you have to spent 10 minutes, sometimes more, taking sightings. This is because you do not know exactly your position so you do not know accurately the time of the meridian passage. So you have to start well before and watch through the sextant the altitude growing. The time of standstill is few miuntes, depending on latitude and declination, and then when the altitude decreases you know that you got the meridian passage and have the latitude nearly for free. According to my friend there is a publicly available GPS device with higher accuracy that can be used as indicator if a yacht drags her anchor when anchored. The important question of dragging an anchor when anchored may be solved in another way by using lead and line. Slawek At 09:01 AM 3/26/99 EST, you wrote: >Dear All, > >Last year there was some discussion on the List concerning >possible changes of accuracy of GPS at the time U. S. was >staging air attacks in Africa & Afghanistan, but as I recall, >there was no difinitive opinion on that matter. In light of the >current U. S. war in Jugoslavia, can anyone confirm any >differences in GPS accuracy? > > >Tom McHugh > >[EMAIL PROTECTED] > >R. R. 1, Box 896 >Fort Fairfield, ME 04742 >USA > >46° 45' 13" >67° 48' 42" > Slawek Grzechnik 32 57.4'N 117 08.8'W http://home.san.rr.com/slawek
