Dear Doug and Roger, You are both right to doubt claims for 0.1 arc-second precision in azimuth using a hand-held GPS receiver but there is more to be said before dismissing GPS outright.
Appropriately translated, 0.1 arcsec precision is equivalent to a 2km baseline with the end points known to 1mm. Unlikely! What you CAN do is set out a baseline of about 50m and, at each end, place matched up-market GPS receivers which are connected to the main kit close to the mid-point of the baseline. Typically, the system is left to run for 6 hours or so. All this time, both receivers are averaging their perceived positions. For any given pair of satellites all each receiver can do on its own is note the perceived difference in the clock times, which translates into a difference in distance. The locus of all points which are nearer to one satellite than another by some fixed amount gives a surface. By knowing the ephemeris data of each satellite, and taking several pairs, the intersections of these surfaces leads to a best estimate of position. In good conditions, you will end up knowing the positions of each receiver to within one or two metres. This is still pretty hopeless but now comes the clever bit... What the kit in the middle does is not merely compare perceived clock times but actually looks at the phase differences of the carrier waves from the different satellites as picked up by the two receivers. This way, they can determine the relative time differences to times corresponding to a fraction of a wavelength. This doesn't improve the estimates of the absolute positions of the two receivers at all but their RELATIVE positions can be determined to about 5mm. This is starting to look good. You are talking about 5mm in 50m or 1mm in 10,000mm which is about 20 arc-seconds or one-third of an arc-minute. With your theodolite you are (at the moment) getting an error of the order of 0.08 degrees which is about 4.8 arc-minutes. You should be able to get this down to 2 arc-minutes with practice unless there is lots of slop in your instrument!! GPS does rather better and doesn't need a clear sky. It DOES need a good view of the sky though and won't work well at street level with high buildings all round. You will need to set it up on something very solid high up. Concrete buildings don't sway as much as steel ones!! The snag is the cost of the kit. This is serious professional stuff costing between $50k and $100k. Specialist contractors who use this kit all the time are available for hire. I have used three different such contractors for big (and expensive!) sundials over the past 10 years. Frank King Cambridge, U.K. --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
