Ken Hornstein wrote: >> I'll be interested in seeing if there's any response to this on the >> list. My views of GPS on the canals are well know, but as keen hill >> walkers Mrs Steve and I have been thinking of investing in one for >> our trekking trips. But If at best they're 500 ft out of kilter (and >> that's before degrading) it hardly seems worth buying one. We can >> read our OS maps wrongly ourselves without technological assistance, >> thank you very much. > > Here is my limited understanding of GPS. > > - Selective Availability was turned off in 2000, and as far as I know > there are no plans to ever turn it on again (especially considering > how much GPS is being used in civilian life). > > - GPS accuracy depends on a lot of variables. One of the variables > are the satellites themselves. At a minimum you need 3 satellites > to calculate a 2D position (4 satellites for a 3D position). But > one of the factors is the position of the satellites in the sky; a > clump of satllites close together is less accurate than satellites > spread over the sky. Additional satellites improve accuracy (but > there is a practical limit, of course). I generally see 6-8 > satellites if my GPS has had enough time to find them.
Also Add.... a.. Receiver Clock Errors - The clocks in receivers are not very accurate. To make it accurate, it would have to have an atomic clock (very big and very expensive so this will not happen in the near future) a.. Number of Visible Satellites - The more there are, the more accurate the triangulation becomes. It is also worth noting that generally GPS units cannot receive a signal underground, underwater, indoors or become inaccurate under thick glass as the signal travels slower through glass a.. Ionosphere and Troposphere Delays - These are the delays in the GPS signal passing through the atmosphere. This is somewhat counteracted by a model that the system uses to calculate these delays and correct them but is uses averages so it it not too accurate. a.. Satellite Geometry - The wider the angle between the satellites the more accurate the GPS will become a.. Signal Multi-path - this is where a signal gets bounced off buildings or cliffs causing slight errors a.. Orbital Errors or Ephemeris Errors - These are caused by the satellites slightly mis-reporting their locations Ron Jones Process Safety & Development Specialist Don't repeat history, unreported chemical lab/plant near misses at http://www.crhf.org.uk Only two things are certain: The universe and human stupidity; and I'm not certain about the universe. ~ Albert Einstein
