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On Fri, 4 Apr 2014, Eion MacDonald wrote:
On 03/04/2014 23:06, Julian Hall wrote:With a SatNav it doesn't matter if you can't see anything as long as you can see /it/.AND you do not move except very cautiously. SatNavs for civilian use are not accurate within 6 feet (deliberately). So easy to walk over an edge. USA military accuracy is 10cm!
I think your information may be somewhat out of date there...The signals that the satellites broadcast are the same for civilian and military use. (and always have been). Part of the issue early on was the electronics and DSPs to do the signal processing - simply too expensive for civilians but now they're cheap, it's down the the processing power vs. electrical power available (and heat dissipation - important in a battery operated hand-held device) not to mention the antennae size - you might not want something 150mm in diameter on a hand-held device! (Bigger antennae => better signal quality, less processing, more accuracy)
Also early on (up to the year 2000) there existed something called selective availability (SA) - this deliberately degraded the signal to reduce the accuracy - unless you had the kit that knew about the degradation to correct it... (This is possibly what you're referring to above)
Then some bright spark came up with the idea of putting a GPS receiver at a fixed known location, working out the difference between that fixed known location and what the GPS satellites were saying and then transmitting that difference to the surrounds (a few 100 miles radius - inside the local SA region for example) So then suitably equipped GPS receiver that took the correction signal in knew exactly where it was.
(and as an example, all the dive boat skippers jumped on it almost overnight and the good ones were then able to shot wrecks with such accuracy that some of them would ask which boiler you wanted the shot-line on!)
What the military folks do have access to is the 2nd frequency and signals that the sats. broadcast, (PPS) this allows them to correct for atmospheric disturbances, (the signal has the same positioning accuracy) however this is an open standard so I guess its only a matter of time before its available to us all.
So (US) GPS along with the russian and european systems - put them all together in one unit and you'll have something very accurate indeed. (and a battery life measured in minutes ;-)
If you need it. The main issue right now is the accuracy of the maps, not GPS. Gordon -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/listfaq