In search of accuracy…GPS
In the post below on On Street I wrote this paragraph:
As for the GPS — It is accurate to the inch. Well maybe foot. However the military doesn’t really like that technology to ‘get out’ so they let those with commercial usage get down to what 30 feet. And you are right, 30 feet isn’t close enough. But this is a political decision, not a technical one. Give it a couple of years and Garmin will have you car located down to the width of a 2×4.
I was concerned about the accuracy so I went to my source, Michael, who knows all about this stuff. Here is his response:
Whoever wrote that para is out of date about a decade. It used to be that there were two grades of service; a ‘Selective Availability’ service for civilian use, and a ‘clear’ service for the US and NATO militaries.The SA service was intentionally degraded to about 200 metres in normal use, but clever people found a way around it by providing ground receivers in known (surveyed) positions, which broadcast by radio special ‘Differential Correction’ signals to improve civil receiver accuracy. It got so silly that the US Coast Guard (and my old outfit The Canadian Coast Guard) spent millions of tax dollars to build differential correction systems to make civil marine GPS sets more accurate — defeating the millions spent by the USAF to provide only degraded signal for civil use. Because of that, and because the growing and extensive commercial use of GPS was creating huge pressure, the SA service was discontinued in 2000 by Executive Order and the full accuracy GPS has been available for all users since.
There are a number of factors at play, but in today’s standard civil sets like a car GPS the accuracy in motion is always less than accuracy stationary, but is generally better than 100 Metres and often better than 20 metres. The normal is now about 20 Metres or better for standard low cost GPS taking multiple readings in a fixed position (e.g. parked) when at least four satellites are visible. But with survey equipment which is more stable, (and expensive,) receives more satellites at once (or at least processes more of those it can see,) and is left in position for perhaps 30 minutes to let it average over many readings, I believe that accuracies of less than 2 metres are now normal.
As stability of the satellite signals has improved, and stability and processing power of low cost GPS sets has improved, so has accuracy. But there are too many variables to expect measured accuracies of “the width of a 2 X 4″. Well, perhaps 2 X 4 metres in normal use, but that will be about the limit without various other ‘Augmentation Services” such as the broadcasting of differential correction data from nearby locations such as every cell tower. And don’t forget that we just don’t have the ability to model the actual shape of the earth to that kind of accuracy, which is important because ‘position’ always is relative to something known, to have any practical meaning.
The major difference now with mil spec equipment is the ability to process position accurately from six or more satellites (of about 34 up there) with full accuracy in three dimensions at high speed and high rate of change — e.g. fighter aircraft, helicopters and missles. In fact, equipment capable of providing accurate position / altitude / track / Rate of turn info at high speed is still considered military restricted gear and not exported. (Except, of course, between NATO allies).
Not to overcomplicate matters, but there is also another whole issue one needs to consider when looking at a nav system like GPS, or LORAN, etc, and that is the difference between ‘absolute accuracy’ against the standard geoid (model of the shape of the earth, which keeps changing,) and repeatable accuracy, which is the ability to repeatedly return to the same place and get the same readings. Some low cost systems like in a car will get confused and give slightly different readings if you wwere to drive around a route perhaps a mile or two long and return to the identical place in your driveway —- the system could give a reading that’s a few metres different then last time — but you’ll never know whichv (if either) was correct.
Hope this sufficiently fuzzifies and obfuscates the already barely comprehensible!
My comment: I was 10 years out of date but HE was my source. He wrote back:
Well, I wrote my reply last night from memory, as I wasn’t expecting to be quoted somewhere, but if its any comfort, in a previous life I was responsible for all Nav Systems at the Cdn Coast Guard (He was actually head of the Canadian Coast guard for a time) when GPS became the dominant marine nav service, so was quite involved in the issues around Differential Correction for GPS, impact on previous generation systems like LORAN C and Decca, etc. (Also wrote the article on Marine Navigation in the Canadian Encyclopedia, but that was a long time ago.)
So there — Technology can get the car located down to what, six feet, and of course, that will become more accurate as time goes by. However politics and military secrets, will keep the technology out of private hands for a few more years. Remember 40 years ago the technology you use every day on Google earth was so top secret that the classification itself was classified.