[LINK] limits of technology in finding someone

[Adam Todd]

At 03:35 PM 9/12/2006, Stewart Fist wrote:
>Roger writes:
> > I've asked a couple of people to use GPS to nail down the height
> > difference between my house and the trig on the ridge about 250-300
> > metres away as the crow flies, and 600 paces in 5 mins 40 secs +/- 10
> > secs.
>
>What do people think about the accuracy of GPS -- in the best circumstances,
>and with the best gear.

Mines within about 90 cms.  Although at times it can bounce as far as 40 
meters.  If you keep the receiver still your accuracy diminishes the longer 
you stay put.  Constantly moving it in circles around a 45cm or greater 
area brings the accuracy to very good levels.

>I remember when the USA had dithering on the system (to deliberately reduce
>the accuracy to a few hundred metres, maybe even more).  At that time they
>boasted that their military accuracy, with no dithering, was a matter of
>centimetres.

Clinton changed this in his sitting.  The US Military use a better, more 
reliable, and more accurate sat system now.  I can't recall the web site 
that has all the details but it's around.

Accuracy is now set to 30 cms.


>In fact, I distinctly remember one claim that it was measurable in
>millimeters, and that it was more accurate for house-block surveying than
>the conventional terrestrial triangulation methods.

Could well be, but you'd need a really good antenna system I suspect.

>The question of exaggerated claims to accuracy in these matters is
>interesting because of the claims to millimeter accuracy in measuring the
>sea-levels from satellites.

Sitting at a set of traffic lights the altitude can go from say 173 feet to 
190 feet or anywhere in between.  Although again, when driving, it 
"appears" stable and reasonable.

Lets just say I'd not fly a plane dependant on GSP altitude.  It might tell 
me 300 feet, but I think I'd sit at 500+ just in case!

>The satellites are perturbed by gravitational
>anomolies they pass over, and they are pushed around by solar winds, and
>they are flying over an Earth which is spheroidal measuring the distance to
>water which has a surface changed by waves, surges, tides, atmospheric
>pressure lifts, etc. and land masses which are rising and falling tidally,
>as well as tectonically.

Gee is that all?

>I just can't see that they could possibly have accuracies in their satellite
>sea-level measurements better than a hand-span or so -- but I've never been
>game to say so in print because I don't have any direct experience or
>evidence.

Welcome to play with mine :)

Or you can download Oziexplorer for your laptop, buy a $60 Bluetooth GPS RX 
and off you go!

>I remember back in the Cuban Missile crisis the US claimed that their
>satellites could read the headlines of a newspaper or the number plate of a
>car from space.  And most people believed them.

Not up to 30 years yet, but I've witnessed the reading of fine print on a 
mans watch and the Sydney Morning Herald, whilst the man was sitting in 
Hyde Park.

>It later turned out that they best photos they could ever produce made the
>bodies of the missile look like fuzzy maggots on a sheep carcass
>photographed by a box brownie at 100 paces.

That was then :)

>I suspect that false claims of accuracy are endemic to the US military, NASA
>and to NOAA.

You mean like the claim that if you look up at the sky the Satellite can 
see you face, but if you look down or to the horizon they can't.

Unless, duh, the satellite is on the horizon.