GPS Selective availability turned off

[Robert Smallshire]

Hi folks,

Well, I've spent day one of the post-selective availability era out mapping
in south-eastern Utah. The EPE (estimated position error) reported by my
Garmin GPS 12XL which always used to be at least 15 m is now consistently
down at 4 m. Long term position averaging with the Psion suggests that
repeatability of position is ± 3 m. Amazingly, the quality of the GPS
altitude data seems to be consistently well within 10 m vertcially of the
value given on the USGS 10 m spacing DEM. All in all I've been hugely
impressed by the performance so far of non-SA GPS - it is certainly good
enough for locating cave entrances now - and probably good enough for fixing
extrances (at least in xy).

I spent a large part of the day re-fixing points I've measured over that
last two weeks. Most position averaged SA points were at least 20 m from
their 'new' positions - such as much as 80 m.

More data on GPS performance when I have it.

Cheers,

Rob Smallshire.
ULSA

> -----Original Message-----
> From: cave-surveying-admin@survex.com
> [mailto:cave-surveying-admin@survex.com]On Behalf Of Wookey
> Sent: 02 May 2000 08:43
> To: surveying forum
> Subject: Re: GPS Selective availability turned off
>
>
> On Tue 02 May, Robert Smallshire wrote:
> > Good ol' Bill Clintons done the decent thing and turned off selective
> > availability
>
> Indeed. We (the BCRA CSG) were working on setting up a european DGPSip
> station to supply differntial GPS info over the internet. This worked
> usefully over a 3000mile baseline, and would be a useful adjunct to the
> two US stations. All you need is a permanent net connection within 150
> miles of a DGPS transmitter and about 400 quid-s worth of DGPS
> receiver+transmitter.
>
> This would have provided better GPS services for cavers anywhere in
> europe (so long as they could get access to the net), however this change
> may have made the idea largely redundant as non-SA GPS is probably good
> enough for most cave-location purposes. I'm waiting to find out what the
> expected differential numbers are under the non-SA regime, especailly
> under long-baseline condiditions. Anyone here know?
>
> Wookey
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>
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