[MLB-WIRELESS] OT: increasing GPS accuracy

[David Ashburner]

On Oct 16, 2006, at 7:38 PM, Steven Haigh wrote:

> Quoting Rowan 2006 <rowan2006 at sensation.net.au>:
>> Just a quick OT question as I know many of you fiddle with GPS: if 
>> you had
>> a reference GPS in a fixed location with long-term averaged 
>> co-ordinates,
>> could you use the difference between the current and long-term reading
>> from that GPS, to correct another (portable) GPS unit nearby?
>>
>> eg, say the long term position is x,y. At 09:01:30 the current 
>> reading is
>> x+3,y-2. Can this offset be retroactively applied to the reading 
>> saved at
>> the same time from a portable GPS?
>
> I think what you're looking for is Differential GPS (DGPS). This uses
> a ground based trasmitter as well as sats to get a VERY accurate
> position (I hear values of 10cm quoted by some people). If you really
> want to get one and play with it, make sure you're sitting down when
> you ask for the price of one of these units ;)
>
It's not the cost of the DGPS receiver, they were relatively cheap ( 
say $200 ) but you need to have a DGPS base within range.
I used to use one sailing in San Francisco Bay and the accuracy was 
really ( 1 metre ) good, but when Selective Availability was turned off 
the need for it in many cases went away.  You will find DGPS 
transmitters all along the US coasts and probably in Major and Minor 
port areas in other countries. - Just looked at West Marine, you can't 
even buy them now so they must be phasing it out of service. Nowadays 
with WAAS you should get down to 3 meters accuracy if you don't have 
obstructions.