[aprssig] DGPS?

Tapio Sokura oh2kku at iki.fi
Mon Apr 7 16:18:39 EDT 2008

Steve Noskowicz wrote:
> Too bad the DGPS data stream is so complex or it would be easier to tap into
> the data for an average hacker without one of the DGPS receivers.

It really isn't that complex, it's just obscured from the naked eye due 
to the bit-based encoding. Receiving the marine dgps (300 kHz) band 
signals also isn't that hard, it's signal structure is a lot like rtty. 
You can also receive some dgps reference stations' data stream via Internet.

>    I suspect averaging only minimizes the error to the location which is
> defined by the limit of the basic system accuracy.  DGPS would be needed to get
> to the" true" location.

Nothing is "true" when you talk about measuring something, like a 
position. There is always some error left, even with survey-grade 
receivers and post-processing. Averaging a non-dgps position over a long 
period of time results with great probability in a "more true" position 
than a single non-dgps position fix. Averaging a non-dgps fix can also 
result in a better fix than a single dgps (RTCM SC-104) position fix.

Generally, with a good GPS receiver and a reference station close by, 
RTCM SC-104 can get you down to about 1-2 meters accuracy. Originally it 
was designed for mitigating SA for maritime use, it is suboptimal for 
real precision/survey work.


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