[clug] GPS, GIS, GTS, SDI and FOSS

Brad Hards bradh at frogmouth.net
Wed Mar 2 20:49:36 MST 2011

On Thursday, March 03, 2011 10:34:32 am Scott Ferguson wrote:
> On 03/03/11 07:21, Brad Hards wrote:
> > On Thursday, March 03, 2011 12:27:51 am Scott Ferguson wrote:
> >> * Another solution to ionospheric distortion is internet sourced
> >> corrective data - any suggestions/experience there?
> >> A local base station is unlikely to work due to the local topography.
> > 
> > I wouldn't have expected that there would be much variation in
> > ionospheric delay across the ACT (or anywhere roughly north / south).
> > The variation is strongest at dawn / dusk. Can you just put something on
> > the roof?
> > 
> > Brad
> Even that tiny variation adds significant error. The best non-military
> GPS systems on the market cannot achieve less than 10m absolute accuracy
> without corrective data. I can't quickly locate the figures - but after
> local signal reflections the largest error factor is ionospheric
> distortion (40%?). Of course the DSE sales rep will disagree with me... :-)
The local signal reflections (multipath) are difficult to assess without seeing 
the environment, so in best academic tradition, I'll assume those are zero.  
Install choke rings around the antenna. Stay away from aircraft hangars and 
Stobie poles. 

The ionosphere does all kinds of bad things to GPS signals, but the primary 
effect is delay change, which adds error to the pseudo-range measurements. 

[For those playing along at home, GPS is essentially a trilateration system - 
four distance measurements from satellites (in known positions) to a receiver 
gives you a 3-D position plus an accurate time. Those measurements are 
basically transmission time, which you turn into a distance by multiplying by 
an estimate of the velocity of transmission (roughly the speed of light). The 
ionosphere makes it hard to guess what that velocity is though.]

> I was probably (late night post) a little unclear - my requirements are
> for reproducible 1/2 meter to 1 metre absolute accuracy in the ACT with
> sub-2 minute warm starts at less than $1K per unit.
In real time? Are you OK with post-processing in the corrections?

> http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_Syst
> em#Atmospheric_effects
> Not sure what you mean by "put something on the roof"... antennae?  If
> so it is a means of reducing signal reflection - doesn't correct for
> lens effects.
If you calculate the pseudo-ranges from a known location (say, the roof of 
your office), and can figure out the error in each pseudo-range, then you can 
just post-process that error out later, since most of the ACT will see 
basically the same ionospheric effects, especially if you can work during the 
nice parts of the day (or night).

It turns out that there are some services / data freely available:

So if you can post-process guess the problem is back to a receiver that will 
log the pseudo-ranges, and some software to apply the corrections.


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