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

Robert Edwards bob at cs.anu.edu.au
Wed Mar 2 17:04:40 MST 2011

On 03/03/11 10:34, 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... :-)
> 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.
> http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#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.
> I'm happy to discuss this further, time allowing, especially as it is a
> learning curve for me, but probably best either off-list or under an
> [OT] prefaced subject-line (lest I become a hypocrite!)
> Cheers

Lets keep this on the list for a little longer...

I think Brad was suggesting building your own base-station with an
antennae in a fixed location (on the roof). Then you can derive your
own differential data and either transfer it to the GPS you are trying
to correct for (eg. over 3G?) OR use your differential data to correct
the data you have collected in a post-processing step, by aligning the
time-stamps etc.


Bob Edwards.

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