[Openstreetmap] Inner city accuracy
Christian van den Bosch
cjb at cjb.ie
Thu Jan 26 16:13:29 GMT 2006
Erik Johansson wrote:
> On 1/26/06, Lars Aronsson <lars at aronsson.se> wrote:
>>How difficult would this be to achieve? What would it cost? Is
>>it realistic? Has anybody done it already?
It's a very nice idea. However, I'm not sure how easy it is to generate
DGPS information; I suspect a dedicated survey-grade GPS unit with a
DGPS output is required.
Another concern would be that WLAN is very sensitive to line of sight,
and as such might not be very appropriate to dense urban environments.
RS232 over 433MHz is much lower tech than WLAN, but might be more
appropriate - http://www.google.ie/search?q=433MHz+RS232
In this version, you'd have two radio boxes - one transmitter with
serial in from the DGPS provider, one receiver with serial out to the
mobile GPS. Advantage: the mobile unit is small (no computer required).
If you wanted a less hardware-hacky method, you could always access DGPS
over IP via GPRS - either with a PDA phone linked to a separate GPS,
perhaps via bluetooth, or a GPS PDA linked to a separate phone. Three
boxes would start to get a bit bulky...
> Since our GPS:es are some what like black boxes you have no idea
> what kind of magic goes on in there to get us a good fix.
> To be able to do the calculations on DGPS you need to know what
> satellites you are looked on to and the errors of those satellites.
> So you can't just take the x/y diff and get a better value.
Indeed. NMEA provides some information about the signal from each
satellite, but I suspect not enough for you to be able to use DGPS for
calculations on a computer connected to the GPS; aiui, the DGPS info has
to go back to the GPS unit, so it can apply that to its fix and give you
a revised location.
Random search terms to try on this: DGPS, DGPSIP, GPSD, WAAS, NMEA
Christian / cjb
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