[FOSS-GPS] High accuracy positioning with low cost GPS devices:a
john at coyotebush.net
Fri Oct 10 13:37:48 EDT 2008
I did a zero baseline test with the LEA-4T, and the carrier phase looked
pretty good. The LEA-4T needs some interpolation tricks to get things to
work. The other inexpensive receiver is Thales AC-12. This one provides
carrier phase within a couple mm. My understanding it is the same GPS engine
which is used in the Promark II for doing single frequency RTK. (BTW, is
there a cheaper LEA-4T than the u-blox demo unit?)
I sort of got burned out trying to implement a kinematic float solution. I
eventually concluded it is very difficult to detect or correct phase errors
with float solutions. You either have to be stationary and use a Kalman
filter, or you have to "fix" your solution so you know your position well
enough to catch the inconsistencies. I'm curious to know your approach - and
I'm really, really glad you've taken on this piece of work.
(It was also good to hear portions of my precision-gps software were of
There is a whole infrastructure needed to create a usable positioning
service. Most of it is technically straightforward once the positioning
software is working. Maybe this is an area where the community can assemble
the pieces to create a complete project.
Some possible pieces:
A small, remotely managed microcontroller to interface GPS base stations.
A GIS field collector
A broadcaster to disseminate real time data streams.
A database server to manage raw data for postprocessing or "near
A service to do the processing.
A service to provide Rinex and Trimble data for external postprocessing.
Internally, I would suggest sticking with RTCM 3.1(RTK), ntrip and SOA. (I
wish the RTCM didn't charge so much for their documents)
I've done bits and pieces of the software, and I'd be glad to donate the
"precision-gps.org" domain name to the cause. Anybody interested? If nothing
else, maybe we can come up with creative names for the various pieces.
- John Morris
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