High precision GPS?
Dear Paolo,
I somehow disagree with Phantom.
Let us look at the possible sources of error in the GPS UERE:
edu-observatory.org/gps/gps_accuracy.html
Multipath can be very high (e.g. urban environment) and will be hard to beat.
But if this is not your case then the main errors left are atmospheric errors and ephemeris/clock errors. To estimate the atmospheric errors normally there are three options:
- using dual frequency receivers (real time),
- use the ionospheric grid information carried by SBAS (in Europe EGNOS) messages (real-time)
- use the meteorologic information from the ground segment of GPS (post-processing, see IGS IONEX)
While dual frequency receivers are very expensive, most receivers receive SBAS and apply its ionospheric corrections.
The ephemeris/clock contribute can be mitigated by using precise products (quasi real-time, e.g. from IGS) or implementing some sort of PPP (Precise Point Positioning) which will have long convergence time (2-3 hours) but you can afford that.
There is free software supporting PPP (see RTKLIB) both in real-time and post-processing and a recent uBlox module (NEO-6P, 40EUR/piece) also implements "in hardware" a "light" version of it.
In my opinion if you want low cost cm accuracy you must look at L1 only receivers which are low cost and output raw measurements, so you can post-process them with ionospheric grids and ephemeris/clock precise products ...effectively mitigating the major contributors to an error that does not average out.
The lowest cost receivers I know are the Skytraq S1315F-RAW, followed by the NVS NV08C-CSM (GPS/Glonass) and the uBlox NEO-6P/T (embedded PPP/timing). They respectively come at _about_ 20EUR and 30EUR/piece in quantities.
Hope this helps!
Best regards,
Michele
