SparkFun Forums

[bonelli]

Hi everybody,

I'd like to build a very-low power gps tracker for "off-line" application (no need for real time positioning). I need a fix every 1 to 15min (user-dependant), and a precision of 10m would be acceptable (5m good, 2m wonderful).

First idea: buy a low-power GPS chip (Skytraq Venus 6 or u-blox 5)
But they have a long long long cold start time to download ephemeris.
I made a PCB with Skytraq Venus 6 GPS IC and I'm able to store position on a memory and transfer logs from card to a PC.

Second Idea: store only raw data and post-process it in combination with ephemeris downloaded from a web-server. This solution soons very good for me but it going hard...

I have some questions:
- Could this second solution works?
- Assuming I will use a u-blox LEA-5T GPS receiver, and store raw measurement every 5 or 10min, how often are refreshed the RXM-RAW fields? How much time should I wait between GPS start-up and first complete raw measurement with all sats?
- What post-processing software should I use (I'm looking for a free DLL or free C/C++ source code)

Thanks!

[dnchayes]

Logging the raw data and processing it later is doable but a significant amount of work. Most of these modern "one chip" and "module" GPSs do the processing for you. The newer ones have very fast "cold start" times so the power isn't such an issue. I suspect that successful off-line processing will still require the receiver to be on long enough to figure out which satellites are up and to track them.

If you want to do it the hard way, there is a fair amount of documentation in the form of published papers and reference material, including serious books. You could start your self-learning process with these links:

A high level article in GPS World magazine (the technical trade rag of the GNSS business:
http://www.gpsworld.com/gps/the-softwar ... ?page_id=1

A PDF paper describing a Software Defined Radio (SDR) GNSS (Global Navigation Satellite System e.g. more than just GPS satellites)
www.sdrforum.org/sdr08_papers/1.1/1.1-1.pdf

-Dale

[astrodanco]

I'm doing this with an EVK-6T (LEA-6T chip) and RTKLIB which is free and open source (google it). I'm logging data to a file with u-center. I run both u-center and the RTKLIB applications on Windows 7 64-bit. RTKLIB has a converter to translate files containing u-blox messages into RINEX. It also has a program to post process the RINEX files. You just need to log RXM-RAW and RXM-SFRB. The RXM-RAW messages are transmitted at the rate you select with the CFG-RATE message. The RXM-SFRB messages are transmitted as the actual data is received from the satellites (several per second). I post process using data from CORS and SOPAC sites.