SparkFun Forums

[VStar650CL]

Got an intriguing question from a different department for all you GPS junkies. The OP in a different thread (here's the link, for those who might want the background: viewtopic.php?t=13805) wants to track freshwater turtles using a GPS/cellular combo. He chose the Telit GE-863-GPS, but it has power-hunger problems. After doing some preliminary engineering, the OP came across a tracker already being made by NavSys that uses an interesting trick for acquiring very fast GPS data. Instead of processing a full-blown fix and being "awake" for many seconds, it simply grabs a ~60mS "snapshot" of the satellites in view and saves or re-broadcasts that, rather than processing the data onboard. This of course results in huge battery savings.

The question is, does anyone know of a GPS or GPS/cell module that can do the same thing (or be "taught" to do it)?

[theras]

Hi,
Have a look at Geotate: http://www.geotate.com/en/products
Their hardware appears to be along the lines of that described by Maxim in this app note: http://www.maxim-ic.com/appnotes.cfm/an_pk/4237
It looks fairly storage hungry, as you need to store 128kB of data per 'fix', then you need some meaty processing to translate that into location coordinates. You also need to have a record of past GPS data.
It's an interesting concept. I'm not aware of any open-source applications that'll do the post-processing for you.
Hope this helps.

[VStar650CL]

Theras... just took quick look, but that Hopi reference design seems very interesting! Big thanks for the heads-up!

[jbeale]

Open source code does exist for GPS post-processing, I've used it... could refer you to a program if you want. It's designed assuming you have many minutes to hours of data, though, not a single fix.

No way do you need 128k for a single point fix without averaging (eg. only real-time GPS accuracy). I'd guess it is under 2k. You just need pseudorange, phase and delta-clock measurements from each of your satellites in view. You can download the absolute clock, real-time broadcast ephemeris and/or precise orbit info later.

GPS post-processing is almost always done from observables stored in RINEX format files, so I'd search for software that handles that format. A few years ago I played with one program "Kinematic GPS" which may no longer be available:

http://www.bealecorner.org/best/measure ... -test.html

This code still is online: "tdxl" TRIPLE DIFFERENCE CARRIER PHASE, STATIC
http://home.comcast.net/~dmilbert/softs/tdxl.htm

regards,
John