Comments on: Real time gps data logging

Question: Real time gps data logging

445supermag — Thu, 23 Nov 2006 13:43:07 -0800

Is there a program (preferably free) that will allow me to log real time speed, time and (possibly) altitude data from my gps on a laptop pc, and save this data as text?

I can log realtime time and long./latt. data from my garmin 12map on my pc laptop with delorme's topo 3D program, but I want the other data that is also recorded, like speed and altitude. Plus I want it in a text file so I can plot it later. The reason for this is to be able to get auto performance data, for instance, if you plot the speed vs. time of a car after you put it in neutral and let it drift to a stop, you can calculate the drag due to wind resistance (goes with the square of velocity) and the rolling resistance of the tires and other parasitic drag forces (1st and zero order with velocity). Altitude data is nice to see if the road is angled (hard to find a perfectly flat road).

By: flabdablet

flabdablet — Thu, 23 Nov 2006 15:13:09 -0800

The raw data stream that comes out of your GPS is already in text form, so you should be able to save it with any serial terminal program. If your laptop's running Windows, you already have HyperTerminal.

By: Jimbob

Jimbob — Thu, 23 Nov 2006 16:54:42 -0800

This python script can probably be hacked pretty easily to return the data you want in a nice, clean format. However, I should note that for your purposes, I don't know if a non-differential GPS is accurate enough. A car put into neutral and let roll will be going at a fairly low speed - with a normal GPS only accurate to 5-10m, I don't know if you would get a useful speed reading.

By: nomisxid

nomisxid — Thu, 23 Nov 2006 17:34:49 -0800

FWIW, unless you have a really old GPS system, it likely uses one of the private systems that help supplement and overcome the 'natural-non-military' GPS inaccuracy. When I go driving with mine, its speed reports match what I'm clocked at by those "your speed is xyz" roadside radar stands; my car's speedometer, on the other hand, reports my speed as 5-15 mph faster, less accurate at higher speeds.

By: schwa

schwa — Thu, 23 Nov 2006 18:34:17 -0800

Garmin's don't speak NMEA unless manually switched to that mode. Newer Garmin's only speak the garmin specific protocol.

But anyway, try http://www.gpsbabel.org/

By: flabdablet

flabdablet — Thu, 23 Nov 2006 19:08:40 -0800

445supermag's Garmin does, according to its manual.

By: 445supermag

445supermag — Thu, 23 Nov 2006 19:18:56 -0800

However, I should note that for your purposes, I don't know if a non-differential GPS is accurate enough. A car put into neutral and let roll will be going at a fairly low speed - with a normal GPS only accurate to 5-10m, I don't know if you would get a useful speed reading.

The technique is to bring the car up to speed and then let it drift. For instance, if you bracket the speed you are interested in, say 65 mph, you take the car up to 75, put it in neutral and record the time it takes to get from 70 to 65 and 65 to 60. With some math you can get the total drag and the horsepower required to keep the car at this speed. I'd like to fit all the way from 75 to 0 so I can figure out which part of drag is due to rolling resistance and which is due to wind drag.
I got hyperterminal to work, but the data needs a lot work (altitude in meters, speed in knots?!). I was going to write an AWK script (yes, I know I'm a dinosaur), but I'll check out that perl script.

By: flabdablet

flabdablet — Thu, 23 Nov 2006 19:21:40 -0800

Garmin documents its proprietary formats, including proprietary extensions to NMEA, here.