garmin gps 72 and gpsmap 60csx, sensitivity and accuracy

[jazzist]

We have a Garmin GPS-72 that works as expected. We are considering purchasing a newer unit of the same vein, e.g. gpsmap 60cs/csx alike. I am connecting the gps 72 to a laptop via a serial cable. We're using the GPSD software in Linux, and configuring the GPS-72 to output NMEA.

My question relates the the sensitivity of the GPS-72 versus newer units. We experienced problems with a new USB GPS mouse from Globalsat based on the SIRF III engine, whereby it seems to report it has lock when it doesn't have much confidence in that lock - perhaps a caveat of the much increased sensitivity. The problem with this is that when we plot a track, based on the recording of the NMEA data stream, we get a large deviation from the actual location - the track plotted looks 'noisy'. This led us to trying our GPS-72 for the same task. It was better, as it only seemed to report it had lock when it really did, so we didn't get the lock on/off noisy erroring caused by the the GPS receiving signal from satellites that it really shouldn't (reflected from the environment/buildings?). We got 2D lock more than 3D lock in urban canyons, but the data plotted was much more accurate and "clean". We were thinking of changing to a more modern version of a Garmin handheld GPS as we thought that perhaps we would get the same stable behaviour with an increased sensitivity of a newer GPS engine, i.e. less gaps in our track plot caused by momentary total loss of lock. The newer units have a different GPS engine than that contained within the GPS-72, the same SIRF-III as our Globalsat mouse. Would any increase in sensitivity cause similar problems that we saw with the SIRF-III based mouse device described, or do all of the Garmin units use a similiar type of filtering before output to stop erroneous measurements (perhaps this is not filtering but just the insensitivity of the GPS-72 seeing only good in line of sight satellites which led to better accuracy but less lock consistency?).

[ernieb]

I'm not qualified to answer your question about outputing from a 60CSx, however, when I imported my track log into my Anquet software the track was very very detailed - confusing even. I then reset the GPS to make a track point every 100metres and the effect is dramatic. The track is very clear and consistent with the paths I have walked as indictaed by the OS 1:25K mapping I have installed on my PC. I connect to both my laptop and PC using the USB interface. I do not have a serial lead so have not tried this output.
The track points can be set for distance or time from the menu. The GPS is very sensitive and gives good locks in all sorts of difficult places.

ernib

[jazzist]

very interesting, thanks.

I will have to take a look through the manual PDF and see if it states whether or not this can be set for the serial output.

[jazzist]

It doesn't seem like you can do this for a serial output.

One thing I noticed when configuring our GPS 72 for gpsd was the advanced options of the serial output.

You can choose from garmin, nmea, etc, but there is a sub menu for some including nmea, this allows you to choose particular things to output with the nmea sentences. One of these had to be checked to get it to work, i believe it was the top box which was called track/way points or something along those lines. When you check it you get a warning that if you enable this option you'll only get updates approx once a second.

Does the 60csx have the same options? The screen shot of the serial output settings menu where you select the output type, looks the same from the picture in the 60csx manual, but I wonder whether or not if it has the same advanced sub-menu as the 72.

Perhaps it's averaging over the second which the globalsat mouse doesn't (I guess it is just spitting out what it can..), and perhaps this is why my plotted track is so much better.

Hmmm!

I would rather not purchase the 60csx in order to find out, but garmin are not replying to emails.
I will try faxing and calling them tomorrow, I guess.

Or, I could buy one and then modify our software to take a downloaded track file that you describe, then I can change the distance between points.

hmm!!

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