GPS SOG and COG

[Chris_Robb]

With the switching off of Selective availability last year I expected to get an accurate Speed over Ground and Track over ground, as a result. My GPS still displays a wildly fluctuating track and speed. Following Vyv Cox's comments on an earlier thread - saying that it should give good and reliable readings, I contacted Garmin, to ask whether the unit could be faulty. (Garmin 120XL 4 years old) This was the reply:

On the older units, the SOG and COG were not averaged but dispalyed the result of each calculation. VMG (Velocity Made Good) however is averaged, and consequently shows a steady reading.

This explains why I get readings varying every second, whilst the VMG shows a steady figure.

Solution is easy - Get a more modern unit!

 

[vyv_cox]

Far be it from me to argue with Garmin, but I'm not sure this is correct. I have had a Garmin 75, bought about 10 years ago, GPSMAP 225 about 5 years ago (I think that's correct, the monochrome one) and now a GPSMAP 215. Both of the previous models could be set to provide various update averaging times. As standard when first received, the SOG and COG varied considerably, but by changing the averaging period ("not recommended" in the manual) I was able to achieve a reasonable result, although it was obvious that both figures took some time to come back up after a change of direction.

On the GPSMAP215 this function is not present, so I'm stuck with the averaging period that is built in. I spoke to Garmin in UK about it at the time of purchase and they confirmed that this is fixed. The effect in practice is that in a seaway the COG jumps about considerably but the SOG is pretty constant. Sailing downwind in big waves the variation can be seen but otherwise the update period exceeds the speed variation, so it reads a constant figure. Since the average COG is shown as a trace on the map it is not too much of a problem unless relying on the cockpit repeater.

 

[vyv_cox]

Garmin 120 XL update averaging

I just downloaded the manual for your GPS from garmin.com. Instructions for changing the averaging period are on Page 42. From memory I made the update time 2 minutes, although to be fair it was five years ago. Suggest you experiment to find a reasonable figure.

 

[vyv_cox]

Re: Garmin 120 XL update averaging

Sorry, meant 54 not 42 - Speed filter.

 

[Chris_Robb]

Re: Garmin 120 XL update averaging

Thanks for the info.
Main reason for wanting more accuracy was to check the log. I'll have a go this weekend - probably could average over 15 seconds with the increased accuracy from SA switch off.

Thanks once again