GPS magnetic compasses in the cold

[DougPaul]

Prompted by the discussion of a recent accident where the victim's GPS compass is reported to have "frozen" in the cold (http://www.vftt.org/forums/showthread.php?t=38528) and some private discussion, I did a bit of research on the topic and found the following:

Calibrate the electronic compass outdoors when you
first use the GPSMAP 60CSx. To ensure continued
accuracy, calibrate after installing new batteries,
after moving more than 100 miles (160 km), or
experiencing a temperature change greater than 60 deg F
(20 deg C). The accuracy of the compass is also affected
if the unit is not held level or you are near objects that
affect magnetic fields, such as cars or buildings.

You should calibrate the compass after
moving long distances, experiencing
temperature changes, or changing the
batteries.

Note: the 60CSx has a 2D magnetic sensor and the 62s has a 3D magnetic sensor.

So Garmin is implicitly stating that the magnetic compass systems are temperature sensitive. However, a recalibration after any significant temperature change should fix the problem.

Garmin gives an operating temperature range for the original eTrex Vista, the 60CSx, and the 62s of 5F to 158F (-15C to 70C), so this range probably applies to all or most of their hiking GPSes.

The Magellan eXplorist 710 has a rated temp range 14F to 140F (-10C to 60C). (I didn't check the manual to see if there was any additional relevant info or check the specs on any other Magellan GPSes.)

The DeLorme PN-60 has a rated temp range of -4F to 167F (-20C to 75C).

I expect that the manufacturers' position on use at colder temps is that proper operation is not guaranteed. (Neither gives storage temp ranges which are often wider.)

I have no info on what model GPS the accident victim was using, nor do I know the temp at the time of the reported GPS compass malfunction.

FWIW, while all my GPSes have magnetic compasses, I rarely use them. My preferred method is to transfer a numerical heading to/from my GPS and my mechanical compass (a Silva Ranger, but any good compass should do) and use the compass for sighting.

ALSO, remember to keep your compass away from magnetic materials (including the GPS) while using it...

While I have never observed this problem myself (and probably never will if I continue to use my mechanical compass for sighting), the person with whom I discussed this issue believes he has seen it when moving his GPS between a warm car and cold outdoors and in some at-home freezer tests. Recalibration seemed to fix the problem.

So I suggest that you recalibrate your GPS's magnetic compass after big temperature changes or use your mechanical compass to avoid this problem.

Doug

Edit: DeLorme PN-60 temp range added.

 

[DougPaul]

An informal experiment...

I just tried a freezer experiment to test the temperature sensitivity of the magnetic compass in my newer 60CSx:

Experiment 1:
* calibrate the compass at room temp (~68F, ~20C)
* put the GPS in the freezer (~0F, ~-18C) for 2 hours
* remove from the freezer
* rotate GPS 360 degrees noting the accuracy of the compass (test 1)
* calibrate
* rotate the GPS 360 degrees noting the accuracy of the compass (test 2)

Experiment 2:
* (immediately following exp 1) allow the GPS to warm up in the room for ~2hrs
* rotate the GPS 360 degrees noting the accuracy of the compass (test 3)
* calibrate
* rotate the GPS 360 degrees noting the accuracy of the compass (test 4)

The GPS stayed on continuously during the entire procedure so there was no question of whether power-cycling affected the calibration. NiMH batteries were used.

Tests 1 and 3 evaluated the accuracy after a 60 to 70F temp change from the calibration and tests 2 and 4 evaluated the accuracy after calibration at the current temp.

The results were "recorded" simply by watching the compass rose as the GPS was rotated through a full circle. The compass was hand-held during the rotation--while I tried to keep it level, I cannot guarantee that it stayed exactly level.* The deviation from north was ~+-20 degrees for tests 1 and 3 and ~+-5 degrees for tests 2 and 4. At no time did the compass rose "freeze".

* Holding the GPS on a flat level surface would have kept the GPS more level, but would have been a less realistic simulation of outdoor use. Calibration was also done hand-held. (The 60CSx has a 2D sensor and is sensitive to being tilted during calibration and testing.)


So 60 to 70F changes in the temperature did reduce the accuracy of the internal magnetic compass, but did not cause the compass rose (direction indicator) to freeze. These results may or may not extend to other GPSes in similar conditions.

Doug

 

[Remix]

At cold temperatures the LCD responds so slowly, it may be possible that the user thought that it froze.

As far as recalibration, that is to compensate for gain and offset changes in the circuit arising from high gain amplifiers.

 

[Trail Bandit]

I realize this is a bit off topic, but a really cheap "magnetic" compas does not need to be recalibrated, updated, or need new batteries and it will continue to work at temperatures outside the range of the average hiker. Sorry, but ti is a thought.

 

[DougPaul]

At cold temperatures the LCD responds so slowly, it may be possible that the user thought that it froze.

The accident in question happened on Nov 27. The minimum temp on Mt Washington that day was -2F and the min was ~20 degrees higher in Whitefield and Berlin. So the temp at the accident site was almost certainly higher than -2F.

My tests of my 60CSx showed the response of the LCD at 0F to be crisp and fast--essentially the same as at room temp. And the LCD display will not freeze as it gets cold--it will simply fade from one image to the next.

As noted in the accident thread, a quick search of LCD display specs shows lower temp limits from -13F to -67F. We do not know what model GPS the victim was using, so we do not know what its low temp limits were, but it is likely that the display would still work at 0F if it was a hiking GPS from one of the major manufacturers.

As far as recalibration, that is to compensate for gain and offset changes in the circuit arising from high gain amplifiers.

It isn't hard to design amplifiers that are stable over wide temp ranges, but it is possible. It is also possible that the magnetic sensor itself (most likely a flux gate or Hall effect sensor) is temperature sensitive.

Doug

 

[DougPaul]

I realize this is a bit off topic, but a really cheap "magnetic" compas does not need to be recalibrated, updated, or need new batteries and it will continue to work at temperatures outside the range of the average hiker. Sorry, but ti is a thought.

This strategy was mentioned in post 1 of this thread. It is also useful in situations where one wishes to get days to weeks of battery life by turning the GPS off between fixes.

And in a sense, a magnetic compass does need to be calibrated--the declination needs to be set on the compass (if you use true headings) or factored in externally (if you use magnetic headings).

Doug