Does your GPS get confused?

[lw395]

My Garmin GPS 60 has made mistakes in the past. I was once sailing at about 4knts a few miles off the Great Orme in North Wales, when my Gps was showing my track at 2300Knts over the Balearic's.
A few moments later it showed me doing 140knts a hundred miles off the west coast of Ireland.!!

Meanwhile my Garmin 292 was showing my correct speed and position.

I still have no idea why it did that. It's been reliable ever since.

That could well be jamming. The newer receiver might not be affeected due to tracking more satellites.
Or a circuit fault in the receiver, something causing the crystal reference to go slightly off frequency momentarily, like a dry joint or cracked component.
Possibly as simple as a bit of moisture inside?
Or even strong AC magnetic field?

 

[Hydrozoan]

I do not rely on the laptop plotter (or any other) GPS, and frequently check it against the fixed GPS - with which it agrees closely. Very occasional spikes I can attribute to a glitch, but in the case I mentioned the plotter showed a smooth turn with several updated positions on the new track. It was a good few years ago and the GPS puck has since been upgraded (and the plotter software of course, many times), so as it has never recurred it just remains a slightly niggling curiosity.

 

[Danny]

What's weird is that some people navigate with dysfunctional junk like this!

Time to confess: I wasn't navigating! I was on a cruise ship waiting for dinner time. I managed to get the GPS dongle to track (just) by sticking it out on the cabin balcony but it was dropping out occasionally. I suspect this was due to the proximity of all the metalwork and the restricted view of the sky! It was interesting that I still got a largely reasonable set of track points although when I looked at the NMEA trace data (it's quite boring at times on a cruise ship) I could see that there was considerable "jitter" both in position and SOG. The huge anomalies shown on the trace occurred over a 6 minute period and corresponded to a period when the GPS was dropping in and out for whatever reason.

In view of the similarity with the OP's symptoms I'd guess the problems were just due to poor reception rather than any equipment malfunction.