GPS Weirdness

[Roger Long]

When I installed my Garmin GPS 3205 chartplotter, I ran short of time before a cruise and just installed the antenna on a board strapped to the top of the wooden battery box cover next to the engine and under a cockpit seat. I expected some performance issues and figured I would just pull it out in the cockpit if I wasn’t getting a good signal. To my surprise, I had no problems with it all the first season, even in rainy weather with other people on board who were often sitting on top of it. I had a few lost satellite alarms but my handheld on the steering stand went off at the same time so it was the GPS system.

Two or three times this summer, I had strange behavior in good weather while on the boat alone. The GPS location would snap back and forth up to a quarter mile every 30 – 60 seconds creating a sawtooth track. Very disconcerting; especially when I glanced over at it once to see that I was in among shoals and rocks only to have it snap back to my real location. My handheld, running the identical Blue Charts version, was steady as a rock.

Anyone have an opinion on whether the antenna, which worked fine all last year, could be the culprit or is it more likely that something is going flaky inside the unit? The problem has occurred so infrequently that moving the antenna outside to see if it goes away isn’t a quick option.

 

[Witzend]

The loss of signal should not re-position your vessel, it should just not update your position. Once communications are re-established it will jump to the new position. I would suspect the plotter may be to blame, not the signal strength, unless of course your tacking or jibing every 30-60 sec and going 12-24 knots.

 

[kloudie1]

Interesting, Roger..Suspect it may have been momentarily losing one satellite of the several it was using which goofed up the accuracy?? Software update lately?? The little Garmin in the car sometimes shows a max speed of 275 MPH as it repositions from where it thought it was to where it is! Doesn’t happen often..

 

[Chilidog]

It may be an issue with the base chart software, especially if you are right on the edge where two paper charts join up. There are a few locations near my house where the GPS in my car does that. I suspect the issue is related to map projection errors that result in a poor match-up with the digital data.

 

[Rich Stidger]

Roger,

I had almost forgotten that I had exactly the same problem this summer with my Garmin GpsMap 492. I checked my ship's log but I failed to record the event and the date/time. Maybe I missed the entry. I'll check again more closely.

The event was just as you described. For a period of about two minutes I had the chartplotter screen jump back and forth. It was very disconcerting. Then as suddenly as it started, it stopped and the display was stable. I think it happened as I approached North Haven or Stonington from the east. The event never repeated.

My GPS antenna is mounted on my stern rail, and I have used this chartplotter for about 5 years. This anomaly has never occurred before now.

I also thought that I had emailed Garmin but I can't find a copy of that email- maybe it was on their website only.

 

[Roger Long]

I think it happened as I approached North Haven or Stonington from the east. The event never repeated.

Interesting. It happened to me going into Rockland and Swans Island which just about brackets where it happened to you. I think those were the only two events. Sounds like and underlying Garmin software issue to me. It's not the Blue Charts because my handheld running the exact same charts was stable.

 

[Tim R.]

I was in that area in September and using my newly installed 3206 with antenna temporarily mounted under the cockpit coaming. I did not see any issue though.

Make sure your firmware is up to date Roger.

 

[Rich Stidger]

Roger,

I went into Mapsource and pulled out the track and the point data. See the attached pictures. It occurred when we were entering Stonington.

This event began on 9/6/10 at 4:51:06pm at track point #526 and it ended at 5:02:02 with track point #545.

Does this match the time of your event? Maybe the GPS system had a burp? You said that your handheld was normal, but does your handheld show a map that would jump? If it wasn't showing your position on the chart you might not have thought it was affected.

 

[dparilla]

Possibly a time code error caused by a new constellation almanac. It is possible that one or more of the sattelites time code transmissions was in error either because of bad almanac data on your gps, or terrestrial jamming. If the bad code falls within a specified time delay (50ms) the gps will not disregard the erroneous information and thus a bad position may be plotted. Could have been any number of things to cause it, as exotic as a hull/isogonic interference pattern (electromagnetic field fluxuation caused by the boats motion over a large iron deposit) or as mundane as someones cell phone ringing or leaning against a wire on board. I would check to see if the chart says "unusual magnetic disturbances" in the area in which you were sailing, I seem to recall some, but don't have the charts handy.

 

[Bill Roosa]

GPS jamming

Is there a naval or air force base close to the area where it happened?
We get this on weekdays in the Bay when the navy tests missile guidance systems for "GPS spoofing." Seems the bad guys have figured out that GPS can be jammed with $10 worth of radio shack equipment and they want to make sure their missiles don't land where they are not intended. Check your NTM or google GPS and the date to see if testing was going on. Also the GPS system is getting old and satellites do occasionaly "loose sync." the Air force has assured the DoD that the problem is well in hand so we clearly have nothing to worry about.:Liar:
Which is why you don't want just one source of navigational data. Alas, LORAN has been decommisioned.

 

[Roger Long]

Maybe the GPS system had a burp?

Your track looks exactly like mine but I saw this in the last half of September.

My handheld is my primary track recorder and doesn't show anything strange. I also looked at it immediately when I saw the big unit jumping around.

 

[Rich Stidger]

This anomaly looks like it will be hard to solve. Different times, different places, conflicting GPS responses. And intermittents are always a nasty.

I'm pretty sure that there are no military installations near Stonington and I don't recall any magnetic disturbances either. I don't recall if the problem simply stopped or if I did a power reset on my GPS. I do know that my GPS feeds waypoint info to my radar and the bearing to my waypoint was also jumping around. So clearly bad data was being displayed and outputted.

I've been through this area probably six times with this gps/chartplotter and this is the first time I have had a problem.

How likely is this a common software problem with two different Garmin units of different ages? If there was a bad connection with the antenna I would expect to see the problem more than just once.

Have you contacted Garmin yet?

 

[TimCup]

Rich- here's the real question-

looking at your log, I'd like to know the secret to getting a 40 footer to go 25 mph!!


cup

Roger,

I went into Mapsource and pulled out the track and the point data. See the attached pictures. It occurred when we were entering Stonington.

This event began on 9/6/10 at 4:51:06pm at track point #526 and it ended at 5:02:02 with track point #545.

Does this match the time of your event? Maybe the GPS system had a burp? You said that your handheld was normal, but does your handheld show a map that would jump? If it wasn't showing your position on the chart you might not have thought it was affected.

 

[Rich Stidger]

looking at your log, I'd like to know the secret to getting a 40 footer to go 25 mph!!

Forget it. No way I'm giving up my racing secrets......

 

[Bill Roosa]

NTM: check the special notices section

http://www.navcen.uscg.gov/?pageName=lnmDistrict&region=1

there could also be a transmitter nearby that is throwing off out of freq signals that is interfering with the GPS. those sat signals are pretty low power and, as we found in a few of the latest war efforts, pretty easy to foul up.

The NTM has these GPS warnings quite frequently.