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Old 03-09-2017, 17:06   #1
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Random spikes in GPS track

Has anyone else experienced a random spike in their GPS track? In my case the spike is a line from my present position to a location hundreds of miles away - over land or across oceans - and then back. It represents a split second deviation. The primary GPS is a Garmin 19X NMEA2000 and is two seasons old. I disconnected it and used only the built in GPS in the Garmin 7612 but this did the same thing. Because both GPS instruments produced spikes I've eliminated them as a problem. If this was a network problem I think I should have other devises on the N2K network with corrupted data, but I don't see this. Does anyone have any suggestions or solutions. Thanks, Rich
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Old 03-09-2017, 17:16   #2
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Re: Random spikes in GPS track

Check out this thread.
AIS..how accurate is yours???
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Old 03-09-2017, 18:38   #3
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Re: Random spikes in GPS track

Deep friz, Thanks for the link to the other thread. Their posted screen shots are the same as what I see using Coastal Explorer software. However, the spikes I have extend for hundreds of miles and back to the current position. Since it happens with my two completely different GPS units something else is going on. It's not a result of GPS placement or a bad wire. Unlikely both are bad in the same way. If the problem was a defective satellite then commercial shipping would be raising hell and it would be front page news. In addition, my inReach mapshare page showed a similar spike this summer. I'm at a total lose here.
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Old 03-09-2017, 22:43   #4
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Re: Random spikes in GPS track

I've seen the same thing (hundreds of miles jump for a single fix) a couple of times over the years, I suspect that a combination of unusual atmospherics and satellites low to the horizon may be an explanation.
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Old 04-09-2017, 16:25   #5
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Re: Random spikes in GPS track

While I am not particularly knowledgeable or well informed, it is my understanding that the satellites providing the GPS information belong to the United States or Russia. Each nation has retained the ability to "mess with" (my term) the signals from its own transmitters. Supposedly, and allowing for the occasional denial of the policy, this will only occur in case of armed conflict (I guess anywhere from a war to a skirmish). Nonetheless, it is difficult to believe that this does not occur at times, either by the sponsoring nation, or by those experimenting and testing ways to interfere with the systems for their own purposes. Space aliens are another possibility. Oh, did I mention old fashioned glitches, and even semi-permanent interference at selected locations and areas? No, I am not attempting to explain or excuse any recent incidents involving US Navy vessels....
All of that said, I have encountered/observed several anomalies -unexplained, but definitely there- over maybe 20 years (since I gave my Magellan Loran to a friend who had more power available) while using various GPS devices. There is the possibility of others that did not come to my attention, but that would be speculation.
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Old 04-09-2017, 16:48   #6
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Re: Random spikes in GPS track

Where is your GPS antenna? Keep it low so it doesn't measure any rapid pendulum or rate gyroscopic movements.
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Old 04-09-2017, 18:16   #7
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Re: Random spikes in GPS track

GPS is located on stern rail. I found that the Garmin 19x NMEA2000 has a Speed filter that is by default set as "Auto". The speed filter or smoothing only kicks in if the boat is moving fast. For a sailboat it means the smoothing will never turn on. I can go into the setup in the Garmin 7612 and change the setting from Auto to ON. I wonder if this would eliminate the transient spikes? Will try this next time on the boat and report back.
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Old 04-09-2017, 21:32   #8
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Re: Random spikes in GPS track

Does anyone here actually believe GPS oddities only occur on boats? Not automobiles, aircraft, drones, missiles, and perhaps earth movement sensors as well? The forum is about boats, but real life experience for some of us is broader than that, and perhaps tends to confirm what occurs to us on bodies of water. Does anyone think anomalies do not occur and are readily explained by the lay person when identified as such? Although I frequently used cell phones (so called smart ones) and have had GPS on boats for over 20 years (handheld and installed) and built into cars for over 13 years, I have only knowingly observed the plainly false information on boats (installed and handheld) and in cars (on handhelds), and then only briefly and inexplicably (not poor satellite angles).
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Old 07-09-2017, 18:57   #9
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Re: Random spikes in GPS track

This is a software bug in your plotter's software. I'm guessing you jump to a position off NW Africa and back again?
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Old 14-09-2017, 09:39   #10
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Re: Random spikes in GPS track

Every once in a while my boat starts to accelerate to a few hundret knots and take off to somewhere... sea or land doesn't matter. This happens after the GPS has been powered up for several hours and lasts for a few seconds to minutes

I notice this because I get high wind alarms on the instruments: last time I did 460kn in 475kn of true wind

I checked position & course on the DSC VHF which is also connected to the GPS puck. Same readings. My tablet's GPS is giving reliable position at the same time. So I think the problem is within the GPS puck.


I ordered a new AIS transponder with integrated GPS to replace the existing GPS & AIS receiver. Will fit in two weeks and see what happens
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Old 14-09-2017, 11:17   #11
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Re: Random spikes in GPS track

If you dig around online you'll find the specs for the civilian signals on the GPS system. The system is NOT intended to be 100% accurate 100% of the time, but rather "within xx meters ##% of the time". Yes, outliers are normal from time to time. This is why people sometimes drive off ferry docks, they don't understand the normal limits of the system.

When LORAN-C was a "new" and terribly pricey thing for sailboats, we were racing in LI Sound one afternoon. Skipper hollers at me, what's the course to the next mark? Because he needs to figure out how to call the rounding. I said "Wait a minute" and he says don't tell him to wait, tell him the damn course to the next mark.

"Okay, the LORAN says bearing 270 degrees, distance about a thousand miles. I think that's somewhere in Kansas. Now, will you wait a minute?"

These things are normal. You have to know when to suspect there's a problem. And what do they say? The prudent mariner never relies on just one source of information, if they can do better?

Kansas...yeah. Going to be a l o n g tack.
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Old 30-09-2017, 07:08   #12
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Re: Random spikes in GPS track

Yes, the spike I see extends from the East coast to off the west coast of Africa. Another common spike is straight south for 4000nm. During my summer cruise I saw several spikes in the local area for 50nm in a triangular configuration ending up back at the boat. Since this happens with two different GPS units I'm thinking it might have to do with the NMEA 2000 gateway or some other offending device on the network that corrupts the GPS's PGN.
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Old 30-09-2017, 07:15   #13
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Re: Random spikes in GPS track

Thanks, Interesting! Please let me know if the new VHF/AIS/GPS unit solves the problem. This is a problem that takes a very long time to troubleshoot because the phenomena does not happen frequently enough or on demand. One mistake I made was not keeping a log for each change I made and the result it produced. Now I find myself repeating what I already did a few months ago - chasing my tail to try and find the offending device on the network.
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Old 09-10-2017, 10:27   #14
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Re: Random spikes in GPS track

When this happened to me, I found that my software was rejecting a particular fix and replacing it with zeros for the lat and long. Thats why we always spent a millisecond under the bulge of Africa at 0 lat 0 long, and then back again, the SOG was impressive, we clocked 2400 knots at one stage.
Problem resolved by enabling checksum under OpenCPN.
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Old 09-10-2017, 10:36   #15
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Re: Random spikes in GPS track

The radio signal of GPS satellites has to cross the ionosphere. That layer is under the influence of earth magnetic fiels and solar radiation, read solar flares. This can delay signals crossing the layer, but it is also possible signals get reflected and you get loss of signal or double inputs in your GPS equipment.
Conclusion: spikes are part of GPS life
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