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Old 19-03-2009, 09:30   #1
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"No GPS Fix" Alarm

This appears to be an issue not yet addressed on this forum.

Twice in the past several months my Furuno Chart Plotter has sounded "No GPS Fix" alarms. Sometimes the alarm condition persists for several minutes and sometimes it clears (i.e. fixes resume) right away after pressing the alarm clear button. The first time this happened we were in very rough seas (8+feet) so I attributed the problem to loss of clean signal because of antenna motion (I saw a similar problem on one of my submarines at periscope depth in a hurricane trying to copy a navigation satellite [pre-GPS] pass). The second episode occurred Friday, 3/13/2009 in the relatively sheltered lagoon off Belize City with only 1-2 foot waves.

Has anyone experienced anything like this? If so, what was the problem and how did you fix it?

Any other ideas from you electronics wizards out there?

I have not started serious troubleshooting yet because, other than checking for loose connections or damaged cables, there isn't much I can think to do without replacing very expensive components one at a time.
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Old 19-03-2009, 10:11   #2
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Where is your antenna located?
It doesn't take a lot to kill the signal.
One layer of duct tape or even your hand over the antenna is usually enough to do it.

Steve B.
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Old 19-03-2009, 10:17   #3
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John,

Loss of GPS fix can occur for several reasons, some of them not involving your onboard setup.

For example, the USG conducts "GPS receiver-jamming tests" periodically. Here's what they say about this:

Quote

Overview of the US Federal Government's Policy on Activities Which May Cause Interference to GPS:
On occasion, the US Federal Government is required to conduct GPS interference tests, exercises and training activities that involve jamming of GPS receivers. These events go through a lengthy coordination process involving the Federal Aviation Administration (FAA), the US Coast Guard (USCG), the Department of Defense (DoD) and other government agencies.
Unquote


Timing errors can occur with an individual satellite, which can cause erroneous fixes or loss.


RFI, some generated by solar activity, could interfere with the received GPS signal.


And, of course, your onboard equipment could suffer a malfunction.



If you have checked your installation and are sure that:


1. you have clean power to the unit (tight connections, adequate voltage, etc.); and


2. your antenna connections are also clean and tight, and your antenna is in the clear (has a clear view of the sky all around) ...


then, you've done about all you can do onboard.


It would be helpful if you had a second GPS aboard (as everyone should have IMHO...they're cheap enough these days) to check and see if it, also, experiences signal loss. If not, then you know it's your main GPS setup. If so, then not to worry -- some things are happening which are beyond your control.

Just, maybe, it's time to polish up your piloting and navigational skills...you know...all that ancient stuff mariners used to fool with before GPS dumbed us all down :-)


Bill
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Old 19-03-2009, 10:30   #4
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Have the same problem with my new raymarine system only it is regular. Message with a beep says 'fix lost' and you press a button and sail on for anything from 1 to about 15 minutes then the fix comes back. This happens 4 to 5 times in a 3 hour passage. Am dealing with the agent who says it is usually a power problem while the installer says it's a faulty GPS. However, the other day I spoke to 2 other people at the marina and they have the same problem. They think it is a satelite problem. Waiting to resolve it one way or another.
Garry
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Old 19-03-2009, 11:00   #5
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My old raymarine RL70 system does the same thing. Since the GPS and chartplotter are separate devices talking across a buss one has to wonder if there is a problem with the communications. I'll look at the satellite status on occasion when this is happening and it sometimes shows 7 or 8 "In Use". It only requires 4 for a 3D fix or 3 for a 2D fix. You would think that since my boat seldom gets much above sea level that three would do.
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Old 19-03-2009, 12:27   #6
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I second Bill's advice. You should also consult your manual - there might be a clue to what gave the "No Fix". Some systems will default to "No Fix" when there's poor geometry. To be clear, just because 7 or 8 sat's are visible doesn't mean they'll give a good fix. The worst case is when they are all clumped together overhead or all in a line. If the angle between satellites is too narrow, then the LOPs (or Spheres of Position in 3D) overlap each other - which results in a vague position. These situations can clear up quickly or linger for some time - just depends on which satellites are visible and what their relative motions are.
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Old 19-03-2009, 12:51   #7
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Ha! To all the raymarine users: check if your GPS or GPS-sensor is a model that is currently sold by RM. If it's not, you have a flawed unit (the whole type is flawed, not just your unit). I went through a RN-300 and three (3!) sensors before giving up ans installing a cheap Garmin and a very good Furuno GP-32. Problem went and never came back.

The strange thing with the RM units is that they work much better when fed 24V instead of 12V. This might well be why some never have trouble (24V house battery system).

Now, the way to check what's going on is to switch to the satellite info page. This is every GPS with a LCD but also on many plotters if you have just a GPS-sensor without LCD. Here you can see what's happening. Most RM units show satellites are there, signals are okay but fix is lost. It is a firmware bug. The last firmware they published solved that bug but would just switch off my RN-300 unit instead of loosing fix. I think the reason was that the CPU couldn't handle the firmware and they dis-continued the product, replacing it with a type with faster CPU.

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Old 19-03-2009, 13:02   #8
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I must add that neither my Garmin nor my Furuno ever ever lost their fix. I first used just the Garmin next to the RM and the Garmin would just continue updating my position while the RM went into No Fix every couple of hours. This would take up to 15-30 minutes but when you reset it, it's giving you a fix within a minute again!

Also, RM acknowledged the bug.

cheers,
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Old 19-03-2009, 14:42   #9
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I had this type of problem and fixed it


It might NOT your GPS but you DGPS alarm.

Differential GPS is not used anywhere but the USA so those in Belize or AU will get it at times.

Look under the Alarms and untick the DGPS

Hope this helps because its annoying and damn scary!!!!


Mark
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Old 19-03-2009, 15:52   #10
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Yes, I have experienced the same issue with my Raymarine GPS. I have Raystar 125 connected to an RL80c. It only happens occasionally and it has not persisted for longer than 20 minutes. The challenge is what happens when I am on Autopilot and this happens. No fancy tracking!
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Old 19-03-2009, 16:23   #11
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Our primary GPS is an old 1996 vintage Autohelm (now Raymarine) one which we have operating for around 150 days a year. Very occasionally, about twice a year, it reports a loss of fix but that has always been due to poor satellite geometry and it corrects itself within a few seconds by aquiring a different set. Most often it occurs if we are tucked up among steep hills so less of the sky is visible. For reasons I don't know, unless probability related, this has never happened while we have been underway.

We have never suffered from any of the following but all other causes are on board problems such as interferance from other equipment not meeting radiated emf requirements expected of marine equipment (eg active antennas for domestic entertainment equipment such as TV), poor cabling practices (if NMEA follow the recommended NMEA wiring practice, many don't), poor installation (eg antenna does not have a clear view of the sky) including electrical supply, faulty GPS receiver, faulty instrument displaying data from it or a user GPS set up problem giving spurious alarms (eg along the lines MarkJ has suggested).
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Old 19-03-2009, 21:04   #12
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Right there with you on the Raymarine Loss of Fix with my C-series. Quick fix? Turn the GPS sensor/plotter off and back on, and your fix will be back a few seconds later. We have had this happen many times, and I'm glad (well, you know what I mean ) to see other Raymarine owners with the same issue. By the way, we're running the latest greatest version of code, so it's not fixed there. Now, if I could just get rid of the flickering screen...
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Old 19-03-2009, 21:43   #13
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The same thing happened to me three times sailing out of Placencia in January. Maybe there is something about that location? It has never happened to me anywhere else.
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Old 19-03-2009, 21:44   #14
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Oh and Labatt's solution worked for us too. Turn it off and then back on and it was OK though it didnt work right away every time
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Old 19-03-2009, 22:01   #15
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Get any decent handheld GPS (if nothing else, that can be your spare against the day you take one lightning strike and fry all the main electronics, or used for the ditch bag) and when the primary system goes belly-up, turn on the handheld.

That will tell you if the problem is in your equipment, or the GPS "system" in general. As will the satellite information page. If the handheld shows there are birds in sight and working, but the primary system says there aren't--then the primary system is not getting a signal, and the problem is in the primary system (antenna, wire, firmware, whatever).

Third and fourth generation units will come up from cold to a fix in all of about one minute, making them easy to keep powered down cold but quickly available to confirm whether the primary is working, or someone has knocked down a dozen satellites.

If both units say "no birds" then there are still two possibilities. The satellites might really not be covering your location (shouldn't be a problem unless you are in the polar regions!) but it might also be local interference. GPSes work with an extremely weak signal and IIRC there are a lot of devices in the 2-GHz range that can easily interfere with them, typically someone with a Wifi, wireless phone, several types of radios, other transmitters that may have been pushed too far, i.e. by another sailor tweaking them for too much range, or simply defective equipment. THAT kind of stuff may be very hard to track down--but if you can take the handheld 1/4 mile away and it starts working again, you know it is local interference.
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