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Old 04 August 2008, 17:08   #1
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GPS wipeout by ferry!

Just spent 2 glorious weeks cruising the west coast of scotland. One thing I noticed (it happened 3 times) was that when I passed close in front of a ferry or other large ship, the GPS stopped working!

It's a lowrance 3500C Plotter with the remote LCG-2000 antenna (which is the actual GPS receiver). When I got near the front of said ship, I got a position lost message followed by a "cannot communicate with antenna"

I can only assume that the radar on the ferry caused the antenna to lock up! I know they have powerful radar sets but never thought they could cause sufficient disturbance to cause latch-up of a GPS antenna!

Anyone else seen this problem?
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Old 04 August 2008, 17:56   #2
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Normally when you get too close to something big you start loosing satellites and might lose lock if you get too close. Radar and GPS operate on widely differing frequencies, so interference should not occur, although mounting a gps antenna directly in the beam of a radar is not good, but this is then more down to massive RF power beiing forced in!

Never seen a unit say it cannot communicate with antenna as you are seeing unless there was a fault and then it tends to be permanent.

If you get the opportunity to try this again, get the satellite page up prior to getting close and see if the unit is just giving a duff message and should really be telling you that it cannot see where it is. You should see the same thing if you get up under a cliff or sea wall, but without the possibility for interference, if you do not the I suggest you speak to Lowrance and ask their opinion.
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Old 04 August 2008, 18:05   #3
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I don't think I was close enough for the superstructure of the boat to cause any sat visibility issues and each time (it happened 3 or 4 times), the unit stopped working when I passed in front of the boat / ferry.

If it was the boat blocking the sats this would indeed cause a loss of position but would not cause the communication problem between the Antenna and GPS. Doing a power off / on solved the issue which is why I think it was the radar effectively causing the antenna to lock up. I believe that ferries introduced some form of low looking radar as well as longer range so they could see boats close to the bow and assume this must be the culprit (ie radar zapping the antenna).

Interesting!
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Old 04 August 2008, 18:07   #4
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I remember there being a case in one marina where nobody could get sat lock - turned out to be an active tv aerial............
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Old 05 August 2008, 06:56   #5
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Is the LCG-2000 getting the voltage directly from the 3500c or from the battery? You can see that by the wiring.

I have the 7000c with the same problem. Questions asked at several forums in the USA and Lowrance. Answer was: the new areals have the voltage directly from the battery. The older ones (my areal is the LGC-12W) from de Lowrance unit itself. Sometimes the voltage for the areal drops to 11 volts which is not enough.
New areals don't fit the older units.

So I cleaned out all the connections, sprayed contact spray (not WD40!) and made a direct connection from battery to 7000c.
The voltage in the 7000c is now 13,9 and on the connection to the areal 11,87. And that works fine.
When the voltage on the screen of the 7000c drops to 11,6 the connection is gone. The areal gets even less at that moment.
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Old 05 August 2008, 07:13   #6
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Quote:
Originally Posted by al40 View Post
when I passed close in front of a ferry or other large ship,
Nothing to do with GPS, but can't help commenting that this is not generally recommended - it's just not a good place to be
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Old 06 August 2008, 14:17   #7
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Originally Posted by MarkM View Post
Is the LCG-2000 getting the voltage directly from the 3500c or from the battery? You can see that by the wiring.

I have the 7000c with the same problem. Questions asked at several forums in the USA and Lowrance. Answer was: the new areals have the voltage directly from the battery. The older ones (my areal is the LGC-12W) from de Lowrance unit itself. Sometimes the voltage for the areal drops to 11 volts which is not enough.
New areals don't fit the older units.

So I cleaned out all the connections, sprayed contact spray (not WD40!) and made a direct connection from battery to 7000c.
The voltage in the 7000c is now 13,9 and on the connection to the areal 11,87. And that works fine.
When the voltage on the screen of the 7000c drops to 11,6 the connection is gone. The areal gets even less at that moment.
That's a very good point! The voltage drop on the instrument itself (through it's own wires) is significant - 0.4v between bright and dim on the display so I guess the antenna will also suffer. Mine is connected directly to the instrument. I'll maybe have a quick chat with Lowrance UK before doing this but it's a good suggestion
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Old 06 August 2008, 14:23   #8
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Originally Posted by SeaSkills View Post
Nothing to do with GPS, but can't help commenting that this is not generally recommended - it's just not a good place to be
It's all relative! Ferries are big so anything seems close. I agree that being directly in front of a ferry is not a good place to be but it's always been what I would deem to be "safe" proximities. When entering or leaving busy harbours or ports (like Oban for instance), the chance of meeting a ferry in the relatively narrow channel is high and even if off to port or starboard, you can still end up "ahead" of the ferry in terms of it's low level radar (without crossing in front of it's bow)! As mentioned, this happened 3 or 4 times. One of them I was on a morring eating fish&ships when the ferry passed in front of me!
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Old 11 August 2008, 11:39   #9
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Radar sets, especially larger units, have been been known to scramble (and in some cases, kill) GPS antennas. The older Lowrance units (early LGC-2000's especially) were prone to this (later ones were hardened a bit, I think; then they went to LGC-3000's.)

Don't know if any other manufacturers were subject to problems from this.


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Old 12 August 2008, 08:16   #10
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Originally Posted by jyasaki View Post
Radar sets, especially larger units, have been been known to scramble (and in some cases, kill) GPS antennas. The older Lowrance units (early LGC-2000's especially) were prone to this (later ones were hardened a bit, I think; then they went to LGC-3000's.)

Don't know if any other manufacturers were subject to problems from this.


jky
I wouldn't be suprised - it can be a pretty powerful signal - look at the way TV broadcasts are often distorted when the reporter is near a warship or whatever.
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