GPS Coverage (Or lack of it)

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AndCur

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Hi Folks

Was sailing yesterday afternoon and switched on the GPS which picked up satellites as usual. Around 7pm repeater was showing no GPS signal so nipped down it to the cabin and the GPS had lost all satellites so switch it off and on but it would not pick up any signal. Thinking it could have been the GPS I got out my back up handheld and it would not pickup any satellite either. Around 7.30pm both sets sprung into life picking up around 8 satellites each. Was any one else out yesterday and experienced the same thing or could explain the loss of signal.

Thanks

Andrew
 
Hi
When reporting something like this it would be constructive for you to give your location as you might have been somehwere other than at home at Bangor.

The reason for this loss could have been due to local anomolies,weather, ie heavy rain clouds can effect signals, or system failure.

There is also concern re jamming hence the importnce of stating where you were!!!!

These strength of these signals have been compared with the energy generated whaen you stroke your pet rabbit so it takes very little to disturb them.

John
 
I have lost signals approaching Foulness point on the east coast and had very strange disruptions as well on a couple of occasions. Could the military operations be anything to do with it ?
 
FWIW try to approach within around a mile of any large American warship and your GPS will go completely haywire. Probably deliberate to jam incoming GPS guided hardware. Get in too close and the US Marine Corps will come and tell you where you are - and where to go - at gunpoint! /forums/images/graemlins/shocked.gif

On this occasion it was more likely your soft Irish weather? /forums/images/graemlins/crazy.gif You can get the same problem with Sat TV in bad conditions at times and they have much bigger more sensitive aerials than GPS.

Its just a reminder that brilliant though GPS is, it is not be relied on as the sole means of position fixing.
 
Chaos at times along the East coast...and even the VHF radio...Radio 2 etc. can be very poor.
The military does use some high powered stuff!

Phil
 
East Coast ...

When off Eat Coast north of Great Yarmouth .... the sweep of the Early warning Radar array can be seen clearly on a ships radar screen as severe spoking and "cloud" on screen ... plus the VHF has the constant cyclic hum as the antennae sweeps across you ... etc. etc. This was on commercial ships gear !

Close to a Warship when it has it's arrays etc. up and running can lead to similar.

As another says - heavy rain, snow etc. can disrupt signals - rare but can happen.
 
Re: East Coast ...

I have never experienced GPS signal loss in bad weather, thunderstorms or whatever. I have frequently watched for bad weather signal interruption but as long as I'm using a well located external antenna, no signal loss...
 
Re: East Coast ...

Bad weather shouldn't wipe out GPS reception- used it in some severely nasty stuff. Also, it's a big sky up there and the sats are not exactly close, if there's no signal in Bangor, I'd suggest, there's a far larger area without cover, but I'd be very surprised if this was the case. I can't remember the sites, but there are web sites with constellation/ephemeris histories that will show if there was a period of no coverage. Try google, I would, but am staying at folk's place and on dial-up. Aaagh.
 
[ QUOTE ]
FWIW try to approach within around a mile of any large American warship and your GPS will go completely haywire. Probably deliberate to jam incoming GPS guided hardware.

[/ QUOTE ]

Been there a lot and never had GPS problems. Besides - what do you think they use to navigate the ship?

Kevin
 
The military will still be using the GPS system BUT they have access to the P (precision) code which we mere mortals do not; so the C (coarse) code could be jammed but leave them knowing exactly where they are.
 
The P code is carried on both the L1 and L2 carrier signals. The C/A code is superimposed on the L1 carrier. If they jam the C/A on the L1 frequency, that will also affect the P-enabled military GPS systems. The US DoD could flash-up Selective Availability (SA) again without notice, but that would cover a segment of the globe, not just the area around the grey ships. The USN ships navigate with GPS; their helicopters and F/W aircraft use GPS; their remotely piloted vehicles and some of their weapons use GPS. So the chance that they jam it deliberately is pretty remote. I've spent enough time within a few hundred yards of major combatants to know that the SPS49 air search radar interferes with most electronics on the bridge, but have never experienced a GPS interruption. Maybe they've got some new kit that does a number on nearby GPSs - but I would bet they would have to shut it off when working with aircraft. IMO.

Kevin
 
Interesting article, but it doesn't really explain the mechanism to jam the C/A code alone. It still seems to suggest that the L1 carrier would be jammed and the military receivers would use the L2 carrier solely. Sorry but that will disrupt PPS - PPS uses the two simultaneously-transmitted P-code signals (on different frequencies) to calculate the actual refraction of the signal through the atmosphere. SPS, using only one frequency uses tabulated corrections to account for the refraction. That is why SPS is no-where near as accurate as PPS. Even if PPS receivers were able to determine that the L1 signal has been compromised and they switch to using the L2 signal alone, the accuracy will drop off by a factor of ten.

Kevin
 
one thing I'm pretty sure of, is that they are not going to go into details

On another note, have you seen the p receivers available commercially that are not for sale in US, that don't use p encryption explicitly, but work around the encryption. I've read descriptions of how they work, but still haven't totally come to terms with it.
 
P code is available but with no commercial guarantee for the future unlike c/a. This means they can (and will if they need to) change the code structure and you and I will no longer have a clue how to decode them. So we'd lose access
 
I've heard of commercial sets that use a trick to mimic PPS. I don't know how it's done, but if I was to guess I would say they take both the L1 and L2 signals, strip the C/A off of L1, use it for the rough position, then compare the remaining P signals (which are scrambled with the same cipher) from both L1 and L2 to determine the refractive error; apply that error to the C/A and you have a near-PPS position.
Of course, once the Yanks start putting GPS III sat's in orbit (2009?) then who knows what tricks they'll have.

Kevin
 
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