GPS signal

[Mac3]

I was out on the weekend and my GPS took about 30 minutes to get a fix, and that was only after restarting the gps via the menu a couple of times. I have a Northstar 557 and my pal was in his boat with the same model and commented that he couldn't get a fix either. After that, no problem. The nest day I was in the working port area and the signal actually dropped out. We went out on our trip and came back and as I passed the same area the signal dropped out again. Can your government switch the birds on and off at will?
cheers
David

 

[NYBo]

Re: GPS signal

I don't know about turning them off completely, but I know they can reduce the accuracy in some manner. The method escapes me at the moment. :redface:

 

[ssobol]

Re: GPS signal

The US military can degrade or shut down the GPS signal. The system was originally designed for military use. However, since so many things now rely on GPS for navigation it is unlikely that this was the cause of the signal loss.

The signal is very low power. It can be affected by solar activity and local RF interference. In the US there is a company called LightSquared that provides internet access via radio signals. This has been demonstrated to interfere with GPS reception.

Depending on the satellite constellation arrangement (visible satellites) and having some satellites out of service for maintenance (or failures) there are times where the satellite coverage may be poorer than other times for a given location. In some remote places were aircraft relay primarily on GPS for navigation (e.g. Alaska) there a recognized times where flights are not allowed to fly into certain places due to reduced GPS reception because of the constellation configuration.

Even with poor reception once the receiver locks onto the GPS signals the connection is fairly robust. The trick is acquiring the signal in the first place. If you continue to have problems you might consider a GPS unit with an external antenna. A separate antenna is much larger than the built in antenna on most GPS units.

You can check the status of the GPS constellation at http://tycho.usno.navy.mil/gpscurr.html or http://www.navcen.uscg.gov/?pageName=GPSSOI

 

[bruceb58]

Re: GPS signal

The satellites are not in a geo synchronous orbit so later in the day, they would be in a completely different position and likely be different satellites altogether. Sounds like your unit is failing or there is some sort of interference nearby. Although cellular frequencies are a completely different spectrum, it is possible that a bad cell station could be putting out harmonics that could interfere with the GPS signal.

 

[Mac3]

Re: GPS signal

I already have an external antenna and I would have suspected that my unit is failing except that my friend's did the same thing at the same time. When we were in the port area we were by a large cruise ship which probably has all kinds of electrical emissions but that didn't explain Saturday's failure to lock on, anyway thanks for all that I'll check out the satellite positions. I can guarantee at 40 degrees south in the middle of the southern ocean we are not in a high priority coverage zone.
cheers
David

 

[O-fishal]

Re: GPS signal

If you were near a large cruise ship you were being blanketed by their radar waves.

 

[bruceb58]

Re: GPS signal

I can guarantee at 40 degrees south in the middle of the southern ocean we are not in a high priority coverage zone.

GPS satellites orbit the earth and I really doubt they turn them off selectively by location.

How many satellites are shown as visible when you have trouble locking?

 

[Mac3]

Re: GPS signal

I'm sure they don't! I get about 10 come up but they show as red 'no signal' or blue 'not used'- what's it doing up there if it's not used??
When it locks on eventually they all go to green in a quick sequence and I noted I got co-ordinates with just 3. If the cruise ship caused the problem then perhaps they were in the vicinity the day before, we were in a secluded channel then and this was a separate outing, we were about 30 miles from the port. But, here's another one- we have had 3 trips in the past 3 years to the west coast of Tasmania which is virtually uninhabited and the gps did not pick up a signal on all occassions. I can't think of any man made interference there. Check this area out and see what you think. 42 20S 145 19E
cheers
David

 

[bruceb58]

Re: GPS signal

You have anyone that can loan you a handheld GPS that you can see if it is having the same problems?

 

[Mac3]

Re: GPS signal

Bruce I have a Garmin Etrex which also didn't pick up a signal on the west coast. The other issues have only just happened and I was wondering why, generally the thing works faultlessly.
cheers
David

 

[bruceb58]

Re: GPS signal

You guys must have missed paying your 2012 GPS tax.

 

[CaptainKickback]

Re: GPS signal

Agree that your friend's gps faure indicaproblem is likely not your unit. But I am not totally clear from your first post where all the failures occured. Was it failure to connect out on open water, followed by 2 dropouts in port in the same spot?

BTW, my understanding is the satellites are in geosynchronous orbit, strategically placed for best overall coverage and consistent coverage. It takes 3 signals to triangulate. One or two won't do it. More than 3 are not necessary, just increases the chances you can get 3.

I used to anchor in Port Everglades and watch the cruise ships go out late afternoon (as many as 7 in an hour). Never had gps interference from them.

 

[jhebert]

Re: GPS signal

...my understanding is the satellites are in geosynchronous orbit, strategically placed for best overall coverage and consistent coverage.

You have a misunderstanding. GPS satelllites are in a low-earth orbit.

 

[bruceb58]

Re: GPS signal

It takes 3 signals to triangulate. One or two won't do it. More than 3 are not necessary, just increases the chances you can get 3.

More misunderstanding! You actually really need 4 and more satellites are used for greater accuracy.

 

[Mac3]

Re: GPS signal

Wow, this is getting exciting! Looks like stuff just happens. The first drop out on Saturday we were about 30 miles from the port of Hobart in a large open bay. On Sunday it was when we were inspecting the ship at a distance about 300 metres and then again when we returned past the ship from our outing at the end of the day. Still doesnt explain the regular non performance on the west coast.
cheers
David

 

[CaptainKickback]

Re: GPS signal

I stand corrected. Sorry, meant mid-earth orbit. But 3 satelites gives a 2 dimensional fix and has worked for me a number of times (lots of tall buildings on the ICW).

I'm beginning to wander if you have 2 separate issues. In the bay, you had startup problems. I have had times on multiple Garmins where it takes forever to get a fix. Its like the gps didn't hsve a clue where it was and takes a while to figure it out, ad opposed to most of the startups when it assumed a starting position where you last shutdown.


The second (dropouts) are more likely a specific radio frequency being transmitted at that location, at least according to all the reports I've been reading.

 

[azboater2]

Re: GPS signal

It would seem a bit unlikely that the cruse ships Radar would completely kill your GPS, those things are considered when designing both devices. Cruse ships use GPS also and the Radar antenna is sitting right on the ship...however, GPS uses signals in the K-band 1575.42 Mhz and sometimes 1227.6 Mhz (most often Military) and while Radar uses a variety of frequency?s depending on the mission, they also use K-Band, many times for traffic control, so it could be the problem, at least at that time, in that place, with that Radar operator. The more sats the merrier, 2 will give you a reading, 4+ and I'd be happier. The sats position does change over time, but once acquired and your units tables are updated it should know which ones to look for after a few hours no problem. Military: On May 2, 2000 selective availability was turned off by the military. If all the sats your GPS 'sees' are in the same area, say the south-west, your in trouble, your fix may be off by 100 meters or so. In no case do any of these concerns cause your GPS to not receive any sats at all. If you and your buddy had the exact same problem at the same time, I vote for interference, or a parallel universe cross convergence spectrum mismatch

 

[Mac3]

Re: GPS signal

I'll vote interference.