GPS question

[Tdog]

I am a relatively new GPS user - - I have a Garmin 660 that I have used successfully on a couple of trips. However, on my last outing, it began to act up. My wife & I were over on the gulf coast - - we intended to spend a few days poking around beaches in Alabama & Florida. For the most part I just wanted to view the map as we drove along. Yesterday, when we were just on the east side of Destin, it lost satellite connection. It would come on for a few minutes & then lose the satellites again. Then driving home today, its operation was intermittent again. I wish I had had the patience, once it started working again, to turn around & retrace my route to see if the malfunction was related to geography. The other option, of course, is that my gps unit is kaput.

I値l call Garmin痴 1-800 number tomorrow to see if they can be of any help. At least, it痴 still under warranty.

I知 just curious if any other gps users have had similar experiences.

Thanks,

Jack

 

[RayH]

My Garmins have lost sat coverage at times but its always been clear why they lost it, such as a tunnel, heavy vegetation or high structures close by. The vehicles metal roof will block signal, say if you had the GPS unit in your lap as you drove instead of mounted in the windshield. Sometimes the weather seems to "weaken" the signal but that usually just effects the accuracy. Ive never seen them just drop sat. signal for no apparent reason. Doesnt sound good for your GPS unit.

 

[RonMar]

Has it always been mounted in the same place on the dashboard? If you are using the internal antenna, it needs to see as much of the sky as possible thru the front window so it needs to be as far forward on the dash as possible. Geography should have no real effect on performance unless you are way down in a narrow canyon with a smaller view of the sky. The GPS sattelites are polar orbiting, so even down in a canyon, you should be able to get occasional low grade fixes as the sattelites pass overhead.

I am not sure how your particular model displays the information, but there should be a sattelite information screen showing what sattelites are being tracked. All the Garmins I have used including my handheld E-trex show this info on one screen in two ways. First as a ringed display like a bullseye showing the sattelites angular(elevation and bearing) relationship to you. The second is a bar graph display showing signal level from each sattelite and wether the sattelite is being used for navigation. You need at least 2 sattelites for basic nav, and 3 or more for precision positioning with altitude.

This is a low power line of sight sattelite signal going to a very small low-gain antenna that fits nicely into the receiver/display package. That antenna unfortunatly is not the most efficient way to receive the signal and things getting in the way(metal roof, buildings, heavy folliage/tree cover) will easilly block the signal.

If you are not getting a good signal in the car, perhaps it is from blocking by the roof. Move outside the vehicle with it and see if the tracking performance improves in the open air. If it does, you most likley have a blocking issue. It appears that they make an external mount antenna for the 660 so if yours is working OK in the open air, an external antenna might be a good idea. The antenna goes on the roof with a good view of the whole sky and allows you to place the unit on the dash for best viewing, not best reception. I know to use my handheld E-trex in the car, i have to place it way out on the dash where it can be hard to see and harder to manipulate the controls.

If the performance dosn't improve out in the open air, you may have a receiver problem.

 

[MrJimi]

I used to have those reception problems till I got an external antenna and my reception is great now
And Ron is right on 100%

 

[dmccarty]

Did you pass any power lines?

My GPS is ancient so maybe things have gotten better but years ago when I was using it to track my kayaking milage I would loose the signal near trees close to shore or if I was near transmission lines.

Never had a problem with it near the Nuke plant though. Guess it boosted my reception.

I listen to a radio station that is barely in range as I get close to work. There are spots at intersections that if I move a foot or so I can hear the station. Otherwise I get static or another station. Best I can tell its the power lines. And I swear if it is foggy and some sunrises the reception is worse.

The military can also deny access to GPS over a specific area. I would be surprised if that was the case though.

Later,
Dan

 

[jdbower]

I notice that Eglin AFB is close to Destin, the military used to scramble the GPS signals close to key potential targets which may affect your reception as well. Usually this just means that you're not as accurate but the mapping software will snap you to the nearest road anyway, but if the reception's weak it could get confused. I'd also recommend an external antenna to help with this, the more satellites you see and the stronger the signal the better you'll be able to lock onto a location.

One of the few nice things about the expensive factory-installed systems is that my GPS has a "dead-reckoning" mode where it will take input from the speedometer and steering sensors to plot out where I am - mine even works in tunnels thanks to that

 

[RonMar]

I notice that Eglin AFB is close to Destin, the military used to scramble the GPS signals close to key potential targets which may affect your reception as well. Usually this just means that you're not as accurate but the mapping software will snap you to the nearest road anyway, but if the reception's weak it could get confused. I'd also recommend an external antenna to help with this, the more satellites you see and the stronger the signal the better you'll be able to lock onto a location.

One of the few nice things about the expensive factory-installed systems is that my GPS has a "dead-reckoning" mode where it will take input from the speedometer and steering sensors to plot out where I am - mine even works in tunnels thanks to that

That scrambling you are refering to was known as Selective Availability or SA. It used to be turned on worldwide, or at least where ever the sattelites were transmitting. SA degraded the signal and fix accuracy down to that of the best inertial nav systems of the day so it was of no particular benefit to an enemys guided weapons. You had to have the right receiver and code info to decode the precise nav data. Because of this many governments, including our own developed differential GPS, a series of beacons that refined/corrected the SA errors for more accurate domestic use around major ports and waterways.

The sattelites are not particularly special. They have an omnidirectional antenna and just transmit precisley timed signals. All the magic is in the receiver that compares the received time delays from the known transmit delays to determine position, basically triangulation by time. Since the sat signal is basically available for use wherever it can be seen, it is as far as I know not possible to degrade only a small areas reception. SA being on, degrades positioning over a very large area. As far as I know, SA has been turned off worldwide for quite a long time now, our gift of precise navigation information to the world...

Due to it's low power line of sight signal, GPS is easilly interfered with, that is why we still have LORAN, at least for domestic navigation. It is virtually impossible to interfere with. GPS is the kid standing on the street corner with a small transistor radio. Roll up your car window and you can no longer hear it. LORAN is the kid in the tricked out car with the very loud base amped stereo that pulls up beside your car going thump, thump, thump. It comes from everywhere and nowhere and there is nothing you can do to stop it from rattling your fillings

 

[Tdog]

Thanks for the replies. I was hoping that someone would tell me that severe sunspots had caused my gps to go stupid. Doesn't look to be the case. I thought about Elgin AFB, but the unit also messed up miles north when I was driving west on US 84 through Alabama & Mississippi. The unit is mounted on a swivel-bean bag accessory that rests on the dashboard & has always worked fine there. I did not go to the satellite screen to determine which satellites it was seeing, but I gather it was blind at the time - - it sure behaved that way.

I’m afraid I can’t blame anything but the gps unit itself. Oh well.

I’m trying to contact Garmin now - - I’m sure I’ll have to go through 10 levels of screening before I find any help.

Jack

 

[orezok]

That scrambling you are refering to was known as Selective Availability or SA. It used to be turned on worldwide, or at least where ever the sattelites were transmitting. SA degraded the signal and fix accuracy down to that of the best inertial nav systems of the day so it was of no particular benefit to an enemys guided weapons. You had to have the right receiver and code info to decode the precise nav data. Because of this many governments, including our own developed differential GPS, a series of beacons that refined/corrected the SA errors for more accurate domestic use around major ports and waterways.

The main reason the government stopped using SA is that some genius figured out that if you have a ground station near the target, you just read the GPS signal and tell your incoming missile to offset the difference between the reading you're getting and the exact location that you know that you are at.

 

[RonMar]

The main reason the government stopped using SA is that some genius figured out that if you have a ground station near the target, you just read the GPS signal and tell your incoming missile to offset the difference between the reading you're getting and the exact location that you know that you are at.

And that is exactly what differential GPS does...

 

[shaley]

I found this on a GPS site and I've been hearing it for 2 years now. Some of the marine GPS units say they have L2-c but I'm going to hold off for a year on a hand held. L2-C will be +- one foot accuracy. Current 3 place decimal is 6 ft b/c there is 6070 ft in a N mile.

"Recently, a new GPS satellite was put into orbit. This satellite is capable of broadcasting not only on the L1 (L1C) frequency, but also a new L2 frequency. (L2C actually, the C denoting civilian). Some people have indicated to me that consumer level GPS receivers will not start hitting the market until there are 24 functional satellites broadcasting L2C information."

 

[orezok]

And that is exactly what differential GPS does...

True, but the government still controls the differential signal which it can switch off at any time. Accuracy is improved to about a meter with differential. We set up our own base station and can improve accuracy to less than 1/8" and it can't shut down by others.

 

[RonMar]

True, but the government still controls the differential signal which it can switch off at any time. Accuracy is improved to about a meter with differential. We set up our own base station and can improve accuracy to less than 1/8" and it can't shut down by others.

Well concidering the government(DOD) still controls the GPS signal, and can shut it down for a whole global region whenever it feels the need, the USCG shutting off a diff beacon is not that big a deal. With Selective availability disabled now, I don't think too many are going to the trouble of installing or using DGPS beacon recievers anymore anyway. The base accuracy of the signal is plenty accurate without it.

What are you doing that needs 1/8" accuracy?

I escorted a USGS surveyor to one of our remote communications site so he could setup a receiver on a geodetic marker. They were comparing markers on both sides of the cascade range to see how much movement is being caused by the subduction zone in the pacific northwest. He had a specially machined $16K antenna coupled to a $3k receiver. No differential, just a high quality signal refined and averaged over a 20 hour period.

 

[orezok]

Survey for grading and improvements. All points need to be within +/- .001 foot (1/8") both horizontal and vertical. Vertical accuracy is almost imposible without an onsite base station.

 

[Tdog]

As a follow up, my gps problem apparantly has nothing to do with military bases being in the vicinity. Or at least the Garmin technician did not think so. He had me do a Master Reset & clear all my user preferences. Also he wanted to be sure I had the latest software, which I think I do. So far it has not screwed up, but I had used it OK for a couple of months before it messed up in the 1st place.
Whatever - - the Garmin techical help people were relatively easy to reach. Surprised me.

Jack

 

[archerynut]

I have my second 660 right now. One day for no apparent reason I got into my truck and the darn thing would not work. It was on but would not track satellites at all. Drove 25 miles and it never picked up a signal. Returned it to the store where I had bought it and have a brand new one that hasn't had any problems. Here's my thought, it was full force hot days (90+) here in PA and I think the thing just overheated something inside and it went dead, battery wouldn't charge either. Since then I never leave my GPS on the dash when the truck is parked. Always stow it in the center console.

 

[Tdog]

archnut, I never leave the unit on the dash. I try to take it down & stow it prior to stopping. Too easy a target on the dash for thieves.

When my failed last weekend, I was driving with the AC on - - & I had been traveling for several hours. It was not especially a hot day, maybe upper 80s. I don't think my problem was heat related. I'll be using it on local trips for while just to give it a chance to mess up. I suspect it won't do that till my next real road trip.