GPS Antenna(s)

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phil-t

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We've had our current Tiffin 36LA for nearly a year - the Kenwood stereo/navigation sytem has never made a gps connection - so useless for that. Yesterday while going through all the systems, preparing for a trip, I decided to pull the Kenwood system and take a look at the back, Oh oh, gps antenna cable was pulled from the connector. Got on line and found a few sources for a new antenna with the proper termination.
Anyone have any ideas on what/where the best mounting location for that new antenna might be? Been reading lots of stuff on the subject.
The original is buried somewhere under the dash on the passenger side.
 

John From Detroit

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The best mount.. Rooftop, but if your roof is mostlyu plastic/wood/rubber.. Just get it above metal. Metal will block the signal.. Generally higher in the vehicle is better. but you can put it on top of your house radio or any that' "on top" of stuff... If your phone's GPS works.. then the GPS receiver will work.
 

Ex-Calif

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If you have a good sticky mount I would mount it high on the windscreen for ease of install and route the wire under the mouldings.
 

boatbuilder

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We mount them under fiberglass decks or hardtops on boats all the time. For all but fringe areas they work fine. We can even get reception in a metal building if the overhead doors are open and the antenna can see a couple satellites.
 

Isaac-1

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GPS receiver sensitivity has came a long way in the last 10 or so years, I would start with trying to mount it just below the dash behind the windshield and see is that works ok, before trying anyplace hard to get to.
 

Ex-Calif

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Yes, I been reading about the metal ground plane so not sure windshield is the best place.

My phone is my GPS and is sits on a mount in front of the vent. As others have said with a fiberglass RV signal strength shouldn't be such a big issue.

It also depends on what kind of antenna it is. Also I would be surprised the antenna supplier doesn't have a suggestion.
 

Lou Schneider

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Yes, I been reading about the metal ground plane so not sure windshield is the best place.
There's really little need for a groundplane on a recieving antenna. Of greater importance is letting it see as much of the sky as possible so it can lock onto the GPS satellites as they zoom past. Much like the Starlink satellites - the more sky the antenna can see, the better the system works.

When I had XM satellite radio I glued it's little puck antenna to the inside of a vent lid to get it just above the roof and it worked great.
 

phil-t

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Ex - they do have a suggestion - they have a metal base plate with adhesive to use as a mounting surface. They say high, under dash should work well.

Found this - The GPS satellites circle the Earth at an altitude of about 20,000 km (13,000 miles) and complete two full orbits every day. The GPS satellites are not in a geostationary orbit, but rise and set two times per day.
 

Lou Schneider

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Bobtop46

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I would look to mount it up front on the roof of the front cap. This mostly skips all the roof structure and sub roof (joists). No ground plan needed. Amount of sky is what's important. Satellites orbit the earth in 11 Hour and 58 minute orbits. The system is minorly vulnerable to local jamming, hacking the system is very difficult, hacking your unit is another story. 1 nano sec of error in the system equals 3 feet of error on the ground. Ground GPS is more accurate because it puts you on the face of the earth, while air GPS has to determine your altitude, which means less satellites to determine your position.
Most commercial GPS units only have two satellite trackers that time share tracking 2 satellites each. Civilian accuracy is advertised at 10 meter circle, but is typically better. GPS is a giant Time, Speed, Distance problem. T - time for signal to reach receiver from satellite, S - speed of light, = Distance from satellite, now triangulate 4 globes and you end up with a 10m sphere that you are in. As mentioned above by placing you on the face of the earth you end up with a 10m circle you are in. There are unclassified means to improve accuracy.
Here endith the unclassified lesson.
 

phil-t

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Thanks for all the comments and suggestions - I have learned a bunch about GPS stuff today.
Up front, on the roof would be great. Thinking I'm going to give the under dash position first, then, if not satisfied, on top of the dash . Using the included metal mounting plate as noted. Don't think I want to get into going through the roof material.
I use my android phone and a Garmin 770 on the dash and never see any issue.
I'll know come Thursday evening, for sure. :cool:
 

DonTom

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Aren't GPS satellites geostationary?
Nope. Unlike TV satellites, GPS satellites move in respect to the earth. And that is why my Spot Messenger sends out the signal every five minutes. You could be down in a canyon with an emergency with no satellites above. You can wait for the satellites to come to you and that can change enough to work or not work in five minutes. It tries to send every five minutes until the batteries go dead.

Two moving satellite systems involved here. The GPS very high main system for the location and the much lower GlobarStar system the Spot Messenger sends to.

-Don- Reno, NV
 
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Ex-Calif

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Ground GPS is more accurate because it puts you on the face of the earth, while air GPS has to determine your altitude, which means less satellites to determine your position.

This is the first time I have heard this. Every GPS I have ever owned will provide altitude if three satellites are visible.

You don't get more or less satellites AFAIK. When you have 2 signals you get 2D accuracy, with 3 or more you get 3D accuracy.

Thanks for the reminder on satellite orbits. I probably knew this at one point or another.
 

Wasillaguy

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The old Garmin handheld GPS units would show you the # of satellites it was receiving. I can remember as many as 7 at once.
Every satellite was assigned a number, and it would even show a display of where each was in the sky.
Military still has control of accuracy. They can skew the civilian signal so adversaries can't aim their stuff, yet still have a signal that's accurate for our boys.
 

DonTom

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When you have 2 signals you get 2D accuracy
IIRC, GPS units normally require at least three, but you can use two if you enter in your known elevation but then have 2D. IIRC, four required for 3D but you can get 3D with three if you enter in your elevation. At least I think that is how one of my hiking GPS units work.

IOW, you can usually use a known elevation in place of one satellite. But if the satellites are too close together, it still may not work.

But it's rare to not get many more satellites on the better GPS units these days. So it's not much of an issue, except in very poor GPS locations such as down deep in a canyon.

-Don- Reno, NV
 

Larry N.

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This is the first time I have heard this. Every GPS I have ever owned will provide altitude if three satellites are visible.
That's interesting, Dan. Every reference I've found shows that four satellites are needed for altitude, three to get horizontal location. And a presentation a number of years ago by the GPS labs at Colo. Univ. at Boulder showed the same. My experience indicates the same. On receivers where I looked at the number of satellites as the receiver searched, no position was shown until 3 satellites were in view, and it took four to get altitude.

At Finding Latitude and Longitude with GPS - How GPS works they say, "The system needs at least 3 satellite to obtain a latitude and longitude fix. Four satellites allows your height to be determined as well and the more satellites that can be received, this higher the accuracy of the results."
 

Old_Crow

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When I had XM satellite radio I glued it's little puck antenna to the inside of a vent lid to get it just above the roof and it worked great.
My XM antenna is magnetically fastened to part of the framework of the cabinets over the dashboard. This puts it under the fiberglass front cap and I never have any problem receiving a signal (excepting tree cover, tunnels or steep sided canyons). The one in my Jeep is stuck to a piece of sheet metal I bolted under the footman loop on the inside of the windshield frame. Doesn't matter if the hard top or soft top is on. My stand-alone automotive GPS hangs off the dash and has no problem with a signal. My handheld Garmin that I use for Geocaching will pick up 7 satellites anywhere in the coach.
In fact, the only satellite device that has trouble receiving a signal inside my coach is the "atomic clock" I have mounted to the wall above the entry door.
 
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