Antenna

[jergalea]

Hey everybody,
Was just wondering if any of you have ever used re radiating antennas with your gps.
I have an old magellan that i use together with discover australia maps on my lap top and while in the bush it often loses coverage.
Will an external re radiating antenna help improve coverage?

Cheers

[away]

Hey everybody,
Was just wondering if any of you have ever used re radiating antennas with your gps.
I have an old magellan that i use together with discover australia maps on my lap top and while in the bush it often loses coverage.
Will an external re radiating antenna help improve coverage?

Cheers

I use one. When I purchased my Panasonic Toughbook PC to use as mapping tool, it came with a built-in GPS pod and a re radiating antenna. I use it with OziExplorer and Natmap.

It seems to be 100% accurate so far as lat. and long. go, but it gets confused and gives incorrect elevation readings. As an example, I was parked on a wharf, 4 metres above sea level and both my built-in GPS and my Magellan told me I was 34 metres below sea level. I got out of the vehicle with the Magellan and it then told me the correct elevation.

I've been told that the receiving antenna and the re radiating antenna should be :

a) as close together as is possible and
b) vertically above one another

in order to get accurate elevation readings.

Personally I'm not bothered about it as I only need to know where I am, not how high I am.

[jergalea]

Thanks for that away!
Do you get good reception with it?
And would you know of any shops that sell them? I tried dick smith but they dont have them.
Any ideas anyone? Or just bye off ebay?

Cheers

[maca82]

GPS by nature is inaccurate as far as elevation goes.

Satellite are a long way up, and it's touhg to judge 1 or 2 metres of elevation from 1000's of kms away.

The external antennas don't make your GPS any more accurate. They simply enable them to recieve better singal by being higher and outside of your car.

That's why we put them on the roof - much clearer view of the sky than on your dash...


~MACCA~

[away]

Thanks for that away!
Do you get good reception with it?
And would you know of any shops that sell them? I tried dick smith but they dont have them.
Any ideas anyone? Or just bye off ebay?

Cheers

I get good reception and track between 8 and 12 satellites at a time. Ideally, position the antenna on the roof of the car where it will get the maximum view in all directions. As maca82 said in his post, it won't make your GPS any more accurate (what I meant by 100% accurate in my first post was that it allows the GPS's to work to their full potential, which in my case is about 5 metre accuracy - plenty good enough for me.)

Having a GPS on the dashboard limits the number of satellites that you can track and when going through heavily treed areas, could even result in a momentary dropout.

A re radiating antenna will alway do a better job than a stand-alone GPS inside a vehicle will, provided you choose where to put the external antenna with care. And they're not expensive either. Mine was $145.00 complete.

[bigjedd]

Know someone who is running a reradiating arial in a discovery. She uses it with a magellan meridian platinum gps. Was told when she runs it to place the internal end of the reradiating arial as close to the back of the magellan as possible and wrap it and the gps in foil so there is no interference from car electrics etc. She runs it in conjunction with a lap top and hema maps and it works spot on . I had the same gps without an outside arial on a trip we did through butcher country and caledonia river. I lost the satellites in the caledonia but she picked up enough to register the whole trip onto the hema maps and downloaded the trip when she got home.

[woolgoolgaoffroad]

i definatly 2nd to what 'away' said, run one on my 126 garmin, the satilites stay constantly in range, and always have the 12 running at 'any' given time specially in our thick bush here on the north coast.

[michaelcarey]

Know someone who is running a reradiating arial in a discovery. She uses it with a magellan meridian platinum gps. Was told when she runs it to place the internal end of the reradiating arial as close to the back of the magellan as possible and wrap it and the gps in foil so there is no interference from car electrics etc.

This technique is more to stop signals from the re-radiating part of the setup from being picked up by the GPS antenna outside the vehicle and causing an RF feedback loop.
In the same way you get that nasty high-pitched squeal with PA systems when the microphone picks up too much audio from the speaker, GPS re-radiating systems can suffer from the same thing (the amplifier is built into the GPS antenna and it's on a much higher frequency but the end result is the same, the GPS doesn't like "listening" to the result very much (-: ). It's best if the outside antenna is some distance away from the coupler on the GPS receiver and there is some metal between them.
Of course the best thing to do when GPS shopping is to work out if you need the facilities of an external antenna connection and then buy a GPS that meets that requirement.
Regards,
Michael.

[michaelcarey]

The external antennas don't make your GPS any more accurate. They simply enable them to recieve better signal by being higher and outside of your car.
~MACCA~

Not receiving more GPS satellites from across the sky, horizon to horizon can make your GPS inaccurate.
Here are a simple couple of diagrams :-
http://www.cmtinc.com/gpsbook/chap7.html
If your GPS is on the dash, looking through the front windscreen at about half the sky, the satellite's it can see to determine a position fix are in a limited part of the sky, there might be plenty of them, but they are all in one "half" of the sky. This will increase the DOP (Dilution Of Precision) and decrease the "potential" accuracy of the computed position. The further apart the sat's are in the sky, the lower the DOP and the better the computed position.
In basic terms, your GPS receiver knows where each GPS satellite is in the sky and it works out the distance from each satellite to you by using the time it takes for the signal from the GPS satellite to reach you. Using these two bits of information from each satellite it computes where you are.
http://www.cmtinc.com/gpsbook/chap2.html
The more satellites and the more spread out they are in the sky, the better the computed position.
So... having an external antenna exposed to more of the sky can and does improve the accuracy of your GPS.
This page is a good read and it's not too techy...
http://www.cmtinc.com/gpsbook/index.htm
Regards,
Michael.

[maca82]

Mike, you're mis-interpreting my quote mate.

What i meant was: having an antenna will not make your 10m accurate GPS a 5m accurate GPS. That's why i said "They simply enable them to recieve better signal by being higher and outside of your car."

You obviously know your GPS (I wish there were more people who did) are you a surveyor?


cheers

~MACCA~

[michaelcarey]

Mike, you're mis-interpreting my quote mate.

What i meant was: having an antenna will not make your 10m accurate GPS a 5m accurate GPS. That's why i said "They simply enable them to recieve better signal by being higher and outside of your car."

You obviously know your GPS (I wish there were more people who did) are you a surveyor?

cheers

~MACCA~

Nope, not a surveyor. I work in marine electronics, been using GPS in many forms since 1989 when it only worked a few hours a day, was terribly inaccurate, GPS receivers were $18000, weighed a ton and consumed about 7 amps.
Here is the first GPS I ever saw/worked with, it's a JRC JLR-4000, we only sold two of these dinosaurs. It has a five channel sequential receiver, but it no longer works as it didn't handle the GPS week rollover properly.

http://www.users.on.net/~michaelcarey/pics/JLR-4000/

People still complain about how expensive GPS receivers are, imagine paying $18K for one!