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View Full Version : External Antenna's - Do they cause a position shift?



mlenser
28th September 2007, 08:40 PM
This may be a stupid question but when I install an external antenna on my GPS is it reporting the position of the GPS or the position of the antenna? I know that they are only 3 meters apart but I was just wondering.
Thinking about how gps's work via very accurate timing signals from satellites it makes me think it might be the position of the antenna.
Hmmm on a tangent re-radiating antenna's shouldn't change the gps postion so maybe wired antenna don't either?
Any thoughts ... or better knowledge on this?

Regards
Mike

Nev
28th September 2007, 08:55 PM
Mike

I guess you could extrapolate this and imagine the impossible situation of your antenna connected to a very looong lossless coaxial cable placed say in New York.
Where would your GPS think it was?
No different if your receiver was a TV.
Thinking out loud. :scratchch

Neville

mlenser
28th September 2007, 09:50 PM
I guess you could extrapolate this and imagine the impossible situation of your antenna connected to a very looong lossless coaxial cable placed say in New York.
Where would your GPS think it was?Interesting thought there - I suspect the GPS would have a heart attack as the timing signals would still take a measurable amount of time to reach Australia from New York but the satellite coverage would not match the GPS's logic for that length of time as those satellites aren't in view of our sky's.
Then again I have no knowledge in the electronics and how the signals are treated once the antenna picks it up ... would the time taken to travel along the cable be added to the the timing signals picked up by the antenna itself?

No different if your receiver was a TV.I don't think you can compare a gps with a TV in this regard can you. TV & radio waves just need to receive a strong enough signal for them to work. They don't rely on the time taken for any signal to reach them to work ... theoretically you can watch tv on the moon or mars several minutes after the transmission occurs?
Although just as you said Nev ... only thinking out aloud, I have NO knowledge about this, thats why I asked.

Regards
Mike.

Nicko
28th September 2007, 11:23 PM
They do because the Antenna is the receiver of the multiple signal from which the GPS calculates it's position.

Imagine being in a room, blind folded, and there being four speakers, one in each corner. If you stand in the middle Led Zep would appear to come from all four corners at the same time and of same loudness thus you know you are in the centre of the room. If you move you will note that the sound appear louder in the direction you are travelling thus you are no longer centre to the room. Because you are so clever you can determine within metres how far away you are from the original position even though your ears are off centre to your brain by a few centimetres, too small to register.

On returning to the centre of the room you remove your ears and place them in the same spot you had walked towards. The sound now will now appear to be louder than the others from one of the four sources of sound. Even though your brain (GPS engine) is the bit that deciphers what it hears, sees and smells it is the ears that actually pick up the sound and transmit the signal to the brain.

However, because your brain can only determine it's position accurately to 6-8 metres typically the ears are not far away enough to make it excessive for the level of accuracy expected. External Antennas should not be longer than 5 metres otherwise excessive signal loss affects performance of the system.

mlenser
28th September 2007, 11:54 PM
Thanks Nicko & Nev for your quick replies,
Something to consider when placing my antenna for collecting track files for the OpenStreetMap (www.OpenStreetMap.org) project.

Best Regards
Mike.