# triangulation

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#### Kane2oo2

##### New Member
hello
How could you find where an object is .. say in a small room with triangulation and radio signals?
I know that this works in GPS because there a long distances involved so the time taken can be calculated, but how could it be done over a small distance?
If not by radio signals how else?

Kane

#### crust

##### Member
It could still be done with radio signals over short distances if you measure the phase shift of each signal. You might have to do more than one frequency depending on the area you are covering to get an absolute position. If this sounds ok, i could go into more detail about how this might be done.

GPS is a little different, the signal includes the time/position of the local oscillator on the satellite. The receiver gets signals from at least three satellites and when they arrived at the receiver and calculates the position.

#### Kane2oo2

##### New Member
It could still be done with radio signals over short distances if you measure the phase shift of each signal. You might have to do more than one frequency depending on the area you are covering to get an absolute position

could you explain further....

thanks alot
Kane

#### crust

##### Member
well, as a simple example, consider if you had a 100MHz local oscillator and then two reference oscillators at 100.01 and 100.02MHz respectively. Your receiver will see both signals, you can mix the local oscillator with these signals (and a bit of filtering) to get the intermediate frequencies. The phase shift from the RF signals is now in the IF. Then you can perform a Fourier transform on the signal, extract the phase shift of the two components and hopefully (unless I neglected something) figure out how far (in terms of wavelengths) you are from the transmitter. If you know their positions, you should be able to compute your position.

#### spuffock

##### Member
if you have two aerials, and arrange to switch between them at an audio frequency, and feed the result to am fm receiver, then an audio tone will result due to the difference in phase between the aerials. The aerials can be moved until the tone disappears, and at this point the aerials are equally distant from the transmitter. A line at right angles to the line joining the aerials will pass through the transmitter. This is quite sensitive, experiments I have carried out would distinguish a movement of the transmitter of 10cm at a distance of 30 metres.

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