RF distance measurement

Why have a re-transmitter?. Makes the system twice as complex as it needs to be, adds an extra delay, and requires use of two frequencies. Just use a retro-reflector. This is then radar... The biggest problem is just measuring time accurately.

Best Wishes

and

and

Yes.

choose time of flight pulse or continuous modulation/measure phase(easier)

Your problem statement is too broad, narrow it down some more so real solutions can exist.

Of course.

Well, at 5 meters, I think you will need a short pulse and very fast RF switches unless you use a different frequency for the return. This is pretty difficult. You will need a lot of special equipment to get it up and running. I don't think you will succeed.

Use ultrasound (see hexamite.com).

If you MUST use RF, I would suggest using a wide-bandwidth FM homodyne type radar. The distant target would be a retro-reflector or a phase-locking transponder. This still won't be trivial, and you would have a low probability of success, IMO.

--Mac

RF distance measurement can be greatly simplified by using linearly SWEPT frequencies that result in an audio difference.

Originally used on the APN-1 radar altimeter.

HI All,

I want to send data from transceiver A and receive it in transceiver B and retransmit back to A - I want to time the round trip.

TRx(a) =>TRx(b)=>TRx(a).

I want to time the round trip to calculate the distance between TRx(a) and TRx(b).

Hence - I want to measure the distance between 2 transceivers between 5 and

This method of distance measurement seems thwart with danger.

Questions Q1 Has this been done before? Q2 What problems (challenges) should I watch out for? Q3 Do you have any thoughts on this subject.

Thanks in advance. Joe.G

For measuring time accurately, you might consider buying an old HP 5370A or 5370B time interval counter. That has a resolution of 20ps, and absolute accuracy under 100ps can be achieved. A 5370B sold on eBay a couple of days back for $199. They cost >$20,000 in their day.

There is a manual on it on my web site.

Well, you can buy the gadgets that do this for about $20 these days at the local hardware store - not sure if the cheapy ones go to 100 metres tho

David

"Joe G (Home)" wrote:

I agree. This is, in fact, what I meant when I said FM homodyne. I just left out the "linear" part.

A linear FM homodyne radar transmits and receives simultaneously, and mixes the TX and RX waveforms together. The mixer output is LPF'd to give the audio frequency you are talking about.

I encountered a certain amount of flack in this newsgroup once when I suggested that downrange resolution is given by:

R = C / (2 * BW)

Where R is resolution, C is the speed of light in the relevant medium, and BW is bandwidth.

This is really the definition of range, but there are radars, or radar-like devices (FM CW tank level indicators) out there which provide much more precise range estimation than this. I think this is only possible when it is known in advance that there is only a single scatterer, or a single scatterer in a certain area, and when the signal to noise ratio of the return signal is pretty high.

--Mac

Yes - one common use is secondary radar (derived from IFF).

Secondary radar uses a 3 microsecond delay in the transponder. The normal radar range calculation takes this 3 microsecond delay into account.

The return time at 5 metres is ~ 30 ns; at 100 metres it is ~ 600 ns. Using radio transmission will give a real headache getting accurate results. Ultrasonics would give much longer transmission times and ease measurement.

As above ;-)