# RF distance measurement

Discussion in 'Electronic Design' started by Joe G \(Home\), Aug 8, 2005.

1. ### Joe G \(Home\)Guest

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
100 metres

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.

Joe.G

2. ### Roger HamlettGuest

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

3. ### PeteSGuest

1. This has been done before extensively for both the radar (from
Roger) and re-transmitted. There are things called transponders
(transmission responders) which may re-radiate the transmitted signal
or radiate a different signal in response (This is how we get IFF,
inceidentally)

As noted, the simplest way is to just reflect the signal. If this is
not possible (because you don't have a clear line of sight), then a
transponder is appropriate.

The pitfalls are ensuring a known and repeatable time between reception
and retransmission / new transmission of the signal.

Radar range is very simply
Distance = C t / 2 where C = velocity of electromagnetic radiation [n
free space], and t is the time between transmission and reception. The
division by 2 simply shows that the pulse travels twice the distance
(there and back

Things to watch for.
Minimum range. This is set by the transmitted pulse (you can't receive
while you are transmitting, but this may not be so for a separate
receiver at perhaps a different frequency, such as a transponder).
Distance (min) = C t / 2 where t in this case is the pulse width of the
transmitted signal.

Maximum range.
Set by the pulse repetition rate. If there is some time t between
pulses, then the maximum time to wait for a single pulse (with simple
techniques, anyway) is the amount of time between the pulses. (Once
another pulse starts, we reset the range - this can lead to aliasing).
So Distance (max) becomes simply C t / 2 where t in this case is the
time between pulses.

Cheers

PeteS

4. ### night soil dalitsGuest

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.

5. ### DaveGuest

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.

http://www.g8wrb.org.uk/useful-stuff/time/HP-5370B/

6. ### quietguyGuest

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

7. ### MacGuest

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

8. ### Don LancasterGuest

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.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email:

Please visit my GURU's LAIR web site at http://www.tinaja.com

9. ### MacGuest

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

10. ### Jim BackusGuest

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 ;-)