detecting distance between beacon and sensor/receiver

I have an idea that depends on being able to detect a distance between a static sensor/receiver and a mobile beacon/transmitter roughly in plane. My question is what method is best for doing this? Is there a ToF measurement method (like ultrasonic) that could be used to measure distances witching a couple of meters? Or is there some IR, RF, or alternative method that would work better with this application(and if so, how would that work in this setup)? I appreciate the help anyone can offer. Let me apologize in advance if my jargon isn't up to snuff yet.

Please provide more details:

Indoors or outdoors? What kind of resolution do you need? 2-3cm? 2-3 inches? etc. How fast is the mobile part moving? How often are updates needed? Can line of sight be maintained the whole time? Do you need just distance, or distance + bearing/heading?

Over limited distances you can use ultrasonic ToF. Take two ultrasonic range finders. Remove the receiver from one and the sender from the other. Use a radio link to trigger both at the same time and read the 'echo' pin on the receiver to measure the ToF distance. No need to divide by 2 since it is not a reflected signal.

Using two receivers at known locations you can triangulate position as well as distance.

Unless you add hardware capable of measuring fractions of a nanosecond (multi GHz clock) you won't be using electromagnetic waves for accurate distance measurement.

Over longer distances, differential GPS might be your best hope. Have the beacon send its GPS location over a radio signal and have the base station compare that to the base station GPS location.

Indoors. preferably less than 2cm if possible, if not I can improvise. The mobile part is held in hand while writing(if that gives you an idea of speed). A fairly high refresh rate would be needed, something like 60 Hz would probably work. Line of sight will be maintained and bearing isn't necessary but would be useful.

@johnwasser what sort of accuracy, refresh, and range is expected from that kind of ToF measurement?

Maybe look at this too:

•Unit size (HxWxD): 40 x 48 x 20 mm (1.6" x 1.9" x 0.8")
•Weight: 22 g (0.77 oz)
•Resolution: 1 cm
•Accuracy: +/- 2.5 cm at distances greater than 1 meter. Refer to operating manual for complete operating specifications.
•Range: 5 cm to 40 meters 
•Update rate: up to 500 Hz

CrossRoads: Maybe look at this too:

The LIDAR will give you distance to whatever you point it at. Then your problems becomes keeping it pointed it at the moving object.

May need 2 pronged attack then - beacon from the mobile part for tracking, and LIDAR for accurate distance.

This thread has some notes on a one way ranging experiment. The approach was to use a radio link to synchronize an ultrasonic transmitter (beacon) and ultrasonic receiver to measure time of flight. One limitation of this approach is that the ultrasonic beam is directional, which limits the relative position of the receiver to be somewhere within the transmitter's beam.

An alternative to using RF to synchronize the ultrasonic sender and receiver you could try IR. The cheap HC-SR04 ultrasonic range finders (under $2 each) use a 40 kHz ultrasonic signal. You could tap into the sender electronics to have the beacon send a 40 kHz modulated IR signal at the same time it is sending the ultrasonic pulse. It is possible that the receiver can use a simple IR receiver to demodulate the IR signal and use it to trigger the receiver. If all goes well the receiver can measure the ultrasonic ToF.

The rangers typically only send around eight cycles at 40kHz - best hope a typically 38kHz IR receiver can lock onto that and produce an output.

AWOL: The rangers typically only send around eight cycles at 40kHz - best hope a typically 38kHz IR receiver can lock onto that and produce an output.

Well, I did say "if all goes well". :)