View Full Version : RF distance calculation,can it be?

03-21-2007, 06:35 PM
I was wondering if we could calculate the distance between two Parallax 433 MHz RF modules. Any one who could is explain how or why not..

03-21-2007, 06:43 PM
I'm not sure about the Parallax 433 MHz units, but some of the longer range (and more expensive) units I've used do a calculation and make that number available in a register. It is more of signal strength number than an absolute distance measurement.

How accurate a measurement do you need?


Tom Sisk


03-22-2007, 02:13 AM
If you can do distance measurements with a laser you should be able to do it with roundtrip timing of a radio signal. That said, I don't know how.

- Stephen

Bruce Bates
03-22-2007, 03:10 AM
Gents -

LASER beams are beams of LIGHT. Light can be reflected from solid objects. Radio signals are RF (radio frequency) and that passes THROUGH objects and in general is not reflected the way light is off of solid objects.


Bruce Bates


03-22-2007, 04:05 AM
Bruce, I was talking about the timing of the round trip not reflecting the beam. I was thinking that if the master sent a pulse and the slave pinged a response back you could compute the time it took for this to happen and that, minus a fixed time for the computer to do the tx/rx.

- Stephen

Robert Kubichek
03-22-2007, 04:20 AM
It would only work if the main unit had an atomic time source, and the response time of the slave was calculated in..
But rf travels so fast, and atmosheric conditions are a large variable that attenuates rf.
Thats why GPS units need a master site that transmits a correction based on the time hack refrenced to the known location of the "master"
site to be acurrate to sub inch measurements... "GPS Surveying" is a major reason for this, as well as navigation of planes/boats....


03-22-2007, 07:07 AM
Hi All;

While conceptually it is possible, it is not practical to measure RF transit time in the range of a few hundred nanoseconds. And even that would require the detection mechanism to have a resolution (think bandwidth) capability of a few tens of nanoseconds, translating to a few tens of feet. Longer ranges are theoretically more practical, although still not accurate, but the cited modules are of low power and only have a range of a few hundreds of feet.

With some very complicated and exotic methods employing spread spectrum modulation of the RF modules (so now they no longer ARE those very modules), similar to the GPS satellites, it may be possible to get a resolution of 50 feet.

Precisely determining the "edge" of a pulsed RF carrier is very difficult (especially at microvolt levels) because the "edge" is coming at you at almost the speed of light. So to get any significantly accurate resolution, you need sub-nanosecond response in the detector, and this is the territory of NASA and the like.

For all intents and purposes, this is not a practical or viable concept for the hobbyist.


Peter (pjv)

03-22-2007, 06:43 PM
Before I·dig·deeper·about my questions. I·want·to share what was I planning about the distance calculation etc..I am planning to make a GPS like project using RF devices that is available in the market.. I was wondering if we could implement it in a building so we could have a realtime monitoring of the·people inside,it may be by RF ID or anything thats why I need some advice.tnx···

Robert Kubichek
03-22-2007, 07:49 PM
The only feasable way to do that, would be by placing several doppler recievers on each floor,
the more the better for accuracy, and use the output to triangulate the position.
To verify the floor, you would need detecters at every avenue of ingress to monitor traffic in and out.
A high power pasive rfid tag system comes to mind in this application.
The rf tags can also be used for secure access to controlled areas like server rooms ect..


03-23-2007, 09:31 AM
Can you give me a illustration to that?what is a doppler reciever?hhehehe sory for being so novice.

Robert Kubichek
03-23-2007, 09:45 AM
Heh "Google" is your friend!! Do a Google for " doppler RF".
The idea is, that even rf waves will reach antennas at certain points in time, kowning the arrangement of the antennas allows
one to find the direction the rf wave originated, if you have 2 or more doppler arrays, you can use triangulation to pinpoint location..

Loran is one of the main uses of this teqnique. But GPS has pretty well eclipsed that by being better and more accurate..

Bob http://forums.parallax.com/images/smilies/scool.gif

03-23-2007, 08:29 PM
One possible technique is to combine RF with ultrasound. The RF and sound pulses are send at the same time. The receiver starts a timer as soon as the RF is received. When the sound is received the timer stops. The period of the timer gives you the distance.