# RF range finder doesn’t need to see to calculate distance

Instructables user [Jones Electric] has been quite busy lately, building a radio-frequency range finder. Built as part of a German youth science competition, he and his partner built a pair of transmitter/receiver modules that can be used to measure distances of up to a mile (~1.5km). Their argument for radio-based rangefinders is that laser rangefinders are obviously limited to line of sight, whereas their range finders are not.

To determine the distance between the two stations, the base station is triggered, which starts a counter and sends a 433 MHz signal to the second station. When the second station receives the signal, it in turn broadcasts an 868 MHz signal, which is received by the base station. The total distance between the points is then calculated based upon the round trip time of the two radio signals.

[Jones Electric] claims that the range finder is relatively accurate, with a deviation of up to 5 meters per measurement, and that the accuracy could be increased by adding a higher frequency crystal to the timing circuit.

We are pretty sure using these two frequencies in the US without a license is not allowed, though we are unsure of the usage laws in Germany, where this was constructed.

## 35 thoughts on “RF range finder doesn’t need to see to calculate distance”

1. Nippey says:

Great project.
These frequencies are free here in germany ;)

2. Tom says:

In the most of Europe, 433MHz is at low powers unlicensed and used for lots of remote operated equipment, including RF based power switches and such.

3. kernelcode says:

868 MHz is smack bang in the middle of CDMA BC0… So I reckon that’s probably frowned upon in the US.

4. macw says:

While it would probably take a much more powerful (and faster!) MCU, doing phase-comparison or rising/trailing edge detection on the signal can get you time-of-flight accuracy to a thousandth of a percent or better. That’s what GPS receivers are doing to measure distance to a few meters when they’re 20,000km away ;)

5. superlopez says:

433/868 are the european equivalent of 315/915 Mhz in US.

6. Kemp says:

To my knowledge, 433MHz is allowed in most places, and 868MHz is allowed in Europe (isn’t 900MHz or thereabouts the equivalent in the US?).

7. Kemp says:

8. Miroslav says:

Might be answered in the article (will take a look when a get more time), but how did they compensate for the “offset” or latency caused by the remote station (time to receive, detect, transmit). Given the speed of light, every nano second of additional delay will cause error (one feet (0.3m) per ns).
I suppose that delay should be fairly constant, so some calibration can take care of it.
Great idea!

9. I’m good on the 433mHz band in fact it appears a lot of devices use that.

The 866mHz band however is land mobile. I suppose my commercial radiotel license might cover me but why risk it?

10. Miroslav says:

@macw
Possibly I missed your point with detecting the phase detection, but not sure it’s applicable here.

Remote “station” does not have a refference to compare the phase with, and if you do that on the main station’s side, it’s not te same signal anymore.

GPS does it quite differently as they do not compare absolute dalays (what this device does), but relative delays between several (many) refferential points distributed arround the (visible) sky. Also all refferential points (satellites) are using high level of syncronization (interal clocks), that one can’t aford with a home brew :)

11. strider_mt2k says:

Elegant.

-and who doesn’t like a good RF hack?

12. macona says:

Pointless. Laser range finders dont need to be in two places at once like this does. Thats really nice when you are trying to blow up the thing you are measuring the distance to. And there is a RF range finder, its called radar.

13. Ken says:

Why does he need 10 digits??? Looks like fun anyway!

14. Williwaw says:

^^Dont be a delta bravo macona!
*******************************
Great project and congratulations on 1st prize. Your passive coordination sounds reminisce of LORAN, unlike that of an active RADAR.

15. Homer says:

@macona I wouldn’t really call it useless. It says it’s part of a youth science competition, and I take this to mean they’re less than 18 years old (not professional engineers). While not very efficient as opposed to commercial alternatives, it is a very good exercise and project for young minds.

16. lwatcdr says:

Actually this sounds a lot like DME. As to being useless. Not really was used for decades as a navigation system and still is until VOR/DME is retired.

17. therian says:

Im god and I let all electromagnetic spectrum to be used as you wish

18. joshfee says:

I could see some fun Geocaching applications :) :)

19. Greg says:

They are using a 30Mhz counter and they are getting measurements that are +-5 meters?
How is that possible (the light travels in a 30 millionth of a second 10 meters)?
What about other delays, how are they compensating for temperature changes?
I thing this is great work, I am just a little bit skeptical regarding the precision.

20. Marco says:

Nice project, but I doubt that this actually scored 1st place in the jugend forscht nationals. They tend to be a lot more competitive than the regionals.

21. qwerty says:

Two thumbs up for the interesting project but one thumb down for posting it at Instructables.

22. Grovenstien says:

Very nice, but way to inaccurate and cumbersome to be really useful.

I like the multiple numeric displays.

23. Jan says:

868MHz is used for alarms, RFID and similar low power devices in Europe. So the rangefinder is probably fine in Europe.

OTOH, the various FM wireless microphones (bugs) transmitting in the 88-108MHz band that I have seen here are completely illegal in Europe. If someone complains of interference, you are in for a hefty fine.

Jan

24. Dax says:

If they took four measurements and calculated the average, they would in effect get one bit more information, which would mean that they would double their resolution.

So, all they need is 16 times supersampling, and they’re down to 1 m accuracy, assuming that the deviation is random.

25. holly_smoke says:

Jan I think you will find European law not as simple as you make out.

Low power FM transmitters are allowed. Infact my Nokia N97 handset has one built in which allows me to listen to MP3 files playing on the phone via my car stereo.

The range is limited to a few meters, due to legal requirements.

26. Awesome project, and impressive results.

27. Jon says:

Good to see an RF hack.

Too bad this project can’t be used in the US (unless using different frequencies).

28. @greg I agree on the calculating delays, but what has temperature have to do with it? afaik EM waves keep traveling the same speed regardless of the temperature..

29. Kyle says:

Meh, I saw a project write-up very much like this in an electronics magazine in the early 90’s. I don’t remember which magazine, maybe Circuit Cellar?

30. Tom says:

@Jan You are wrong. There’s a European guideline that had to be implemented in the whole Eurozone which allows you to run a FM rig on every frequency in 88 – 108MHz but only on low power (I thought it was 25 or 50mW EIRP).

31. Thomas says:

Can anyone tell me how to do this with Arduino or perhaps some other microprocessor(s)?

The processor speed would be the limiting factor for precision. I really think this can be done WAY simpler.

What if you use VxWorks or something

32. Greg says:

@woutervddn
Temperature is affecting the frequency at which the crystal is oscillating. This adds a significant error to the measurements.

33. Demyke says:

This is a good work, keep on, I’m this knowledge of yours can be greatly employed where you neva thought. Thanks.