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# using RF to measure distance

## using RF to measure distance

(OP)
Hi, I am trying to measure the distance between a stationary point and a moving point up to at most 1200ft.  I think RF is the best(or only way) to do it.  Wondering if anybody knows an effective, cost efficient, method of doing this. The distance is not always line of sight either.  Thanks.

### RE: using RF to measure distance

What is the intent?  RF needs line of sight as well.  Otherwise, you're not getting lineal distance.

Why not use DGPS?

TTFN

### RE: using RF to measure distance

(OP)
Ok lets make it so that it is always line of sight.  I wanna us RF as a cheaper alternative to GPS.

### RE: using RF to measure distance

By continually failing to provide adequate information, how do you expect to get a serious answer?

what accuracy?
how much money?

TTFN

(OP)
I am looking for an accuracy of around +/- 5 feet or better. Price wise of course as cheap as possible but lets say around $100. This is just a guess for price. I am trying to see if its feasable to do. ANy more info? That should be enough ### RE: using RF to measure distance Just one? NRE? temperature range? environment? TTFN ### RE: using RF to measure distance (OP) It is going to be used in a flat feilds with tree surroundings. Going to used on normal outside temps no less than 0 degrees C. There are going to be more than one used within a close proximity(lets say around a 50 to 200 feet from eachother). Not sure what NRE is. Sorry, i am new at this. Thanks for you patience. ### RE: using RF to measure distance Think radar gun- You can hack one up to use non-doppler ranging. Best of luck and have fun. Cheers, Rich S. ### RE: using RF to measure distance (OP) i was considering using radar but worried about hitting the wrong point but i will look into it some more. Thanks ### RE: using RF to measure distance You could send out a radio burst and an audio burst at the same time, measure the time between receipt of each, and use the speed of sound to infer the distance. Better to measure the temperature also, and compensate for that. The audio could be ultrasonic, but it will need some power, and you'll have to characterize the response time of the receiver, e.g. how many cycles it needs to reliably distinguish a burst from random noise. If the moble unit can measure the distance traveled and its compass heading, it could infer its position by dead reckoning. If you could arrange two stationary sources of bright light or continuous RF, the mobile unit could use a rotating directional receiver to measure the bearing to each source and infer its position. Absent interfering trees, bushes, people, etc., you could just keep a string taut between the units, and measure the extension of the string. Mike Halloran NOT speaking for DeAngelo Marine Exhaust Inc. Ft. Lauderdale, FL, USA ### RE: using RF to measure distance Radar on a non-Doppler basis could be difficult. A 35-GHz radar with a 6-inch antenna will have a beam width of about 8 degrees. At 1200 ft, the beam footprint would be about 165 ft across. Without Doppler, there would be no target discrimination. Bushnell has a number of laser rangefinders that sell for less than$200:
http://www.nextag.com/All--PRICE-Under-200--zzbushnell+laser+rangefinderz1zB3mz5---html

and without beam size issues.

TTFN

### RE: using RF to measure distance

We bought a range finder off the shelf from a hunting supply shop the other year. Binocular look-alike.

It uses a IR laser diode and works well at ranges beyond your needs. (has been tested at 1500 feet or so)
As this is Europe, the range comes in meters, though

### RE: using RF to measure distance

Strongly suspect that you'll find the laser rangefinder is a better solution, but if you're expected to do a feasibility study, it would be nice to offer alternatives that work.

A.

### RE: using RF to measure distance

(OP)
Thanks for all the input guys.  I am a recent grad so it is nice to hear some advice from some pros.  I am trying to keep the price as minimal as possible. I am also interating it with a microprocessor and an LCD display for my own specialty functions.  I think the Rf seems to be the best way to go.  Keep the feedback coming as it is very helpful.

Cheers,

mark

### RE: using RF to measure distance

Use two transceivers (probably UHF) that ping back and forth with as little latency as possible. By measuring the overall cycle time you should be able to get a relationship between cycle period and distance. There will likely be an offset due to the zero-distance minimum cycle time, and this offset might vary with environmental factors.

The cycle time is obviously going to be in the RF range, so you're going to have to use very fast hardware to count it. The system will have a minimum distance increment probably in the several meter (or tens of meters) range.

Or just use DGPS.

### RE: using RF to measure distance

Nice, clean, to the point, I like it VE1BLL.

### RE: using RF to measure distance

(OP)
Thanx VE1BLL, I had your idea in my head to begin with, it seems to be the most appropriate way to do it.  One question about actually calculating the distance.  Do I use the general (speed of light)*time?

Cheers

### RE: using RF to measure distance

With any radio system there is always going to be the possibility of multi path reception. The receiver may possibly detect several signals all slightly displaced in time. The most direct and shortest signal may not always be the strongest either. Spurious reflections from buildings and vehicles may be a problem with any simple radio system.

GPS combined with some sort of data link may still be the most reliable way to do it.

### RE: using RF to measure distance

I had considered suggesting RF modems, but the synchronization is non-trivial, particularly when you consider that a 5-ft error is incurred with a 10-ns timing error

TTFN

### RE: using RF to measure distance

(OP)
there is gonna be no problems with cars and buildings interfering.  It will not be used in that environment and it will always be line of sight.  I will aslo gave each carrier a specific "identity" so it doesn;t mistakingly pick up other signals.

### RE: using RF to measure distance

O/k fair enough. Do a bit of a search into telurometers and how they work. These are surveying instruments that can measure over very large line of sight distances to extreme accuracy.

The basic idea is you modulate a microwave (or optical) carrier with a much lower frequency, and measure the phase of the modulation at the other end. The lowest modulating frequency may have an effective wavelength of a mile or more. That would tell you very roughly where you are within a whole mile.

You then switch the modulating frequency to a shorter wavelength. Perhaps a tenth of a mile, and measure the phase of that. You just keep switching ranges down to successively finer resolution.

It is like reading a clock where you have hour, minute and second sweep hands. Each clock hand just by itself is pretty useless, but with all three readings you can easily get down to a few seconds resolution in 24 hours in one quick glance.

Telurometers are a bit more complex, but that is the basic concept in its crudest form.

### RE: using RF to measure distance

You don't specify requirements for endurance, data rate, latency, resolution, area over which the system has to operate, rate of movement of the moving object, or any exclusion rules.

If none of these is demanding, you might get away with borrowing a GPS and some string to help you paint a few range lines on the field, then paying some kid with a phone to stand beside the moving machine and tell you where it is while you type the figures in at the keyboard (or even just write it down for later analysis)

Only make it complicated if it needs to be complicated?

A.

### RE: using RF to measure distance

(OP)
Thanks for the input zeusfaber.  Yes the reason why there are no requirements cuz i am new to all of this so i don't know what to look out for but i am sure those wil come oup during the design process. Here are some more details: The object that is moving will be stationary when the measurement is taken place. THe key here is to make it much more feasabel than GPS.  That is why i am seeing if there any other ways to do it.

Cheers

### RE: using RF to measure distance

You have an interesting problem here.  You're trying to measure an inconvenient distance to better than half a percent accuracy.

Half a percent is quite feasible, but is well into the territory where most measuring devices are going to have to be made first, then calibrated.

If you buy, borrow or steal proper surveying gear, a laser rangefinder or a GPS, someone else will already have done this calibration for you.

If you make your own kit, you'll need to calibrate it yourself - for which you'll need to obtain an accurate means of measuring distance to start with, taking you right back to one of the bought-in options.  You've got to ask yourself what added benefit the homebrew gadget will bring to outweigh the added error, hassle and expense.

This benefit could be real - for instance, you may only need to hire the expensive gear for one day instead of the whole trial period, and once calibrated, the new toy might be quicker and easier to use - but all these depend on exactly what you want to do.

"Systems" thinking preaches the discipline of understanding your requirement really thoroughly before you get stuck into design work (the politicians who spoil my day job reckon we ought to be spending up to 15% of the project budget before we even start development).  Might be a bit doctrinaire, but it helps keep you and your money out of the blind alleys.

A.

### RE: using RF to measure distance

(OP)
Perhaps i am missing the big picture here.  I was planning on using to transceiver chips.  To measure the distance i was just going to measure(with a uP) the time it takes to send and recieve a signal and put that into d=ct equationg to get the distance.  Could you maybe explain the need for calibration in this matter?  Thanks

### RE: using RF to measure distance

VE1BLL hinted at this earlier on:  The linearly distance-variant bit (about 2.5 microsec) is only part of the cycle time.  There will also be the time it takes the transponder to discriminate the incoming signal from the background noise, decide to reply, and build up transmitter power.

Some of these things are likely to vary with distance (for instance, the further you are away, the weaker the received signal is and the longer it is likely to take for the receiver to recognise that it is there.  If you just use d=ct, you will overestimate distance - probably increasingly so as your target gets further away.

The way round this is to calibrate the device by recording output at a variety of accurately known distances.

As IRStuff has already pointed out, you'll need to measure your cycle time to within 10 ns.  I'm not sure how easy that's going to be just using a uP.

A.

### RE: using RF to measure distance

(OP)
Cool, I understand what you are saying now.  Thanks so much.  Well I found a transceiver chip designed for uP control.  It has a CTS(clear-to-send) pin that goes low whenever an incoming signal is received.  That is what i am going to use the counter on cuz soon as that bit goes low, i know that the signal is receivee however i will make sure that what is received is what i sent out.  Anyway i am gonna give it a try.  It is experience for me which i really need so i can't lose either way(new grad from engineering).  Thanks for all the help guys and i will keep you posted.

### RE: using RF to measure distance

I can't think of a single processor in the entire universe that can respond reliably and repeatably within a time calibratible to 10 ns.

In a standard rangefinder, ALL of this is done ONLY in discrete or FPGA logic.  Even then, massive temperature calibration is required to assure accuracy, since the detection logic chain is usually multiple gates, which means that your logic timing must be known to better than 1 ns per gate.

TTFN

### RE: using RF to measure distance

(OP)
frick, didn't even think about that.  This is slowly turning into a lost cause.  Ah well, i shall keep reading.

Cheers IR

### RE: using RF to measure distance

The processor itself may not have to. External hardware would make the actual phase/time measurement, the processor could crunch the numbers produced at a much more sedate pace.

Ten nS time resolution only requires a 100 mHz clock or sampling rate, which is by no means fast these days. But as has been previously pointed out, analog circuit delays may be highly variable with signal strength adding a most worrying uncertainty to the whole process.

### RE: using RF to measure distance

Frankly, when one is faced with a technical obstacle, it behooves you to review your premises.

Your desire to find a "cheaper alternative to GPS" is, I think, based on a fallacy.  I've yet to find even the simplest laser rangefinder that's less than $100, while I just bought a GPS with mapping software for$79.  The reason is that GPS is ubiquitous and mass produced, while rangefinders are less so.  RF rangefinders are generally useless in ground applications, so there is essentially zero market and you'd have to roll your own completely from scratch.

Given the range involved, you could buy two GPS receivers, one as a basestation, RF modem the remote GPS to the base station and brute-force a differential GPS for less than the cost of a decent laser rangefinder with RS-232 output (the cheapest I found was running over \$500).

TTFN

### RE: using RF to measure distance

Have to agree with IRstuff.  Off the shelf mass produced hardware is pretty hard to beat for value.

Quite often the whole professionally built system can be manufactured cheaper in Asia than just buying the bare components over the counter to build it yourself here. Plus you get all the packaging, development expertise, software, and reliability for free.

### RE: using RF to measure distance

(OP)
I think you gentlemen are right.  I was looking for a cheap alternative for GPS but after i did do some research on GPS and saw that it is a reasonable price with tones of cool features which makes it really hard to beat.  I will keep research for an education stand point but as far is development, well........maybe not.  Once again, appreciated all of the input.

Mark

### RE: using RF to measure distance

It's one of the perversities of engineering that something "simple" such as measuring the time of flight, as was attempted by the ancients would prove to be more complex and expensive than launching 24 satellites with rubidium time standards and using 12-channel all-in-view receivers.

Go figure...

TTFN

### RE: using RF to measure distance

(OP)
Ha, too true and quite funny.  Well it is nice to have these forums where one can share his ideas and get realistic feedback.  I hope in the futur I can do the same as you guys have done for me.  Happy engineering!

Mark

### RE: using RF to measure distance

Such a system was described by Hugo Gernsback in 1912 in his book Ralph 24C41+

It was developed for practical use in the late 1930s

Now, you can get a GPS engine a whole lot less expensively than a radar unit and you can adapt APRS (Check on ARRL.ORG about APRS) to have the remote unit radio its position on a VHF transceiver.

Good Luck with yourexperiment.

I remain,
The Old Soldering Gunslinger

### RE: using RF to measure distance

SiRF makes some very interesting GPS chips.  (Disclaimer - my cousin works there. But they're still very interesting chips...)

In case you wander back to the pinging architecture:

Multipath shouldn't be a major issue because the FIRST signal to arrive is the right one. The multipath can cause interference, so you need to get your measurements done quickly. I've seen this exact concept done (prototype) to eliminate skywave interference from MF navigation systems - grab the phase BEFORE the skywave arrives later (taking a longer path).

To eliminate other issues you put a tiny flag of some sort into the signal and you detect the flag, not the raw RF that might bring issues as mentioned by zeusfaber.

Before GPS there were all sorts of interesting methods of (for example) guiding an oil rig back onto the well (within inches) where the nearest fixed landmark was hundreds of miles away (over the horizon).  Tricky business...

Still - GPS wins hands down for most applications.

### RE: using RF to measure distance

(OP)
That is what I was thinking VE1BLL.  The first signal has to be the correct one.  However with all the other technology out there it is only worth me doing just for the "heck" of it.  Those rangerfinders IRstuff reffered me to do just the trick and they are cheap.

Cheers

### RE: using RF to measure distance

Let us know how you get on with the homebrew.

In the meantime, does anyone fancy a guessing game over the application for this?

A.

### RE: using RF to measure distance

I would guess a potential application would be controller feedback for an automated crop circle making machine :)

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

### RE: using RF to measure distance

(OP)
Wow, you nailed it on the first try analog....or maybe not lol

### RE: using RF to measure distance

Wow!  I agree, Analog is dead on.

I thought the device would be placed on the Bull so you could go cow-tipping whithout the needless worry of accidently bothering the bull far from the pasture fence!

I remain,

The Old Soldering Gunslinger

### RE: using RF to measure distance

I think I have an idea.  What you do is get the lowest fz osillator you can and plug it into a syn. binary counter.  When you press a button it sends the voltage to the counter to start and the tranmitter.  Then, when the reciver gets the signal after it comes back it sends the signal to the counter to stop.  You then have the counter send to final count to a BASIC stamp chip that you have programed to calcutate the distance.  It then sends that to a Binary-to-BCD converter and then to a 7-segment LED display.

### RE: using RF to measure distance

(OP)
Nice one stargate, that is pretty much what i have in mind.  Except i will use and LCD instead.  Gonna get around to it pretty soon.  Cheers

### RE: using RF to measure distance

Stargate3216 - Congratulations, you've just invented 'The Digital Timer' (but you'd want the highest frequency feasible).

### RE: using RF to measure distance

MikeHalloran, you just described my EE senior design project!  I made 3 receivers, collected delay data from them and triangulated a position.  The idea was to locate someone with a "panic buzzer" using sensors on the blue light emergency call poles in many campuses.

The implementaiton of "listening" to a buzzer and timing it to within a few milliseconds proved to be harder than I thought, but I did it!  I used a tight bandpass filter to listen for the sound, and measure the time difference between the RF and the sound.  But that didn't work very well...

So I repeated the RF and sound at about 2 Hz.  I then took my magnitude output signal from the bandpass filter and bandpass filtered that at 2Hz. After a few itterations it gave resonable results.  We ended up with a map of campus and a dot following around a person with a very annoying buzzer.  Accurate to maybe 5 or 10 meters.  We Won best overall project from the IEEE.

### RE: using RF to measure distance

Congrats, JBender!

Now I'm wondering if I could pick up the 'bang' and the RF from my lawnmower ... and make it autonomous.

Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

### RE: using RF to measure distance

If you scew it up, poor kitty... :(

### RE: using RF to measure distance

Think of it as population control for felines.

Alternatives exist.

A friend has a Rhodesian Ridgeback, a big, strong dog with a distinctive textured stripe on her spine.  Ridgebacks were bred to hunt lions, so they don't much care for cats.

She doesn't kill 'em directly, though; she field- strips 'em, by peeling back their pelt from neck to tail, leaving them to hobble home without a skin.

Women hear about that, and say, "Eeewwww, that's awful".

Guys say, "What a cool dog."

Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

### RE: using RF to measure distance

I'll take two!   :)

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