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
Why not use DGPS?
TTFN
RE: using RF to measure distance
RE: using RF to measure distance
what accuracy?
how much money?
TTFN
RE: using RF to measure distance
RE: using RF to measure distance
NRE?
temperature range?
environment?
TTFN
RE: using RF to measure distance
RE: using RF to measure distance
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
RE: using RF to measure distance
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
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://
and without beam size issues.
TTFN
RE: using RF to measure distance
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
Cheers,
mark
RE: using RF to measure distance
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
RE: using RF to measure distance
Cheers
RE: using RF to measure distance
GPS combined with some sort of data link may still be the most reliable way to do it.
RE: using RF to measure distance
TTFN
RE: using RF to measure distance
RE: using RF to measure distance
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
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
Cheers
RE: using RF to measure distance
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
RE: using RF to measure distance
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
RE: using RF to measure distance
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
Cheers IR
RE: using RF to measure distance
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
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
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
Mark
RE: using RF to measure distance
Go figure...
TTFN
RE: using RF to measure distance
Mark
RE: using RF to measure distance
It was developed for practical use in the late 1930s
It's called RADAR.
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
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
Cheers
RE: using RF to measure distance
In the meantime, does anyone fancy a guessing game over the application for this?
A.
RE: using RF to measure distance
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
RE: using RF to measure distance
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
RE: using RF to measure distance
RE: using RF to measure distance
RE: using RF to measure distance
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
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
RE: using RF to measure distance
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