Ranging using radio
Ranging using radio
(OP)
Hello. I am fairly new at the whole circuits thing, I'm trying to get into using microcontrollers and such to control everyday applications and what not. I do have an engineering degree but my 3 credit class in engineering instrumentation didn't prepare me for everything I want to know.
I was wondering if it's possible to send out a radio signal that is recieved by a reciever and then sent back to the original transmitter for purposes of getting range? I know it's possible because that's sort of how RADAR works, but I want the return signal to be able to be processed in order to determine what the range is for a given object in a field of say 20 or 30 objects. I know there are addressing issues and stuff like that, that's fine. I'll deal with that later. Right now I'm just looking for the basics at sending out a pulse and having one sent back so that the reciever knows the time of flight and what object sent it.
Hope this isn't too confusing.
T
I was wondering if it's possible to send out a radio signal that is recieved by a reciever and then sent back to the original transmitter for purposes of getting range? I know it's possible because that's sort of how RADAR works, but I want the return signal to be able to be processed in order to determine what the range is for a given object in a field of say 20 or 30 objects. I know there are addressing issues and stuff like that, that's fine. I'll deal with that later. Right now I'm just looking for the basics at sending out a pulse and having one sent back so that the reciever knows the time of flight and what object sent it.
Hope this isn't too confusing.
T





RE: Ranging using radio
This is something that requires custom electronics and counters, not really suitable for a newbie project. Your typical microcontroller would have a system clock on the order of 100 nanoseconds.
If you're stuck on doing something like this, an acoustic rangefinder may literally be more to your speed. The speed of sound is around 340 m/s. so 1m range resolution is much easier to deal with.
However, your desire to discriminate objects requires significantly more complex hardware, so you might consider simply concentrating on getting range from a single object for the time being.
TTFN
RE: Ranging using radio
T
Just in case you are wondering, I am kidding.
RE: Ranging using radio
Short distance ranging can be performed with some of the impulse radar and other ultrawideband location technologies (usually referred to as MIR for micropower impulse radar). Some of the patents within this area describe applications similar to your described problem. Because signal reception with MIR is equivalent-time sampling based, resolution between signals with only picosecond differences can be performed with a timing source of microsecond resolution.
RE: Ranging using radio
What about 2 or 3 transmitters transmitting unique carrier wave type of signals and a reciever that can determine what direction the individual signals are coming from. Once again a triangulation thing.
That way each individual reciever needs to worry about itself instead of one reciever having to worry about multiple inputs from different sources.
My current thought is this:
2, 3, or 4 fixed transmitters transmitting unique frequencies.
a reciever located somewhere in between the transmitters at an unknown location.
The reciever gathers direction information from the transmitters every 5 minutes or so and then sends the information of to a different source that collects and analyzes the data.
The eventual goal is a GUI that will show the user exactly where the object is within the given box.
Any suggestions?
T
RE: Ranging using radio
1) The "receiver" will actually need two receivers to get any sort of angular accuracy, multiplied by the number of unique sources.
2) Instead, have one transmitter at the unknown location. Two dual-channel receivers located roughly orthogonal positions. Each dual-channel receiver accurately measures direction of arrival (DOA). The intersection of DOA's is the location of the transmitter.
BUT, either is quite non-trivial, since you effectively need to accurately measure signal phase across two receivers.
TTFN
RE: Ranging using radio
It can be done with RADAR, but for a person wanting to use UP's for a project, you are jumping into the fire in trying to work with radio tracking/location at this point in your development.
Keep in mind that if the project you choose is too hard to implement, you will never get to the UP because of all the frustration of trying to become an expert in an unrelated field.
Good Luck!
LCM
RE: Ranging using radio
RE: Ranging using radio
Transponder systems are not all that accurate and pulse transponders like aircraft TACAN for example, are limited to 0.1nm accuracy. You must measure time to 3ns accuracy to resolve 1m and that is generally done by triangulation using phase difference between two accurate signals of in the case of GPS, by correlating 2 or more signals modulated with maximal length pseudo random sequences. You can easily get 1m accuracy using this method. I think GPC C-code uses a 10 bit sequence. Over short distances you will not have a lot of difficulty with variations in atmospheric velocity factor (due to temp, moisture and pressure), changing the speed of lightlocally but you will, over 20km or more.
RE: Ranging using radio
The idea is this, a "course" can be laid out with one or more "foxes", radio transmitters that keep transmitting their own ID, and all the compettitors start out at the same time to find the hidden transmitter. You can take several strategies. The first is to find the direction of the transmitter, and keep moving in that direction. Another approach is to take one direction reading (bearing) write it on a map, then move at right angles to the possible location, take another bearing, and with two postions marked ont he map with two bearings you can project a final location.
The receivers can be simple amplitude affairs with rotatable antennas, or they can be relatively fixed antennas that are omni directional, with electronic phase shifting to steer the beam. They can work by finding the direction of the strongest signal, or by finding the weakest location. Some antennas have a very sharp null only in one direction, which is convenient for direction finding.
Problem: RF signals only travel directly on a straight line in outer space. Objects, the shape of the terrain, buildings all affect the signal, and several signals can take different paths from transmitter to receiver. There is not necessarily a single, strongest path. But if the situation you immagine is simple, like flat asphalt with no obstructions, it might work.
RE: Ranging using radio
I have been assuming I need to make a very narrowband, very stable preamplifier, and then diode detect the output to trigger my time stamp. Is the the best way? How would I construct the preamp or where would I buy it?
Anyone got any design tips?
RE: Ranging using radio
Get a copy of AIRBORNE RADAR by STIMSON where you'll probably find as good an explanation as you're likely to get.
RE: Ranging using radio
While you say that you'll time stamp the reception, how do the receivers know when the pulse was sent? You're doing the GPS problem in reverse, which means that you'll need 4 receivers to solve for the 3 position unknown and the 1 time unknown. If it's all in the same horizontal plane, it might be possible to use only three receivers.
Since you'll have to use GPS anyway, it would seem to be easier to use DGPS in the first place, which is all off-the-shelf.
TTFN
RE: Ranging using radio
Sorry this is so general but it's 17 years since I did this.
RE: Ranging using radio
RE: Ranging using radio
The first problem is the detection circuit for triggering a 4 ns resolution time tag repeatably after the 433 MHz hits each receiver.
The second problem is the counters and data latches them selves need to work at 250 Mhz.
Any recommendations?
RE: Ranging using radio
The recievers can cost several hundres dollars or more and can be constantly time syncronized to each other to within a few ns via rf because the delay times between them are fixed and known.
Given this information, if i can just time tag the cheap transmitter bursts to within a consistant 4 ns window i will know the relative delay distances.
This system should not require any fancy clock stability, just detection delay stability over a large variation of signal strength.
RE: Ranging using radio
Even calibration every second requires stability of 2.5 parts in 10^8, which still requires oven stabilization.
TTFN
RE: Ranging using radio
The relative time includes any drift in or differnces between the transmitter clocks + propogation delay + detection errors + receiver clock errors.
However, assume a perfectly accurate clock in each transmitter. Variations in radio propogation delay due to weather will give you more than 4ns shift in a 20km circuit over a day or so. This radiolocation system needs to estimate it's position based on the time difference between received signals and known transmitter locations, determine the error circle size (caused by time measurement, propogation errors and receiver clock errors etc), shift it's clock, or by some other means it's estimate, until the error circle is down below 1m in size. That's where the Kalman filter comes in.
When the transmitter clocks have uncorrelated individual errors you are not in the race. Also you will not measure range accurately over any useful distance using simple pulse technology. Some form of pulse compression is needed and PRS modulation is as good as it gets.
RE: Ranging using radio
I am only ranging up to 500 meters so weather is less of a problem and yes I will have fixed, known position transmitters to perform periodic range calibration at least once per second.
The transmitter clocks or what ever encoding i send dosent matter for anything in this ranging scheme, only having a clearly defined onset of the 433 MHz carrier.
I am afraid I dont understant pulse compression and PRS modulation...How do they help me reliably timetag?
RE: Ranging using radio
Thank you guys for your help.