newb trying to build an RS-LPS, please advise.

[OfficerMaggot]

I will preface this by saying, I have absolutely zero experience with radios, and am just starting to learn, with a very specific goal in mind.
Since this is my first post on this forum, I'll introduce myself: I am 29 years old, I recently moved from my birthplace of New Bedford Massachusetts to Orlando Florida. I work as a rent-a-cop at the Orlando International Airport. I have a B.S. in criminal justice, and no idea what I'm doing with my life.

I had an idea for a product I think I have a half decent chance at putting to market some day. But I'm still working out the technical requirements.
Among other things, I am interested in learning about, and perhaps creating a proprietary ground based Local Positioning System using Radio Pings or a Solid Tone instead of a conventional satellite GPS system. (not trying to put to market mind you) the RS-LPS in question is a requirement for something else not radio-related. I'm not trying to reinvent the wheel because I think I can make a better LPS than what ever is already in use. I just have a very specific application in mind, and I don't know if a product exists that can do what I need it to do.

Correct me if you've heard this one before...

The basic idea, is to have 3 transmission towers equal distance apart, emit a signal, and use an RF meter to measure how much the signal has decayed, subtract the difference, and use that number to determine how far away the transmission tower is. 3 towers is the minimum needed to triangulate a position.

More towers could make more triangles, and more coverage, but for the sake of keeping this simple, 3 is good enough for now.

Think of it like; comparing the impact speed of a bullet to the muzzle velocity it started at. The deeper it penetrates the target, the faster it was going, the faster it was going, the closer the gun is to the target.

- One of the challenges I have to overcome, is the signal needs to noticeably decay over short distance in order to create a spectrum the LTS can use to gauge distance to the transmission tower.
- In addition to that requirement: to reduce error; the signal needs to penetrate trees, buildings, and other topography, so that distance is the determining factor of the signal's decay, by an order of magnitude large enough to negate the error that obstacles would cause.
- Some obstacles like mountains may not be negate-able, and so the tower must be placed above such obstacles. I consider this an acceptable compromise.
- All that matters, is that the LTS be accurate down to 50 centimeters.

Theoretically, this should be possible. But practically speaking, the level of precision I need may be legally, or technically too expensive depending on the requirements.

Not knowing if this is even practical, I'm more or less just using this as an excuse to learn about radio waves.

Any advice, tips, or references would be GREATLY appreciated.

 

[mmckenna]

Interesting approach.

Not picking it apart, but here's a few issues you'd need to overcome:

RF doesn't play by our rules. Signal strength is going to be impacted by the materials it passes through. Some building materials will attenuate RF more than others.

RF at higher frequencies really likes to reflect off things, it's not going to always take the straightest path. You might get signals arriving from the transmitter that arrive at slightly different times and at slightly different strengths that will throw things off.

Interference would hose things up quickly. You'd need to have a very quiet section of the spectrum and you'd need to run a signal that would be complex enough to be distinguishable from background noise.


And interesting project, none the less.


There's already pretty good competition in this field. There are systems that use Bluetooth beacons that can get you location down to a few feet. Between that and GPS, there's some pretty stiff competition.

 

[OfficerMaggot]

Not against the idea of using existing tech. Going in blind, I figured this would be the most efficient method, assuming it was even possible.
All this nonsense is an attempt to get around the fact radio waves travel at near the speed of light. So timing them for triangulation at close range would require incredibly sensitive equipment, which for the application I have in mind would be too problematic.
So I made an even bigger problem out of it, trying to find an alternative way to measure distance via radio waves.
If nothing else, I hope to learn alot about radio waves.

 

[OfficerMaggot]

Addressing the issue of RF reflection, and obstacles adding error. Assuming these cannot be negated in post, what do you think the chances of success would be if we used multiple triangles to average out the errors?

So, originally I was keeping it just 3 reference points for the sake of simplicity... but...
If a building or reflection was throwing off the RPS by a significant amount, maybe another signal from a different direction will bypass said interference.
And vise-versa.
Put simply, an average of many triangles will be more accurate than any one triangle.
So instead of 3 emission towers, maybe multiple sets of 3 will fix the problem.

With multiple triangulation's, we can average them all together to get a more accurate reading.

This is meant to be a short range local system, so the towers don't need to be as massive or powerful as a cell tower. Having lots of them isn't a deal breaker.

 

[mmckenna]

A good brain exercise, at least.

Depending on the environment and frequencies used, there will be a LOT of reflections, like from all the transmitters all arriving at very slightly different times. Over bigger spaces, this becomes a bit more manageable since accuracy isn't quite as important. In small spaces (I'm assuming you are thinking of indoors?) it gets to be a huge problem. Isolating the direct signal from all the reflections would be difficult. Each beacon would need to have a unique ID so the system could tell them apart. You'd likely start to run into an issue where the sheer amount of computing power at the user device would be pretty considerable and it would get expensive. Pulling radio frequencies out of noisy environments isn't easy. It's harder when you don't have dedicated spectrum to do it in. Getting dedicated spectrum for something like this would be a huge undertaking, quite literally an act of Congress since you'd be dealing with the FCC and a whole lot of competition.

The Bluetooth beaconing is the easy approach, easy to run very low power. Easy to identify each beacon, cheap solution. It's why it's already been done.

Using a simple beacon at a known point is possible, but you need to identify each signal and have a way of finding direction to them. Radio Direction Finding is an easy solution that's been done since the early part of the 20th century.

Time difference of arrival is another good option, but it's been done since WW2, systems like LORAN did it pretty well but accuracy was measured in hundreds of yards, not inches.

GPS uses a lot of satellites using dedicated frequencies. Time Differential of Arrival is easy since each satelllite carries its own atomic clock and it's sending out a time signal. The frequencies in use lend themselves well to this. Accuracy isn't hard to get with modern electronics.

There are those working on systems using known locations of existing transmitters. There's been some effort made to do location finding using existing TV/Broadcast transmitters.


I think you have an interesting idea, but it's not a new one, and the challenges are many. It would be an interesting project, but finding funding for such an undertaking might be difficult since there are already systems that do this better. Not trying to discourage you...

 

[popnokick]

These two links should provide you with both information on an existing technology, as well as how TDOA direction finding / location works -
KrakenSDR Scroll down this page to get to the RDF section and how it works
Object Tracking Using Time Difference of Arrival (TDOA) - MATLAB & Simulink

 

[OfficerMaggot]

thanks!