Is there a secure way for two people to know how far they are from one another?


I was wondering if there was a way for two people to know how far they are from one another, without a 3rd party being able to listen in and triangulate their distance?

I was thinking of something like the following:

…where the sender sends a wave of some sort that increases in amplitude the further it travels.

The receiver would use the amplitude of the wave to roughly estimate the distance from the sender.

However if a spy where to analyze the same signal (from somewhere in between the sender and receiver), they’d be unable to triangulate the distance between the sender and the receiver (as the amplitude of the signal would be smaller than the amplitude from the receiver’s location).

I know there are ways for two parties to “ping” one another and determine the signal strength between themselves (which could be a way of roughly determining distance), but I need a way to establish a rough estimate of distance without a 3rd party (ex. a spy) being able to triangulate the distance between both parties.

Any help or suggestions would be greatly appreciated.

Thank you,

You’ve got a hotchpotch of conflicting ideas here - and you also use terminology in a strange way.
First - triangulate is a very specific type of function, and uses basic maths - trigonometry mainly. The old system where you can find somebody by taking a map and two separate bearings from different places, which when plotted on a map cross in the general location the person is. You then could go in closer and repeat the process, eventually pinpointing the location.

Signal strength is an unreliable means of determining distance because radio signals ALWAYS degrade with distance - it can never increase without additional energy being put into the system. You can do a kind of triangulation with signal strengths from different receivers, which will produce areas on a map where the signal strengths correlate. Once you get close you can use a signal strength meter to head towards the emission. There is no way to prevent a third party analysing your transmissions - they may be unable to make sense of them, but they could hone in on them. Two parties in communications have no simple way to determine distance. It would be quite feasible to configure a system to receive then return the transmission, which could then be analysed for time delay. GPS, for example, can measure time delays very accurately to determine position by triangulation. If you have a radio that is duplex, then there will be a delay in receiving then re-transmitting, and the distance to and from the distant radio would change in time as distance increased. The third party spy with the right equipment could also analyse the delays.

This all takes time and complex equipment. Pinging is usually a technique when the sender initiates a data packet, and waits for the remote end to respond back. Mobile phones are duplex, so continually receive, process and send data - but I have no idea how I as a user would instigate such a process.

You’ll have to provide much more specific data for a proper answer. Are we talking phones, walkie talkies, HF radio or what? The answers can be varied.

Signal strength, however is not a reliable method because obstructions to radio signals can absorb, reflect or diffuse the signals, so a distant station at the top of the hill could easily be stronger than the person very close, but inside their home?

Hello Paulears,

Thank you for the detailed reply.

Regarding signal degradation, if a sender and receiver both send a signal to one another, will the signal degrade by the same amount (as long as the same sort of signal was sent by both parties)?

In other words, if the sender’s signal has to go through a mountain to arrive at the receiver’s location, can we assume logically that if the receiver sent a signal to the sender, that the receiver’s signal would also have to go through the same mountain?

Basically, I’m working on an encryption algorithm (on the PC), but need a way to pass some data from a sender to a receiver that will seed / initialize certain parameters within my encryption algorithm (this only needs to be performed once).

Once initialized, the sender and receiver can send data back and forth and decrypt the data using my encryption algorithm.

I was trying to use the physical location of the sender and receiver (and possibly their surroundings - all of which will be very difficult if not impossible to perfectly duplicate by a spy) in order to seed / initialize my encryption algorithm, but was having difficulty coming up with something that a 3rd party wouldn’t be able to deduce / calculate.

I like the idea of a signal degradation, as different environments will degrade the signal differently, so if a sender and receiver were sending data back and forth from California to Utah (for example), a spy in Nevada would not be able to deduce the signal degradation accurately enough (though they may be able to get close).

The seed / initialization data is not important, in other words, I don’t care what data is sent back and forth, as long as the data is unique enough such that someone snooping from in-between the sender and receiver, or from another country, wouldn’t be able to deduce / derive the same information accurately enough to be useful.

Basically the whole idea of the seed data is that it pertains to the physical location of both the sender and receiver. So that for someone to snoop in on the conversation (and successfully decrypt the data), they would need to be either at the exact same location as the sender and receiver or extremely close (which would likely tip off the sender and receiver that they were being snooped on).

Thank you for all your help,

Wow - you have a pretty steep RF learning curve to overcome here. Between two fixed antennae, the path between them exists in a bi-driectional form. It’s actually measured in dB, and there are a number of pieces of software that can determine this from topographical data, and of course in free space, without obstacles, signal strength drops at a linear and quite predictable levels. Your idea seems on the surface to be able to make use of this. In practice, however, RF is NEVER as simple as this. In the real world things are never predictable once RF gets closer to the ground.

Part of what I do is radio microphones in theatres and other event venues. Nothing guarantees RF gets even just a few metres. You can have a totally solid signal with the signal strength meter showing full, and then the person with the transmitter moves 1 metre to their left, and the signal strength vanishes totally! Yet another transmitter three times the distance away also shows maximum. Our receivers are actually twin units, and they have dual aerials. We tend to place one high up, and another somewhere else, often perhaps 5 to 10 m away. The receivers automatically switch to the strongest signal - and they do this constantly.

What causes these weird phenomena is something called multi-path interference. If you looked down on a typical radio base station aerial from space, and could see the RF, it would look vaguely like a donut with the aerial at the centre when the aerial system is omni directional, which is the most common, if you don’t know where your other users will be. As perhaps 359 degrees of this is wasted, you need higher powers to get further, and once you get to a certain point, increasing the power doesn’t really do very much. If you know exactly where the two points are who need to communicate, you can use a ‘beam’ style aerial which funnels most of the available signal in a certain direction, and with something the same the other end, the two points can be further apart. Sounds good. What can wreck your idea though is that some of this RF can reflect of things in the way, and this bouncing means that it is possible for the two signals to arrive at the destination out of phase with each other, because it has gone further, and the two can cancel out - a little (usually ok) but it can be complete - which is what happens with my radio microphones. These ‘black holes’ are almost impossible to control, and frequently change as the area has things in it that move.

More in a bit

Hello Paul,

Thank you for taking the time to explain everything.

Unfortunately I was looking for a way to pass data to another party in a random manner (which couldn’t easily be deciphered by a 3rd party), but should’ve stuck to more common methods (such as RSA), as passing data in what seems like a random manner doesn’t seem to be possible.

Apologies for taking your time,

Why not just use almost any digital radio and encrypt it?

If you’re talking amateur radio in the USA, it’s illegal…

Which brings me to this question, K6CPO, I use DMR. The available DMR radios have the capability to encrypt signal. Being that the technology and system was originally designed for use by “business band” users, it was/is perfectly legal for them to encrypt.

So comes the question: Why is it illegal for us, as Amateur radio operators to encrypt our signals? Other than, that is, “Because it’s the law.”

Thanx in advance.


That’s easy. Amateur radio is a community, licence users and of course radio listeners are in the hobby of sharing. Encryption robs these users of the ability to listen en clair - which is the core reason virtually all administrations world wide encourage it.

If you encrypt, you could have had a business licence.