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How to Protect Radio Signals Over Short Distances? 93

Posted by Cliff
from the broadcast-security dept.
anth_007 asks: "I'm a software developer seeking to delve into the hardware world. I am trying to create a prototype for a new product that would broadcast signals over a very short range (100s of meters). The important part is that only authorized users (ie. no hackers) would be able to broadcast signals. Well, a more accurate description would be that anyone could broadcast (it's just a radio transmitter after all), but receivers would be able to differentiate between authorized signals and those which are not. I've been looking to find information on Satellite Radio (XM, Sirius, etc) and how they protect their signals, but I haven't had much success. I realize that I'm talking about two different problems here (I want to stop unauthorized people from broadcasting, satellite radio is trying to stop unauthorized people from receiving), but I need to start somewhere. Any ideas? How do the big boys protect their signals? Is there any readily available technology out there that would allow me to accomplish this?"
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How to Protect Radio Signals Over Short Distances?

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  • Idea (Score:3, Funny)

    by addaon (41825) <> on Wednesday March 30, 2005 @04:52PM (#12093010)
    Use shielded signal guides. You can even get ones that work up to optical frequencies these days!
    • Re:Idea (Score:3, Funny)

      by CXI (46706)
      The parent post is currently marked as informative by suggesting a "shielded signal guide"? Sure, you mean a WIRE?! Come on people, I'd agree with funny, but informative?
      • Actually, I was going to suggest the same thing: use wires. There's no way to prevent other people from broadcasting on the same frequencies. Sure, you can probably do some kind of key-signing to authenticate the signal, but that doesn't solve the problem of someone else simply drowning out authenticated signals with sufficient noise.

        Authenticating and "protecting" the signal are two separate problems. If you use the aether to carry your signal, only the former is soluble (unless you can invoke the gove

        • the answer is timedomain

          rather than using a sine wave - it uses square waves at predetermined intervals - which intervals are known only to the secure pair.

        • Yeap. You should use the latest technology trends:
          - Coax cable in a "Bus" topology.
          - BNC connectors and T-shaped spliters for each device.
          - 50 Ohm rezistive terminators at the end of the Bus
          - And use the latest transmission devices using the industry proven 10Base2 transcievers which can send or receive data at up to 10 Mbit/sec ! They use a 20Mhz carrier so you don't get interference with wifi. The range is up to 200 meters.

          You can even run IP over this advanced data transmission setup.
      • A shielded signal guide could include coaxial cable, microwave waveguides and other conductors, but "wire" would not qualify as shielded or a "signal guide". It's a simple conductor.
  • Signatures (Score:1, Informative)

    by Anonymous Coward
    You could use some sort of signature (GPG, PGP, your own, whatever) to see if the broadcaster is authorized.
    • The poster is asking if s/he should re-invent the wheel, and while using gpg/pgp would provide some level of code re-use, there is a better way...

      As there is no "scramble proof" radio technology, the best you can do is authenticate the sender and reciever. You can't prevent anyone from sending or recieving the radio signal. Although you can prove the authenticity of teh sender, and the authorization of the reciever. The best way is to use 802.11x to secure your radio transmissions. All the hard work is alr
  • Tinfoil (Score:5, Funny)

    by MarkGriz (520778) on Wednesday March 30, 2005 @04:58PM (#12093086)
    Tinfoil........lots of tinfoil.
  • but public/private key encryption would do exactly what you're looking to do.
  • Buy an FCC license (Score:3, Informative)

    by duffbeer703 (177751) * on Wednesday March 30, 2005 @04:58PM (#12093099)
    If you buy a license to broadcast over specific wavelengths, the Feds will happily track down unauthorized broadcasters, arrest and possibly even fine & imprison them.

    If you are using some unlicensed spectrum like the 2.4GHz band, you have no recourse. Any unlicenced radio communication is required to not interfere with other people's use of the spectrum, and accept any interference without recourse.

    • by dougmc (70836)

      If you buy a license to broadcast over specific wavelengths, the Feds will happily track down unauthorized broadcasters, arrest and possibly even fine & imprison them.

      Correct in theory, though in practice not so much.

      You can complain to the FCC, but they're unlikely to send somebody to investigate unless it's an extreme and repeated violation.

      More likely is that you'll need to find the offender yourself, then give the FBI your evidence, and maybe they'll send them a nasty letter that they mu

    • If you buy a license to broadcast over specific wavelengths, the Feds will happily track down unauthorized broadcasters, arrest and possibly even fine & imprison them.

      Yeah, and instead of using a secure encryption scheme, you can just depend on the DMCA to bust people who crack your code, right?

      Get real.

      • Secure encryption does not prevent jamming by any malicious third party. This problem is one of the main reasons why the FCC exists in the first place.
        • I see... By making something illegal, you can prevent criminals from doing it. That's genius.
          • Certainly, anyone implementing some sort of wireless communication system should exercise due diligence in encrypting and/or cryptographically signing traffic, using directional antennas to the maximum extent possible, and using some spread spectrum technology to avoid narrowband interference, but guess what? It will still be vulnerable to jamming. This is a fundamental characteristic of radio communication, and the only practical way to get around it is to make the person jamming your transmissions stop
  • 128-bit encryption

    Transmitting signals into the air leave it open to anyone within line of site of the transmitter to receive the energy and do what ever they want to with it....

    There are antennas with very narrow beamwidths, but new DSP radio receivers can really pull signals out of the mud
  • Spread spectrum (Score:5, Informative)

    by spaceyhackerlady (462530) on Wednesday March 30, 2005 @04:59PM (#12093103)

    With the right despreading key, you get signals.

    Otherwise, you get noise.


  • by cbiffle (211614) on Wednesday March 30, 2005 @05:00PM (#12093115)
    Radio waves are radio waves. Once your frequency, modulation system, etc. are documented, anyone can fake your radio waves.

    It's far easier to secure the higher layers (in the OSI sense) -- the data you're sending over the radio waves.

    Authenticating this is pretty trivial, from a crypto standpoint. The simplest approach is to encrypt your data with a shared key; more robust approaches might include signing unencrypted data with a private/public keypair, etc. This would also introduce some noise resistance, since you could tell if the data's been corrupted by checking the signature. (Though for non-malicious interference, you're probably better off using a redundancy check of some sort.)

    Good luck!
    • That covers most of what I planned to say, but I do want to add that the best data protection is "end to end". Don't try to protect every link you use or even the least secure one. Protect the entire data path end to end and you'll have far fewer problems. You'll still have to worry about traffic analysis, but that's an expensive one to stop.

      There are plenty of existing solutions for this (SSL, ssh, etc). You've probably already used them.

      Also, there is no such thing as a short-range radio signal.

    • Radio waves are radio waves. Once your frequency, modulation system, etc. are documented, anyone can fake your radio waves.

      Not really true.

      It's far easier to secure the higher layers (in the OSI sense) -- the data you're sending over the radio waves.

      Except that does nothing to protect you from DOS attacks.

      This guy was asking for a system where no one else could transmit but authorized users. Obviously you can't actually prevent someone from building a transmitter, but you can design your modulat
      • timedomain uwb largely prevents dos. the jammer would need to introduce overwhelming noise at most if not points in time on all frequencies. this inherently problematic and sure to be noticed.

        digital radio provides the option of infinite channels consisting of a shared time base, and any psuedo random sequence of signal delays.

        with two or three antennas - it would be possible to cancel a rouge source based on spatial diversity

  • IIRC
    Since Sirius are digital broadcasts, they can encrypt their signals to whatever encoding they'ed care to use. No?
  • This is a very vague and probably not all that interesting idea, but would it be possible to set it up so it's not all that practical without a repeater? If so, then you'd have a central place to moderate the signals. That sort of make sense?
    • Pardon the computer terms, but there is absolutely no difference between the client -> repeater -> server interaction and the client -> server interaction. All the repeater is is a server that rebroadcasts. Any authentication you could do on it would be just as easy to do on your server. You can't "firewall" using a repeater either, because all an attacker needs to do is pretend to be a repeater. So, to answer your question, no it doesn't make sense.
      • "Pardon the computer terms, but there is absolutely no difference between the client -> repeater -> server interaction and the client -> server interaction."

        Well, the point was that the central computer (repeater) would 'own' that frequency/channel. The devices wouldn't accept commands from anybody else. (I did not express this clearly.)

        I'm not holding onto it as a good idea, rather just suggesting something I remember from back in my ham radio days.
  • by Naikrovek (667) <jjohnson@ p s> on Wednesday March 30, 2005 @05:03PM (#12093155)
    ... trying to make radio not transmittable is like trying to make water not wet. (ripped off from jwz i think, and transformed to fit your question)

    the only way to actually attempt to control who transmits on what bandwidth would probably require some sort of licensing from the FCC. that way you have some sort of legal recourse if someone does decide to walk on your airwaves.

    there are lots of techies out there who know how to build transmitters and recievers that can send and recieve at any frequency they want, so trying to put a technical barrier to the actual airwaves won't get you anywhere, as you mentioned.

    an elegant solution would consist of some sort of stream verification, so that you can verify the sender of a signal via fingerprint, encryption, hidden messages, etc. Something that you control and that is difficult to reverse engineer would allow you to differentiate your signals from someone else's.

    I think XM and Sirius encrypt their streams, or at least apply some sort of proprietary DRM to their streams to keep idle listeners from tuning in. They also use a fully digital stream, so encryption is pretty easy. And if I remember, they use an odd frequency that must be licensed from the FCC to use. The combination of those things seems to work great for them, a quick search on satellite radio hacking revealed almost nothing useful.

    • Welll, you COULD stop people from transmitting with a large chain link fence. If you fence off the entire area within 1 mile, you can be sure that nobody will transmit within 1 mile...

      OK. With that sarcastic answer out of the way, the parent is completely correct. You CANNOT keep other people from transmitting on your frequency. But what you CAN do it know if the received signal is from the desired source by using cryptography (assuming that your information is digital). Of course, jamming is still a
    • ... trying to make radio not transmittable is like trying to make water not wet.

      The key is not to make radio waves not transmittable, but to make it easy to reject any radio waves other than the ones you don't want.
    • The most effective means to deal with an unauthorized transmission is make it very easy to see when someone is not performing the authentication right, or there is transmissions from a source or type or format that you do not expect. Then the person can go looking for the offender with an appropriately sized stick (literal, or otherwise).

  • by inio (26835) on Wednesday March 30, 2005 @05:04PM (#12093171) Homepage
    There is no practical way to provide secure RF transmissions. However, there are many many ways to protect the data carried on those transmissions. What you're interested in is public-key cryptography as it applies to message signing. It works like ths:

    1. Send block of data
    2. compute fingerprint ("hash") of the transmitted data
    3. encrypt that fingerprint with the transmitter's private key
    4. transmit that

    on the receiving end, you do this:

    1. receive data
    2. receive encrypted hash
    3. compute hash of received data
    4. decrypt received hash using sender's public key and compare to actual hash of data

    This process is called signing, and is used in many many places. The use you've most likely encountered is to verify the identity of online stores. (the store's encryption certificate is signed by the trusted certificate authority that issued it. Your browser has all the authorities' public keys built into it, and uses it to check the signature against the certificate contents.)
    • Of course, what happens if you move your reciever around or something else causes a period of affected reception? The transmitter has no way of knowing what packets of data you did or did not recieve, unless your reciever has a transmitter of its own... and one powerful enough to transmit back to the reciever, without interferring in its broadcast (or the other recievers) so that it would know whether to resend content - and to which recievers which packets should be resent.

      I suppose it depends on what con
    • There is no practical way to provide secure RF transmissions.

      Sure there is.
      Take military GPS for example.

      Try and explain to me how you're going to spoof a military GPS signal......

      What this guy needs to do is research something called Electronic Counter-Countermeasures (ECCM). This is a common military problem.

      For example:
      Joe's shooting a radar-guided missile at Steve. If Steve can send back enough fake radar pulses to throw off the missile's tracking, Joe will never be able to shoot him dow
  • by marcus (1916) on Wednesday March 30, 2005 @05:07PM (#12093203) Journal
    All you need is an LED at the transmitter and a telescope with a photodiode at the receiver. No one outside the field of view of the receiver would be able to interfere. If you want even better protection, put a telescope at the transmitter too.

    A telescope is effectively a very high gain "antenna."

  • You want the receiver to distinguish between an "authorized" broadcast and an "unauthorized" broadcast.

    So the broadcast must be tagged in such a way as to allow this identification to be made (since potential transmitters cannot be controlled -- as you observed).

    Encrypt the broadcast, or transmit a hash (or equivalent). Be sure to consider "replay attacks". An old message can be recorded and played back to the receiver at a later time.

    And that's really all that you can do. Be sure to choose a cipher or h
    • If I understand ... You want the receiver to distinguish between an "authorized" broadcast and an "unauthorized" broadcast.

      I suspect he also wants to counter hostile jamming. Redundancy and spread spectrum will help towards that goal.
  • The usual means to prevent unauthorized broadcasting is to have a lawyer send them a letter. Even better if the lawyer works for the FCC! However, if the short-range broadcasts meet the FCC part 15 rules (in the US anyway), you're out of luck in preventing anything.

    Beyond this, the "big boys" recognize that it is most difficult to prevent the dedicated and motivated from transmitting before the fact. They deal with the problem in two ways:

    The first is jam-resistant modulation schemes - so the miscreant

  • anth_007 asks? (Score:1, Insightful)

    by Anonymous Coward

    Can't you just ask Q?

    Public key cryptography solves this. Only transmitters get the private key, they sign the communications with it, and anybody with the public key can verify that the private key signed it.

  • by stevew (4845) on Wednesday March 30, 2005 @05:11PM (#12093272) Journal
    The guy who mentioned "Despreading" had the right idea. Use of "Spread-spectrum" direct sequence modulation is probably the most secure you're going to get. Essentially, a "key" is used to control the spreading code. If you don't have the key, you can't receive the signal.

    Further, the transmission by it's very nature is somewhat covert. Now - if you're the NSA, perhaps you can figure out, but it's likely to be able to put together a system that is for all intents and purposes "private" using this type of transmission.

    Now -FCC licenses, and spectruma are your REAL problem. The technical problems have solutions.
  • Sounds like you need to use GNU Privacy Guard, or some streaming equivalent to encrypt the source stream. You need a few ingredients:
    • A source stream of data
    • A stream of random keys
    • Block encryption to secure the stream in transit
    • An RSA encoded stream of the above keys, encoded for each applicable user
    • Broadcast the block encoded data, and the RSA'd key streams on a shared channel
    • Decoding of the key stream at the user
    • Decoding of the data stream with the keys

    This makes the same compromises Phil Zimmer

    • I read it, understood it, and still replied to the wrong aspect of it. Take the above, and use RSA to SIGN the keystream. Leave the data stream encrypted, Don't just count on a hash. It'll force everyone to decode it, but then you don't have to worry about hash collisions.


  • WCDMA (Score:2, Informative)

    Spread spectrum such as WCDMA (Wide band code division multiplex) is probably the best way to prevent jamming. However there are a couple of difficulties: 1. You need a wide spectrum to spread the energy over. I don't know the details of spectrum allocation in your juristiction but it is unlikely to be available anywhere (except for the military). 2. Whatever the bit rate you transmit at (slow is best) a jammer will always be able to swamp your signal by raising the noise floor sufficiently. The best wa
  • Buy a license from FCC [] and then use their power to prosecute anyone who transmits on your licensed frequency. They have the power to shut down anyone who squats on licensed spectrum.

  • Broadcast and receive your signal directionally (with a dish, for example). This limits the places where the bad signals can be. There's no magic way that a good signal can be distinguished from a bad one, but you can make it harder to receive signals from directions that you don't want to receive from.

    Encrypt your traffic. This prevents people from sending you crap that you might mistake for good stuff, and it prevents others from reading your transmission.
  • Isn't authenticating a group of broadcasters and receivers to work together and disclude anyone without the right authentification exactly what task groups like 802.11i have been working on for years? Why not just borrow the ideas they're using and customize them for your own needs?
  • The only way to make an electromagnetic wave secure--short of sticking your entire system inside of a Faraday cage--is to make the information it sends secure. Higher level encryption is the way to go, here; most likely either redundancy, a checksum, or the like.
  • But since you're new to the hardware side of things, maybe this will provide an easy transition: [].

    Basically, it's a software-controlled receiver/transmitter, which makes it easy to pull signals out of the air, store and analyze them, and generate your own signals for broadcast.

    I saw a nifty demonstration of it once. So I've already told you about everything I know.

    As to your actual question, I agree with the general consensus: It's a crypto problem, not a ha
    • It's a crypto problem, not a hardware problem.

      Partly. Spread sprectrum is at least in part a hardware problem (in that it requires certain hardware).
      • True. On the other hand, the goal of spread spectrum isn't just to keep people from listening in. It's also trying to make the signal difficult to jam. If he wants to solve that problem as well, SS may be an option for him.
        • the goal of spread spectrum isn't just to keep people from listening in. It's also trying to make the signal difficult to jam.

          Quite right. I interpreted the original Ask Slashdot question, where the asker said "I want to stop unauthorized people from broadcasting" as meaning he wants to prevent jamming, since you can't really stop people from transmitting a radio signal.
  • First of all, you need to define what part of the world you are planning on doing this in. Different parts of the world are governed by different agencies, and different laws. Information that may be valid within the US may not be valid in the UK, for example.

    That said - let me examine your statements from the context of a project to operate within the US.

    First, there is the question of what frequency you want to be operating in. If you are wanting 100's of meters, and yet you are planning on operating in
  • hop frequency (Score:3, Insightful)

    by imsmith (239784) on Wednesday March 30, 2005 @05:43PM (#12093641)
    if you want to make sure that the RF is only being sent and recieved by you, you have to set up a system where the frequency "hops" - changes very rapidly in what appears to an outsider as randomness. You do this by using cryptography to establish a connection and share a seed value for the variable that initiates the hopping sequence and a synchonization pulse for their internal clocks.

    Then you need to build a transmitter and a reciever that will be able to change frequencies very very fast (hundreds or thousands of times per second), keep in sync with one another, and send packetized digital data between them.

    If you are really paraniod, you encrypt the signals going to the transmitter, allow the transmitter to encrypt them again, the reverse the process at the receiver side.

    If you are really, really paranoid, you iterate that cascading pattern over several layers of the network, multiplex the signal to the radio, and broadcast a solid stream of encrypted information, filling it the gaps between "real" information with garbage so that there isn't any variation in the RF between when you are saying something and when you aren't.

    If you are more paraniod than that, you unplug everything, use very low tech methods executed by fanatically loyal zealots who would rather die horribly painful deaths than bring shame on their families, betray the cause, or endanger their eternal soul.

    If you want to stop the paranoid, really paraniod, and really really paraniod folks from communicating, you build your own frequency hopping device, add a modulation hopping function, connect it to a really big tesla coil mounted on an electronically isolated & shielded truck, and broadcast megawatts of RF into the atmosphere while driving around in arbitrary paths.

    If you want to stop the zealots, pull a Keiser Soze on them and kill them and everyone they've ever known, their dog, their neighbor, the guy who sold them coffee this morning, etc. Iterate until there aren't any more zealots.
  • Basically, the concept is that you use an array of antennas and cross-cancel signals for areas you do not want to recieve (or send to). See [] among other sources.
    Transmit a broadband signal to all recievers. Have the receiver narrow it's coverage to your area and send an authentication request. Then you are "in" if you pass, and if not the receiver decreases signal reception and transmission to your geographic area, and could even pass that information to other
  • How do the big boys protect their signals?

    You are worried about others transmitting on your bandwidth? Well don't worry there is a federal agency just for this. It's called the FCC. I'm sure that your company is paying a very large amount to license the spectrum that you want to prevent others from transmitting on. Well, just identify others that are transmitting on your licensed spectrum and the FCC will go out and if they don't stop transmitting on your licensed spectrum space they will be fined $10,000
  • To answer your question, we really need more information. For most situations, an encrypted link between the sender and reciever would do what you need; this can be done in hardware fairly cheaply and would work well. Directional antennas would also work.

    Still, there are situations where these would be unworkable: severe cost restrictions might rule out encryption, or a moving transmitter might rule out a highly directional antenna on the reciever. What is needed is more information: what are your limita

  • Depends ... (Score:2, Informative)

    by a2wflc (705508)
    It depends on the type of data and on who else will be broadcasting.

    I assume it's a digital signal you are sending, but if it's analog audio you can do some cool things by adding something that sounds like white noise to the transmitter, then subtracting it on the receiver. A "sync" signal will need to be sent to get them together (or transmit the "white noise" on another frequency if 2 tranmitters is feasable). If the transmitter isn't adding the "white noise", but you subtract it, then the audio wil
  • Is there some reason SSL won't work for you?

    You trust your credit cards, personal information, and bank account to it, so why wouldn't it be good enough here?

  • You need to provide more information to get a coherent answer. You mention a range of 100's of meters, are the endpoints fixed with line of sight between them for antennas or are you going through concrete and steel walls? What bandwidth do you need? Is this an intermittent signal like polling for data or a continous stream of audio? These questions have implications on the frequency, power, bandwidth and modulation scheme of the signal generated. Give more info on the application and why you can't have "ha
  • "ssh" pun (security? sound of static on radio?)
  • by chinakow (83588)
    to stop people from transmitting you would have to restrain them but from what I have read DSS could help avoid jamming of your signal.
    check here for a brief overview: tm []
  • ultra-wideband
    supposed to sound like static
    only readable/interpertable if you know exactly what you are looking for.

  • by nullset (39850)
    Check out Zigbee, it has some support for encryption in the protocol stack. [] has some info that may be of use. Zigbee is a "reliable, secure, low power wireless communications protocol based on IEEE 802.15.4" --buddy
  • a wonderful new RF transmission system that's almost impervious to outside interference. It's called coax cable.
  • At the very least get an Amateur Radio License. It is an easy way to learn the basic technology behind radio, and will also aquaint you with the legal aspects of broadcasting.

    You should be able to get a no-code license in a few months through a local club. The Amateur Radio Relay League [] is a good place to start.
    • Excellent advice (glad someone thought of it). There are many digital modes available in the Amateur Radio Service. It would provide an excellent learning foundation on Spread Spectrum and other modes as people have already listed. The commercial nature of the traffic, or it's encryption, cannot be broadcast in the spectrum allocated to amateurs, but you will sure find lots of people with encryption experience from the commercial radio side within your hobby. Maybe look up your local amateur radio club
  • spread spectrum is pretty cool, but to be /really/ effective, i'd suggest throwing in some steganography. for example, instead of choosing a single frequency at a time, you pick three and simultanenously transmit on all three channels. you choose one of those channels as the 'true' channel, and (using a stochastic process of some sort, i'd imagine), manufacture plausible signals on the other two channels that are totally ignored receiving side. joe average will pick up that you're using spread spectrum, but
  • realize that it's RADIO WAVES.

    Everyone's "obvious answer" is encryption of some sort, but no one realizes that no matter what you do, someone can come in and broadcast out noise on your frequency to pretty much kill your broadcast in all regards. And THAT is what you need to really protect against. The encryption only makes sure someone isn't receiving when they shouldn't be.

    To get it into your hard wired minds... you can encrypt anything and send it over a cable, but if someone hooks up a vacuum cleaner'
    • If I use direct sequence spread spectrum with a high chip rate, you are going to have a difficult time detecting that I am transmitting a signal, and the power requirements for a brute-force jammer can be made impractical, not to mention that your jammer is going to attract a lot of unwanted attention.
  • Perimeter-based: Use triangulation, antenna arrays or similar tricks to check that the source is located within an authorized area. See how the SETI guys can differentiate transmissions from ground sources, satellites, our galaxy, infinity and beyond.

    Active denial: Microwave the intruder's rig.

    The Right Way: Use crypto, of course.

  • by cinc (207139)
    As long as the other guy has enough power to drown out your signal there is really nothing you can do about it. Anyone can broadcast on your freqs (in a technical sense) and you would be essentially helpless until they stop, or they get shut down
  • 1. Spread Spectrum.
    2. A public key style encryption.
    3. Highly directional signal if possible.

    You could combine them as well to make it as secure as possible. Have fun.
  • Make it line of sight, like microwave links. Of course only if that's suitable for the intended task.

    Encryption will help, but it's still defeatable with brute force.
  • Designing a decent waveform framing and authentication system is a serious, multi-year commitment, both for the theory and hardware, and there is no reason you should have to do it. What you want to find is an analogous requirement and use what exists.

    Sirius and XM Satellite radio are probably not the best place to start, since they use proprietary waveforms, and they are designed to avoid dropouts as you drive around in your car. They do this by combining two satellite signals and hundreds of local ter

  • Thanks for all the feedback so far. Allow me to provide some more info as it seems my original question was a little ambiguous.

    First, I am not really transmitting much info. A simple "signal is on" is ok... perhaps a bitmap, but that's it. I'm not transmitting audio/video/data at all. In terms of the broadcast stations, they must be mobile.

    There can be no line of site.

    I don't care who picks up the signal or what they do with it... the data is unimportant.

    I just want to make sure no one can b
  • Digitally Controlled Squelch using PLs
    Basically a signal broadcast at the same time as the communication signal that opens the squelch up on the receievers. This is what most Police/Fire/Safety Departments use on their radios. You may also try using a (Digitally) Trunked System.

    How PL Systems Work []
    What is Trunking? []

What is wanted is not the will to believe, but the will to find out, which is the exact opposite. -- Bertrand Russell, "Skeptical Essays", 1928