Electronic Warfare and Walkie Talkies

[Shroom Man 777]

I am basically clueless on this and need a whole lot of enlightenment.

Anyway, I know certain forms of electronic warfare can jam radio transmissions. Now, I have heard (I cannot remember from where) that while some types of jamming can adversely effect long-ranged communications by radio, short-range communication by walkie talkie can still be possible despite the jamming.

Is this because long-range radio communications and short-range walkies work on different frequencies? I know that the power of the jamming devices are also a factor in this... (sort of like how certain Russian airplanes have radars that can burn through extensive jamming due to their raw power, and something-something vacuum tubes).

I would also like to know how militaries circumvent this. Counter-countermeasures? It's probably not as simple as Star Trek, changing frequencies and such.

Now, I know there are a lot of factors to this. Electronic warfare encompasses a lot of methods, countermeasures, counter-countermeasures, advanced technologies, and other stuff I am totally ignorant about (hence I'm asking folks way more knowledgeable than me ).


I probably got this in my head from watching Transformers, when their tactical military radios wouldn't work and so they had to resort to walkie talkies. What was up with that?

[Knife]

Terrain is an issue. Long range comm. tends to be retransmitted from various places that are high up to give good range. That's where you snow it with jamming.

Walkie Talkies on the other and usually have extremely limited range, like serious LOS. In rough terrain, and due to range ( less than 100 meters) you might as well shoot the guy as try to jam his radio.

[PeZook]

Walkie Talkies and long-range comms simply work on different wavelenghts, that's all. And jamming signals are limited by exactly the same factors as comms - that is, you can't just set up a short-wave transmitter 2000 km from the battlefield and expect it to effectively jam short range communications - you need to jam short wavelenghts locally.

From what I know, some counter-countermeasures do involve changing frequencies, and also hiding your frequencies from the enemy by scrambling your signals so that they look like white noise. I don't know much more, though. Then there's power, and there's the brilliantly simple solution of blasting any significant signal source with anti-radiation missiles

[The Jester]

Anyway, I know certain forms of electronic warfare can jam radio transmissions. Now, I have heard (I cannot remember from where) that while some types of jamming can adversely effect long-ranged communications by radio, short-range communication by walkie talkie can still be possible despite the jamming.

Radio Communications 101:

Radio carriers occupy fixed frequency bands. In order to effectively jam your communications, your enemy would need to transmit a noisy signal which occupies part of the same band you're using to communicate. If the noisy signal has sufficient power at the receiver, it can either corrupt part of the information received, or completely blanket it out. The key here is the ratio between the received signal power and the noise generated by the jammer (within the band of the receiver as out of band noise can just be filtered out). The objective of the jammer is to therefore direct as much of his available power into the frequency band of the receiver. Trying to cover the whole radio spectrum at the same time is a colossal waste of power resources for the jammer, especially when you consider that the path loss for higher frequencies is greater than for lower frequencies.

In the case you give, it is likely that the walkie talkies will operate on a much higher frequency than the long-range radio as they're not as concerned over path loss due to the shorter distances involved. Unless the jammer is operating over a very wide spectrum, he is unlikley to affect both signals with the same jamming signal. Of course, if he were to know all the bands you're using to communicate with (which he can do by passively listening in to your communications), he could focus his power into those bands and given sufficient power, block out your communications. The walkie talkies would, however, be more resistant to the jammers attacks due to the fact that they're operating on a higher frequency and with lower power (so their signal may not be detected by the jammer, who is typically further away), and are relatively much closer together.

Is this because long-range radio communications and short-range walkies work on different frequencies? I know that the power of the jamming devices are also a factor in this... (sort of like how certain Russian airplanes have radars that can burn through extensive jamming due to their raw power, and something-something vacuum tubes).

If you can maintain a high enough signal-to-noise ratio, then the jamming will be ineffective. To do this, you can always just increase your output power (or just get further from the jammer). In the case of radar, you will have very high gain directional antennas involved, which make more efficient use of your power compared to your enemy who would probably need to broadcast his jamming signal over a wider area.

I would also like to know how militaries circumvent this. Counter-countermeasures? It's probably not as simple as Star Trek, changing frequencies and such.

Your options depend on whether you're using an analog or digital system. Digital systems are significantly better for reasons I will go into in a moment. In both cases you can always just increase your transmission power (supposing you have the power available), or implement higher-gain directional antennas which allow you to use power more efficiently (though you obviously need to know in what direction to point your antenna). You can also implement a frequency agile system, which changes frequency bands very rapidly, making the jamming far more difficult.

With digital systems, you can improve signal to noise by implementing heavier error detection/protection (though this will reduce your data rate), or by using a modulation scheme with fewer symbols (this also reduces your data rate). The best solution that I know of though is to use Code Division Multiple Access (which was, surprisingly enough, developed for military purposes and is also used in 3G communications). The idea behind CDMA is to spread the information over a very wide spectrum using a spreading code. This yields two advantages against jamming. One, your signal is less susceptible to in-band noise (error detection/correction comes into play here to compensate for whatever noise you do get in-band). And two, since you're transmitting over a very wide spectrum, your signal will look like noise to any passive listener. So if your enemy doesn't know your spreading code (very important!), he will have a far more difficult time trying to figure out on what band you are transmitting. Of course, you can combine CDMA with a frequency agile system to make the attackers job even more difficult.

[Stuart]

Anyway, I know certain forms of electronic warfare can jam radio transmissions.

That's correct. The two generic types here are COMINT (COMmunications INTelligence) and Active Jamming. COMINT is passive, it relies on the interception of enemy radio transmissions. In a perfect world, we could decrypt and translate those transmissions so that we can understand what the enemy units are transmitting and act accordingly. Of course, the world isn't prefect and we can very rarely do that. Even in a "perfect world" where we have cracked the enemy cyphers, we are unlikely to be able to understand more than 10 - 20 percent of the intercepted transmissions.

However, understanding them is not necessary. What we do is use direction finding to get a cut on where the transmission was made from. That locates the transmitter and we have several options, one of which is to drop artillery fire onto that location. That's the crude response. The subtle way of doing things is to stick that location along with all the others that we have into a database. Then, we plot signal densities frome ach locations and note responses to signals sent out. The faster the signal gets a response, the more likely it is to be a higher rank source. We keep on doing this, noting who transmits, who responds, how fast, how often, what happens aftera signal etc etc etc. This is called Traffic Analysis and if its done right, we end up with a map of the enemy command network.

That's when life gets really fun; now we know what the command network looks like we can start to take it apart. Blowing things up is one response but playing the network against itself is a better one. Its possible to insert false messages into the net etc. Chaos reigns. Now, the other side don't like that so they have to defend themselves. There are quite a few dodges to do that. The more common one includes the use of dummy transmitters that broadcast misleading data (the intent isn't so much to deceive as to bury the real transmitters in the noise), the use of frequency-agile radios to make monitoring hard, the use of landlines for important messages and, as a last resort, using messengers etc.

Frequency-agile radios are very important bits of the arsenal. In fact, non-frequency agile sets are pretty much useless these days. What a frequency-agile set does is hop from one frequency to another at set intervals (at most a second or so, usually much more often; 600 hops per second is quite common according to the manufacturer's brochure for one such system I have here). This is supposed to make it impossible to intercept such transmissions on a prolonged basis. It doesn't quite work like that but frequency-agile is a valuable tool. The problems with it are that the band width over which the system hops is limited. The big difference between cheap frequency-agile radios and expensive ones is that the expensive sets have a much larger available band and can thus hop to more destinations.

Now, there is an obvious problem here. If the transmitter is hopping all over the place, how does the receiver follow it? There are two solutions to this. One is to have the next link embedded in the message itself so the tranmission will include a code that effectivley says "when this ends, hop here". This has a plus, its simple, it has a minus, its vulnerable and easily cracked. Most military radions have what is called a "Fill Gun" which inserts the day's hops and codes into the radio itself. This has the plus that its much more flexible and puts everybody on the same system, it has the minus that if somebody captures a radio, they can download the codes and crack the system.

Traffic Analysis also works with frequencies. As a rule of thumb, range is inversely proportional to frequencies. UHF radios are used for tactical work, VHF for operational level, HF for strategic and MF for grand strategic. That is a rule of thumb, there are a lot of exceptions to it. UHF radios are pretty much line-of-sight; they really don't work very well in built-up areas or heavily-forrested ones (you may remember Operation Market garden had severe radio problems due to inappropriate frequency choices; that wasn't actually a UHF problem but it illustrated several important lessons, one of which was the deleterious effects of trees and buildings - chosing frequencis used by other radio stations didn't help either. But, Frequency choice is a pointer to how important a given sender is. If on our computer display, one of our radio signals sources suddenly makes a long HF transmission, that's a pointer that said source is a pretty important command post (or that its a complete fake).

OK, we've gone into COMINT in some depth because its the basis of everything else. We can't jam something unless we know what it is we're jamming. So, COMINT is the wellspring from which everything else flows. (By the way, some sources confuse SIGINT and COMINT. The difference is quite simple; COMINT deals with communications; SIGINT deals with all electronic signals.) So, lets look at active jamming.

Now, I have heard (I cannot remember from where) that while some types of jamming can adversely effect long-ranged communications by radio, short-range communication by walkie talkie can still be possible despite the jamming. Is this because long-range radio communications and short-range walkies work on different frequencies?

Exactly; a short-range communication (say a personal radio/walkie-talkie) is likely to be in the UHF or high VHF band, a long range communciation will be in HF or even MF. So, jamming orientated against one will not affect jamming aimed at the other. As an initial bounce-off point we're looking at what is called barrage jamming. This means flooding the airwaves in a given frequency band with so much energy that the radio message simply gets drowned out. Its the sledgehammer approach. This is the primary step in the dance and its a good start. Note, we are jamming whole bands so it doesn't really matter whether the radio is a Frequency-agile radio or not; the whole band is swamped with energy. Oh grief.........

I know that the power of the jamming devices are also a factor in this... (sort of like how certain Russian airplanes have radars that can burn through extensive jamming due to their raw power, and something-something vacuum tubes).

This is indeed the problem. Now, the effectiveness of barrage jamming is directly proportional to the ratio of the strength of the jamming signal as opposed to the transmission signal at a given location. Now, the nearer that location is to the jamming station, the stronger the jamming signal, the nearer it is to the transmission station, the stronger the transmission signal. So, as we get further away from the jamming station and nearer to the transmission station, the jamming gets weaker and the transmission gets stronger and eventually the transmission signal breaks through. Oh happy day. So, the effect of barrage jamming isn't necessarily to prevent signals getting though but to reduce the range at which the transmission can be received. That's why airborne electronic warfare aircraft are so valuable, they can put the source of the jamming right over the transmission source.

I would also like to know how militaries circumvent this. Counter-countermeasures? It's probably not as simple as Star Trek, changing frequencies and such.

As I said, barrage jamming is just the start of the dance, its Step One. One of the problems of barrage jamming is that its spreading the available transmission power over a wide band; the wider the band, the less power at any spot within the band. The defense can exploit that. What they do is go to a spot frequency - a very tightly defined frequency and then put all their available transmission power into that spot. It's like using a bright spotlight in daylight; the spot and even its beam can still be seen despite the general illumination from the sun. So, that's the second step in the dance. The defender determines that his radios are subject to barrage jamming and goes to using spot frequencies. Of course, there is an answer to that - the offense picks up those spot frequencies and simply concentrates all its jamming power onto that frequency and - zowie - the spot frequency goes down. That's step three.

So we do the Star Trek and change frequencies. (Step Four) That's very effective and gives us about one millisecond of clear air before the jamming comes down again (assuming the jammers are asleep at the switch, it should be a lot faster than that). So, we switch again, very fast, over and over again - we back to Frequency-agile radios. They're boundinga ll over the place trying to get the message through while the offense is trying to track and jam them. The problem the defense faces is that no random selection is truly random; there's always an underlying pattern to the transmission frequencies; it takes the COMING boys time to figure out that pattern but they will figure it out - and then they can match their spot jamming to the defenses spot transmissions. That's Step Five.

However, the dance still isn't over. The offense is now using spot jamming, concentrating all its jamming energy into the spot frequencies used by the defense. So, the defense goes to what is called spread frequency transmissions. In other words, instead of concentrating all its energy into a spot, it spreads it out over a whole band so that even if some parts of teh band are jammed, others are not and the message gets throught. That's Step Six. Spread frequency is countered by barrage janmming and we're back to Step One, repeat as necessary.

The upshoot of all this is that no radio transmission will stay unjammed very long. So, its essential we get the max message in the minimum time. Transmitting long speeches is decidedly unwise. So, we use a thing called burst transmission. Essentially what this does is record the message, compress it and then transmit said message in the shortest possible burst (usually a millisecond or so). The receiver picks it up, stretches it out and plays it to the listener. The plus of this system is that the very short burst transmission may go unnoticed or be so short that the COMINT boys are unable to get a cut on it. The minus is that its very prone to distortion and causes misunderstandings. The Germans were first to use burst transmissions (a system called Kourier) in WW2 although Kourier was very crude by modern standards.

I probably got this in my head from watching Transformers, when their tactical military radios wouldn't work and so they had to resort to walkie talkies. What was up with that?

Technically, that doesn't work although its possible that the civilian radios were on frequencies that military ones don't use so the COMINT on the other side was caught flat-footed. However, its probably better to assume the scriptwriters were simply unaware of the technical details. EW is a black art, so much of it is classified that what we can talk about is severely limited and the rest tends to be distorted. If you're seriously interested in EW, the best US source is the Association of Old Crows (yup, I'm a Crow) who are the professional association for EW specialists. They do a lot of handbooks that take one up from basic information through to advanced technology. Their magazine, Journal of Electronic Defense (JED) is the absolute required professional reading for anybody interested in EW. A guy called David Adamy has published two books EW101 and EW102 that are compilations of basic technology articles from JED. They're really good.

The other excellent source was a Brassey's EW Handbook. Note the was. The UK MOD saw the unpublished draft, threw fifty purple fits, had multiple conniptions and went into spasm. Then, they cut out everything that they considered to be a security breech and the book went down from 540 pages to 107. Needless to say, those who have pre-production copies printed before the great slash treat them like gold (the selling price is around six figures these days). Even the cut-down version is a goldmine of data.

A word of warning, EW is addictive and everybody who dabbles a toe in that water gets hooked. Then, the victim goes very strange indeed and isn't in quite the same world as everybody else. They get a strange remote gaze and fall into the habit of staring intently at unfamiliar antennas. They also get arrested at regular intervals for doing that.