unjammable guns next?

[ancientintegra]

This question is probably for Jim, or maybe DS can ask his engineers.

What is to stop a laser gun mfg from having the gun emit its pulses at random intervals? This would make it impossible for a jammer to predict when the next one was coming so the current approach to jamming would be useless (look up table or sync). The time of flight is so short compared to the interpulse interval that there is a huge amount of time to play with and still keep the average pulse rate high enough to get reliable speed readings (as reliable as any laser gun anyway).

The only way I see to jam such a gun would be to emit a high frequency, randomly timed set of pulses, but that would be a very clear signal to the gun that it was being actively jammed and would, I think, be able to generate an error code that would rarely occur falsely. Thus, such a jammer would be useless where they are illegal and highly suboptimal even where legal.

[ancientintegra]

Couldn't a different gun test for active jamming by having regular pulses, but randomly dropping one and looking for a return flash (where none was sent)? Again, this would be a pretty clear and reliable signal of active jamming, and in this case the jammer probably wouldn't even notice (since I assume they don't manage to detect every laser pulse) even after it was too late (obviously impossible to detect the impending lack of a pulse). Thus the gun could drop several pulses a second to get a reliable jamming indication.

[Ford_Focus_05]

well from a electronics stand point random disorder is really not possible... you can pause you intervals and what not but to be completely random well thats un computer like!

Thats my take on it...

Stealth mode with the Laser Atlanta works like that... they send 2 pulses waits a bit then sends 2 more so on snd so on... typically the dectors and jammers wait for 3 pulses before they start jamming.

Well I am no Jim but thats a stab...

[ancientintegra]

well from a electronics stand point random disorder is really not possible... you can pause you intervals and what not but to be completely random well thats un computer like!

OK, this is true but irrelevant. Computers don't generate random numbers but they can generate pseudorandom numbers that would be impossible to predict. If the pulse rate needs to average 200 Hz the interpulse interval needs to average 1/200=5 ms. Since the gun timing is obviously good to, say, 0.02 microseconds (3 meter distance * 2 ways divided by 3E8 m/s), this would give the gun at least 250,000 different intervals it can choose from. Without being an expert, I'm pretty confident you can't guess the pattern of pseudorandom numbers with a few hundred samples (a long, ticketworthy 2 seconds of lasering) out of 250,000. Especially given that the jammer has to deal with the changes in interval due to changes in distance which it is not aware of (it could assume you are moving at constant speed but I think you can see that a subtle change in speed would cause it to pick a different number and hence a different pseudorandom number pattern). Even if you can, I would suspect you are now incorporating a super computer in your jammer to figure this out in real time.

Thats my take on it...

Stealth mode with the Laser Atlanta works like that... they send 2 pulses waits a bit then sends 2 more so on snd so on... typically the dectors and jammers wait for 3 pulses before they start jamming.

Well I am no Jim but thats a stab...

That is hardly as difficult to jam. If the 2 pulses are evenly timed, then you can at least try to jam the second one (after you've seen a pair or two to figure out the stealth mode you're dealing with). It's even easier if the interpair interval is constant, in which case you can try to jam the first one too. Although the car is moving and the intervals are therefore changing, I think that since the gun can't wait too long (or else the car will drive away), the intervals don't change by too much. And this is presumably why these guns can now be jammed.[/quote]

[jimbonzzz]

The gun's processors arern't fast enough to measure pulse timing absolutely in order to determine speed. They currently use some external timing tricks in order to pull the high resolution measurements off, like timing the discharge of a capacitor of a known value. I don't know how well these tricks would work with a "random pulse" scheme, or if it would make implementation of something like that especially difficult. But I have no doubt though that such a scheme is possible.

But I think there are other things they could do too. For example (though it would present other challenges) a gun could modulate or encode the pulses in a manner which the gun could attempt to recognise it's own pulse returns and discern them from any jamming pulses received.

Jim

[nvr2fast]

Remember fellas, you're needing randomness, but it doesn't need to be crypto-grade.

[ronsc1985]

This question is probably for Jim, or maybe DS can ask his engineers.

What is to stop a laser gun mfg from having the gun emit its pulses at random intervals? This would make it impossible for a jammer to predict when the next one was coming so the current approach to jamming would be useless (look up table or sync). The time of flight is so short compared to the interpulse interval that there is a huge amount of time to play with and still keep the average pulse rate high enough to get reliable speed readings (as reliable as any laser gun anyway).

The only way I see to jam such a gun would be to emit a high frequency, randomly timed set of pulses, but that would be a very clear signal to the gun that it was being actively jammed and would, I think, be able to generate an error code that would rarely occur falsely. Thus, such a jammer would be useless where they are illegal and highly suboptimal even where legal.

You have that exactly right. This would be relatively simple to do and probably will be done if and when laser jammers become a problem for law enforcement. You have to remember the random part only appears random to the jammer. The gun knows exactly when it sends out pulses.

Generating random number is a really simple process and the sequence cannot be detected by analysis in real time by anything that can be implemented in a consumer based jammer. Even in the crypto world breaking the random number sequence usually takes powerful computers and is not in real time,

[stalker]

it wont matter what a gun does, the def 2 has pulse master software..it watches any pulse no matter how random it is...

[HighVoltage]

Its relatively simple to generate a true random number. You just need to seed the psuedo random gen with a random number, with a point of entropy outside of the system (i.e the gun). Easiest way is to grab some atmospheric noise via radio or they could even sample audio outside the gun (traffic sounds, birds, chatter, etc)

[jimbonzzz]

it wont matter what a gun does, the def 2 has pulse master software..it watches any pulse no matter how random it is...

Sure, it can "watch" any pulse it wants.

And it might be able to jam a gun if the pulses are a different random pulse rate every time the trigger is pulled, like 100pps on time, 270pps the next, 240 the next etc.

But if the pulses are truly random, there's no way to jam except for the high freq method mentioned above used by the old LE-10 and Target LE-850.

The only way any kind of "pulse link" technology could possibly work is by predicting the next pulse based on the previous pulses received, and then sending a jamming pulse slightly before it thinks the predicted pulse will be received so that it reaches the gun before the reflected predicted pulse does so that the jamming pulse is accepted as the guns own pulse. If the jamming pulse doesn't make it into this "window" it will have no jamming effect whatsoever. Since the pulses in such a scheme would be random, there would be no way to predict future pulses based on past pulses received, and no way to ensure jamming pulses get into the "window".

Pulse linking would have to work as described above, because there's no way for a jammer to receive an individual pulse from the gun and then get a jamming pulse out before the received pulse reflects off the vehicle and back to the gun. Unless a jammer has the ability to bend the space time continuum or utilize wormholes, I don't see this happening. But somehow I wouldn't be surprised if you were to tell me that DS already has such a feature in development