How the heck do laser jammers work

[mildlyamused]

So after reading too many posts on here, I got curious and started hunting down patents both on the Lidatek LE-10 (which I have, and have never used) which I believe has a flaw in the description, and on lidar units themselves. In particular I was always rather curious how a lidar gun is able to so accurately measure distance. When you send a pulse out and it comes back, light is traveling effectively 150 million meters per second, so that means to get 1 cm accuracy you'd need roughly a 1.5GHz counter, and to get mm accuracy you'd need more like a 15GHz counter. While this is doable, its not really in the realm of reasonable for relatively cheap mid-90s commercial technology.

While reading through LTI's patents, I was actually quite impressed (something I rarely am when reading patents since most of them appear to be total garbage). Besides explaining at least roughly how they deal with the above issue they explicitly or implicitly mentioned that they have the following capabilities:

1) Ability to randomize when they send out a pulse for the purpose of fooling laser jammers that want to try to memorize their pattern. (This was explicitly mentioned as being intended to counter laser jammers).

2) Ability to measure and alter the exact length of the pulse they send out

3) The ability to dynamically adjust the "window" in which they observe the receiving pulse. In otherwords, if a jammer sends out a pulse and its not received while the window is open, the pulse is completely rejected -- and they explicitly state they do adjust the window after each pulse received.

Here's the deal... Let's say you have a vehicle moving at 100 meters per second (a little over 200 mph), and you are sending out 100 pulses per second, that means that the vehicle can move up to 1 meter between pulses, which means the time of flight should change no more than 6.666 ns from the last pulse. So if the lidar unit sends out a pulse, and sees a reflection after say 10 microseconds, it knows the next time it sends it out it should be 10 us +/- 7 ns. If the next time it sends it out, it sees the time of flight was 9.999 us, then it can be reasonably sure the third pulse will have a time of flight of 9.998, but you could even open up that window to be 9.997 to 9.999. Afterall, even a Lamborghini is going to have a hard time doing 30 to 60 in .01 seconds.

If they chose to use windows like that, in order for your jammer to have any effect whatsoever on lidar gun you'd have to have your response timed to within a few nanoseconds. And remember, the lidar unit can randomize when it sends out the pulse. So say instead of every 10 ms, it was every 10 ms +/- 100 microseconds. You'd have to guess ahead of time when its going to send out the pulse to get your timing right, within a 15 nanosecond window of 200000 nanoseconds. Not real likely.

But then there's the fact that the gun has the ability to measure and alter its pulse width. So the gun sends out a random pulse width, and is looking for a pulsewidth of the exact same length. Again, if it randomizes the pulse width on each firing, you would have to have foreknowledge of the pulse width. Not real likely either. It can simply reject a pulse that is of a different length than it fired.

Even if the lidar unit didn't filter out the false pulses with its windowing technique, it still has the capability to observe multiple return pulses. If it looks at the data in totality over multiple pulses, and its seeing one return that is saying the target is at an almost constant distance but changing slowly and predictably, and a bunch of others that are erratic, it shouldn't be too hard to pick out the correct return.

So the only way I see a lidar jammer as working is if the unit, 1) doesn't effectively use windowing (relatively easy), 2) doesn't randomize the pulse stream (easy), and 3) doesn't do good signal processing (relatively difficult, although getting easier with every passing year), or if the lidar jammer were some how able to send out false returns before it actually received the first one (nearly impossible unless its constantly operating, which as far as I know, none do).

[Laser-InterceptorUSA]

See here: http://www.radardetector.net/forums/...mmer-work.html

Cliff aka ARS W4NJ

[mildlyamused]

I presume you are referring to Jim's posting. Unfortunately, Jim doesn't address ANY other issues I mentioned:

1) LTI's patent explicitly claims that they can vary the pulse timing to make it more difficult for jammers. This rather simple technique seems like it would defeat all non-brute force jammers on the market. Are you (or Jim) claiming they simply don't do that, even though they know it will work?

2) LTI's patent also claims that they dynamically adjust the window. Jim makes it appear that the window is relatively wide and fixed (e.g. 6 microseconds from when the pulse is fired). LTI however, explicitly claims they change the window based on previous pulses. That is to say, if they fire the first pulse and get a return within 1 microsecond, its reasonable to assume the next time they fire, the pulse will be received at 1 microsecond +/- 10 nanoseconds, and so the window only has to be 20 nanoseconds long, not the 6 microseconds Jim implies.

Add a 20 nanosecond window to +/- 100 microseconds of intentional jitter, and you have to be firing in a 20 out of 200000 ns, or about a 1 in 10 thousand chance of getting in the window. Based on reading the patent, which was written 15 years ago, they clearly know all this stuff. I'm not contributing any new techniques they were unaware of. Do they know all this, and just not do any of it?

[Laser-InterceptorUSA]

1. VPR LIDAR are in use and have been defeated.

2. Higher speed processors can address the time on the fly issue.

3. LCM Manufacturers, like their counterparts in the LIDAR business, are all very aware of this.

Cliff

[category4]

I don't think the jammer companies are going to come here and tell their secrets just because inquiring minds want to know, anymore than the lidar manufacturers are going to tell theirs!

[mildlyamused]

I don't think the jammer companies are going to come here and tell their secrets just because inquiring minds want to know, anymore than the lidar manufacturers are going to tell theirs!

Actually, LTI did spill a lot of their secrets in their patents. I think the reason that people aren't spilling their secrets is because they don't have any secrets to spill. If a pulse train is truly random, it can't be predicted. That's what random means. Maybe the reason that Cliff and other LCM manufacturers are able to deal with this is because the lidar manufacturers aren't using truly random pulse rates, and are instead using predictable ones (whether variable or not). And if the answer is, the lidar manufacturers aren't using truly random pulse rates, then thats the answer, but I haven't heard someone say that.

In terms of the dynamic windowing, LTI explicitly states that they use dynamic windowing based on time of flight of the previous pulse(s). The only way you could effective jam a lidar unit with random pulse rates would be with brute force. If they are able to get a good reading which they can base a dynamic timing window off of, there is absolutely no reasonable way you could brute force jam it. So maybe the answer is, they need several pulses before they can get a good enough reading to establish where to put the dynamic window. If that's the case, a fast brute force jammer may actually be able to effectively jam it. If not, I don't see how a brute force jammer could work -- and I have doubts that even a good predictive one could work.

As for Cliff's claim that both the lidar and LCM manufacturers are aware of all this stuff, whether or not its true (and I know LTI knows it, cause I got all this information from them), its not really relevant. The RSA public key encryption system has been around for 20+ years and survived intense scrutiny, and no know has ever even claimed there is a significant vulnerability in it. Just because the LCM manufacturers know what the lidar manufacturers are doing doesn't mean there is any way of defeating them (or vice versa).

[category4]

How long have you worked for a lidar manufacturer? Why all the interest here all of a sudden from you guys?

Guess there are new toys getting ready to hit the market. New toys for you, new toys for us!!

[mildlyamused]

How long have you worked for a lidar manufacturer? Why all the interest here all of a sudden from you guys?

Guess there are new toys getting ready to hit the market. New toys for you, new toys for us!!

I'm an analog IC designer and Extra class Amateur Radio Operator, that works for a major semiconductor manufacturer. I have never worked for any LIDAR or RADAR manufacturer nor any company that manufactures countermeasures. I got curious as to how they the LIDAR units were able to so accurately measure the time of flight since a brute force high-speed clock and counter approach was not practical for the sort of distance resolution they are capable of. So I looked up the patent. While reading through the patent, I was rather impressed as to how much capability the units have (at least if they actually incorporate everything in the patent, which is hardly guaranteed). I also looked up the patents on the Lidatek LE-10 (since I actually have one), and so when I put the jamming technique used by the Lidatek (and the one described by Jim) together with the information in LTI's patent, it made certain questions come to mind.

[category4]

All I can tell you is any system a man creates another man can find a way to defeat it. It has and always will be that way.

Stealth technology was supposed to make planes invisible to radar, but the Serbs shot one down. When you make better mouse traps the only real result is you evolve smarter mice!

[Laser-InterceptorUSA]

Well, the Amateur Radio Community is well represented on this Forum. Jim, is also a Ham, as am I.

With that said, we welcome your curiosity and input and are just cautious when newcomers, especially as of late, not usually so well versed in IR and LIDAR and LCMs, suddenly show up with posts that seeming have the flavor of wanting to work against the LCM community. Call us very jaded.

Welcome to the forum and enjoy your stay.

Cliff - W4NJ