How do fighter jets deal with friendly radar noise? - Page 1
Forum section: electronics / general

author: MrOmnos
author_rank: Frequent Contributor
timestamp: December 13, 2016, 02:33:46 pm
content: So, I suspect the same model of f-15 would use the same radar which would probably transmit in same bands. How do they differentiate between transmissions?

author: StillTrying
author_rank: Super Contributor
timestamp: December 13, 2016, 03:16:52 pm
content: I would think that the pulses sent are very short, less than 1 us, and then the receiver only listens for about 50us afterwards for the return, any received pulses outside that 50us can be ignored. You'd have to be very unlucky for 2 radars timings to be synchronized.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: December 13, 2016, 03:39:13 pm
content: Quote from: StillTrying on December 13, 2016, 03:16:52 pmBut it is a possible scenario, right? So, when people are designing radar do they take it in consideration or ignore it?

author: StillTrying
author_rank: Super Contributor
timestamp: December 13, 2016, 03:51:18 pm
content: I dunno, encoding a small amount of info in the pulse, random info would be good, and having each radars pulse frequency slightly different would fix it.Are you designing one.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: December 13, 2016, 03:56:05 pm
content: Quote from: StillTrying on December 13, 2016, 03:51:18 pmI am designing one short range FMCW radar. I am still a nooob but I am learning a lot. I was wondering how they do it. Might come in handy in the future.

author: LazyJack
author_rank: Frequent Contributor
timestamp: December 13, 2016, 03:58:36 pm
content: Quote from: MrOmnos on December 13, 2016, 03:39:13 pmYes, it is possible, and it does happen from time to time. I don't know about fighter jets, but on marine radar, you get a spiraling dots on the screen if you are illuminated with an other radar. This makes sense as your antenna is rotating, but get the short pulses from somewhere else. I'm not sure how they avoid this, as usually there is a function on radars for interference rejection, which can be turned on or off. Having it off is actually sometimes useful, as you will know that something with a radar is out there.I think the way the rejection works is by having a time window when the receiver accepts signal also by comparing multiple rotations, as interference would be random, while normal targets usually move a little only. Also, the tuning of the receiver can come into play, as (at least magnetron based radars) the transmitter frequency changes over time and the receiver is automatically or manually tuned to the transmit frequency. So other radars may be slightly off in frequency.

author: John Coloccia
author_rank: Super Contributor
timestamp: December 13, 2016, 04:11:24 pm
content: Turns out I used to do a little radar work...There are many different things you need to consider when doing radar work. What you're referring to is "running rabbits" interference. If you're in a friendly environment, you can simply switch to a different frequency. Human operators usually don't have much trouble figuring what's real and what's not, but if there's enough of it the screen just turns into a mess and you can't see anything anymore. You can also use various gating techniques. It'll depend on the environment and how much integration there is between various units.You also need to consider multi-path, which is probably going to be more of a headache for you than running rabbits if you're doing some sort of automated tracking. That's when you get multiple returns from the same object due to reflections. Sometimes it's obvious because the angles involved often make the multi-path return move very quickly (i.e. if you see an airplane doing 5000mph, it's probably multipath). Other times it causes an annoying static return in an inconvenient spot (buildings, for example, making a big return in the middle of a runway) and you just need to know it's there and ignore it. Sometimes you just need to know that if you have a target HERE that it sometimes causes a multipath return THERE, and you need some logic to ignore it. And sometimes you get fooled.If you're just playing around, I would forget about running rabbits and study up on multi-path.

author: retrolefty
author_rank: Super Contributor
timestamp: December 13, 2016, 04:14:58 pm
content: Quote from: StillTrying on December 13, 2016, 03:51:18 pmModern combat aircraft have multi-mode radar integrated with Identify friend or foe (IFF)data interrogation to help prevent friendly fire type incidents.

author: coppice
author_rank: Super Contributor
timestamp: December 13, 2016, 05:01:32 pm
content: Fighter radars have extensive features to minimise the possibility of being jammed, and to lower the possibility of their signal being intercepted. These features also minimise the effects of the radars in other aircraft in a battle group. The biggest single feature is probably frequency agility. The frequency hopping sequences are random(ish), so they shouldn't be able to synchronise, even between several planes with the same model of radar.