BAE Systems Inches Out In Public On Electronic Warfare

[eloise]

assuming the kind of jamming used is barrage noise so factors like PRF jittering and frequency hopping , pulse compression can be ignored

Actually, pulse compression will reduce J/S by 30dB
Pulse compression


Frequency hoping



PRF jittering

https://books.google.co.uk/books?id=foY ... &q&f=false

[eloise]

for example : if APG-81 detect a target at 100 km , the SNR has to be at least 13dB ?

The require SNR ratio for detection actually depending on the probability of detection (Pd) and probably of false alarm (Pfa) ,Western standard for radar detection is 90 Pd and 10^-6 Pfa , so that measured to about 13 dB SNR

[garrya]

That very informative post, i didnt realize pulse compression reduce J/S ratio that much , but wouldnt pulse compression and frequency agility counter the other ? since radar practically spreading out its energy over a while bandwidth in pulse compression instead concentrated in a narrow frequency ( longer the pulse the wider the bandwidth)

[hornetfinn]

That very informative post, i didnt realize pulse compression reduce J/S ratio that much , but wouldnt pulse compression and frequency agility counter the other ? since radar practically spreading out its energy over a while bandwidth in pulse compression instead concentrated in a narrow frequency ( longer the pulse the wider the bandwidth)

Yes, pulse compression is a huge thing for radar performance in many ways. Pulse compression and frequency agility do not counter or affect negatively to each other at all and actually are almost always used together. There are many pulse compression techniques used, but they all have some things in common. They transmit a relatively long (in time) pulses which are divided into smaller parts which can be though as subpulses where pulse length is much shorter. The frequency or phase is usually changed between these so called subpulses to get more information from the target to improve radar resolution and resistance to jamming. Basically the pulse compression system transmits long pulses to have a lot of average power in the pulse. While receiving, it handles the smaller parts (subpulses) on their own and can thus get very high range resolution as that is inveresely proportional to pulse length. Then the pulse frequency can be changed when the next long pulse is sent. Quite often modern systems use phase coding within pulses (frequency stays the same) or frequency is changed within a narrow bandwidth between subpulses and make bigger change in frequency between long pulses. Another advantage with pulse compression is that radar can use very powerful pulses that are not easily detectable for outsider who does not know how the radar signal is coded. In modern digital pulse compression systems the frequency and/or phase modulation is pseudorandom. This way powerful long pulses do not show up for outsider as the power is distributed in phase and frequency. Without knowing the code, pulses don't show up from background noise. Modern jammers and ESM systems try to use some form of partially matched filtering (educated guess about coding scheme) to detect such radars. Radar itself knows what it transmits and can use fully matched filtering and gets a big processing gain compared to EW/ESM systems.

Actually the wide bandwidth can be had with short pulses as there can be a lot of them. A long pulse can also have wide bandwidth with chirping where frequency is changed within pulse.

And thank you eloise for those very good posts about the issue.

[hornetfinn]

That can be achieved by having huge amount of RF power which is bad because you become a very bright radar beacon for everybody.

to be fair though , the same thing can be said when aircraft using their radar , even though to lesser extent. I know that there are LPI radar and what not , but still , if the small GPS antenna in mobiles phone , JDAM can detect GPS signal from satellite hundred thounsand miles aways then i think modern RWR should do well again radar at distance 200 km or less.

The detectability difference between jammer and radar is huge. Radar transmits very narrow and short lived beams which makes detecting them very hard. Another thing is the modern pulse compression techniques can make it very hard to detect the radar signals from background noise even if the radar is transmitting with a lot of power. This is because the signal is spread in both frequency and phase as I noted in previous message. Without knowing how this is done, it can be very difficult to detect the radar signals.

Jammer on the other hand must transmit much wider beams to do effective jamming and most jammer systems transmit very wide beams. Jammers also must use waveforms that maximize the jamming effect on enemy radars and this can only be achieved with easily detectable signals. Same as GPS signal can be easily detectable as the signal was designed to be just that. F-35 for example will have nice advantage as it has very low RCS and thus power required for effective jamming is very low also. Besides using high gain antenna of the radar for jamming will also make it harder for most sensitive enemy systems (dedicated ESM platforms) to detect the jamming signals.

[hornetfinn]

I've been wondering about this: http://www.codeonemagazine.com/f35_arti ... tem_id=182

“This jet isn’t just about the weapons — it’s a game-changing capability. The Tornado GR.4 can't just stroll into a double digit SAM MEZ [Missile Engagement Zone]. In the F-35 I can generate a wormhole in the airspace and lead everyone through it. There isn’t another platform around that can do that. This isn’t all about height and supercruise speed — it’s the ability to not be seen,” added Beck.

I think this means F-35 will just get close with SAM systems or other engagement radar systems and do powerful and precisely targeted stand-in jamming from close range and makes the system unable to engage even unstealthy aircraft. As can be seen with stand-off jamming burn-through equation garrya posted in previous page, the range where jamming is done makes a huge difference. If F-35 can sit at say 50 km away and legacy stand-off jammer has to sit 200 km away, F-35 requires 16 times less power to achieve the same jamming effectiveness to protect the other aircraft if all else remains unaffected.

[popcorn]

I guess the jamming is part of the story. Also being able to detect and exploit holes in the IADS.

[garrya]

Yes, pulse compression is a huge thing for radar performance in many ways. Pulse compression and frequency agility do not counter or affect negatively to each other at all and actually are almost always used together. There are many pulse compression techniques used, but they all have some things in common. They transmit a relatively long (in time) pulses which are divided into smaller parts which can be though as subpulses where pulse length is much shorter. The frequency or phase is usually changed between these so called subpulses to get more information from the target to improve radar resolution and resistance to jamming. Basically the pulse compression system transmits long pulses to have a lot of average power in the pulse. While receiving, it handles the smaller parts (subpulses) on their own and can thus get very high range resolution as that is inveresely proportional to pulse length. Then the pulse frequency can be changed when the next long pulse is sent. Quite often modern systems use phase coding within pulses (frequency stays the same) or frequency is changed within a narrow bandwidth between subpulses and make bigger change in frequency between long pulses.
.

I mean if the compressed pulse consist of different frequency , shouldn't the matched filter allow signal/energy from much wider frequency range get into receiver ? the compressed chirp pulse seem to have very wide band width too

[garrya]

Quite often modern systems use phase coding within pulses (frequency stays the same) .

Wouldn't number of possible phase combination is much more limited compared to frequency ?

[hornetfinn]

Quite often modern systems use phase coding within pulses (frequency stays the same) .

Wouldn't number of possible phase combination is much more limited compared to frequency ?

Yes, phase combinations are very limited. In binary phase codes it's as name suggest binary with two states. Polyphase codes can have more states, but it's still limited to fairly small number of states. Anyway, in phase coding a long pulse is divided to extremely short subpulses which are coded by varying the phase. Bandwidth of such binary phase coded signal is B=1/τ where τ is the code subpulse length. So the shorter the subpulse length, the wider the bandwidth of the signal. Resolution is also similarly affected and short subpulses mean higher resolution. It's basically like binary data with 0 and 1 being used to code information and stringing more of them together means more data can be transmitted.

Using phase modulation system it's possible to transmit more information than using frequency modulation as it allows more efficient use of signal bandwidth to transmit information. It's rather complicated and very math-intensive area and very difficult to describe. Of course both phase and frequency can be modulated at the same time or frequency can be hopped with each pulse but pulse compression done with phase coding (pulse frequency stays the same). Together these will give radar very significant processing gain advantage against jammer and thus increase J/S ratio for successful jamming significantly. Of course there are jamming methods to reverse that and DRFM is the name of the game in modern jammers. However even DRFM and such methods will benefit from low RCS just as much as basic noise jammer. Also having high gain jamming antenna does the same. F-35 has both which makes it really great jamming platform.

[eloise]

Forms of pulse compression :

[hornetfinn]

I mean if the compressed pulse consist of different frequency , shouldn't the matched filter allow signal/energy from much wider frequency range get into receiver ? the compressed chirp pulse seem to have very wide band width too

That's true in chirp type radar, although the effect of increased processing gain and the need for jammer to use wider bandwidth to counter this far outweight the negative effect.

[hornetfinn]

Thanks eloise, very nice table!

[garrya]

That's true in chirp type radar, although the effect of increased processing gain and the need for jammer to use wider bandwidth to counter this far outweight the negative effect.

Iam not sure if processing gain can really counter the negative effect of spreading signal over wide frequency range though.For example : if radar can operate between 8-12 Ghz ,it operating frequency jumping randomly 4 Ghz total bandwidth , unable to predict pattern , jammer has to distribute energy over the whole frequency range, for a normal pulse with bandwidth of only 1 Mhz ,then only 0.025% of jamming power will get into radar receiver. For a compressed pulse with 1 Ghz bandwidth ,about 25% of jamming power will get into radar receiver. That quite a big different of around 30dB

Yes, phase combinations are very limited. In binary phase codes it's as name suggest binary with two states. Polyphase codes can have more states, but it's still limited to fairly small number of states. Anyway, in phase coding a long pulse is divided to extremely short subpulses which are coded by varying the phase.Using phase modulation system it's possible to transmit more information than using frequency modulation as it allows more efficient use of signal bandwidth to transmit information

According to this http://www.radartutorial.eu/08.transmit ... on.en.html phase coded pulse compression seem to have very low compression ratio ,maximum value is only 13 ( or is that only for Baker code ? )

[garrya]

Forms of pulse compression :

What is a passive phase coded compression ?