First, I’d like to know if I understand LPI radar properly:
There are a few ways in which a radar can be LPI. It can use wideband, frequency hopping or continuous wave. However, the term “LPI” is usually reserved for radars which use those techniques but in addition also transmit at very low power across many frequencies at the same time; AESA radar.
An LPI radar emits at very low power so that, at the chosen distance, the signal’s amplitude is lesser than the noise floor’s variation. E.g.: If the noise floor is at an average of 100mV and typically ebbs and flows between 50 and 150mV, a 10mV signal will be difficult to detect, especially if frequencies are switched between pulses. If the LPI radar keeps switching frequencies, it becomes very difficult to notice when a frequency is being used to transmit a signal or if tiny mV increases are just the usual ebb and flow of the noise floor.
If the signal is smaller than the noise floor’s variability, this means that the return signal should be extremely faint. E.g.: In the same situation as described above, the return signal might only be 1mV if it bounces off a target.
The radar’s signal processing will compare the average mV value of all frequencies and pulses received from a direction to the average mV value that would be expected if there were only pure noise and no return echoes across all those frequencies and pulses. The lower the CFAR setting, the more the signal processing will require a large difference between the actual received mV value and the expected pure noise mV value.
Is this accurate?
Couldn’t a PESA be LPI with the disadvantage of requiring more time? The main advantage of an AESA is that it can transmit over many frequencies at the same time, right? If a PESA can transmit over 1 frequency per pulse while an AESA can transmit over 10 frequencies per pulse, then the AESA should require 10X less time thant he PESA to detect a target when both are operating in LPI mode, correct?
Is is accurate that a radar in tracking mode will emit more power in a given direction than in search mode and more power still in targeting mode? If this is accurate, doesn’t this imply that AESA radar is less LPI in tracking mode and even less so in targeting mode?
Can we expect AESA radar to be much less LPI if you can sneak up on them? I.e.: If the transmitted power is tailored to be smaller than the noise floor’s variability at 100km, it will likely be greater than the noise floor’s variability at 10km. Thus, if a stealthy/pop up/jamming-protected target can get closer to the AESA radar than the AESA expected, the AESA may not be LPI anymore.
An LPI radar can be expected to be only intermittently detected. Doesn’t this make DRFM jamming nigh useless against LPI radar? Most of the time, the DRFM will send back nothing which should make its deception easy to distinguish from the true return.
In the same vein, can DRFM can more than those 3 things against a frequency hopping radar 1) make the jammer look like it’s further away fromt the radar than it really is 2) create a false target that looks like it’s further away from the radar than the jammer 3) mess with the radar’s velocity gates, thus possibly throwing off a tracking or targeting lock.
No. The difference between a PESA and AESA is the number of formed beams, not the number of frequencies. An AESA system can simulate a number of separate arrays, and thus track more than one thing at a time. A PESA is limited to one beam. Either can use as many frequencies as they like - that is a function of a different part of the system.
Ignore the question of PESA versus AESA. The core component in thinking about these things always come back to simple thing. Shannon. You have a defined bandwidth and a defined signal to noise. This defies your information rate. So if you need more information, you need to trade off - the time you take to acquire it, so more or less time taken, the bandwidth - so how wide the frequency bands you transmit on are (which is directly related to the pulse shape and sequence you send) and the the number of bands you use - and the power level you use (which effectively defines the signal to noise ratio). Everything you want to know comes from working out what the mix of these tradeoffs is.
Probably, but with an active beam forming capability things are not as clear cut. Search mode may use a wider beam. Target mode may have a higher signal on the target, but less anywhere else. It may still be below noise at the target.
Because of the inverse square versus inverse fourth power advantage given to the jammer, this is always going to be true. But the ability to dynamically modify beams gives some advantage back.
It think you may assume this is one of the big selling points. If not the biggest.
Almost certainly. You might reasonably assume that this is a never ending escalation of creative ideas and workarounds. Knowledge of the internal operation of one will cause changes in the other. In the end there is only one desirable outcome. Make the missile miss. You could try phenomenology on it. It might work. Once.*
I’m trying to understand how AESAs are more LPI than PESAs. Do AESAs use wider frequency bands or a greater number of bands than PESAs?
Are AESAs more LPI than PESAs because they can send several beams at the same target even if all those beams (which use different frequency bands) have a signal below the noise floor whereas the PESA can only send one beam (of one frequency band) at a time?
With the development of stealth and AESAs, it seems that warfare between well-equipped militaries might involve a lot of units being within range of each other but 1) being completely unaware of the other’s presence 2) catching a glimpse but being unable to get a good track 3) getting a good track but missing. Mr Magoo warfare.
Do you have some idea how large the range gates are when tracking an aircraft? Are we talking tens of meters, hundreds, kilometers?
I would say that as a first approximation they are orthogonal issues. Active beam forming is a more modern technology and may simply be associated with more modern systems in general. But it affords more flexibility, so allows more options. But AESA versus PESA is all about what the antenna is. It doesn’t say a great deal about the rest of the system.
Nope. It is limited by Shannon, in that the width of the gate is a component of the information rate - tighter the gate the more information you need. So in order to tighten the gate you need to put more into those things you would prefer not to - power, bandwidth, time. You would probably find that the numbers are classified. You would assume that the numbers are operationally useful.
Not much to add to Francis Vaughan’s good posts. As to the above, gates are in the time domain, not distance domain. With a factor for range. The faster your processor tech, and the higher your freq, the tighter your gates. You can make some rough calcs if you like.
Everything going on now is about processing. Logic is everything; radio is nothing. The various *ESA’s are all about adding more computing to make up for less radiating. Think hard about what that means. Don’t get hung up on categorical labels. What really is the difference between a 3rd gen & 4th gen PC for example?
Ok. I’d read about AESAs specifically being LPI and I thought that both PESAs and AESAs could form narrow beams.
So, the main advantage of AESA is that if a radar has a PRF of 10 000 and wants to do 10 tasks, it can allocate 10 000 pulses/second to each of those 10 tasks but a PESA would have to allocate an average of 1000 pulses/second to each of them?
Cray becomes the king piece? Each side can see a lot more but with a lower refresh rate because of comms & processing delays? Were you thinking of other implications?
Sort of. But back in the day an awful lot of the limitations of radar was in the radio engineering. We knew we wanted to make a radio do *this *and that; but nobody could make it work with the required freq stability or pulse width or whatever.
Software defined radio has been a game changer on so many fronts.
Compute in these radar systems is often a box filled with very high end FPGAs. You don’t get anything like the performance you need from any ordinary computer.
I’ve been butting my head against this. I have no doubt that I can think hard but here, it seems I just don’t have the necessary background knowledge to see the implications. Can someone explain what the quoted segment means?
It is a neat way of summarising what I was talking about in the abstract. Shannon - limits to communication in a noisy channel.
The trick with LPI radar is to be indistinguishable from background noise. This implicitly means you need to reduce your power levels. But it isn’t just about pure radiated power. The bandwidth you spread that power over and the manner in which you modulate your power matter too.
An simple radar just blasts out a shaped pulse at one frequency. It does this with enough power that the return signal is useful. As the return signal drops off with the distance to the target to the fourth power, you can’t be too mean with the power. The signal being useful is the trick. You need to get enough information back. And that information content is determined by the product of the bandwidth of the signal times the signal to noise ratio times the length in time of the pulse. That product is everything. You can play about with the mix to your hearts content, but the final value is unvarying.
So, if you have enough compute power you can start to really have fun with the mix. First thing is clearly to spread the signal over a wide bandwidth. Old school radio really could not usefully do this easily. Software defined radio system can. Now already your signal is much much lower in power at any individual frequency slot. So it is harder to detect. If you spread our signal about over a wide range of frequencies you can do so with a psuedo-random sequence. If only you know the generating seed for the sequence it becomes extremely hard to detect your signal, as it looks even more like background noise and anyone trying to correlate the signal they do see has no idea where to look. Better, your signal is now very difficult to jam. But whilst with a fixed radio transmission this is not too hard (this technique is used in just about all digital radio systems - ie mobile phones, WiFi etc) when you have a radar system you are trying to correlate a very wide bandwidth set of signal returns that are arriving with different delays and doppler shifts and trying to sort out what is actually out there. Moreover, you are still trying to keep your power levels so low that it is still impossible to distinguish it from noise. And you need to do it in real time. You are going to need some impressive signal processing capabilities.
