On Subject of "Radar Bandwidth. Which bandwidth and what are they ?"

[stealthflanker]

Greetings.

So yeah as in title. This might be more appropriate with a question.

The subject of Radar bandwidth especially in discussion seems not to be properly treated. I have seen people claiming "this radar have more bandwidth to this" But the definition seems to be vague. Are they discussing about the frequency bandwidth where the radar operates (e.g 9500-10000 MHz) or some other bandwidth e.g Receiver bandwidth ? Which have to match the emitted pulse (Thus why the rules of thumb 1.2/pulsewidth in microseconds and the bandwidth is in MHz so 1 microseconds of pulse will have 1 or 1.2 MHz of bandwidth).

The confusion become more of a problem when discussing ECCM capability of a radar. That i have seen people actually think that AESA Radar with "wide bandwidth" can just hop into whatever frequency they desire or even "pulse to pulse change". Without considering what being used to electronically steer the radar beam. Current modern and operational AESA radar are still using phase shifters. This actually limits the frequency bandwidth of the antenna. With the limit of about the same as the beamwidth. Thus an antenna with 2.5 degrees of beamwidth will have operational frequency bandwidth of 2.5% meaning that it can shift frequency of that amount of percentages from its center frequency. The Radar Handbook 3rd Edition makes more elaboration by differentiating the operational bandwidth by the radar's types of emission the Pulsed emission have about twice the bandwidth of CW emission.

The wider operational frequency bandwidth offered by the Transmit Receive module can only be fully accessed with application of True time delay beamsteering. This true time delay beamsteering is for my knowledge applied by optic fiber delay lines. Seems to be no operational application today. Especially in small fighter aircraft radar. But i would like to hear everyone's opinion on this.

On the jamming case, well actually the frequency hopping is the most basic yet in EW-101 by David Adamy, considered to be the easiest to crackdown, as when the sequance of hopping is known, jammer can be simply made to "follow" that change thus known as "Swept spot jamming" situation. and to say nothing on "LPI" Capability as it can be applied even without phased array radar.

Radar had to have such sequence as the signal needs to be Coherent to allow doppler effect and thus speed of target to be measured and clutter to be filtered. Thus why radar cannot really "spread" its signal far and wide or make changes every pulse. Otherwise it needs another way to provide target speed or clutter filtering. There is however pseudorandom sequence but as the "pseudo" part suggest it is not a true random noise.

and now the Pulse bandwidth. Using the rules of thumb of 1 or 1.2/pulsewidth one may notice that Long pulse e.g 100 us will have lower bandwidth to shorter pulse. I usually just take it for granted. As in my field of interest the radar range modeling, Pulse bandwidth or (B) Are ignored for the reason outlined by K.Barton's book "Equations for Modern Radar 3rd Edition" Namely they are difficult to define.

Using pulse bandwidth without elaboration may cause ridiculous case where the range resulted from the calculation can be near infinite as the bandwidth is lowered toward zero which equates to non operational receiver and very long pulsewidth. Thus why one can see there are radar range equation without the B but instead replaced with simply KtF (Boltzmann constant, system temperature and receiver noise figure) which indicates a matched filter approach where the receiver bandwidth is always matched with pulse bandwidth. Another impact is that "B" constant makes the equations unapplicable when Pulse compression is considered.


I found that proper understanding is really necessary, especially when the time comes to explain things to other people. I found this bandwitdh topic to be very confusing and often only used as technoblabber to amaze people on certain topics, without any practical use or any allowance to translate the topics into simple mathematical model or logic people can use to see how things work.

Mentioning "my anti ship missile seeker have 1 GHz bandwidth" is not enough to claim strong ECCM capability or LPI instead an expert may ask

"Isn't 1 GHz bandwidth too much ? Because that means your seeker may just head into Less vulnerable part of the ship, as it have way too fine of range resolution,"


Yeah.. i guess that's my point. I wonder if you guys have anything to add on the bandwidth subject.

 

[Maury Markowitz]

That i have seen people actually think that AESA Radar with "wide bandwidth" can just hop into whatever frequency they desire or even "pulse to pulse change".

I'm a dilettante at best, but this is indeed my understanding.

Current modern and operational AESA radar are still using phase shifters.

Are they?

I am under the impression that modern AESA's like the APG-77 and -81 consist of entirely individual T/R modules that directly create their transmissions from software-defined radios, and can indeed change their entire output within the limits of the transmitter amplifier with every pulse.

There are no phase shifters in these systems. Beam steering to a given angle can be accomplished at any wavelength that the antenna can handle, and the software selects output parameters that result in the same steering angle on a pulse-to-pulse basis even at different frequencies.

Even the MAR-1 had some of this capability, and that was in 1963. That was indeed bandwidth limited, and did indeed use phase shifters, but modern GaAs has come a very long way since then.

Article with details.

 

[Ronny]

I am under the impression that modern AESA's like the APG-77 and -81 consist of entirely individual T/R modules

The phase shifter is inside the T/R modules

 

[stealthflanker]

I'm a dilettante at best, but this is indeed my understanding.

Also with "pulse to pulse" change, i wonder how they can keep the transmission coherent for any form of doppler effect utilization. The more correct term which you can find in G.Wiley's 2005 edition book "ELINT" Was "pulse train to pulse train".

Are they?

I am under the impression that modern AESA's like the APG-77 and -81 consist of entirely individual T/R modules that directly create their transmissions from software-defined radios, and can indeed change their entire output within the limits of the transmitter amplifier with every pulse.

There are no phase shifters in these systems. Beam steering to a given angle can be accomplished at any wavelength that the antenna can handle, and the software selects output parameters that result in the same steering angle on a pulse-to-pulse basis even at different frequencies.

They are. Phase shifter is still in part of current fighter aircraft radar. Integrated of course within the module Might not be obvious but they are in the back of the TRM's preceeding all the amplification and receiving parts. in PESA might be more obvious as they are right behind the radiating elements.

What you are describing are element level Digital Beamforming. something which a holy grail for AESA but haven't yet to be practical at least in fighter radar or high frequency radar above S band. But that because cost and difficulty of implementing Analog to Digital converter in TRM level. It may change but certainly not in today's AESA radar.

 

[OliverSedlacek]

I've got no experience of radar but I've done plenty of signal processing! I seem to spend a lot of time working out how to filter signals to minimise bandwidth around the signals of interest and to filter out noise or interference. I'm sure modern systems have impressive frequency agility, which doesn't seem to difficult with software defined radio chips. Pseudo random sequences are sufficiently unpredictable in practice to be uncrackable, so that's not a limitation either.
The challenge to me seems to be the pulse flight time, which means the receiver needs to be ready waiting for the return echo for ages. This is a system bandwidth limit that I'd be curious to know if anyone has found a way to cheat.