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Lengthy answer here. The other answers have lots of good information, and I even saw another answer that was downvoted with some good information, so hopefully this will provide some good insight.
Both AESA and PESA radars are (generally) pulsed radars. AESA as well as modern PESA both are frequency agile and can frequency hop over different frequencies at different times. Both can have narrowband or wideband mode, and both can be used for ECM, passive scanning, beam-forming, etc. The primary difference is in the source of the high-power RF signal.
How is a radar pulse formed? A digital pulse command is created by the radar computers which tell it to send a pulse out in some direction. The receiver/exciter (REX) creates the baseband pulse, which is really just a square wave, which is used to modulate the Intermediate Frequency (IF). Signal processing is applied to the IF pulse to compress it, shape it into efficient Gaussian forms, etc. Then the IF pulse is used to modulate an oscillator that is operating at the final RF frequency. This refers to the final up-converted frequency that will be used for transmission over the air. This modulated RF signal goes through high power amplification and is sent to the antenna for transmission.
It is only this last step, the high-power RF generation/amplification that differs between AESA and PESA.
- In PESA, there is a single high-power transmitter source, often an older device like a
or a
. These devices can amplify RF signals at microwave frequencies up to very high powers and then there is a single antenna horn radiating the signal out. After the signal is radiated there is an RF “lens”. An RF “lens” is an array of thousands elements that can selectively delay a portion of the RF signal. So by delaying the radiated RF signal in a particular shape, beam shapes can be formed that allow the beam to be steered or spoiled to serve specific purposes.
- In AESA, the thousands of phase-shifting elements are also themselves transmitters and antennas. The IF signal from the REX is fed to each of the AESA elements, along with a digital “command” which tells the element how to delay the signal to form a particular beam. The individual element does the RF up-conversion and power amplification, along with phase shifting in order to form and steer a beam. Each radiating module is much less powerful than the Klystron or the TWT, but the sum of all of the AESA elements allows for high total power levels.
Contrary to other answers posted here, the difference in
capability between AESA and PESA is less than you might think.
- Both AESA and PESA can be low probability of intercept (LPI). Being LPI isn’t determined by anything special the modules are doing. It’s just due to the fact that you can change your frequency with every dwell, so that over time the average power level on any particular frequency channel is kept very low. Both AESA and PESA can do this. It’s more typical to do this on AESA because the oscillators and tuned amplifiers are more modern solid-state devices, but you can do the same thing with modern PESA as well. Also, the suggestion that you can change the frequency with every single pulse is true, but not very practical. Few radars do that. Unless the radar is looking up at the sky, a radar signal will be contaminated with clutter (energy returns from other objects in the scene). Typically this clutter is not moving, so Doppler processing is used to separate out the clutter from the moving signals. This is particularly important with ships doing horizon search or with airplanes that are looking downwards (strong ground return). In order to do Doppler processing, you must send out a series of pulses all at the same RF carrier frequency. This “dwell” may last up to several milliseconds until the required number of pulses is received, and then the radar can switch to a new frequency for the next dwell. Different methods and waveforms can be used if an airplane is trying to be stealthy, but radar situational awareness is also minimized in those circumstances.
- Both AESA and PESA can utilize large bandwidths for a single pulse. “Wideband” modes are typically created with separate hardware paths in the system that utilize more advanced wideband modules that are linear over a much larger range of frequencies. Wideband pulses can be compressed hundreds of times tighter than narrowband pulses allowing extremely detailed range resolution. This feature is generated by the REX at the IF and then is up-converted to RF by either an AESA or a PESA transmitter.
- Both AESA and PESA can be electronically steered so that a “flat panel” antenna can have a wide viewing angle. These antennas can only be steered so far, though. After a certain point, the phase shifting is no longer practical to form a coherent beam. So on ships, for example, you’ll see 3 or 4 faces of flat panel antennas around the ship. Of course an antenna can receive energy from any direction, but the mainlobe of the antenna can only be steered to within moderate limits and the mainlobe is what you want for directional reception. Energy received through sidelobes will be significantly reduced in power, and you won’t be able to determine what direction it is being received from, so very rarely is sidelobe energy used for target detection. Sometimes, special sidelobe pointing is used to cancel sidelobe interference out of the mainlobe.
- Both AESA and PESA can do jamming detection, ECM, and passive scanning. All you need to do is turn on the receiver and don’t send pulses out. The PESA array can be steered and formed just like the AESA array. Special signal processing once you get back into the digital electronics gives you the real advantage in terms of doing advanced jamming detection.
- Both AESA and PESA can track multiple targets the same time. Sometimes hundreds.
The list goes on. AESA does have the ability to form multiple beams at different frequencies at the same time, but this is less common. If you are splitting up your beam into a beam for Freq-A and a beam for Freq-B, then each beam is now half as powerful and will get you (1/2)1/4=84(1/2)1/4=84 of the range you would get if you were using the full array. Divide that into 4 sub-beams and now you’re down to 70% of your maximum range. You also lose monopulse processing which allows you to fine tune your angle measurement. Without monopulse, your angle measurement is only going to be as accurate as your beam width, and now with only half of an array, your beam just got wider. You typically don’t need this feature where you do two frequencies at the same time, because the radars are pulsed. Radars are emitting thousands of pulses per second. And after each dwell, you reset to a new target, re-steer the beam, and then send more pulses. Both AESA and PESA can track hundreds of targets at the same time.
The primary advantage of AESA is logistics and SWAP (size, weight, power). With a PESA, you have a single high-power amplifier like the Klystron or TWT. These are older devices, require extensive cooling, and very prone to breakdowns. And when your single source of RF amplification goes down, your whole radar goes down and you are blind. They are expensive, fragile, and a single point of failure. With AESA, your “high-power” amplification is now split up between thousands of solid-state devices. Several of these AESA elements can fail, and the overall AESA performance will be essentially unchanged. The modules are circuit cards that can be manufactured much more easily, and technicians can easily switch modules in and out.
With PESA, you require a precision set of waveguides in order to get the high power signal from the common amplification source to all of the phase shifters. This ultimately makes the radar larger, it has special space constraints, it is heavier, and it is more difficult to manufacture. AESA radars only require a flat panel with all of the elements installed. Think of it as a frame with a bunch of circuit cards plugged in. The panel can be separated from the REX and connected only with cables allowing it to be more easily integrated onto different platforms.
AESA also allows the use of solid state devices for RF generation and amplification. Single solid-state devices were never capable of generating the power needed at a single source for a PESA radar. But when split up over thousands of elements, now you can use solid-state, and you end up getting much better radar efficiency, and can take advantage of modern solid-state advances in silicon technology
, D
efense engineer, designed and tested phased array AESA pulse Doppler radars
