Kersi D wrote:I understand that one requires a physically large antenna, about few m length, for L and S band. (Example THD 1955, DW 04 etc). So how can an aircraft radar say 1 m in diameter work in L and S band.
I suppose that AESA can do search, track as well as missile guidance simultaneously ?
A googly (according to me) Can a AESA radar work as a SAR or a ISAR ?
Pl educate us by giving soem good websites to understand these radars
Long time no see... primarily because I was AWOL for the past few years.
I guess I could answer your SAR related question (thats my current area of research)....
In general, an AESA radar (or even any array radar) is not useful for SAR. In fact, and this is the most interesting aspect of Synthetic Aperture Radars - unlike in the usual array-based radars, where we want the SMALLEST beamwidth possible in order to get the best resolution, SAR, requires the WIDEST beam possible.
What? How? What do you mean, you ask...
Well, simpull explanation wonlee (or not, considering I took a month to understand it)....
You see, a SAR works by artificially creating a large aperture, so that the effective resolution is really really sharp. How this works is, to put it in very very rough (and inaccurate) terms, its like stitching up many overlapping images together. and because they overlap, if they are coherently added up together, the image is more accurate than each individual component. (The technical reasoning for this is a bit involved, especially when talking about chirp waveforms, but for now, I guess this is a reasonably rough explanation)
So, the more the number of overlapping images we have, the better the accuracy we get. Now, we get these images as the aircraft flies along the flight path, at regular intervals. If we have a narrow beam, then the field of view (called the Ground Scene) is quite tiny, and moreover, is not going to overlap a lot.
Think of this as a circular torch beam on the ground, which is moving in a line, and we switch it on and off at regular intervals as we move. If the torch beam is small, then each interval we move, we light up an entirely different part of the ground. On the other hand, if we had a really wide angle torch beam, we will be able to illuminate a particular part of the ground for much longer time / number of on-off intervals (the on-off intervals are the radar pulses, and this longer time is called dwell - typically quantified as angular dwell, in terms of view angles that the radar takes - best case scenario is when our radar always has that point looked at, ie, an angular dwell of 360 degrees, or 180 degrees - I still get confused about which of the two it is)
So, if we want a LOT of overlapping images to get a higher resolution, we want to have a large number of overlapping pulses. Thus, we need AS WIDE A BEAM as possible. The resolution limit is approximately given as
resolution = antenna size / 4
whereas a real-aperture radar (AESAs etc) have
resolution ~ r x lambda / antenna size
(~ - proportional to)
So, to answer your question - can an AESA do SAR or ISAR?
ABSOLUTELY.... this link
How this will probably work is to use what is known as "spotlight" mode SAR. This mode does away with the problem of a fixed, non-steerable beam that was assumed in my explanation above (that mode was the earliest SAR mode, called "Stripmap mode" SAR). Instead, it increases dwell time on the ground scene through electronic beam steering. Which is something that the AESA radar is pretty good at. So what we'd need to do is to keep a few beams from the AESA targetted constantly at the ground point, and make sure we're constantly focussing on it.
Its quite tricky though, and far more difficult than it seems. Firstly, the waveforms need to be coherent, and we need to have an unbelievably accurate phase history, else they'll destructively interfere. What this means is that we need to know PRECISELY how long the waveform took to travel to the ground point and back. because thats how we identify which ground point it is. So every little motion and environmental artifact needs to be compensated precisely - roll, pitch, atmospheric effect, even tiny changes in speed and heading. Everything.
Secondly, the tracking and beamsteering will need to be seamless and extremely precise.
The major limitation that an AESA radar will have when compared to a dedicated SAR radar is that because its a multirole radar, the ground scene we're imaging is going to be quite small, even if we use multiple beams in a cellphone cell type grid fashion. Doing that could also have the problem of overlapping sidelobes causing phase history errors. So I'd imagine that the AESA wont be able to image a large swath of ground very well, but will be able to image a small area, but pretty accurately.
Oh and also, SAR imaging is insanely computation heavy. Which is why high accuracy SAR processing is usually done offline, at ground stations, because they have the big heavy computers to do the crunching. Its unlikely that a fighter platform can afford to lug along that amount of computation power within its airframe.
Hope the explanation helped.
Edit: The RBE-2
description says High resolution imagery modes (SAR) - Designations
under the Strike mission heading. This suggests the limitation I mentioned, in that the high-resolution SAR mode will be mostly useful to identify the specific target in a small area of ground identified from other modes, to improve the accuracy of munitions delivery.