Silly noob question regarding SARH AAM's.

[pathology_doc]

Okay, so let's say we have a fighter with a proper airborne-intercept radar, not just a ranging set for a gyro gunsight, and the radar is locked onto the target. AI.23 on a Lightning F.2, for argument's sake, or AI.18 on a Sea Vixen. The radar set is transmitting energy and receiving echoes that provide directional information regarding the target. Why can't a SARH missile work with that? Why does it need a special illuminator set, and what's so different about the signal the illuminator puts out?

 

[SOC]

Continuous wave SARH illuminators gave the missile a fairly constant signal to guide on, I assume in part preventing it from needing to make larger, more severe (and therefore potentially energy-wasting) course corrections. Your basic old AI radar might have been a monopulse set, so giving it a separate CW illuminator, or a built-in CW capability, would've been needed for SARH. Oversimplified and ignores things like having the missile seeker tuned to the CW signal, beamwidth differences between tracking and CW signals, PRF differences, etc. but there it is.

 

[Fluff]

If we use your idea, of the main search radar, what would happen if there were 2 or 3 enemy aircraft, or 2 enemy and 1 friendly. Each is giving off the same amount of radar reflection. Your missile will bounce from target to target, and probably end up missing them all, if they are close together.

Different signal, so you hit the right target. FYI laser designators are also coded, so you also tell the bomb which code to follow. This means you can have multiple aircraft, and bombs in use, in the same area, and time.

 

[pathology_doc]

If we use your idea, of the main search radar, what would happen if there were 2 or 3 enemy aircraft, or 2 enemy and 1 friendly. Each is giving off the same amount of radar reflection.

Isn't the radar in track mode going to be more specific than that?

 

[Fluff]

Is this a war thunder question? As we seem to be bouncing around Era's.

Tracking if your system is capable of it, is looking for the same or similar signal next time the radar comes past. Continues ilumination is just that, continuous. If you only have one transmitter/receiver, you can only do one or the other. Newer radars with beam guidance, LPIR, agile beams and goodness knows what else, are different, but only in service a few years, and not using CWI, because it would be like manning a lighthouse, to the enemy.

Also perhaps the opposite point, if you have one system, if you have to illuminate the target for say 15 seconds, you are blind to anything coming at you, for those 15 seconds, aircraft, missiles, etc. So not a great place to be - not that I flew them!!

 

[overscan (PaulMM)]

One major reason is that pulse radars have a low pulse repetition frequency.

Pulse radars emit one pulse, then wait for that pulse to travel to maximum range, and back, before sending another pulse out.

Therefore most of the time, the radar isn't transmitting. Typically it's transmitting less than 10% of the time - 4% might be a typical value for
a pulse radar (this is called the duty cycle). This is not good for the missile to be able to hit the target - it will likely be flying "blind" during the terminal engagement phase as the radar is only intermittently painting the target. Try chasing after a dog with your eyes closed 96% of the time, and see how that goes.

Pulse doppler radars typically use medium and high PRF, which means the radar is transmitting for a much greater proportion of the time, up to 50% for high PRF. On the F-15 pulse-doppler illumination mode is a high PRF mode. The gaps between signals are much shorter.

Second, early SARH seekers are dumb - they receive the main radar signal from a rear antenna and compare it to the signal from the front antenna and directly generate steering commands. When it loses signal, its dead in the water, flying blindly. The target could move a fair distance between pulses.

So it made technical sense in the early days to have a separate CW transmitter that is transmitting 100% of the time for target illumination use.

When high PRF pulse-doppler radars came around, it became possible to do without the CW transmitter. Using the main radar signal has advantages in lookdown/shootdown capability and other areas, it also makes the radar lighter as you don't need the separate transmitter, but it needs more "smarts" in the seeker.

The AIM-7E2 used CW, the AIM-7F could use either CW or PD, the AIM-7M onward exclusively used PD.

 

[TomS]

The target could move a fair distance between pulses.

Also, in a conical scan, don't you have the risk that the pulse will arrive at a different part of the scan each time, so the missile sees a stronger return at different angles for every pulse? Given that con-scan seekers are (essentially) trying to drive the received signal to be equal all around the scan,* an intermittent signal like that is going to play havoc with the seeker and cause it to hunt rather than steer a straight course.

* I know there are tricks designers can do to enable proportional navigation with conical scan seekers, but that's the basic principle.

 

[overscan (PaulMM)]

Yes I think that's probably correct.

 

[Fluff]

The target could move a fair distance between pulses.

Also, in a conical scan, don't you have the risk that the pulse will arrive at a different part of the scan each time, so the missile sees a stronger return at different angles for every pulse? Given that con-scan seekers are (essentially) trying to drive the received signal to be equal all around the scan,* an intermittent signal like that is going to play havoc with the seeker and cause it to hunt rather than steer a straight course.

* I know there are tricks designers can do to enable proportional navigation with conical scan seekers, but that's the basic principle.

For the OP - wiki has a page on fireflash.

In testing it did hit a Fairey Firefly target drone. The launch aircraft had to follow the target, until impact. Not easy when most bombers still had tail guns, Probably ok in terms of hitting a bomber, but I cant see it hitting a manoeuvring fighter.