That was kinda my concern also, if IRST is a major feature of this new aircraft, they must be confident that IR is able to effectively counter VLO.
Beyond just the durability of the RAM coating, operating an aircraft at mach 2+ would create significant heat and lighting up the aircraft for all to see, rendering radar stealth moot?
Some folk have questioned the thermal signature implications of a Mach >2.0 supercruise capability that is being widely attributed to this aircraft. I would add to this a query regarding the durability and maintainability of RAM coatings under such conditions.
The reason why radar is the preferred means of long range object detection is that it is a active scatter method. You send out either a broadband pulse (or many narrowband pulses that have the total area coverage of a broadband pulse) with no assumptions as to where you think the targets are. Then you monitor for return scattering.
As a scatter method, you have information on 1. when your pulse goes out and 2. when the return pulse hits 4. doppler shift of the return waveform 4. distortion of the return waveform.
This has advantages:
1. you do not assume where your target is, you detect your target wherever it is.
2. you have ranging information based on time of flight
3. you have velocity information based on Doppler shift
4. you have orientation information based on waveform distortion
IR in comparison:
1. you must assume where your target is and point your detector in that direction such that the assumed target will be within the field of view of the detector.
2. your detector must change its orientation to match the change in angular position of the target to keep it in view.
3. you only see the 2D projection of the target which provides only angular position, not radial position.
4. you do not have full velocity information since there's no outgoing waveform to compare to
5. you do not have orientation information if the 2D projection is non-unique for a given orientation
Why is the IRST important then? Because of
strategy.
1. China is on the strategic defensive, thus against VLO attacks against fixed targets within friendly territory, there will be VHF radars, military and civilian visual spotters, etc for early warning against VLO targets. An unsupported VLO penetrating attacker is then on a 1 way trip as VHF, spotters, etc vector interceptors with IRST towards it, who
know the rough angle and distance. Even attacking the spotters is a bad idea, since they are easily replaced and reveal position anyhow.
2. Opponent is assumed to be on the strategic offensive, with disparate fixed and low speed sites within mostly neutral territory that cannot be made friendly (ocean). There are no spotters and no terrain to hide VHF radar. There is nobody to vector interceptors in a hidden way. All possible detectors are themselves high end assets.
