Could we say "radar warning systems are also passive radars"
Or passive radars can identify further frequencies than usual radar signals which fighters' radars emit?
View attachment 30970
Yes RWRs are passive (receive only).
Their antenna can be phased array these days....but their channelized receiver and related architecture tends to be optimized to dealing with missile threats and pulse-doppler signals given thats all there generally is room for (on say fighters) and these continue to cover a large bulk of threats found today.
They generally wont be as capable as a larger AESA (with far more advanced channelized receivers and algorithms) on listen only mode....but the important thing regarding them and DAS is you can orient them to cover different approaches to the platform (unlike the AESA radar which is quite narrow field of operation).
Its not really a case of further frequencies (since the sweeps are all pretty much the same), but the frequency analysis regimen (w.r.t background noise), capacity and sensitivity itself that is lot more robust (esp. given computational power backing it) in dedicated ESA radar than an RWR.
There are some exceptions, as you do get large + advanced RWRs on larger/specialised aircraft (ELINT, SEAD etc)...that do end up being something like a listen-only but very capable ESA one can say.
But you need to commit mass budget for that (i.e the aircraft is specialised role...rather than multi-role fighter).
Generally if you have AESA on board (say a fighter), there is less need for traditional RWR....the AESA will have a mode to commit some TRMs to what an RWR does....and would do it better and you end up saving room + power (and use DAS like in your pic)....room+power you can use for (IR/UV) MAWS instead and cover more bases there. This ends up actually being an extra advantage of having an AESA + DAS.
RWR domain is undergoing lot of RnD lately (given size available on a fighter and given the growing threat of ESA seekers).
They are evolving into even more integrated systems ("sensor fusion") by better multiplex processing with MAWS and LWR on the platform. Conceptually very much like DAS distributing the AESA aperture effectively.