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If yes, the radar needs to be off, how can it detect enemy plane, locked-one and fire missles?

If stealth plane still operates its radar, can enemy planes detect its radar emission and pinpoint it location instead?

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    $\begingroup$ That (duplicated) question is a pretty awful question, but the answers to it are good, and they do address this question pretty well. VTC as dupe. $\endgroup$
    – Ralph J
    Commented Oct 7, 2021 at 0:28

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Yes, having your radar on will make you VERY detectable.

Think of two guys walking around a dark warehouse, light off. Both have a pistol and a flashlight. Whoever turns their flashlight on first loses.

So what do the stealth pilots do? In a competent Air Force, they have an AWACS orbiting 75 miles away, telling you exactly where the bad guys are. If the AWACS can get me to within 25 miles (later models much farther) and pointed in mostly the right direction, I can fire an AIM-120, and lets its own radar take over. The radar in my aircraft never needs to be turned on.

But later.... If we're just 1 v 1 or 2 v 2, 'stealth' is not meant to make you "invisible", just harder to detect. If the better stealth on my jet gives me a 1 minute advantage in detection over your jet, you lose.

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  • $\begingroup$ So how does this work? If the other craft is transmitting known data at a known frequency it should be easy to determine its position using multiple antennas. You just measure when the data arrives at the antennas and use the small time difference in arrival time to calculate its position. But I can’t get my head around how you’d do it for an unknown signal which is only transmitted in pulses and directionally. $\endgroup$
    – Michael
    Commented Oct 7, 2021 at 6:51
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    $\begingroup$ @Michael antennae are typically not super targeted at one frequency, and radar isn’t going to diverge much. $\endgroup$
    – Tim
    Commented Oct 7, 2021 at 8:11
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    $\begingroup$ @Michael Ultra-wideband receivers. UWB radio existed since the original invention of spark-gap radio (that's what spark gaps are: they transmit pulses on almost all frequencies) however processing the signal was impractical until the invention of software-defined-radio. With SDR you don't need tons of decoders and demodulators and filters to process multiple frequencies. You can do all that in software. To process more frequencies in parallel all you need is more CPU power. $\endgroup$
    – slebetman
    Commented Oct 7, 2021 at 9:35
  • $\begingroup$ Receiving the signal seems straightforward, but what I’d like to understand is how you filter it out from all the other (maybe even deliberate) noise and determine its time of arrival. Especially if you are not just interested in the strongest signal (where calculating a cross correlation might be sufficient). $\endgroup$
    – Michael
    Commented Oct 7, 2021 at 10:18
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A notable mention is that some (Russian) planes use IR detectors to detect aircraft (-exhausts) in-front of them. This is notorious because you will only receive (in the most hopeful scenario) a missile launch warning when the IR-seeking missile is launched.

Like this IR tracker on a SU-35:

enter image description here

https://en.wikipedia.org/wiki/Infrared_search_and_track

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Aircraft can (and almost certainly do) use passive radar. This uses a third-party emitter, often a TV or Radio mast.

The radar system, by observing the direct signal from the mast, and the echos from other aircraft, determine their range and direction. By using Doppler corroboration, the speed of the target towards/away from the receiver can be determined. After observing the target for a while its current heading can be calculated.

A phased antenna array allows the direction a signal came in to be determined, without moving the antennae.

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