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The F-4 Phantom has an interesting collection of sensors at the extreme rear of the empennage. What are these sensors for? My daughter calls them them smiley sensors but I suspect that is not the proper technical term. Her guesses as to their real purpose:

  1. Detect other airplanes.
  2. Detect smiles.

Which one is it?

F4 from the rear

F4 from the rear angled

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    $\begingroup$ There are happy ghosts trailing the skies! :D $\endgroup$ – Zizouz212 Jun 5 '15 at 0:45
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    $\begingroup$ @Zizouz212: Tori Amos has got you covered! $\endgroup$ – dotancohen Jun 5 '15 at 0:58
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Your daughter is right as explained here:

The 'bug-eyed' face of the parabrake door mounting the two rear-hemisphere radar warning antennas.

So they do detect other airplanes and probably the smiles of their pilots too as they are getting close to the phantom.

Rear Antennas

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    $\begingroup$ Presumably, it's the "eyes" that are the radar sensors. Any idea what the "mouth" is? $\endgroup$ – David Richerby Jun 4 '15 at 17:07
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    $\begingroup$ Radar warning antennas technically detect the emissions of other aircraft, SAM systems, etc, rather than the aircraft themselves. The "mouth" appears to be some sort of vent, APU exhaust maybe? $\endgroup$ – Joel M. Jun 4 '15 at 17:10
  • $\begingroup$ @DavidRicherby I read somewhere that there is a fuel drain, which is probably vomited by that mouth. $\endgroup$ – Farhan Jun 4 '15 at 17:37
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    $\begingroup$ The 'mouth' is part of the parabrake door and it opens so it can not be a fuel drain nor APU exhaust. My guess is that is equalizes air pressure or vents fumes from inside the fuselage. Note the holes surrounding the parachute chamber in this image. $\endgroup$ – dotancohen Jun 4 '15 at 18:00
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    $\begingroup$ @dotancohen That "open" picture looks like an entirely more frightening mouth. Wish I could unsee... $\endgroup$ – JPhi1618 Oct 20 '17 at 20:04
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Farhan is correct that these "bug eyes" are part of the radar warning system, but unfortunately is incorrect as to what that system actually does.

Radar warning systems are commonly installed on military aircraft. Their primary purpose is to detect the upper-level radio frequencies employed by ground- and airborne-based active (i.e. radiation-emitting) radars, to include those used by radar-guided missiles such as the AIM-120 AMRAAM. Though each antenna is omnidirectional, radar warning systems typically consist of multiple antennas (usually four) each mounted pointing in a separate "quadrant" for maximum coverage and to determine where the signal is coming from.

The F-4 Phantom employed various iterations of radar warning systems over the years, to include the AN/APR-36, AN/APR-37, and ALR-46. The antennas pictured in the question constitute the aft-facing pair of one of these systems; the remaining front facing antennas are mounted one on each wingtip as seen below and captioned in the following image:

Port wing tip with bug-eye RWR dome and position light.

F-4 wingtip-mounted radar warning antenna

However, radar warning systems are almost universally passive in nature, meaning they do not emit radiation of their own, and can only sense radar signals being emitted by something else (and unfortunately not planes per se, nor smiles).

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