How does the target indicator in Jet Fighter Aircraft work? How does it stay on target? Is this queued from the radar, from infrared or is this image classification? What does the x in the target indicator mean? Are the other symbols around it part of the target indicator or related to something else?

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    $\begingroup$ Could you please point to the source of the image? $\endgroup$
    – Federico
    Dec 17, 2017 at 13:52
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    $\begingroup$ It could be any of them, depending on the configuration of the fighter. Some fighters acquire manually and tracks it with the missile's IR detector, mig-21 is such a example. Other more advanced fighters acquires and tracks the target with onboard radar or IR detector then let the missile lock on after launch. But sensor fusion is a thing now so you would expect the latest generation would collect data from all sources available and gets the job done. $\endgroup$ Dec 17, 2017 at 23:32

2 Answers 2


There are two main questions here, mainly how does a fighter jet track a flying target, and questions about the symbols used on the Heads Up Display.

How does a fighter jet track targets

A fighter jet tracks its target(s) using its radar, which (as you are all aware) works by emitting EM waves and detecting the reflected waves bounded off by the target. The onboard computer processes this data and translates it to target information.

There are two modes of radar operation*. The first is a scan mode: the radar makes a sweep across a wide angle to look for targets. In this mode, the radar scope is wide, but target update rate is slow, and target information is coarse. The second is targeted mode: the radar focuses a narrow beam of energy at the target, which provides accurate readings with a high update rate. On the HUD, you can see "200Vc", meaning the jet is closing in on the target at a relative speed of 200 knots. Below that is "1000 FT", the distance to target.

* There are many modes actually, but to keep it simple, let's just focus on two.

A fighter jet does not identify a target optically nor using infrared. Neither of these two methods can provide accurate position information of the target. This may change in the future, but it is what the technology is now.

About the HUD

The HUD is from a F-18 (I am not sure which variant). Let's go over the symbols top to bottom and left to right:

Left column

  • On the upper left is a black square. It indicates that the trigger is pressed at this moment. It is displayed only on the recorded tape and is not visible to the pilot.
  • the boxed number is the indicated airspeed. It is reading 179 knots.
  • Going down is the angle of attack, or "alpha". It is reading 20.3 degrees.
  • Mach speed is 0.36.
  • G-loading is 1.7.
  • The max g-loading on this flight is 7.6 G.
  • Below that is the time in UTC.

Center column

  • On the upper center, the arrow pointing upwards "^" is the needle of the heading indicator. Above it is the ruler marked with heading numbers, which is apparently cropped off in this image.
  • The cross sign below "+" is gun cross. The cross is always displayed at the gun inclination angle as a backup.
  • The "w" sign is the attitude indicator. In the background we can see the numbers: the aircraft is pitched 26 degrees down and banked approximately 70 degrees to the right.
  • "GUN" indicates machine gun is the selected weapon. The cross indicate it cannot be fired, most likely because this is a practice mission and master-arm is not on.
  • "0" is the number of bullets left.
  • Near the bottom edge of the image, a bit to the right of center line, is the flight path indicator, a circle with three bars extending from its left, top and right respectively. It shows where the aircraft is travelling to, and it is useful for landing or flying low near terrain.

Right column

  • Current altitude is 15180 feet MSL.
  • Target closure rate is 200 knots.
  • Range to target is 1000 feet. If the range is longer than 1 nautical miles, it will display in nautical miles instead, up to the nearest 0.1 nautical mile (e.g. 2.3)

Symbol around the target

  • This particular circle is the target reticle for gun mode. A bigger circle is displayed if it is in missiles mode.
  • The center dot shows where the bullet will hit if the gun is pressed at this very moment. The circle moves around the HUD as the jet is maneuvered. For example, if the pilot pulls on the stick, the circle will move towards the bottom of the HUD because that is where the bullets are going.
  • Targets are identified by a square box on the HUD. The square is not displayed when the gun reticle overlaps with the target, which is the scenario in this image.
  • "IN LAR" means In Launch Acceptability Region. "IN LAR" is displayed instead of "SHOOT" because the target is designated as friendly. If the target is designated as hostile, "SHOOT" will be displayed instead.
  • Below the letters "LA", we can see a bold portion from 12'o clock to 1'o clock on the circle, and a mark at 1'o clock. This is the range to target. The mark rotates clockwise as target distance increases. For example, if the bold portion is instead from 12'o clock to 3'o clock, the target range will be 3000 feet.
  • There is a similar mark on the circle slightly pass 6'o clock, but the mark is outside the circle. This is the maximum range of the currently selected weapon. By comparing the bold portion of the circle to this mark, a pilot can tell whether the target is within range of the current weapon.
  • I have no idea what the "x" means.
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    $\begingroup$ Is the waterline mark "w" exclusive to Navy jets? Or how can you tell it's a Navy HUD? $\endgroup$
    – user7241
    Dec 18, 2017 at 4:14
  • $\begingroup$ Identifying a target and tracking it are two different things. The fact that it was classified as friendly just shows the identification part. And that range info is given in the picture means that radar is on in this case. But that doesn't generally exclude optical or infrared for tracking. $\endgroup$
    – user7241
    Dec 18, 2017 at 4:25
  • $\begingroup$ @jjack I didn't say its a Navy HUD; it is the F-18 HUD. The F-18 is operated by many countries around the world, besides the US Navy. $\endgroup$
    – kevin
    Dec 18, 2017 at 4:34
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    $\begingroup$ Yes, ok. But how can you tell it's from the F-18? $\endgroup$
    – user7241
    Dec 18, 2017 at 4:55
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    $\begingroup$ @jjack because only the F-18 HUD looks like this. The HUD of every other military aircraft (that I am aware of) has a different layout. The F-16 for example has a speed tape and altitude tape; the F-15 has neither box nor arrow around the numbers etc. $\endgroup$
    – kevin
    Dec 18, 2017 at 5:16

I think this is an F-22 HUD. I don't know this particular HUD, but I can make some educated guesses:

  • everything is radar based, a computer attempts to track any object in the radar's view

  • the circle means it is an acquired target, which means the computer is satisfied that it has tracked the target successfully and knows its position relative to ownship

  • IN LAR means "in launch acceptable range", that means that if a missile is launched, it might hit the target

  • the "x" probably means the currently selected target; in other words, if a missile is fired, it will target the circled object; it could also simply mean that the target is in range

  • the tick mark inside the circle is probably the radial velocity of the target; in this case the target is moving away from ownship at a moderate rate

  • the tick mark outside the circle is probably the relative aspect to target, but it might also be some kind of velocity indicator

  • the other notations have nothing to do with the target

  • $\begingroup$ It's not an F-15 or F-16 HUD? $\endgroup$
    – user7241
    Dec 17, 2017 at 14:02
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    $\begingroup$ How does the Navy HUD end up in an F-22? $\endgroup$
    – user7241
    Dec 17, 2017 at 14:18
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    $\begingroup$ @jjack Nothing on this screen is cued by an optical sensor. The US uses three types of air-to-air missiles AIM-7, -9 and -120. All three are radar cued. The AIM-9 has an infrared seeker, but that is used only for homing, not target cuing. $\endgroup$ Dec 17, 2017 at 14:23
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    $\begingroup$ @jjack That is misleading. What they mean is "airborne" targets, like balloons, helicopters, and other slow moving stuff that can downed with guns. To a jet such targets are similar to other helpless ground targets. Optical pods are not used on enemy fighters. $\endgroup$ Dec 17, 2017 at 15:32
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    $\begingroup$ Almost everything in this answer (except "IN LAR") is completely wrong. $\endgroup$
    – kevin
    Dec 18, 2017 at 2:35

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