# How does AWACS pick up enemy info like speed and altitude without transponder interrogation?

So basically, IFF can pick up friendly vehicles. I'm assuming that enemy planes will have their transponder off to minimize speed and altitude data. How does the AWACS determine enemy info to pass on to friendly fighters?

• Do you know what that giant disc on top the airplane is? (not being snarky, just trying to establish a knowledge baseline...) Apr 10, 2023 at 22:01
• @MichaelHall yes that's the radar with the IFF interrogator correct? Apr 10, 2023 at 22:05
• That disc does much more than just IFF. Apr 10, 2023 at 22:06
• -1 for not knowing what radar is. I wish I could use more minuses for actually using the word "radar" without knowing what radar is. (I'll admit to snark, but this is special.) Look up "radar" on Wikipedia, or even just in the dictionary, and think about how your question needs updating. Apr 11, 2023 at 7:05
• How does a cop know how fast you're going in your car using a radar gun when your car has no IFF transponder? Apr 11, 2023 at 7:41

Primary radar doesn't require a transponder or IFF module. It works by actually bouncing a radio signal off the skin of the airplane. It doesn't require the cooperation of the plane being detected, so it works even for enemy aircraft. The direction the antenna is pointed when the signal is received tells you the direction to the target, and the delay between transmission and reception tells you the distance.

Altitude can be determined the same way as position. If you know the target's elevation (vertical angle with respect to the horizon) and distance, it's a matter of simple trigonometry to find its altitude.

(Civilian radars normally don't tell you the elevation, because making the radar antenna go up and down in addition to in a circle was considered too much of a technical challenge for early radars, and transponders came along before vertically-scanning radars. But for military use, where the target probably isn't going to tell you their altitude just because you asked nicely, vertically-scanning radars are common.)

(Also note that modern radars can be aimed via solid-state techniques, so just because you don't see the antenna moving vertically doesn't mean it isn't scanning vertically).

Speed is determined simply by tracking how far the target moves between successive sweeps.

• Isn't speed determined by the strength of the doppler effect on the returning signal? This means you can get the speed within a single sweep. Apr 11, 2023 at 8:32
• @LordOfThePigs - that would be the relative speed, surely? Apr 11, 2023 at 8:48
• @MichaelHarvey Right, but getting the current ground speed from the avionics or GPS and adding it to the Doppler result shouldn't be too hard. Apr 11, 2023 at 11:53
• @LordOfThePigs Are you going to get a doppler shift if the target's flight vector is perpendicular to the vector from the radar station to the target at the time it gets pinged? I'm not convinced that a single ping will be sufficient to get a velocity in all cases. Apr 11, 2023 at 12:06
• Also note that military radar showing the (potentially less precise) radar altitude instead of the one broadcast by the transponder can be misleading. This may have been a contributing factor to the 2006 midair collision of a business jet and a Boeing 737 (en.wikipedia.org/wiki/Gol_Transportes_A%C3%A9reos_Flight_1907) - the business jet was at FL370 (on a collision course) the whole time, but the radar altitude fluctuated and the controller didn't notice that what he was seeing was not the "transponder altitude". If he had just seen a dot without any info, it would have been clear. Apr 11, 2023 at 13:18