A spy plane has a transponder so you don't shoot a friendly/neutral aircraft down (or waste time classifying it as friendly/neutral.)
The codes it squawks can be modified in various ways but there will be scenarios when you want your transponder, such as flying in an area where you have air superiority.
Planes have transponders to make it easier to find them, though that's mostly for when they are in the air. The issue is that the military likes to turn them off.
They have transponders. But military airplanes can and sometimes do turn them off. When airplanes broadcast signals they can be detected. The military sometimes wishes to avoid this.
If you can't trust the pilot then you can't trust the equipment either. Civil aviation requires a lot of trust, so there's no real reason to try to lock out the transponder.
Once the pilot is not trustworthy, you've entered the realm of military defense, and a lot of that is built around the ability to track targets that don't want to be tracked.
Civilian planes fly with transponders - something that you can receive with a $20 bit of hardware. Military aircraft do not turn their transponders on unless they want the world to know where they are.
As already pointed out, civilian transponders are not really "IFF", they're a tool for ATC to keep track of you.
Military IFF transponders don't emit unless they get interrogated by a valid code, and then only briefly.
The interrogating plane would typically only interrogate if you show up as a radar contact, and at that point, being able to say "I'm friendly" is very useful. You're not hidden anyway.
Note that the name "Identification Friend or Foe" is misleading; IFF can only positively identify friendlies. A nonresponsive bogey (unknown) may still be a friendly with bent IFF, wrong codes, etc.
My impression was that military aircraft turned off their transponders all the time, especially during sensitive parts of their missions. You don't have to follow any of the OSINT accounts that post flight info to see posts about how some Global Hawk re-emerged on the trackers, after having gone silent some number of hours ago. Sometimes they keep their transponders on, intentionally, to communicate intent and posture to potential adversaries.
Yes, the squawk code is not itself necessarily used to identify individual airplanes and distinguish them from one another and transponders now transmit significant amounts of data on top of the squawk code.
(I don't know how much squawk codes are, or were, used to specifically identify aircraft in the past, but I believe that currently it's not unusual that many aircraft in the same region would be squawking the same number, which would not confuse ATC because of all the other transponder data that's available.)
Eh. Military aircraft would also have a transponder, they just wouldn't necessarily have active broadcasts.
Civilian aircraft do broadcast actively (ADS-B). But they also respond to secondary radar for Mode A/C, which are basically cases of IFF Mode III (okay, maybe not exact term, but the idea applies.) So it's still a challenge-response/IFF, just in this case always responding.
Military aircraft use different modes and presumably don't respond unless interrogated with an appropriate challenge, but the principles are the same.
Protocol is to visually identify the target. In the case of an airliner, they try to make visual contact with the pilots if they don’t respond by radio. There are visual verification methods commercial pilots are trained on.
And airliners have transponders and flight plans. If a civilian plane stopped talking to ATC, the Air Force is likely already involved.
Additionally, we’re not on a high-alert war footing like during the Cold War. As far as I know, we don’t have hostile military aircraft routinely flying with transponders off on our coasts.
Even if we did, I’m pretty sure the larger military radar systems that would be used to track this stuff can read transponders and separate out which plane is which.
Yes. I am astonished that you do not already know that.
On 9-11, the combined maximal efforts of civilian and military aviation were unable to locate four airliners because their transponders were turned off. This occurred near the New York and DC metropolitan areas, some of the most carefully controlled air space in the world.
Without a transponder, an aircraft is just a raw radar return among, potentially, hundreds or thousands of other returns from civilian and military aircraft, weather, birds, balloons etc. Add to that glitches, ghosts and anomalies in busy air space and it is not surprising that a large aircraft without a transponder can hide in plain sight.
You can actually track a large number of military flights on websites such as https://www.adsbexchange.com/ a large amount of the time they fly with there transponder on because they don’t want to hit other aircraft.
> far easier to install one than to wrangle an exception
You are talking about an agency that builds and operates planes that nobody else knows about. They can have any "exception" they want by just shrugging and saying the aircraft in question does not even exist.
All military aircraft have transponders because the air bases have ATC as well and (at least in the US) are near commercial airports they have to work with to avoid collisions during takeoff and landing.
It is worth mentioning that transport aircraft carry Mode S [0] transponders which emit a unique ICAO 24 bit address assigned by aircraft registration. This data is available regardless the pilot-entered 4 octal digit Mode A transponder code. The transponder can still be turned off, however.
This contrasts to small aircraft which typically have Mode A/C only (no mode S).
I've heard that the first few years of flightaware they made most of their money in taking requests for not publishing specific aircraft.
As for military stuff - they have additional capabilities for their transponders that provide for responding to interrogators (think identify friend or foe - IFF systems). They're not supposed to disable transponders in the required airspace - they've got to comply with the FAA rules too. There are situations where ATC transfers control over to the military where they are free to do more things. Like in a hot military operating area (MOA) or when ATC transfers traffic separation to the military (MARSA).
I know absolutely nothing about this so forgive me if I'm asking a dumb question. ADS-B broadcasts so that other planes can figure out where you are and avoid you, correct? Why can't spy planes stop broadcasting it, but then listen for other planes' ADS-B signals, and just make sure to stay away from them? That way the only crash might be between two different spy planes?
The codes it squawks can be modified in various ways but there will be scenarios when you want your transponder, such as flying in an area where you have air superiority.
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