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An aircraft’s transponder transmits signals that announce (at the very least) the aircraft’s identity, altitude, and direction of motion (plus, for most newer aircraft, a whole slew of other useful information); this is what allows secondary surveillance radar (SSR), the backbone of modern air traffic control, to function,1 and what makes it possible for aircraft’s TCAS to see other nearby aircraft and (if necessary) warn their pilots not to collide with them.2

For a long time, I thought that this worked by having the transponders broadcast this information in the blind, in case there was a radar or TCAS within range who might want to know about the aircraft attached to the transponder. I’ve recently learned, however, that most aircraft transponders “speak only when spoken to”, not transmitting their information until they hear a radar or TCAS shouting “Anyone there?” into the deep blue sky (although ADS-B, which works essentially how I thought all transponders worked, is being rolled out, and will soon become mandatory in the U.S. for many aircraft) - and that this transmission is sent only to the shouter, and not to anybody else who might have an interest in the matter.3

I don’t get why most (or, until very recently, all) transponders don’t broadcast unbidden; it’s obviously feasible (otherwise we wouldn’t be able to have ADS-B), and would eliminate the need for those big, expensive SSR transmitters (ATC would just need a small, cheap receiver, plus a primary surveillance radar4 dish for detecting birds, weather, detached aircraft parts, aircraft with broken-or-otherwise-nonfunctioning transponders, and aircraft without transponders [such as most balloons and gliders], and as a backup in case the aforementioned receiver blew up/contracted malware/got stolen by space aliens). So why do, or, at least, did, aircraft transponders have to be interrogated by a radar or TCAS before they spill the peanuts?5

1: Which, in turn, enables controllers (for instance) to stack multiple aircraft at different altitudes in close horizontal proximity to one another without risking a mid-air collision (whereas most primary radars can’t determine the altitudes of their targets - since they can measure the azimuth and slant distance of a target, but not its elevation angle - and, thus, can’t be used for vertical separation of targets), to warn aircraft if they sink dangerously low (same reason), to immediately identify new targets appearing on their screens, and to associate said targets with radio transmissions from said aircraft without having to ask the transmitter to make a 360º turn for identification purposes.

2: I’m sure there are a lot of other things transponders are useful for as well, but that I’ve forgotten to include.

3: For instance, this means that an aircraft has to have an operational transponder in order for its TCAS to work - it can’t just sniff out transponder signals sent by nearby aircraft in response to interrogation by other radars and/or TCASses.

4: Radars that detect targets by physically bouncing radio waves off them and listening for the echoes.

5: Like spilling the beans, except that aircraft serve (and spill) peanuts instead.

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The Secondary Surveillance Radar system provides range in a method similar to primary radar: by measuring the time between the transmission of the interrogation signal and reception of the reply. The range is proportional to the time difference, minus a constant that allows for the aircraft transponder to perform processing.

This would not work if transponders replied without a interrogation.

Azimuth is traditionally found by observing the direction of the SSR antenna the moment this occurs, though some modern systems utilize the time delay between multiple receiver locations (multilateration).

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I don’t get why most (or, until very recently, all) transponders don’t broadcast unbidden

GNSS systems have only been commercially available relatively recently (compared to the timeline of aviation). Also, GPS SA (Selective Availability) was turned off in May 2000 to make available the required accuracy for civilian aviation. ADS-B relies on the aircraft actually knowing where it is.

Basic Mode A/C transponders did not require the aircraft to know where it was. The SSR would determine azimuth, and the Mode A / C replies would provide identity and pressure altitude.

Equipment fitted in certified aircraft changes very slowly. There are still aircraft flying today that were built well before GPS existed. Retrofitting new navigation equipment to an aircraft is expensive.

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    $\begingroup$ To the point of this answer, if I remember correctly, the newest aircraft I've personally flown was built in 1980 and the oldest some time in the 1960s. Every aircraft I've flown is older than I am. $\endgroup$
    – reirab
    Commented Jun 11, 2019 at 18:25
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The main purpose of SSR is to provide information to the specific station requesting it. Radar antennas are directional when broadcasting and receiving the signals. When the SSR system interrogates the transponder it means the antenna is pointing at it, and so a signal can be received. If the transponder responded when the antenna was pointing away there would be nothing to receive it and the information would be lost.

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In addition to the other answers: by waiting for interrogation, you make sure transponders don't transmit simultaneously. This is the big problem with all radio systems: when two transmitters transmit at the same time on the same frequency, both transmissions are unintelligible.

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    $\begingroup$ That's not the reason. If two aircraft are at the same distance and (rough) direction from a radar site, they still transmit at the same time. $\endgroup$
    – Rainer P.
    Commented Jun 10, 2019 at 20:48
  • $\begingroup$ @RainerP. No. They won't. All radios operate at 1090MHz, they will talk over each other and interfere it's basic physics. $\endgroup$
    – Sam
    Commented Jun 10, 2019 at 23:13
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    $\begingroup$ @Sam You misunderstand Rainer's point. If two aircraft are the same distance and direction they will both hear the interogation and both respond. The system has to be able to cope with this (because two aircraft in the same place is exactly when you do not want the system to fail). Given the system can cope with this, rather weakens the argument that challenge-response is to prevent interference. $\endgroup$
    – Martin Bonner supports Monica
    Commented Jun 11, 2019 at 16:49
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    $\begingroup$ @MartinBonner: SSR Mode S solves that, by letting the base station specify exactly which aircraft it wants a reply from. And mode S rather than mode A/C is to prevent interference (though not only this kind), which makes this answer valid for a question that is almost but not quite the one that was asked. $\endgroup$
    – hmakholm left over Monica
    Commented Jun 12, 2019 at 9:59
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    $\begingroup$ I'm not sure about commercial aviation, but in the military there are two antennas involved with IFF - an omni antenna and a directional antenna. The directional antenna is generally what the aircraft respond to. It's unlikely that two aircraft will be at the same bearing at the same distance for very long, I think... The reply would likely be picked up during the next sweep if it was missed or garbled. The response, by the way, are a set of pulses, which are either present or not. There is nothing modulated onto the signal. $\endgroup$
    – Steve
    Commented Jun 14, 2019 at 19:57
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Why do aircraft transponders wait to be interrogated before broadcasting beacon signals?

To measure the distance between interrogator and target, similar to how DME works.

I don’t get why most (or, until very recently, all) transponders don’t broadcast unbidden; it’s obviously feasible

It was infeasible in the pre-GPS days, because aircraft didn't know their own position. Even today, the reliance on GPS is a single point of failure and if it is jammed or shut down, aircraft can no longer announce their own position unsolicited. They have to revert to direct distance measurements amongst each other, which requires a challenge-response style of communication.

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Great question (with a nice follow-on narrative). Also, great answers! I will add something else I have not read here:

Why not transmit unbidden?

Transponders trace their origins back to the beginining of military Identify Friend or Foe (IFF) technology. In this context, transmitting unbidden is like painting a target upon yourself. Even responding to an interrogation carries risks of an enemy triangulating your position. Transponder equipment in both the vehicle and ground both evolved with this assumption in mind, and certain optimizations were made with certain other trade-offs.

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Also note that broadcasting all the time takes a lot more energy than sending a directional signal once in a while. And if the aircraft (rather than a ground station) does it, that energy adds to the takeoff weight. I'm not sure how much it matters, but I wouldn't discount it, particularly for relatively small aircraft.

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