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.