Xiaohua: 欢迎来到这一期的词汇小百科。Now, in this edition we’ll be talking about the word “transponder”, 应答机. This word kept popping up when the media reports the incident of the missing Flight MH370.

John: Right. So, what happened was, there are two ways to know where a plane is. One is through regular radar. With regular radar, with primary radar, you can only tell that there is an object there. However, with so many planes in the air these days, air traffic control, both civilian and military, requires a little bit of extra information. That’s really what the transponder is there for. It has craft number, its unique ID number, and a four-digit code that can mean different things at different times. So what happened with MH370 is that at some point the transponder on the plane turned off.

XH: 飞机上的应答机 transponder 是一种辅助通信系统。那么我们通常所说的radar system 雷达系统指的是一级雷达系统，它只能告诉你比如飞机的位置，但一些更具体的信息，比如飞机的高度和压力，这就需要应答机来发送信号告知了。

JA: And so what it does is the transponder receives a signal from the secondary radar and then it returns a squawk code with its aircraft’s position, altitude, and call-sign.

XH: Transponder 应答机是和地面上的二级雷达 secondary radar system 一起工作的。当地面雷达给飞机发送雷达信号的时候，应答机就会发出一串 4 digit squawk code，也就是一组4位的数字，这组数字可以告诉地面雷达系统飞机的高度以及其它一些信息。

JA: Right, and so with the squawk, the four digit code, it actually runs from 0 to 7. When we look at Flight MH370, one the strangest things is that the transponder signal just disappeared. Now, it is possible, in that squawk code, in that four-digit code, to somehow communicate that there is a problem on the plane, that’s it’s having technical problems, that it’s being hijacked, or anything else. So, basically, anything that the pilots want to communicate, not over radio, but through the transponder to air-traffic control, to anyone who can receive that signal, they can do that. But, however, with MH370, nothing like that happened. Instead, it just, turned off or disappeared or who knows?

XH: 这组4位数字实际上是8进制的数字，也就是说最小的数字是0000而最大的数字是7777.奇怪的是马航MH370如果发生了任何故障或问题，transponder应答机都会向地面发出特殊的信号，but in fact, nothing happened. Then the thing is, what possibilities are there that there are no signals at all from the transponder?

JA: There are some cases, in fact, where air traffic control will ask certain airplanes to turn off their transponder or perhaps just turn it to standby. This happens usually when there are many planes that are close together and so it makes it easier for the air-traffic controllers to actually identify which planes is which. But, in this case, however, with the Malaysian Airlines flight, it would seem that that would have been unnecessary for that to happen.

XH: So, the only reason maybe that either the transponder is broken, or damaged, or the pilot decided to turn it off.

JA: Exactly.

XH: 塔台可以命令飞行员关掉应答机或者把应答机调在备用状态，而在马航的事件中，显然塔台没有发出这样的指令。那只有另外两种可能了，一是应答机出现了故障，而二则是飞行员自己关掉了应答机。That wraps up this edition of Round Table’s Word of the Week.