A piece of radio equipment that stays quiet until it is asked something, and then answers automatically. A signal comes in; the transponder sends a signal back. Nobody has to press a button.

A piece of radio equipment一種無線電裝置 that stays quiet平常很安靜 until it is asked something直到有人問它什麼，and而 then answers automatically才會自動回答。A signal訊號 comes in進來；the transponder這個裝置 sends a signal back就送一個訊號回去。Nobody沒有人 has to press a button需要按按鈕。

The most famous ones are on aircraft. Air traffic control sends out a radar pulse, and every plane's transponder replies with its identity and height — which is how a controller's screen shows not just dots but 'this dot is flight BR225 at 30,000 feet'. When you hear on the news that a plane's transponder 'went dark' or 'stopped responding', this is the box they mean.

The most famous ones最有名的例子 are是 on aircraft裝在飛機上的。Air traffic control航管 sends out發出 a radar pulse一個雷達脈衝，and而 every plane's transponder每架飛機的這個裝置 replies with回覆 its identity and height它的身分和高度 — which這就是 is how a controller's screen shows為什麼管制員的螢幕不只是顯示 not just dots光點，but而是 'this dot is flight BR225 at 30,000 feet'「這個點是BR225班機，高度三萬呎」。When you hear on the news當你在新聞上聽到 that a plane's transponder說一架飛機的這個裝置 'went dark' or 'stopped responding'「失去訊號」或「停止回應」，this這 is the box they mean就是他們指的那個裝置。

But they are everywhere else too: on satellites (a satellite transponder receives a signal from the ground and sends it back down over a wide area), in the little tag on a car windscreen that opens a toll gate or a car park barrier — Taiwan's eTag works on this idea — and in animal microchips and warehouse tags.

But不過 they它們 are everywhere else too其實到處都是：on satellites衛星上也有（a satellite transponder衛星上的這種裝置 receives a signal from the ground接收從地面來的訊號 and而 sends it back down over a wide area再把它送回一大片範圍），in the little tag車窗上的小標籤 on a car windscreen，that opens a toll gate or a car park barrier能打開收費閘門或停車場柵欄的 — Taiwan's eTag台灣的eTag works on this idea用的就是這個原理 — and而 in animal microchips and warehouse tags動物晶片和倉庫標籤裡也有。

It is a technical noun, ordinary and neutral in tone. Engineers, pilots and journalists use it plainly. There is no informal version — in casual speech people just say 'the tag' or 'the box'.

It它 is是 a technical noun一個技術性的名詞，ordinary and neutral in tone語氣平常、中性。Engineers, pilots and journalists工程師、飛行員、和記者 use it plainly都直接這樣用。There沒有 is no informal version非正式的講法 — in casual speech平常講話 people just say人們就直接說 'the tag' or 'the box'「那個標籤」或「那個盒子」。