It listens for signals from whole constellations of satellites in orbit,

它會接收來自整個軌道上人造衛星群的信號，

and by triangulating all those signals and using a lot of maths, it works out where you are and shows you a little dot on a map.

並通過三角測量所有這些信號和運行大量的數學運算，藉此計算出你的位置，並在地圖上顯示出一個小點。

But how do the satellites know where they are?

但是，衛星們該如何知道自己的位置呢？

There aren't any landmarks in orbit for them to refer to.

軌道上沒有任何的地標可供他們參考。

If satellites are in a high enough orbit, well away from the atmosphere, then they can predict their position.

如果衛星在足夠高的軌道上，遠離大氣層，他們便能預測自己的位置。

They'll just keep orbiting in about the same path: there's no atmosphere to slow them down.

他們會繼續在大約相同的軌道上運行：因為沒有大氣層會減緩它們的速度。

But that's not precise enough.

但這樣還不夠精確。

There will be distortions caused by gravity from the sun and moon, or even from mountain ranges down on Earth.

來自太陽和月亮的重力，甚至是地球上的山脈都可能導致軌道受到干擾。

So maybe they could use the other satellite signals, like GPS.

因此他們或許可以像是 GPS 全球定位系統那樣運用其他衛星的訊號。

But if they're all relying on each other, that could steadily drift apart from reality.

但如果它們全都倚賴著各自的訊號，那便會逐漸與實際狀況越差越遠。

Maybe they could track the stars or the features down on Earth, but that's not precise enough either.

也許他們可以追蹤恆星或地球上的特徵，但這樣仍然不夠精確。

At some point, something down here on Earth has to look up in the sky and work out the position precisely.

到了最後，地球上依然必須要有某個儀器能往天空看去，並精準地找出衛星的位置。

And one of those somethings is hidden away in the countryside of southern England.

而其中一個這樣的儀器就隱藏在英格蘭南部鄉間。

It looks like a regular observatory, but it's not looking at the stars.

它看起來就像一個普通的天文臺，但它不並沒有在觀察星星。

We track manmade satellites in various orbits doing various jobs.

我們追蹤在不同軌道上運行各種工作的人造衛星。

They all have to have special reflectors on that enables the light to be returned to us.

它們上面都必須裝有特殊的反射器，讓光線能夠反射回到我們這邊。

Uh, and so we can measure the time of flight, and from the time of flight, derive the distance.

然後我們便能藉此測量光線的飛行時間，並從飛行時間中推算出距離。

You send out a very short pulse of laser light. You do that a thousand times a second.

我們發出的是一段非常短的脈衝雷射光，並在一秒鐘內發射了一千次。

Each of those shots, because you're using such a tiny short-pulsed laser, is 10 picoseconds long, which is about 3mm.

因為雷射發射出去的時間間距極小，每一次發射約耗時 10 皮秒，也就是大約 3 毫米。

That's the limitation on your precision. So each shot is about 3-4mm precision.

而這就是精準度的極限。所以每發雷射約有 3 到 4 毫米的精確度。

This station works around the clock, whenever the sky is clear.

只要天氣足夠晴朗，這個天文台便不分日夜的運作著。

That's why I'm here today: there will be some genuine observations when the clouds break, but right now, I'm not actually disrupting anything.

而這就是我今天人在這裡的原因：在雲層散去之後他們將會進行一些觀察，但就現在來說，我並沒有干擾到作業的進行。

But it does look significantly better after dark.

但是在天色暗下來之後看起來確實更加的棒。

Because it's a laser, it's a very specific wavelength, so you can filter, and that's what lets you work in day and night.

因為它是一種雷射，有著非常特定的波長，讓我們能輕易地濾出它的光，讓我們從白天到晚上都能運作。

The main problem with the atmosphere is: the speed of light in a vacuum is a very well known constant.

而大氣層的主要問題是：光在真空中的速度是個為人所熟知的常數。

The speed of light through the atmosphere is variable according to the mainly the pressure, but also temperature and humidity.

然而穿過大氣層的光速會受到主要為大氣壓力，以及溫度與溼度等變數的影響。

So we measure those things on site, as well.

因此我們也會在現場測量這些數值。

There's probably about 35 active stations scattered around the globe, located predominantly in the northern hemisphere, which is a slight problem from a science point of view.

全球大概有 35 個活躍的站點散播在世界各地，主要都分布在北半球。而從科學角度來說，這是個有點麻煩的問題。

You'd like them evenly distributed all around the globe.

你會希望他們能均勻地分佈在全球各地。

There's a couple in Australia which are really important because they're providing a bulk of the data from southern hemisphere.

在澳洲的幾個站點非常的重要，因為它們負責提供來自南半球的大量資料。

Ideally, we'd have a more even distribution.

理想情況下，我們會想要有一個更均勻的分佈。

Of course, as the light fades, there's one assumption that we've been working on throughout this video: that the earth, the ground we're standing on, doesn't move.

當然，隨著天色漸漸暗了下來，還有一個我們整部影片都依據的一個假設我們還沒提到：也就是地球，也就是我們站在上面的這塊地面，不會移動這點。

And that's not strictly true.

而嚴格上來說，這並不是事實。

In the early days of laser ranging, in the 60s, it was the primary method by which they were deriving those early measurements of the drift between us and North America.

在 1960 年代雷射測距技術開始發展的早期，這原本是拿來測量我們 (不列顛島) 與北美之間地塊飄移的早期量測結果的方法。

So you can see long-term trends, like the ice caps melting and the redistribution of mass from the ice caps to the equator.

因此你能藉此看到長期的趨勢，像是冰蓋溶解導致冰蓋的質量重新被分配到赤道上。

All the data goes off to international databases, from all the sites all around the world.

所有的數據都會從世界各地的站點傳輸到國際資料庫上。

Anyone, any researcher, anywhere can go and grab the international data set, the data from us, from Australia, from North America, from China, from Russia,

任何人，不論是來自何方的研究人員都能獲得國際資料集，獲得來自我們的資料。不論他們是來自澳洲、北美、中國、俄羅斯都行，

and can use that data to derive the orbits and do the research they need to do.

並能利用這些資料來推測出軌道，並用在他們需要進行的研究上。

Thanks very much to all the team at the Space Geodesy Facility. You can find out more about them and their work at the link in the description.

非常感謝空間大地測量設施的所有團隊。你可以通過影片資訊欄中的連結來了解更多有關他們的資訊以及工作內容。