WEBVTT Kind: captions Language: en 1 00:00:00.000 --> 00:00:02.440 If you want to know where you are, you can check on your phone. 2 00:00:02.560 --> 00:00:05.760 It listens for signals from whole constellations of satellites in orbit, 3 00:00:05.760 --> 00:00:08.480 and by triangulating all those signals and using a lot of maths, 4 00:00:08.480 --> 00:00:11.520 it works out where you are and shows you a little dot on a map. 5 00:00:11.920 --> 00:00:14.400 But how do the satellites know where they are? 6 00:00:14.400 --> 00:00:17.760 There aren't any landmarks in orbit for them to refer to. 7 00:00:17.760 --> 00:00:20.520 If satellites are in a high enough orbit, well away from the atmosphere, 8 00:00:20.520 --> 00:00:22.240 then they can predict their position. 9 00:00:22.240 --> 00:00:24.880 They'll just keep orbiting in about the same path: 10 00:00:24.880 --> 00:00:26.440 there’s no atmosphere to slow them down. 11 00:00:26.440 --> 00:00:28.360 But that's not precise enough. 12 00:00:28.360 --> 00:00:31.240 There will be distortions caused by gravity from the sun and moon, 13 00:00:31.240 --> 00:00:33.360 or even from mountain ranges down on Earth. 14 00:00:33.360 --> 00:00:36.960 So maybe they could use the other satellite signals, like GPS. 15 00:00:36.960 --> 00:00:42.000 But if they're all relying on each other, that could steadily drift apart from reality. 16 00:00:42.000 --> 00:00:44.880 Maybe they could track the stars or the features down on Earth, 17 00:00:44.880 --> 00:00:46.560 but that's not precise enough either. 18 00:00:46.560 --> 00:00:48.880 At some point, something down here on 19 00:00:48.880 --> 00:00:52.720 Earth has to look up in the sky and work out the position precisely. 20 00:00:52.720 --> 00:00:56.840 And one of those somethings is hidden away in the countryside of southern England. 21 00:00:56.840 --> 00:01:01.000 It looks like a regular observatory, but it's not looking at the stars. 22 00:01:01.000 --> 00:01:05.040 - We track manmade satellites in various orbits doing various jobs. 23 00:01:05.040 --> 00:01:07.040 They all have to have special reflectors 24 00:01:07.040 --> 00:01:09.600 that enables the light to be returned to us. 25 00:01:09.600 --> 00:01:14.520 And so we can measure the time of flight, and from the time of flight, derive the distance. 26 00:01:14.520 --> 00:01:17.400 You send out a very short pulse of laser light. 27 00:01:17.400 --> 00:01:19.760 You do that a thousand times a second. 28 00:01:19.760 --> 00:01:23.560 Each of those shots, because you're using such a tiny short-pulsed laser, 29 00:01:23.560 --> 00:01:27.720 is 10 picoseconds long, which is about 3mm. 30 00:01:27.720 --> 00:01:32.880 That's the limitation on your precision. So each shot is about 3-4mm precision. 31 00:01:32.880 --> 00:01:35.400 - This station works around the clock, whenever the sky is clear. 32 00:01:35.400 --> 00:01:37.920 That's why I'm here today: there will be some genuine observations 33 00:01:37.920 --> 00:01:41.120 when the clouds break, but right now, I'm not actually disrupting anything. 34 00:01:41.120 --> 00:01:44.880 But it does look significantly better after dark. 35 00:01:44.880 --> 00:01:47.120 - Because it's a laser, it's a very specific wavelength. 36 00:01:47.120 --> 00:01:50.760 So you can filter, and that's what lets you work in day and night. 37 00:01:50.760 --> 00:01:53.760 The main problem with the atmosphere is: the speed of light in a vacuum 38 00:01:53.760 --> 00:01:54.960 is a very well known constant. 39 00:01:54.960 --> 00:01:57.560 The speed of light through the atmosphere is variable 40 00:01:57.560 --> 00:02:00.680 according to the mainly the pressure, but also temperature and humidity. 41 00:02:00.680 --> 00:02:02.640 So we measure those things on site, as well. 42 00:02:02.640 --> 00:02:07.040 There's probably about 35 active stations scattered around the globe, 43 00:02:07.040 --> 00:02:09.200 located predominantly in the northern hemisphere, 44 00:02:09.200 --> 00:02:11.880 which is a slight problem from a science point of view. 45 00:02:11.880 --> 00:02:14.040 You'd like them evenly distributed all around the globe. 46 00:02:14.040 --> 00:02:16.360 There's a couple in Australia which are really important 47 00:02:16.360 --> 00:02:19.520 because they're providing a bulk of the data from southern hemisphere. 48 00:02:19.520 --> 00:02:21.880 Ideally, we'd have a more even distribution. 49 00:02:21.880 --> 00:02:24.120 - Of course, as the light fades, there's one assumption 50 00:02:24.120 --> 00:02:25.800 that we've been working on throughout this video: 51 00:02:25.800 --> 00:02:28.800 that the earth, the ground we're standing on, doesn't move. 52 00:02:28.800 --> 00:02:30.960 And that's not strictly true. 53 00:02:30.960 --> 00:02:33.520 - In the early days of laser ranging, in the 60s, 54 00:02:33.520 --> 00:02:36.000 it was the primary method by which they were deriving 55 00:02:36.000 --> 00:02:40.560 those early measurements of the drift between us and North America. 56 00:02:40.560 --> 00:02:42.960 So you can see long-term trends, like the ice caps 57 00:02:42.960 --> 00:02:46.800 melting and the redistribution of mass from the ice caps to the equator. 58 00:02:46.800 --> 00:02:49.640 All the data goes off to international databases, 59 00:02:49.640 --> 00:02:51.520 from all the sites all around the world. 60 00:02:51.520 --> 00:02:53.120 Anyone, any researcher, anywhere 61 00:02:53.120 --> 00:02:56.720 can go and grab the international data set, the data from us, 62 00:02:56.720 --> 00:02:59.520 from Australia, from North America, from China, from Russia, 63 00:02:59.520 --> 00:03:03.360 and can use that data to derive the orbits and do the research they need to do. 64 00:03:03.360 --> 00:03:06.560 - Thanks very much to all the team at the Space Geodesy Facility. 65 00:03:06.560 --> 00:03:09.440 You can find out more about them and their work at the link in the description.