Singing Satellites

They wanted to listen to Sputnik. Instead, they invented the GPS.

It’s interesting to track an Ola cab coming to you once you’ve hailed it. To start with, the car is several feet wider than the road. It flips round to face the opposite direction several times, while making its way from the parking-slot — without, as far as you can make out, knocking anything over in the process.

Once it’s on its way, your cab may defy convention by travelling six metres to the left of every road, rather than right on top of it. Finally, after leaping over a park or two, it comes to a stop in front of you. Unfortunately, as you realise upon looking up from the phone, “in front of you” is actually three buildings down on the opposite side of the road.

That’s the state of GPS navigation technology today: a little off, with a few glitches, and the ability to almost tell you where you are.

Even that, if you think about it, is quite remarkable.

When humans launched the first artificial satellite, Sputnik 1, into space, they also wanted to track where it went. The Sputnik 1 didn’t have any special tracking equipment — just a radio transmitter sending a regular “beep…beep…beep…”

All over the world, people with radios were tuning in to the signal. Among those people were William Guier and George Weiffenbach, from John Hopkins University’s Applied Physics Lab. Before long, the two junior physicists decided to record this historic signal, using Guier’s tape-recorder.

That was when they noticed the signal wasn’t constant. It kept getting shorter and longer over time.

beep..beep….beep…..beeep.….beep.…beep..beep

This changing is because of the Doppler Effect. Waves — of light, sound, or anything else — seem different depending on whether something is moving towards or away from you. If a screaming siren is moving towards you, the sounds get compressed and sound more high-pitched than normal. When it moves away, the sounds get stretched and sound deeper instead.

The same thing happens with light and radio-waves. Using that, Guier and Weiffenbach realised, they could calculate how fast Sputnik 1 was moving.

Then, they realised something else. They knew where they were, and they knew how fast the Sputnik was moving. With that information, they could calculate where Sputnik was, too! The next two satellites were also tracked in this way.

And then, their boss, Frank McClure, came up with a new idea: what if they could do it in reverse? Instead of knowing where you were and calculating where the satellite is, why not have fixed satellites that help you know your own position?

Thus, the idea of satellite-based navigation was born.

For the people of Australia, songs weren’t just about music. They were so much more than that.

In the beginning, Australian tradition tells us, the Earth was a featureless void. There was nothing on it. It was empty. This was in the beginning of the Dreaming, that period of time when the world was being created.

Then came the Ancestors, from the earth and the sky, and they began to travel across the land. While travelling, they shaped the landscapes of Australia, created living beings, and made the laws for human society.

The journeys those Ancestors took, the routes they walked, are known as “Songlines” — and they’re remembered in detail even today.

The Songlines are remembered in the traditional songs, dances, stories, and artwork of the many tribes of Australia. Different groups have especially detailed knowledge of what happened in their particular area during the Dreaming. They know where in their area the Ancestors walked.

But that’s not all. They also know where they can walk now. That’s because some songs have detailed descriptions of their Songlines. Their notes mark different features of the landscape — a mountain here, a bridge there — tracing the steps the Ancestors once took, but also the steps that can be takes today. By singing those songs in the right sequence, you can travel across the country, through any kind of terrain and weather, without ever getting lost.

It’s just like a map, but one that’s sung instead of looked at.

Satellite navigation works a bit like Songline navigation. But instead of people, it’s satellites doing the singing. Each satellite broadcasts its own special note, and the many notes combine into one signal that tells you where you are.

Of course the signals don’t just magically combine. They have to be calculated.

In 1964, the world’s first satellite-navigation system became operational. It was a set of satellites called Transit, and run by the US Navy to help guide submarines and ships. The Navy knew where the satellites were, and used the Doppler Effect to calculate their relative position.

There are many satellite-navigation systems in use today. The most well-known one is the USA’s Global Positioning System, or GPS. But there are several other networks too, like China’s BDS, Russia’s GLONASS, and India’s IRNSS. Each networks has several satellites orbiting the planet — enough so that, wherever you are on Earth, you’ll always find at least four of them in the sky.

But why four, specifically?

It turns out, the Doppler Effect could only provide so much accuracy. The soldiers wanted more. That’s why today’s systems use a slightly different technique to operate.

Suppose you know you’re 200 kilometres from point A. You can draw a 200-kilometre-long circle around point A on the map, and know you’re somewhere on the edge of that circle.

That’s not very helpful on its own. But if you also know you’re 250 kilometres from point B, then there are only two possible places you can go — the two points where the circles touch each other. And if you also know your distance from a point C, you can be very sure where you are.

GPS uses a 3D version of that principle. With four satellites instead of three, and spheres instead of circles, you can make out exactly where, and how far above sea-level, you’re located.

Ships, missiles, submarines — and now, phones and cars — can look up to the satellites to know where they are.

Finding location by looking at the sky is not new. People used to do it long ago, when they navigated using the stars as a guide. The problem with satellites is that you can’t see them directly, only hear their radio-signals. You can know they’re somewhere in the sky, but not know exactly where.

To get an accurate measurement, you’ll need to find out exactly how far away those satellites are.

It’s easy to find out the distance to a lightning strike. Just look at the flash, and count how long it takes for the thunder to follow.

Light travels faster than sound — and the longer the two travel, the more sound will lag behind. So you can measure the gap between lightning and thunder to find the gap between the strike and you.

Every day, at a very specific time, every GPS satellite starts broadcasting a signal in a very specific pattern. You can think of the pattern as a song, though if you convert it to sound it may not sound very tuneful.

Now what does the GPS receiver do but sing that same song as well! It then listens to the satellites, to hear where in the song they are. The satellites’ singing will always be a bit behind the GPS receiver’s one. That’s because their songs will need to travel, and take some time to reach the receiver. The receiver’s own song, on the other hand, will take no time at all.

Like lightning and thunder, the gap between each song tells you how far away each satellite is.

There’s only one problem with this kind of singing: it has to be timed very precisely. Go even one nanosecond wrong, and it’ll throw the whole measurement off.

That’s why GPS satellites are equipped with atomic clocks. Those are the most accurate clocks in the world — and also the most expensive. But then, what about the poor receiver? How can it have a good-enough clock without becoming super expensive?

Well, it doesn’t. GPS receivers have ordinary quartz clocks, which means their singing often goes out of sync. Luckily, they know how to set it right.

When they hear the songs, receivers will get a measurement of how far the satellites are: a 3D edition of the circles map. When their song goes off, the measurement will, too. The circles won’t overlap properly. But they have to overlap properly!

So, the GPS receiver adjusts its clock back and forth until they do.

It gets even better if there are more than four satellites in the sky. Then, there are more circles to align. The receiver can do more fine adjustment, narrowing down on the possible timings, until every single circle touches at the same spot.

So that’s how GPS receivers know exactly where they are. And, as a bonus, they get to know when they are too!

Have something to say? At Snipette, we encourage questions, comments, corrections and clarifications — even if they are something that can be easily Googled! Or you can simply click on the ‘👏 clap’ button, to tell us how much you liked reading this.

Sources and references for this article can be found here.