GPS III satellite
Courtesy of GPS.gov
Without GPS, we’d be lost. Literally. Thanks to these radio transmissions from space though, we’re able to pinpoint our location and find our way home. Join us as we learn all about satellites, how the robot voice of GPS is created and how atomic clocks hold it all together.
Right-click here for an episode transcript.And check out this video of a GPS satellite being launched into space:
And this excellent video will tell you more about atomic clocks and timekeeper Dr. Demetrios Matsakis:
This episode originally aired April 27th, 2015. Listen to the original here!
How does GPS know where you are?
by MPRAudio Transcript
Download transcript (PDF)
SOPHIA CHOO: You're listening to Brains On, where we're serious about being curious. Brains On is supported in part by a grant from the National Science Foundation.
MOLLY BLOOM: Sophia, where are we right now?
SOPHIA CHOO: According to the GPS, we're on 7th and Minnesota in downtown Saint Paul.
MOLLY BLOOM: And only a few blocks from our studios. Should we go there now?
SOPHIA CHOO: Yeah.
MOLLY BLOOM: OK, let's go.
AUTOMATED GPS VOICE: In 800 feet, turn right onto Wabasha Street North.
MOLLY BLOOM: So GPS is pretty amazing.
SOPHIA CHOO: And it's pretty much everywhere.
MOLLY BLOOM: It's the thing that helps you know where you are. Exactly where you are.
SOPHIA CHOO: It's on smartphones, cars, planes, trucks, ships, tractors, but what is GPS exactly? And how does it work? We'll find out right now. Keep listening, and we'll hustle back to the studio.
MOLLY BLOOM: [SIGHS] And we're back. We don't need GPS to tell us that this is Brains On from American Public Media. I'm Molly Bloom, and my co-host today is 9-year-old Sophia Choo. Welcome, Sophia.
SOPHIA CHOO: Thanks.
MOLLY BLOOM: Sophia is here today to help me answer a question that was sent to us from Casey, who lives in Porter Ranch, California.
CASEY: My question is, how does the car GPS know where the car is going?
MOLLY BLOOM: In order to understand how GPS works, we need to know first what it is.
SOPHIA CHOO: GPS stands for Global Positioning System.
MOLLY BLOOM: And the information that the GPS uses to let you know where you are comes from space.
SOPHIA CHOO: Thanks to GPS satellites like this one.
BLOCK: I'm Block. Pleasure to meet you.
SOPHIA CHOO: GPS satellites are built here on Earth. So first, they need to get into space.
CREW: T-minus 10, 9, 8--
BLOCK: We are launched into space by rockets.
CREW: All three engines up and burning. 2, 1, 0, and liftoff.
BLOCK: Here we go.
[RUMBLING]
Whoa, what a ride.
MOLLY BLOOM: Once they're about 12,000 miles above the Earth, a smaller rocket attached to the satellite gets it placed exactly where it's supposed to be.
SOPHIA CHOO: There are 31 GPS satellites orbiting Earth right now.
SUBJECT 1: Hey, Block. Welcome.
SUBJECT 2: It's so great to see you.
SUBJECT 1: Hey, do you have the time?
[LAUGHTER]
MOLLY BLOOM: Each one takes about 12 hours to orbit around the Earth.
SOPHIA CHOO: And these satellites are basically radios.
MOLLY BLOOM: They're constantly transmitting information via radio waves. Very specific information.
SOPHIA CHOO: The time and their location.
BLOCK: It's 12:01 and 2/10 of a second, and I'm right here. It's 12:01 and 3/10 of a second, and now I'm here.
MOLLY BLOOM: Your GPS, whether that's part of your phone or part of a vehicle, receives that information from the satellite.
SOPHIA CHOO: Radio waves move as fast as the speed of light.
MOLLY BLOOM: Which is very fast. About 186,000 miles per second. So the satellite sends a transmission.
BLOCK: It's 12:01 and 7/10 of a second, and now I'm here.
SOPHIA CHOO: And your phone receives it.
[NOTIFICATION PING]
MOLLY BLOOM: OK, it was 12:01 and 7/10 of a second when you sent that message. But according to my clock, it's 12:01 and 9/10 of a second. Since the phone knows how long it took the signal to travel, and it knows the speed of light, it can use the difference between the satellite's time and its own time to figure out how far away that satellite is from the phone.
SOPHIA CHOO: Imagine our satellite friend Block has a really long piece of string tied to it.
[RISING PITCH]
MOLLY BLOOM: And that string extends all the way back to Earth.
[DESCENDING PITCH]
SOPHIA CHOO: Now imagine you're holding the end of that string. The end tied to the satellite is stuck, but you can still move around while holding it.
MOLLY BLOOM: So I could be here and holding it, or here, or here. We know that our phone is at the end of the string, but the end of the string could be a lot of different places. Or here, or here, or here, or here, or here, or here.
SOPHIA CHOO: It could be anywhere inside a specific area, like a bubble.
MOLLY BLOOM: And that's not that helpful if you're trying to figure out exactly where you are.
SOPHIA CHOO: So in order to narrow down its location, the phone gets messages from four satellites at once.
PEOPLE: (TOGETHER) It's 12:01 and 7/10 of a second, and now I'm over here.
SOPHIA CHOO: Now think about a string coming down to Earth from each of those four satellites.
[DESCENDING PITCH]
MOLLY BLOOM: The end of each piece of string could be anywhere inside its own separate bubble.
SOPHIA CHOO: But the four bubbles created by each piece of string will overlap a tinsy bit.
MOLLY BLOOM: And where they overlap is where you and your phone are.
[NOTIFICATION PINGING]
Ah, here I am. I am here. Thanks, satellites.
PEOPLE: (TOGETHER) You're welcome.
MOLLY BLOOM: This process of using the overlapping bubbles to find a location is called trilateration.
SOPHIA CHOO: It's possible that the GPS will be receiving signals from more than four satellites.
MOLLY BLOOM: And the more satellites, the more accurate it is.
SOPHIA CHOO: But it needs at least four to work.
MOLLY BLOOM: GPS first became fully functional in the '90s, but scientist John Langer told us it's an idea that's been around for a while.
JOHN LANGER: You navigate off stars. If you could have artificial stars in the sky, you could somehow perhaps navigate better with those. So they thought about different ways of using radios in space. So there were a number of programs before GPS that did different experiments. In the '70s, a lot of these programs came together, and they came up with the idea for the Global Positioning System.
MOLLY BLOOM: In order to send messages to your phone, the satellite actually has to be able to see it. There can't be anything big blocking the path between it and the phone.
JOHN LANGER: When we first built the system, we only had four satellites. And that was quite a challenge because they're in different orbits, so they only line up over two particular places. One was in New Mexico, and the other was in Norway. Now we have 31 satellites, so most of the time you have in view 8, 10, sometimes 12 satellites.
MOLLY BLOOM: So in a sense, those satellites can see us.
SOPHIA CHOO: But they're so far up that we can't see them with the naked eye.
MOLLY BLOOM: Now we're going to let these transmissions sink in for a moment and tune into a different frequency. It's time for the mystery sound.
[ELECTRONIC STINGER]
PERSON: (WHISPERING) Mystery sound.
MOLLY BLOOM: Here it is.
[THUMPING, CRACKLING]
Any guesses?
SOPHIA CHOO: It sounds like someone pressing a lot of buttons.
MOLLY BLOOM: That's an excellent guess. To me it sounds like someone running on the sidewalk almost. Like footsteps.
SOPHIA CHOO: Yeah.
MOLLY BLOOM: The answer is traveling to us over a great distance, so hopefully it will arrive by the end of the show.
PEOPLE: Brains On!
MOLLY BLOOM: GPS hinges on both the GPS satellites and our GPS devices here on Earth knowing what time it is.
SOPHIA CHOO: And that time has to be exact, and exactly the same among our devices in order for GPS to work.
MOLLY BLOOM: To do that, we need help from a very accurate clock called an atomic clock.
MARC SANCHEZ: Hey, guys. Am I late?
SOPHIA CHOO: That depends.
MOLLY BLOOM: Are here to tell us about atomic clocks?
MARC SANCHEZ: Yep.
MOLLY BLOOM: Then you're right on time.
MARC SANCHEZ: Yes.
SOPHIA CHOO: Brains On producer Marc Sanchez has been looking into what makes atomic clocks tick.
MARC SANCHEZ: That's right. And as you might have guessed from its name, its atoms. And atoms, they're basically the building blocks of everything around us. All matter. Atoms make up molecules, molecules make up stuff like this water bottle, like my eyeglasses. Atoms are everywhere.
SOPHIA CHOO: How do atoms tell time?
MARC SANCHEZ: Well, technically they don't tell time. Atoms make up this chair and this table--
[KNOCKING]
--but neither are going to tell you the time. Here, give the table a listen. Hang on. Anything?
SOPHIA CHOO: Nothing.
MARC SANCHEZ: It's nothing o'clock, according to that table.
SOPHIA CHOO: [LAUGHS]
MARC SANCHEZ: But in atomic clocks, scientists have figured out a way to isolate specific atoms and get them moving really fast. And this is important-- they figured out how to measure that movement. I called up Dr. Demetrios Matsakis at the US Naval Observatory. He's the chief scientist there, and he's the guy who's in charge of time.
DEMETRIOS MATSAKIS: An atomic clock is a clock that gets the basis of its time from the transitions of atoms. Atoms can exist in what we call states, where they have some energy, and they might be oscillating or vibrating in a certain way. And if we count the oscillations, we have a clock.
MARC SANCHEZ: When something oscillates, it moves back and forth, back and forth, back and forth, like a pendulum on a grandfather clock or something else you might be familiar with-- a swing. Think about when you're at the highest point moving forward on a swing. Your legs are in front of you, and it feels like you just slightly stop when you get there. And then you tuck your legs under you tight and you go back and you do the same thing moving backwards.
One swing back and forth is one oscillation. The atoms in atomic clocks are doing the same thing, only a lot faster. Do you know how many times a cesium atom oscillates in one second?
SOPHIA CHOO: Um. [CHUCKLES]
MARC SANCHEZ: Guesses? Guesses? Come on. You've got to give me a guess.
MOLLY BLOOM: 30. What do you think?
SOPHIA CHOO: 100.
MARC SANCHEZ: In one second, a cesium atom oscillates 9,192,631,770 times.
MOLLY BLOOM: We were way off.
SOPHIA CHOO: I was closer.
MOLLY BLOOM: Yeah, you were closer than me.
MARC SANCHEZ: In fact, we've all decided to base what we think of as a second on that exact number of a cesium atom oscillating back and forth.
DEMETRIOS MATSAKIS: The first atomic clock that was really functioning well was based on cesium, and that was first done in 1955. But we have atomic clocks built on other atoms as well since then. We have ones that are built on rubidium. There's also calcium, strontium, mercury.
MOLLY BLOOM: I have a kind of weird question.
MARC SANCHEZ: My specialty.
MOLLY BLOOM: Where would I even find a place to count atom oscillations? You just kind of shake atoms up in a jar or something?
MARC SANCHEZ: In order to accurately count oscillations, atoms need to be in a very stable environment. So inside of an atomic clock is a vacuum chamber. [SUCKS IN] That's where all the air and gases have been sucked out. Everything is still. The temperature is controlled. Nothing moves. Except for--
SOPHIA CHOO: Atoms.
MARC SANCHEZ: Right. GPS satellites, for example, use rubidium atoms. Scientists like Demetrios Matsakis insert those atoms into the vacuum, and he knows he can get them to oscillate an exact number of times by exciting the atom with radiation.
MOLLY BLOOM: So thanks to atomic timekeepers, all of our clocks tick together.
MARC SANCHEZ: And this is really important when you think about the parts of our lives that depend on GPS. Airplanes use it to fly straight, banks use it to stamp every time you use an ATM or any transaction, power companies use it to keep the electric grids in sync. And we all use it too. Like Sophia, you use it to get to school on time, right?
SOPHIA CHOO: Yeah.
MOLLY BLOOM: Thanks, Marc.
MARC SANCHEZ: No problem. Take it easy.
SOPHIA CHOO: Thanks, Marc.
MOLLY BLOOM: We're working on a couple episodes right now, and we want your help. We're looking into what makes fun fun and what makes gross gross. And we want you to help us explain what fun and gross even are. Like how would you explain what gross is to a robot? Or how would you explain what fun is? Send us your answers by heading to brainson.org/contact.
We'll feature some of your answers in these upcoming episodes. And that site, brainson.org/contact, is where you can send us all your drawings, mystery sounds, and questions too. That's what Jake did.
JAKE: My question is, if there are three types of matter-- solid, liquid, and gas-- what is light considered?
MOLLY BLOOM: We'll answer that question during our Moment of Um at the end of the show. And we'll read the latest group of listeners to be added to the Brains Honor Roll. Keep listening.
You're listening to Brains On from American Public Media. I'm Molly Bloom.
SOPHIA CHOO: And I'm Sophia Choo.
MOLLY BLOOM: Now, the GPS data that comes from GPS satellites is available to everyone. The satellites are run by the government, and their transmissions are public.
SOPHIA CHOO: In order for us to be able to harness the information and find our way around, we need a GPS device or app that can interpret the transmissions for us.
MOLLY BLOOM: When we were working on this episode, you were most interested in finding out about the voice of the GPS. Where did you think it came from?
SOPHIA CHOO: I thought it came from a robot, honestly.
MOLLY BLOOM: Sophia talked to Karin Tuxen-Bettman from Google Maps to find out exactly where it came from.
KARIN TUXEN-BETTMAN: The way that Google Maps builds this voice is a couple of steps. Number one, we start by building a good map. And then the second thing we do is we build a special algorithm. And an algorithm is kind of a fancy word for a recipe. The computer is trying to solve a problem, and so it needs basically a recipe for how to solve it.
After we have a good map, we need to have an algorithm that guides people through there. Now when you're on a computer at home or at school or work, some people will see the instructions to get from one place to another, they'll see it written out as text. How we get from the text to the voice is a pretty interesting solution.
So first, the team here at Google that works on the voice navigation, they record a real person speaking specially designed lines. So basically many different direction words, direction sentences. And they chop up all those lines, they chop up all those different words, and they make a database of very small chunks of speech.
So this is a real person's voice, but it's recorded. And then all those little pieces of vocal speech are all chopped up and made into a giant database. And then when you, Sophia, want to go from school to home using a smartphone, for example, you could then ask Google Maps for directions and then turn on the voice navigation.
And then at that moment, the system searches the database for all the little bits of speech and then glues them together. Turn right here, turn left on Park Street, turn right on Main Street. And then that's what's spoken out to you over the phone. And so there's a lot of stuff happening in just a few seconds.
SOPHIA CHOO: Well, why exactly does the voice have to sound like a robot instead of more like a human?
KARIN TUXEN-BETTMAN: There are some words that the algorithm might come across and it's not in their database as a full word, and so the algorithm is then basically programmed to predict how it would be pronounced. And so that would be done with several different chunks would then sounding out several different syllables, and so it would piece together and glue together multiple syllables to make that word.
And that's when you start getting sometimes that more robotic or unfamiliar sound to certain words. But it's the team's goal to make it sound as natural as possible, so it's just like your friend telling you how to get from one place to another.
SOPHIA CHOO: I would like to know about the new car that doesn't need a person driving it and how it can drive itself.
KARIN TUXEN-BETTMAN: They're working on a driverless car, just testing it on a small scale right now, with the goal that different sensors on the outside of the car are able to not only sense activity all around but also able to control the car and brake if another car in front of them is braking and things like that.
So GPS is a huge part of that car so that the car knows where it is in relation to everything else. It's pretty amazing, and who knows. Maybe if we talk, Sophia, in 10 years, we'll both be driving driverless cars.
SOPHIA CHOO: There are 31 GPS satellites in orbit, but what happens if one of them stops working?
MOLLY BLOOM: Scientists can do things from the ground like send commands and reconfigure them, but if something goes really wrong with it, they can't bring it down to Earth to fix it.
SOPHIA CHOO: And they can't go up to space to fix it either.
MOLLY BLOOM: So the satellites become space junk. Here's scientist John Langer again.
JOHN LANGER: When a GPS satellite wears out, when it's at the end of its lifetime and we can't use it anymore, we have to find a way to dispose of it. We can't bring it down, so what we do is we push it up a little bit. Every satellite carries with it a little tiny rocket that's just used for disposal. We push it up into an orbit that no one would ever want to put something in. And so all around the Earth is this so-called disposal orbit which is full of old GPS satellites.
SOPHIA CHOO: GPS satellites are medium-altitude satellites.
MOLLY BLOOM: There are lower-altitude satellites like the Space Station and weather satellites, and there are geosynchronous satellites.
SOPHIA CHOO: Those ones are so far up that they appear to be stationary in the sky.
MOLLY BLOOM: Those are used for TV, radio, and other communications.
SOPHIA CHOO: I asked John Langer how many satellites have become space junk.
JOHN LANGER: So there are probably tens of disposed GPS satellites. Probably less than 100. In other orbits, like the ones that give you television, there are hundreds and hundreds of disposed satellites up there. That seems like a lot, except that distances are very big.
MOLLY BLOOM: So once a GPS satellite is nudged out of the way--
SOPHIA CHOO: They send up another one to take its place.
BLOCK: Here we go. It's 12:01 and 2/10 of a second, and I'm right here. It's 12:01 and 3/10 of a second, and now I'm here.
MOLLY BLOOM: It's time to go back to the mystery sound. Let's hear it one more time.
[THUMPING, CRACKLING]
Any final guesses?
SOPHIA CHOO: It kind of sounds like someone starting up a lawnmower.
MOLLY BLOOM: Ooh, that's a good one. We'll bring it back to Demetrios, the timekeeper who helped explain atomic clocks. He has the answer.
DEMETRIOS MATSAKIS: That's the sound of a pulsar. Pulsars are neutron stars. They are what remains of the star after it goes supernova. When the star explodes, it pushes the outer part of it out in a blaze of glory, and the inner part condenses to form a rapidly spinning object about 10 miles in diameter and weighing maybe as much as the sun.
Pulsars can spin as slowly as once a second or as quickly as 1,000 times a second. And as they spin around, they have hot spots they give off radiation. When that radiation passes in front of us, when the line of sight, if it goes by like a lighthouse beam, we see a spike or a pulse.
MOLLY BLOOM: So just like scientists use the oscillations of atoms in an atomic clock, there's another kind of clock where scientists can use these pulses to keep time. And the sound you're hearing is these pulses of radiation. It's what microwaves sound like.
[THUMPING, CRACKLING]
Your GPS device or app knows where you are by getting transmissions from satellites.
SOPHIA CHOO: Those satellites send two pieces of information-- the time and their location.
MOLLY BLOOM: Knowing how long it took that information to travel from space allows us to find our location on Earth.
SOPHIA CHOO: There are 31 GPS satellites orbiting Earth.
MOLLY BLOOM: And you need at least four to help you pinpoint your location.
SOPHIA CHOO: That process is called trilateration.
MOLLY BLOOM: And since it all hinges on accurate time--
SOPHIA CHOO: We rely on atomic clocks to keep everything synchronized.
MOLLY BLOOM: That's it for this episode of Brains On.
SOPHIA CHOO: This episode is produced by Marc Sanchez, Sanden Totten, and Molly Bloom.
MOLLY BLOOM: We had engineering help from Veronica Rodriguez and production help from Abby Samuel. Special Thanks to Sam Choo, Colin Campbell, Meg Martin, [? Nikki Tandell ?] John Gordon, Eric Ringham, Mike Mulcahy, Kate Smith, and Curtis Gilbert.
SOPHIA CHOO: You can always listen to past episodes at our website, brainson.org.
MOLLY BLOOM: And you can send your questions, drawings, and high fives to brainson.org/contact. Now, before we go, it's time for our Moment of Um.
[VOCALIZING]
JAKE: Hi, my name is Jake from Lancaster, Pennsylvania. My question is, if there are three types of matter-- solid, liquid, and gas-- what is light considered?
JAMES KAKALIOS: That's a great question. So gas, liquid, solid, different phases of matter all have one thing in common-- they're all matter. They all have mass. Whereas in physics, we study two things. We study matter, and we study energy. This is James Kakalios, physics professor at the University of Minnesota and author of The Physics of Superheroes and The Physics of Everyday Things.
Light is a form of energy, and it's an unusual form of energy. Light can be considered an electromagnetic wave, oscillating electric and magnetic fields. But it's not a wave the way other waves are, like water waves on the oceanfront or on the lake shore. Those waves can exist without the water, whereas light doesn't need anything in order to propagate.
If it's a wave, physicists used to ask, what is it that's waving? And the thing that's waving are electric and magnetic fields which don't actually require anything to exist, which is a good thing for us, because the light coming from the sun has to pass through the vacuum of outer space. And if it needed some medium in order to propagate, we would be all in the dark here, wondering why doesn't someone turn on the lights.
MOLLY BLOOM: These names light up my life. It's the Brains Honor Roll. They're the amazing listeners who have shared questions, ideas, mystery sounds, and drawings with us. They make this show what it is.
[LISTING HONOR ROLL]
PEOPLE: (SINGING) Brains Honor Roll. High five.
MOLLY BLOOM: That's it for this episode. We'll be back soon with more answers to your questions. Thanks for listening.
PEOPLE: (SINGING) Buh-buh, buh-buh, buh-buh, buh-buh-buh-buh Brains On.
MOLLY BLOOM: Sophia, where are we right now?
SOPHIA CHOO: According to the GPS, we're on 7th and Minnesota in downtown Saint Paul.
MOLLY BLOOM: And only a few blocks from our studios. Should we go there now?
SOPHIA CHOO: Yeah.
MOLLY BLOOM: OK, let's go.
AUTOMATED GPS VOICE: In 800 feet, turn right onto Wabasha Street North.
MOLLY BLOOM: So GPS is pretty amazing.
SOPHIA CHOO: And it's pretty much everywhere.
MOLLY BLOOM: It's the thing that helps you know where you are. Exactly where you are.
SOPHIA CHOO: It's on smartphones, cars, planes, trucks, ships, tractors, but what is GPS exactly? And how does it work? We'll find out right now. Keep listening, and we'll hustle back to the studio.
MOLLY BLOOM: [SIGHS] And we're back. We don't need GPS to tell us that this is Brains On from American Public Media. I'm Molly Bloom, and my co-host today is 9-year-old Sophia Choo. Welcome, Sophia.
SOPHIA CHOO: Thanks.
MOLLY BLOOM: Sophia is here today to help me answer a question that was sent to us from Casey, who lives in Porter Ranch, California.
CASEY: My question is, how does the car GPS know where the car is going?
MOLLY BLOOM: In order to understand how GPS works, we need to know first what it is.
SOPHIA CHOO: GPS stands for Global Positioning System.
MOLLY BLOOM: And the information that the GPS uses to let you know where you are comes from space.
SOPHIA CHOO: Thanks to GPS satellites like this one.
BLOCK: I'm Block. Pleasure to meet you.
SOPHIA CHOO: GPS satellites are built here on Earth. So first, they need to get into space.
CREW: T-minus 10, 9, 8--
BLOCK: We are launched into space by rockets.
CREW: All three engines up and burning. 2, 1, 0, and liftoff.
BLOCK: Here we go.
[RUMBLING]
Whoa, what a ride.
MOLLY BLOOM: Once they're about 12,000 miles above the Earth, a smaller rocket attached to the satellite gets it placed exactly where it's supposed to be.
SOPHIA CHOO: There are 31 GPS satellites orbiting Earth right now.
SUBJECT 1: Hey, Block. Welcome.
SUBJECT 2: It's so great to see you.
SUBJECT 1: Hey, do you have the time?
[LAUGHTER]
MOLLY BLOOM: Each one takes about 12 hours to orbit around the Earth.
SOPHIA CHOO: And these satellites are basically radios.
MOLLY BLOOM: They're constantly transmitting information via radio waves. Very specific information.
SOPHIA CHOO: The time and their location.
BLOCK: It's 12:01 and 2/10 of a second, and I'm right here. It's 12:01 and 3/10 of a second, and now I'm here.
MOLLY BLOOM: Your GPS, whether that's part of your phone or part of a vehicle, receives that information from the satellite.
SOPHIA CHOO: Radio waves move as fast as the speed of light.
MOLLY BLOOM: Which is very fast. About 186,000 miles per second. So the satellite sends a transmission.
BLOCK: It's 12:01 and 7/10 of a second, and now I'm here.
SOPHIA CHOO: And your phone receives it.
[NOTIFICATION PING]
MOLLY BLOOM: OK, it was 12:01 and 7/10 of a second when you sent that message. But according to my clock, it's 12:01 and 9/10 of a second. Since the phone knows how long it took the signal to travel, and it knows the speed of light, it can use the difference between the satellite's time and its own time to figure out how far away that satellite is from the phone.
SOPHIA CHOO: Imagine our satellite friend Block has a really long piece of string tied to it.
[RISING PITCH]
MOLLY BLOOM: And that string extends all the way back to Earth.
[DESCENDING PITCH]
SOPHIA CHOO: Now imagine you're holding the end of that string. The end tied to the satellite is stuck, but you can still move around while holding it.
MOLLY BLOOM: So I could be here and holding it, or here, or here. We know that our phone is at the end of the string, but the end of the string could be a lot of different places. Or here, or here, or here, or here, or here, or here.
SOPHIA CHOO: It could be anywhere inside a specific area, like a bubble.
MOLLY BLOOM: And that's not that helpful if you're trying to figure out exactly where you are.
SOPHIA CHOO: So in order to narrow down its location, the phone gets messages from four satellites at once.
PEOPLE: (TOGETHER) It's 12:01 and 7/10 of a second, and now I'm over here.
SOPHIA CHOO: Now think about a string coming down to Earth from each of those four satellites.
[DESCENDING PITCH]
MOLLY BLOOM: The end of each piece of string could be anywhere inside its own separate bubble.
SOPHIA CHOO: But the four bubbles created by each piece of string will overlap a tinsy bit.
MOLLY BLOOM: And where they overlap is where you and your phone are.
[NOTIFICATION PINGING]
Ah, here I am. I am here. Thanks, satellites.
PEOPLE: (TOGETHER) You're welcome.
MOLLY BLOOM: This process of using the overlapping bubbles to find a location is called trilateration.
SOPHIA CHOO: It's possible that the GPS will be receiving signals from more than four satellites.
MOLLY BLOOM: And the more satellites, the more accurate it is.
SOPHIA CHOO: But it needs at least four to work.
MOLLY BLOOM: GPS first became fully functional in the '90s, but scientist John Langer told us it's an idea that's been around for a while.
JOHN LANGER: You navigate off stars. If you could have artificial stars in the sky, you could somehow perhaps navigate better with those. So they thought about different ways of using radios in space. So there were a number of programs before GPS that did different experiments. In the '70s, a lot of these programs came together, and they came up with the idea for the Global Positioning System.
MOLLY BLOOM: In order to send messages to your phone, the satellite actually has to be able to see it. There can't be anything big blocking the path between it and the phone.
JOHN LANGER: When we first built the system, we only had four satellites. And that was quite a challenge because they're in different orbits, so they only line up over two particular places. One was in New Mexico, and the other was in Norway. Now we have 31 satellites, so most of the time you have in view 8, 10, sometimes 12 satellites.
MOLLY BLOOM: So in a sense, those satellites can see us.
SOPHIA CHOO: But they're so far up that we can't see them with the naked eye.
MOLLY BLOOM: Now we're going to let these transmissions sink in for a moment and tune into a different frequency. It's time for the mystery sound.
[ELECTRONIC STINGER]
PERSON: (WHISPERING) Mystery sound.
MOLLY BLOOM: Here it is.
[THUMPING, CRACKLING]
Any guesses?
SOPHIA CHOO: It sounds like someone pressing a lot of buttons.
MOLLY BLOOM: That's an excellent guess. To me it sounds like someone running on the sidewalk almost. Like footsteps.
SOPHIA CHOO: Yeah.
MOLLY BLOOM: The answer is traveling to us over a great distance, so hopefully it will arrive by the end of the show.
PEOPLE: Brains On!
MOLLY BLOOM: GPS hinges on both the GPS satellites and our GPS devices here on Earth knowing what time it is.
SOPHIA CHOO: And that time has to be exact, and exactly the same among our devices in order for GPS to work.
MOLLY BLOOM: To do that, we need help from a very accurate clock called an atomic clock.
MARC SANCHEZ: Hey, guys. Am I late?
SOPHIA CHOO: That depends.
MOLLY BLOOM: Are here to tell us about atomic clocks?
MARC SANCHEZ: Yep.
MOLLY BLOOM: Then you're right on time.
MARC SANCHEZ: Yes.
SOPHIA CHOO: Brains On producer Marc Sanchez has been looking into what makes atomic clocks tick.
MARC SANCHEZ: That's right. And as you might have guessed from its name, its atoms. And atoms, they're basically the building blocks of everything around us. All matter. Atoms make up molecules, molecules make up stuff like this water bottle, like my eyeglasses. Atoms are everywhere.
SOPHIA CHOO: How do atoms tell time?
MARC SANCHEZ: Well, technically they don't tell time. Atoms make up this chair and this table--
[KNOCKING]
--but neither are going to tell you the time. Here, give the table a listen. Hang on. Anything?
SOPHIA CHOO: Nothing.
MARC SANCHEZ: It's nothing o'clock, according to that table.
SOPHIA CHOO: [LAUGHS]
MARC SANCHEZ: But in atomic clocks, scientists have figured out a way to isolate specific atoms and get them moving really fast. And this is important-- they figured out how to measure that movement. I called up Dr. Demetrios Matsakis at the US Naval Observatory. He's the chief scientist there, and he's the guy who's in charge of time.
DEMETRIOS MATSAKIS: An atomic clock is a clock that gets the basis of its time from the transitions of atoms. Atoms can exist in what we call states, where they have some energy, and they might be oscillating or vibrating in a certain way. And if we count the oscillations, we have a clock.
MARC SANCHEZ: When something oscillates, it moves back and forth, back and forth, back and forth, like a pendulum on a grandfather clock or something else you might be familiar with-- a swing. Think about when you're at the highest point moving forward on a swing. Your legs are in front of you, and it feels like you just slightly stop when you get there. And then you tuck your legs under you tight and you go back and you do the same thing moving backwards.
One swing back and forth is one oscillation. The atoms in atomic clocks are doing the same thing, only a lot faster. Do you know how many times a cesium atom oscillates in one second?
SOPHIA CHOO: Um. [CHUCKLES]
MARC SANCHEZ: Guesses? Guesses? Come on. You've got to give me a guess.
MOLLY BLOOM: 30. What do you think?
SOPHIA CHOO: 100.
MARC SANCHEZ: In one second, a cesium atom oscillates 9,192,631,770 times.
MOLLY BLOOM: We were way off.
SOPHIA CHOO: I was closer.
MOLLY BLOOM: Yeah, you were closer than me.
MARC SANCHEZ: In fact, we've all decided to base what we think of as a second on that exact number of a cesium atom oscillating back and forth.
DEMETRIOS MATSAKIS: The first atomic clock that was really functioning well was based on cesium, and that was first done in 1955. But we have atomic clocks built on other atoms as well since then. We have ones that are built on rubidium. There's also calcium, strontium, mercury.
MOLLY BLOOM: I have a kind of weird question.
MARC SANCHEZ: My specialty.
MOLLY BLOOM: Where would I even find a place to count atom oscillations? You just kind of shake atoms up in a jar or something?
MARC SANCHEZ: In order to accurately count oscillations, atoms need to be in a very stable environment. So inside of an atomic clock is a vacuum chamber. [SUCKS IN] That's where all the air and gases have been sucked out. Everything is still. The temperature is controlled. Nothing moves. Except for--
SOPHIA CHOO: Atoms.
MARC SANCHEZ: Right. GPS satellites, for example, use rubidium atoms. Scientists like Demetrios Matsakis insert those atoms into the vacuum, and he knows he can get them to oscillate an exact number of times by exciting the atom with radiation.
MOLLY BLOOM: So thanks to atomic timekeepers, all of our clocks tick together.
MARC SANCHEZ: And this is really important when you think about the parts of our lives that depend on GPS. Airplanes use it to fly straight, banks use it to stamp every time you use an ATM or any transaction, power companies use it to keep the electric grids in sync. And we all use it too. Like Sophia, you use it to get to school on time, right?
SOPHIA CHOO: Yeah.
MOLLY BLOOM: Thanks, Marc.
MARC SANCHEZ: No problem. Take it easy.
SOPHIA CHOO: Thanks, Marc.
MOLLY BLOOM: We're working on a couple episodes right now, and we want your help. We're looking into what makes fun fun and what makes gross gross. And we want you to help us explain what fun and gross even are. Like how would you explain what gross is to a robot? Or how would you explain what fun is? Send us your answers by heading to brainson.org/contact.
We'll feature some of your answers in these upcoming episodes. And that site, brainson.org/contact, is where you can send us all your drawings, mystery sounds, and questions too. That's what Jake did.
JAKE: My question is, if there are three types of matter-- solid, liquid, and gas-- what is light considered?
MOLLY BLOOM: We'll answer that question during our Moment of Um at the end of the show. And we'll read the latest group of listeners to be added to the Brains Honor Roll. Keep listening.
You're listening to Brains On from American Public Media. I'm Molly Bloom.
SOPHIA CHOO: And I'm Sophia Choo.
MOLLY BLOOM: Now, the GPS data that comes from GPS satellites is available to everyone. The satellites are run by the government, and their transmissions are public.
SOPHIA CHOO: In order for us to be able to harness the information and find our way around, we need a GPS device or app that can interpret the transmissions for us.
MOLLY BLOOM: When we were working on this episode, you were most interested in finding out about the voice of the GPS. Where did you think it came from?
SOPHIA CHOO: I thought it came from a robot, honestly.
MOLLY BLOOM: Sophia talked to Karin Tuxen-Bettman from Google Maps to find out exactly where it came from.
KARIN TUXEN-BETTMAN: The way that Google Maps builds this voice is a couple of steps. Number one, we start by building a good map. And then the second thing we do is we build a special algorithm. And an algorithm is kind of a fancy word for a recipe. The computer is trying to solve a problem, and so it needs basically a recipe for how to solve it.
After we have a good map, we need to have an algorithm that guides people through there. Now when you're on a computer at home or at school or work, some people will see the instructions to get from one place to another, they'll see it written out as text. How we get from the text to the voice is a pretty interesting solution.
So first, the team here at Google that works on the voice navigation, they record a real person speaking specially designed lines. So basically many different direction words, direction sentences. And they chop up all those lines, they chop up all those different words, and they make a database of very small chunks of speech.
So this is a real person's voice, but it's recorded. And then all those little pieces of vocal speech are all chopped up and made into a giant database. And then when you, Sophia, want to go from school to home using a smartphone, for example, you could then ask Google Maps for directions and then turn on the voice navigation.
And then at that moment, the system searches the database for all the little bits of speech and then glues them together. Turn right here, turn left on Park Street, turn right on Main Street. And then that's what's spoken out to you over the phone. And so there's a lot of stuff happening in just a few seconds.
SOPHIA CHOO: Well, why exactly does the voice have to sound like a robot instead of more like a human?
KARIN TUXEN-BETTMAN: There are some words that the algorithm might come across and it's not in their database as a full word, and so the algorithm is then basically programmed to predict how it would be pronounced. And so that would be done with several different chunks would then sounding out several different syllables, and so it would piece together and glue together multiple syllables to make that word.
And that's when you start getting sometimes that more robotic or unfamiliar sound to certain words. But it's the team's goal to make it sound as natural as possible, so it's just like your friend telling you how to get from one place to another.
SOPHIA CHOO: I would like to know about the new car that doesn't need a person driving it and how it can drive itself.
KARIN TUXEN-BETTMAN: They're working on a driverless car, just testing it on a small scale right now, with the goal that different sensors on the outside of the car are able to not only sense activity all around but also able to control the car and brake if another car in front of them is braking and things like that.
So GPS is a huge part of that car so that the car knows where it is in relation to everything else. It's pretty amazing, and who knows. Maybe if we talk, Sophia, in 10 years, we'll both be driving driverless cars.
SOPHIA CHOO: There are 31 GPS satellites in orbit, but what happens if one of them stops working?
MOLLY BLOOM: Scientists can do things from the ground like send commands and reconfigure them, but if something goes really wrong with it, they can't bring it down to Earth to fix it.
SOPHIA CHOO: And they can't go up to space to fix it either.
MOLLY BLOOM: So the satellites become space junk. Here's scientist John Langer again.
JOHN LANGER: When a GPS satellite wears out, when it's at the end of its lifetime and we can't use it anymore, we have to find a way to dispose of it. We can't bring it down, so what we do is we push it up a little bit. Every satellite carries with it a little tiny rocket that's just used for disposal. We push it up into an orbit that no one would ever want to put something in. And so all around the Earth is this so-called disposal orbit which is full of old GPS satellites.
SOPHIA CHOO: GPS satellites are medium-altitude satellites.
MOLLY BLOOM: There are lower-altitude satellites like the Space Station and weather satellites, and there are geosynchronous satellites.
SOPHIA CHOO: Those ones are so far up that they appear to be stationary in the sky.
MOLLY BLOOM: Those are used for TV, radio, and other communications.
SOPHIA CHOO: I asked John Langer how many satellites have become space junk.
JOHN LANGER: So there are probably tens of disposed GPS satellites. Probably less than 100. In other orbits, like the ones that give you television, there are hundreds and hundreds of disposed satellites up there. That seems like a lot, except that distances are very big.
MOLLY BLOOM: So once a GPS satellite is nudged out of the way--
SOPHIA CHOO: They send up another one to take its place.
BLOCK: Here we go. It's 12:01 and 2/10 of a second, and I'm right here. It's 12:01 and 3/10 of a second, and now I'm here.
MOLLY BLOOM: It's time to go back to the mystery sound. Let's hear it one more time.
[THUMPING, CRACKLING]
Any final guesses?
SOPHIA CHOO: It kind of sounds like someone starting up a lawnmower.
MOLLY BLOOM: Ooh, that's a good one. We'll bring it back to Demetrios, the timekeeper who helped explain atomic clocks. He has the answer.
DEMETRIOS MATSAKIS: That's the sound of a pulsar. Pulsars are neutron stars. They are what remains of the star after it goes supernova. When the star explodes, it pushes the outer part of it out in a blaze of glory, and the inner part condenses to form a rapidly spinning object about 10 miles in diameter and weighing maybe as much as the sun.
Pulsars can spin as slowly as once a second or as quickly as 1,000 times a second. And as they spin around, they have hot spots they give off radiation. When that radiation passes in front of us, when the line of sight, if it goes by like a lighthouse beam, we see a spike or a pulse.
MOLLY BLOOM: So just like scientists use the oscillations of atoms in an atomic clock, there's another kind of clock where scientists can use these pulses to keep time. And the sound you're hearing is these pulses of radiation. It's what microwaves sound like.
[THUMPING, CRACKLING]
Your GPS device or app knows where you are by getting transmissions from satellites.
SOPHIA CHOO: Those satellites send two pieces of information-- the time and their location.
MOLLY BLOOM: Knowing how long it took that information to travel from space allows us to find our location on Earth.
SOPHIA CHOO: There are 31 GPS satellites orbiting Earth.
MOLLY BLOOM: And you need at least four to help you pinpoint your location.
SOPHIA CHOO: That process is called trilateration.
MOLLY BLOOM: And since it all hinges on accurate time--
SOPHIA CHOO: We rely on atomic clocks to keep everything synchronized.
MOLLY BLOOM: That's it for this episode of Brains On.
SOPHIA CHOO: This episode is produced by Marc Sanchez, Sanden Totten, and Molly Bloom.
MOLLY BLOOM: We had engineering help from Veronica Rodriguez and production help from Abby Samuel. Special Thanks to Sam Choo, Colin Campbell, Meg Martin, [? Nikki Tandell ?] John Gordon, Eric Ringham, Mike Mulcahy, Kate Smith, and Curtis Gilbert.
SOPHIA CHOO: You can always listen to past episodes at our website, brainson.org.
MOLLY BLOOM: And you can send your questions, drawings, and high fives to brainson.org/contact. Now, before we go, it's time for our Moment of Um.
[VOCALIZING]
JAKE: Hi, my name is Jake from Lancaster, Pennsylvania. My question is, if there are three types of matter-- solid, liquid, and gas-- what is light considered?
JAMES KAKALIOS: That's a great question. So gas, liquid, solid, different phases of matter all have one thing in common-- they're all matter. They all have mass. Whereas in physics, we study two things. We study matter, and we study energy. This is James Kakalios, physics professor at the University of Minnesota and author of The Physics of Superheroes and The Physics of Everyday Things.
Light is a form of energy, and it's an unusual form of energy. Light can be considered an electromagnetic wave, oscillating electric and magnetic fields. But it's not a wave the way other waves are, like water waves on the oceanfront or on the lake shore. Those waves can exist without the water, whereas light doesn't need anything in order to propagate.
If it's a wave, physicists used to ask, what is it that's waving? And the thing that's waving are electric and magnetic fields which don't actually require anything to exist, which is a good thing for us, because the light coming from the sun has to pass through the vacuum of outer space. And if it needed some medium in order to propagate, we would be all in the dark here, wondering why doesn't someone turn on the lights.
MOLLY BLOOM: These names light up my life. It's the Brains Honor Roll. They're the amazing listeners who have shared questions, ideas, mystery sounds, and drawings with us. They make this show what it is.
[LISTING HONOR ROLL]
PEOPLE: (SINGING) Brains Honor Roll. High five.
MOLLY BLOOM: That's it for this episode. We'll be back soon with more answers to your questions. Thanks for listening.
PEOPLE: (SINGING) Buh-buh, buh-buh, buh-buh, buh-buh-buh-buh Brains On.
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