GPS satellites are constantly bathing the world in their electromagnetic signally glow,
but how EXACTLY do these satellites find you?
Saying a satellite finds YOU is very Enemy of the State, very NBC TV government drama;
but GPS satellites don't actually track you, they're simply broadcasting a signal that
you pick up. What would be a more accurate way to look at it is, GPS doesn't find you,
but you find it.
The Global Positioning System is network of around 30 satellites, a receiver, and a SUPER
accurate clock; all pulled together with math. The system was created in the 1980s for the
military, but they opened it up for civilian use, and it has since changed how we find
directions, look for pizza, and connect with each other. But how it works, requires a TON
of incredible science.
Firstly, we have to know what time it is. I know that seems strange, but if you don't
have a SUPER ACCURATE measure of time, you can't do GPS! The most accurate measure of
time humans have, is the constant, predictable vibration of an atom. The U.S. Naval Observatory
in Washington, D.C., is the official United States time. They mark time, by measuring
billionths of a second using 9,192,631,770 electron vibrations of a Cesium-133 atom.
This atomic clock is accurate to nanoseconds… which is super important.
Secondly we have to have a satellite network which ALSO know the time. As of October 19,
2015, there were GPS 31 satellites with three atomic clocks on board, as well as transmitters
to send microwave signals out at the planet below. The system is flying at 11,000 kilometers
per hour, about 20000 kilometers above our heads (7000mph/12500mi) and is maintained
by the U.S. Air Force. Those signals? Those are what are telling you where you are on
the planet!
Okay, so. GPS signals are sent at exact intervals. Say… every few seconds. Embedded in microwave
signal is the ID of the satellite, the health of the satellite, the location of all the
satellites in the system, and the precise date and time. VERY precise. Because Tens
of thousands of kilometers away on the ground, the signals will be picked up by your smartphone
once you click, let the pizza app know your location.
Smartphones and GPS units have to have very precise time, because microwave signals travel
at the speed of light, roughly 300,000 kilometers per second, so in a fraction of a second,
the signal travels from each satellite to you! This is why we need atomic clocks, because
20-30 nanoseconds matter! But we'll come back to this in a sec. The receiver can tell where
each satellite is, by the difference in time lag. Using a mathematical process called trilateration
the GPS unit can determine your exact location. Basically, a sphere is drawn indicating the
time lag from each satellite and the overlapping point is your current location! After ALL
THAT, the pizza app knows exactly where you are.
Before you get confused, there's NOT an atomic clock in your smartphone, but as long as the
smartphone clock is pretty good, it can will regular updates from the Observatory clock
and stay on track! The more satellites it can "see" the more accurate the location.
If you pick up three satellites, your GPS can determine your 2D location (latitude and
longitude), but with four it can tell your altitude too! As you move, the time lag from
the satellite to the antenna of each signal will change, and by constantly updating the
trilateration math, the location dot on your map moves.
Even after all this work just to get a pizza delivered, there's a ton more complications.
Satellites need replacement, and only have an operational life of 7-15 years. So they
have to keep launching new ones. We're on our fifth generation now, and they're getting
a lot better, but they're not perfect. The satellite signals can bounce off an ionized
layer of the atmosphere called ionosphere -- AM radio bounces off it from the ground
too. It's not friendly to signals going through it. But on top of THAT GPS satellites are
traveling FAST and far away, meaning Einstein's General & Special Relativity comes into play.
Even if the atomic clocks are super accurate, the ones on the satellites are ticking faster
than the Naval Observatory clock, because gravity is stronger on Earth and they're moving
so fast! SO! The GPS has to mathematically calculate and adjust for the roughly 45,000
nanoseconds per day of difference in time; since being off by 20-30 nanoseconds can make
the system pretty much worthless. This a big deal. It's frickin' crazy.
Thanks to the space program, Einstein's relativistic physics, atomic clocks, and smart phones,
you my friend, can get a pizza delivered to your exact location -- give or take a few
feet. What do you want? We're not perfect. Yet. The next generation should get us within
inches.
And thank you also to the US Air Force for sponsoring this message, and making sure the
pizza guy knows where to come. Every day American Airmen go above and beyond to break barriers
both professionally and personally. The United States Air Force is powered by Airmen, fueled
by innovation. Crazily enough, your BRAIN has GPS too, after
a fashion. A set of cells creates a map of the stuff around you… No, no, I'll let this
super handsome guy with a great t-shirt collection explain it.