HOW GPS WORKS
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How does GPS work?
Not even 10 years ago, those who wanted to travel to
unknown places had to rely on huge maps that were sometimes confusing and could
never be folded back up exactly the same way they came. While these road maps
are accurate, they’re not the most convenient tools to use when you’re trying to
drive, look down to get the right directions, and that’s not even mentioning the
fact that there’s often not enough room to spread out the entire map! While this
was merely an annoyance for the occasional traveler, it became a large problem
for the American military. Such a problem in fact, that in 1973, the American
Department of Defense created a system that would allow them and their vehicles
to travel to unknown parts quickly and easily, and during times when it’s not so
easy to just ask a local. Today, this system known as the Global Positioning
System is available to everyone and for a small price – you can find these
systems for less than $100. But how does it work?
The Satellites and HistoryThe GPS
consists of 27 satellites that orbit Earth. Only 24 of these satellites are
actually used however, the extra 3 are there in case one of the satellites
should fail. The first satellite went up in 1980 and was known as “Block I.” It
was only 3 years after that, in 1983, when the GPS became open to the public.
This was due to the Korean Airline Flight 007 that had gone missing over Soviet
territory only to be shot down. However, it still took until 1994 when all 24 of
the satellites were in place and ready to be used.
Each satellite is solar-powered and weighs
approximately 3,000 to 4,000 pounds. Each satellite orbits Earth at a pace of
12,000 miles. This means that each satellite makes 2 rotations around the Earth
every day! These orbits are coordinated so that on any given day, at any given
time, there are at least 4 satellites that can be “seen” in the sky. The GPS
then determines how far it is from each satellite and using this information,
can locate where it is on Earth. While this may sound simple enough, this
process relies on a mathematical process called trilateration. The GPS uses 3D
trilateration but to understand this mathematical principle, it helps to first
understand 2D trilateration.
Trilateration
Trilateration works
sort of on the concept of process of elimination – eliminating all other
possibilities of where you could be. For example, if you were lost in the United
States but knew that you were 625 miles away from Boise, Idaho, it wouldn’t do
you much good. You would still not know in which direction Boise was and
therefore, not really know where you are. However, if you could deduce that you
were also 690 miles from Minneapolis, Minnesota, you would have a better idea of
where you were. This is because you now have 2 location points to work with
instead of just 1. If you then found out that you were also 615 miles from
Tucson, Arizona, you could then probably determine, fairly accurately, where you
were – you’d be in Denver, Colorado. You would be able to figure this out by
taking all of the information you’ve received and putting them together to
deduce that only Denver fits in with all of those distances from other places.
It may help you even further if you imagine a circle around all of the locations
that you had determined and those circles represented the number of miles that
you were out from the point. These circles would intersect at various points and
the point where all of the circles met would be the point of your location.
Although this may sound complicated, this is still only 2D trilateration and GPS
works on 3D trilateration.
In 3D trilateration, these circles are thought of
as spheres and the distance isn’t between you and various other states, it’s
between you (and the GPS in your hand or car) and the distance to the
satellites. At the same time, the GPS also needs to be able to determine the
distance the satellites are away from each other. So instead of you drawing an
imaginary 625 mile circle around Boise, Idaho, the GPS will use imaginary
spheres around the satellites to determine the distance. The Earth can also be
seen as a fourth sphere when working with a GPS. Then, for example, the GPS
would determine that you were 10 miles away from Satellite A and 15 miles away
from Satellite B and these 2 spheres would intersect. Then if a third satellite
can also be determined, the sphere around this satellite will also intersect at
2 different points with the other spheres. Because you’re using the Earth as a
fourth sphere, one of these points will be on Earth and the other point will be
in space. Therefore, the GPS will be able to eliminate the one in space (as you
most likely will not be there!) and tell you where on Earth you are! Although
this may seem like only 3 satellites are being used by the GPS, the system
actually uses 4 satellites so that it can be even more accurate and precise.
Although 3D trilateration may seem complicated, it’s not extremely different
from 2D trilateration, it’s just a little harder to visualize.
In order for this mathematical principle to take
effect, the GPS must be able to do 2 things. The first is that it must be able
to locate at least 3 of the satellites and it must also be able to calculate the
distance between it and those satellites. The GPS determines all of this by
receiving high-frequency and low-power radio signals from the satellites. Lower
end models of GPS units will only have one receiver to do this while the more
expensive models that are generally of better quality will have more than one
receiver, which means that it will be able to communicate with more satellites
at one time. But the GPS’ job isn’t done yet! To get your exact location, the
GPS must be able to determine how long it took for this radio signal to get to
the receiver and this is also a fairly complicated process.
Calculating Signal Speed
Radio waves are made from electromagnetic energy and
so, they travel at the speed of light, which is approximately 186,000 miles per
second. The satellites will begin to transmit a long digital code called a
pseudo-random code at a particular time, such as at around midnight. At the
exact same time, the GPS will also begin to transmit this code. Once the
transmission has reached the receiver, it will be slightly behind the receiver’s
transmission of the same pattern. How long this delay takes will indicate the
distance from the satellite to the GPS. The receiver uses this time and
multiplies it by the speed of light, to determine how far the signal travelled
and determine the distance from the unit to the satellite.
But a system so advanced needs more than just
ordinary clocks to help them work. They actually need atomic clocks that can be
synchronized to the exact nanosecond, to ensure that the distance and speed
calculated is accurate. The problem with this is that atomic clocks cost
approximately $50,000 to $100,000 and so, they are not very practical for public
use and so, it has its own clock system to make things just as accurate. Each
satellite is still equipped with an atomic clock however individual GPS units
are fitted with a standard quartz clock. Basically, this quartz clock works by
always resetting itself and estimating its own inaccuracy. This means that there
is only one ‘real time’ that the receiver can use. This real time value makes
all of the satellites (usually 4) that the receiver is communicating with to
align at one specific point in space. It will be this time value that the atomic
clocks within the satellite will use and therefore the GPS unit will also
correct itself to this time. This way the GPS unit gets the same accuracy as the
atomic clock, without needing the expensive hardware.
But the GPS still isn’t done yet! To still be able
to get its accurate location, the GPS needs to know the location of the
satellites that it’s communicating with. Establishing this is one of the easiest
components of the GPS. The GPS unit houses an almanac that will tell it the
position of any one of the satellites at any particular time. Although things
such as the shift of the moon or the sun will change the location of the
satellites a bit, the Department of Defense constantly monitors the position of
the satellites. Should they come out of place at any time, the DOD will send out
a signal to the satellites that will automatically send it to the GPS receivers
so that these units always have the correct position of the satellites.
This process of GPS units communicating with
satellites may sound much more complicated than it actually is. Once you ask
your GPS to find your location, you will have it in a matter of seconds – as
long as it takes for the unit to deduce the location of the satellites and
calculate the speed of the signals. Even though this process is relatively
simple, it’s very advanced technology that allows this process to take place and
lets you find your way home faster and easier! With a GPS unit, you can
determine: how far you have travelled and how long you’ve been travelling for;
how fast you are going; the average speed you travel at; and how long it will
take you to get to your destination should you continue to travel at the same
pace. A GPS unit will even show you the route you took to get there so if you
want to get back, it’ll be even easier than getting there in the first place!
A GPS
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