HOW DOES GPS WORK??
GLOBE NY METRO
Modified with permission from AMSTI-GLOBE (www.amsti.org)
• Imagine you are somewhere in
your town but aren’t sure
where. You ask someone for
help and they say, “You are 5
miles from the fire station.”
5 miles
This is nice, but it really doesn’t help
because you could be anywhere on a circle 5
miles around the fire station!
A second helpful person
tells you that you are 7
miles from the library.
•Combine this
information with the fire
station information, and
you have two circles
that cross.
Fire Station
Library
•If you are 5 miles from the fire station and 7 miles
from the library you can only be at one of these two
points
• A third person tells you
that you are 4 miles
from home.
Fire Station
Library
•This circle will cross
the other two circles
at only one point.
You must be at the only place where all three
circles cross.
• GPS satellites
work the same
way to find the
location of a
plane, ship, car,
or a person lost
in the city.
• Since 1994, there have
been 24 functioning GPS
satellites orbiting Earth
at all times and as many
as four spares
• The more satellites your
GPS can lock onto, the
more accurately your
position can be
determined.
• Your GPS can receive up
to 12 satellites at the
same time.
NASA/JPL
This illustration
shows how the GPS
receiver can see the
satellites as they
pass overhead.
A purple line
connecting satellite
and Earth means the
station on the ground
can receive the
satellite signal. The
line disappears when
the satellite is out of
direct line of sight.
NASA/JPL
• By collecting the information from
several satellites, a GPS unit can
determine its exact location on Earth.
• A minimum of three satellites is needed
to locate a point (just like the three
intersecting circles).
• This is called triangulation
“Doing the Math” with GPS
Locating with GPS
GPS satellites send a signal at exactly the same time
as your GPS receiver. Even at the speed of light
(186,000 miles per second) it takes that signal a few
hundredths of a second to reach your GPS receiver.
In this example, the signal arrives 0.06 seconds after
your GPS signal
0.06
http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps02.html
Learning math with GPS
Calculate the distance from your location to the satellite:
• The satellite microwave signal travels at 186,000 miles
per second
•
It took the signal 0.06 seconds to get to your GPS
Velocity x time = Distance, so
186,000 miles/second x 0.06 seconds = 11,160 miles
You must be 11,160 miles from the satellite, but..
http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps04.html
that could be anywhere on the surface of
a sphere with a radius of 11,160 miles
As in the example of your town, you need two more
readings to find out exactly where on this sphere (not a
circle this time)
Data from satellite 2
0.07
• It took this signal 0.07 seconds to get to your GPS
REMEMBER: velocity x time = distance
186,000 miles/second x 0.07 seconds = 13,020 miles
You are 11,160 miles from Satellite 1, and 13,020 miles
from Satellite 2, so you are somewhere where the two
spheres intersect.
http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps07.html
Two circles intersect at two points. The intersection of two
spheres is a curved surface. But where on that surface are
you? Data from Satellite 3 will tell.
http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps09.html
Data from satellite 3
0.05
• It took this signal 0.05 seconds to get to your GPS
REMEMBER: velocity x time = distance
186,000 miles/second x 0.05 seconds = 9,300 miles
You now know that you are 11,160 miles from
Satellite 1, 13,020 miles from Satellite 2, and 9,300
miles from Satellite 3.
http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps07.html
The third sphere
intersects the surface at
two points. You are at
one of them, but which?
One of the two points is always on Earth’s
surface, the other more than 100 miles above it
The GPS makes the easy choice for you!
You are here…
…not up here!
http://www.pbs.org/wgbh/nova/shackletonexped/navigate/gps/gps13b.html