The Big Idea Geometrically:
The Global Positioning System (GPS), is a constellation
of 24 satellites (and several backups) that orbit the
Earth in a satellite-based navigation system. GPS was
developed by the United States Department of Defense for
a variety of military applications, but the system has
been open to civilian use since the 80's.
solar-powered GPS satellite completes two orbits of the
earth a day, at a height of 19 300km (12000 miles) above
ground. They transmit signal at 1228 MHz and 1575 MHz.
Civilian GPS receivers use the latter frequency. Each
satellite constantly transmits a long digital
pattern called a pseudo-random code, which is an
identifying signal unique to each satellite. At the
same time, the receiver plays back a stored copy of
the pseudo-random code to itself while listening for
the satellite’s signal.
When the signal reaches the receiver
the satellite’s pseudo-random code will be slightly
delayed compared to the receiver’s playback of the
pattern. This delay is due to the large distance between
the satellite and receiver, as well as the
signal-slowing effects of the troposphere which the
signal has to pass through.
The signal travel time is the length of the delay.
The receiver merely multiplies this time by the speed of
light (GPS signals are electromagnetic waves, which all
travel at the speed of light) to find out the distance
between the satellite and receiver.
This brings up an interesting point: how does the
receiver know the exact time that the satellite started
transmitting? Wouldn’t the clocks in the receiver and on
the satellite need to be perfectly synchronized? The
answer is, funnily enough, no. While GPS satellites have
a highly accurate atomic clock onboard for timekeeping,
it is unfeasible to put an atomic clock in a handheld
To solve the problem, the receiver uses the signals from
four or more GPS satellites and finds out the inaccuracy
of it’s internal clock. As it has determined it’s own
inaccuracy, it is able to adjust it’s clock to make it
accurate – and the problem is solved.