For as little as $250Aus you can know exactly where you are and where you
have been. For anyone who has ever been lost while hiking in the woods, or
lost in the middle of the ocean, or lost while driving in a unfamiliar
city, or lost while flying a small airplane at night, a GPS receiver is a
miracle. With a GPS, you are never lost. In this edition of How Stuff
Works we will discuss both what a GPS receiver can do and how it works so
that you can learn more about these amazing devices.
Here is a picture of a typical GPS receiver, in this case the Garmin GPS
This is a handheld device weighing about 8 ounces and measuring about 6
inches by 12cm (ignoring the antenna). The antenna is the verical stub on
the right, roughly three inches long. This unit has a small LCD screen and
a set of buttons to activate its different features.
When you turn on a GPS receiver, its first task is to try to find the
radio signals for the satellites it can "see".
GPS satellites top...
GPS satellites live in very precise orbits about 16,000 km up (for
comparison, the space shuttle orbits at about 320km and geosynchronous
satellites orbit at about 38,600km). Because the satellites are so far
away, their radio signals are fairly weak. Therefore, for the GPS receiver
to "see" a satellite, the satellite must be above the horizon and
unobstructed by buildings, mountains, etc. At any given moment at any
point on the planet there are between 6 and 9 satellites above the
horizon. During the process of acquiring the satellites, the GPS display
will look something like this:
On this screen, the larger circle represents the horizon and smaller
circle represents 45 degrees. The dot in the center is straight overhead.
The numbers within the circles represent satellites that are visible, and
the bar chart on the right represents the relative strength of the signals
from the different satellites.
Once the GPS receiver has locked on to 3 satellites, it can display your
longitude and latitude to about 100 foot accuracy. If the receiver can see
4 satellites it can also tell you your altitude. With this information you
know exactly where you are.
Most modern GPS receivers are able to store your track. As you move, the
GPS periodically stores your position in its internal memory. It can then
show you the path you have followed on the display so that you can see
exactly where you have been. Tacks also make backtracking easy.
The GPS system depends on two things to make it work: First, each
satellite has an on-board atomic clock that gives it an extremely precise
time base. The satellites send radio signals to the receiver, and the
extremely precise time bases make it possible for the receiver to
determine exactly how far away each satellite is.