In the early 1970s, the Defense Department needed a
navigational tool that troops on the move could use to
pinpoint their location. As a result, they created the
Global Positioning System, or G.P.S., a hi-tech system of
two dozen satellites, atomic clocks, microwave radio
transmitters and some heavy-duty number-crunching
hardware.
The G.P.S. is now used by millions for fun and profit. Any
time you take a plane trip, receive goods from a commercial
shipping agency or drive a car with a NeverLost or OnStar
navigation system, you are benefiting from G.P.S.
technology.
A receiver on the ground or in the air uses time signals
from four satellites to calculate its position. The
calculation itself is based on a kind of triangulation -- a
math technique used to locate an object based on its
distance from three points. So signals from three satellites
are necessary, although in practice a signal from a fourth
satellite is used to improve the accuracy of the other three
signals.
To calculate these distances, the system uses another basic
high school math equation: distance is equal to the speed of
travel multiplied by the time.
In addition to the time, a signal from a G.P.S. satellite
also includes information about the satellite's exact
location, which is known, tracked and kept accurate by
ground control stations. The time signal is also very
accurate, because each satellite contains several atomic
clocks. These rely on the natural, and very regular,
oscillation frequencies of atoms to keep time.
The end result is that a G.P.S. receiver -- such as a
dashboard navigation system in a car -- can produce highly
accurate coordinates of latitude, longitude and altitude.
This kind of system was used in a recent report in
Yated that discussed the boundaries of Eretz Yisroel
as being north of the 30th parallel of latitude.
Software developers in the commercial sector took care of
the final step, placing your location on a map and providing
directions. Companies provide consumer products that combine
a G.P.S. receiver with map programs that can provide turn-by-
turn directions for drivers. From longitude and latitude
readings that update every second, these programs can
determine the speed and direction, information that enables
them to superimpose a "you are here" arrow on the display of
a road map.
This navigation technology also has implications for the
trucking industry, which has begun to automate its fleet
management systems with customized G.P.S. equipment. In
addition to receiving G.P.S. satellite signals and
calculating location, they also transmit location
information to a central computer that handles route
planning for multiple deliveries. This enables new delivery
and pickup points to be added to a driver's route without
requiring any data entry by the driver.
The G.P.S. does have some blind spots, however, if the
receiver is behind trees, tunnels, power lines and tall
buildings which can block satellite signals.
Another problem is the orbits of the satellites, since not
all parts of the earth have the minimum four satellites
within range at all times.
A greater source of error is atmospheric conditions, which
can ever-so- slightly slow down the radio signals from the
satellites, introducing errors into the system.
To help improve accuracy, an enhancement, called
differential G.P.S., uses stationary ground-based receivers
to monitor the signals from the constellation of G.P.S.
satellites. Differential G.P.S. can provide accuracy down to
a meter or so.
