The
Global Positioning System (GPS)
How Does GPS Work?
The Global Positioning System (GPS) defines a reference grid that
covers the entire Earth. GPS provides a way to exactly identify every spot on Earth. GPS
was developed and is owned and operated by the US
Depart of Defense (DOD).
Civilian use of GPS is free and is provided in
a less accurate position reference system. GPS satellites transmit numerical data that is received by a GPS receiver.
The GPS receiver calculates the data and provides two reference
grid coordinates.
Some GPS receivers are part of a GPS system. A
GPS system refers to a device that uses an embedded GPS receiver
along with a computer and system software ran by that computer.
Based on what features are programmed into the system software, a
GPS system can provide a calculated pair of 2D grid reference
numbers that pin-point the location of that GPS system on Earth.
The two position coordinates can be pin-pointed on a map
using the map's grid reference.
IMPORTANT!
GPS Is Not A Replacement For Map & Compass Use
GPS Does Not Replace A Good Map, Compass And
The Skills Needed!
Know How To Use A Map And Compass!
The GPS System
Operation In Detail
The
basis of GPS technology is precise time and position
information. Using atomic clocks (accurate to within one second
every 70,000 years) and location data, each satellite
continuously broadcasts the time and its position. A GPS
receiver receives these signals, listening to three or more
satellites at once, then calculates its position within a world
reference grid.
By measuring the time interval between the transmission and
the reception of a satellite signal, the GPS receiver calculates
the distance between the user and each satellite. Using the
distance measurements of at least three satellites in an
algorithm computation, the GPS receiver arrives at an accurate
position fix.
The position information in a GPS receiver may be displayed
as latitude/longitude, Universal Transverse Macerator or other
system coordinates. Information must be received from three
satellites in order to obtain two-dimensional (latitude and
longitude) fixes, and four satellites are required for
three-dimensional (latitude, longitude and altitude)
positioning.
Each satellite continuously broadcasts two signals, L1 and
L2. The L1 frequency contains the C/A code which provides
Standard
Positioning Service (SPS) for worldwide civilian use.
The encrypted P-code is broadcast on both the L1 and L2
frequency, resulting in the Precise Positioning Service (PPS)
for military use. The SPS signal will provide a civilian user an
accuracy of better than 25 meters. Because they are so accurate,
civilian GPS receivers using the SPS signal are sometimes
subject to Selective Availability (SA)
interference by the United States Government, to maintain
optimum military effectiveness of the system. When engaged, SA
inserts random errors in the data transmitted by the satellites.
As a result, SPS signal accuracy can be reduced to 100 meters.
However, using a technique called differential GPS (DGPS),
the user can overcome the effect of SA interference and increase
the overall accuracy of the GPS receiver. With DGPS, one GPS
receiver unit is placed at a known location and the position
information from that receiver is used to calculate corrections
in the position data transmitted by the satellites. This
corrected information is then transmitted to other GPS receivers
in the area. The resulting real-time accuracy is in the 10-meter
range. Sub-meter accuracy can be obtained by using DGPS and
post-processing calculations in static positioning.
A newly developed and
approved Wide Area Augmentation System (WAAS) has been
implemented to increase civilian's GPS position accuracy. A
special GPS receiver with the capability to receive and
calculate this new system's signals and information must be
used. Currently, this system is being developed for marine uses
on the Pacifica coast. For more details
Click Here.
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