GPS-The
Global Positioning System, under development by the Department of
Defense for over 20 years, consists of 21 satellites, plus three back-up
satellites in predictable orbits around the earth. The system provides
24-hour positioning information regardless of weather. Launched aboard Delta
rockets and tracked under Air force administration, GPS consists of the
Space Segment (the satellites) and the Control Segment (the network of
tracking stations which monitor and control the GPS satellites in orbit).
Ashtech has recently enhanced the
performance of its GPS positioning systems by combining GPS receivers, with
GLONASS receivers. GLONASS is the Russian equivalent of GPS--by combining
GPS and GLONASS, you have a combined satellite constellation with many more
than the standard 24-satellite constellation of GPS alone, which offers much
better system availability and integrity.
GPS has wide applicability. Linked
to a vehicle, it becomes a tool of navigation. Within the context of a
coordinate system, it is an instrument of surveying. With a cellular phone
or transceiver, it becomes a method of tracking vehicles or people. With a
digitized map base, it provides an all-electronic chart. For weapons
guidance, it is unsurpassed.
GPS works on the principle of
triangulation. By knowing its distance from three or more satellites, the
receiver can calculate its position by solving a set of equations.
Information from three satellites is needed to calculate longitude and
latitude at a known elevation; four satellites are needed to include
altitude as well.
Satellites orbit the earth twice a
day at an altitude of 10,900 miles, repeatedly broadcasting their position
and the time. The atomic clocks aboard each satellite keeps time by the
vibration of atoms and are accurate to one second in 30 years!
In theory, the distance from
satellite to receiver can be calculated by multiplying the time it takes for
the signal to arrive by the speed at which it travels -- the speed of light.
In practice, more sophisticated calculations are required to account for the
fact that receiver clocks are not as accurate as satellite clocks. Upper
atmospheric conditions and solar disturbances can also interfere with signal
reception. All GPS receivers need direct and unobstructed line of sight
access to each satellite.
While a single receiver can
provide accurate positioning to about 100 feet, accuracy to a fraction of an
inch is possible by using two receivers. One is fixed to a spot whose
coordinates are already known. The other, whose location is sought, logs the
same satellite data and the errors are resolved, in real-time for pin-point
navigation or by post-processing for precision geodetic surveying.
Because GPS was designed for
military use, it contains a number of features to limit its use to national
defense. Each satellite broadcasts two signals, one for commercial use
(C/A-Code) and a more accurate one for military use (P-Code). The Pentagon
can encrypt the P-Code signal ("anti-spoof" mode or AS) so that unauthorized
receivers cannot understand the information, however more advanced
commercial receivers such as the Ashtech Z-12 can compensate by correlating
the components of the P-Code for continued use in high resolution
positioning.
Another restriction is "selective
availability", or SA, in which the data transmitted by the satellites
contain deliberate errors for all but military receivers. If SA is turned
on, the accuracy of commercial receivers droops from 100 feet to 300 feet.
(However, using two receivers together in a differential mode can correct
for this misinformation).
Commercial use of GPS has proven
invaluable in many fields. It has revolutionized surveying. It can be used
to track everything from migrating animal herds to the creep of the earth's
crust. Using GPS provides an entirely new way of navigating and piloting, on
land, sea or in the air. It is one of our best space adventures yet.
