spread-spectrum rf signals, 1575.42 MHz (LI-RF)
and 1227.60 MHz (L2-RF). Each signal is modulated
with a unique code sequence and a navigation data
message. The code sequence allows the navigation
sets to identify the satellite, and the data message
provides the navigation sets information about the
operation of the satellite.
An observer on the ground will observe the same
satellite ground track twice each day, but the satellite
will become visible 4 minutes earlier each day
because of a 4 minute per day difference between the
rotation of the earth and the satellite orbit time. The
satellites are positioned so a minimum of four
satellites are always observable to a user anywhere on
earth.
Satellite Signal Structure
The satellites transmit their signals using spread
spectrum techniques. Two types of techniques are
used: course acquisition (C/A) code and precise (P)
code. The C/A code is available to military and
civilian GPS users. The P code is available only to
U.S. military, NATO military and other users as
determined by the DOD.
Since only the P code is on both frequencies, the
military users can make a dual-frequency comparison
to compensate for ionospheric propagation delay.
The C/A code-only users must use an ionospheric
model, which results in lesser navigation accuracy.
Superimposed on both codes is the NAVIGATION-
message (NAV-msg), containing satellite ephemeris
data, atmospheric propagation correction data, and
satellite clock-bias information.
Satellite Ranging
GPS navigation is based on the principle of
satellite ranging. Satellite ranging involves measuring
the time it takes the satellite signal to travel from the
satellite to the navigation set. By dividing the travel
time by the speed of light, the distance between the
satellite and the navigation set is known. By ranging
three satellites, a three-dimensional picture, such as
the one shown in figure 1-12, can be developed. The
distance measurement to each satellite results in a
sphere representing the distance from the navigation
set to the satellite. The point where the three spheres
intersect (X) is the position of the navigation set,
This explanation does not account for errors. For
satellite ranging to provide accurate position data, the
following three sources of error must be compensated
for:
Satellite position and clock error
Atmospheric delay of satellite signals
Navigation set clock error
With these errors compensated for, the GPS can
determine position fixes within 50 feet or less and is
accurate to within a tenth of a meter-per-second for
velocity and 100 nanoseconds for time. This accuracy,
however, requires inputs from four satellites.
USER EQUIPMENT
User equipment is installed in ships, aircraft, and
motorized vehicles. The vehicle version can also be
carried by personnel (particularly SEAL teams and
other special forces units) as a manpack. The most
common manpack version is the AN/PSN-8( ). The
most common shipboard GPS receiver is the
AN/WRN-6. These GPS receivers will be described
later in this chapter.
Signal Acquisition
During operation, navigation sets collect and
store satellite almanac data in critical memory. The
almanac data is normally available when the
navigation set is first turned on and provides
information on satellite locations. Operators may
input information about the navigation set position,
time, and velocity to enhance the information in
critical memory. With this information, the navigation
set determines which satellites are available and
searches for the code sequences that identify those
particular satellites. When the C/A code of an
available satellite is identified, the navigation set
switches to the more accurate P code, collects the
navigation data message, and updates critical
memory.
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