crpf samachar may_ june-2012 - Central Reserve Police Force

precisely timing the signals sent by GPS satellites
high above the Earth. Each satellite continually
transmits messages that include
� the time the message was transmitted
� satellite position at time of message transmission
The receiver uses the messages it receives to
determine the transit time of each message and
computes the distance to each satellite. These
distances along with the satellite’s locations are used
with the possible aid of trilateration (In geometry,
trilateration is the process of determinating absolute
or relative locations of points by measurement of
distances, using the geometry of circles, spheres or
triangles), depending on which algorithm is used, to
compute the position of the receiver. This position is
then displayed, perhaps with a moving map display
or latitude and longitude; elevation information
may be included. Many GPS units show derived
information such as direction and speed, calculated
from position changes. Three satellites might seem
enough to solve for position since space has three
dimensions and a position near the Earth's surface
can be assumed. However, even a very small clock
error multiplied by the very large speed of satellite
signals results in a large positional error. Therefore
receivers use four or more satellites to solve for
both the receiver's location and time.
Structure
The current GPS consists of three major
segments. These are the space segment, a control
segment, and a user segment. The U.S. Air Force
develops, maintains, and operates the space and
control segments. GPS satellites broadcast signals
from space, and each GPS receiver uses these
signals to calculate its three-dimensional location
(latitude, longitude, and altitude) and the current
time.
The space segment is composed of 24 to 32
satellites in medium Earth orbit and also includes
the payload adapters to the boosters required to
launch them into orbit. The control segment is
composed of a master control station, an alternate
ebZ&twu 2012
Technical Feature
master control station, and a host of dedicated and
shared ground antennas and monitor stations.
The user segment is composed of hundreds of
thousands of U.S. and allied military users of the
secure GPS Precise Positioning Service and tens of
millions of civil, commercial, and scientific users of
the Standard Positioning Service.
Communication
The navigational signals transmitted by GPS
satellites encode a variety of information including
satellite positions, the state of the internal clocks,
and the health of the network. These signals are
transmitted on two separate carrier frequencies
that are common to all satellites in the network.
Two different encodings are used: a public encoding
that enables lower resolution navigation, and an
encrypted encoding used by the U.S. military.
All satellites broadcast at the same frequencies
1.57542 GHz (L1 signal) and 1.2276 GHz (L2
signal). Signals are encoded using code division
multiple access (CDMA) allowing messages from
individual satellites to be distinguished from each
other based on unique encodings for each satellite
(that the receiver must be aware of). Two distinct
types of CDMA encodings are used: the coarse/
acquisition (C/A) code, which is accessible by the
general public, and the precise (P) code, that is
encrypted so that only the U.S. military can access
it.
Because all of the satellite signals are modulated
onto the same L1 carrier frequency, the signals
19