User segment:

The user segment includes all military and civilian users.

GPS User Segment consists of the GPS receivers and the user community.

With a GPS receiver connected to 3 (178 antenna, a user can receive the GPS signals,
which can be used to determine his or her position anywhere in the world. The
typical receiver is composed of an antenna and preamplifier, radio signal
microprocessor, control and display device, data recording unit, and power supply.
GPS receivers convert SV signals into position, velocity, and time estimates. A
minimum of four satellites are required to compute the four dimensions of X, Y, Z
(position) and Time.

GPS is currently available to all users worldwide at no direct charge.

b) ( Explanation including major 5 points,)

Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System
that provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10
cm in case of the best implementations.

DGPS uses a network of fixed ground-based reference stations to broadcast the difference between
the positions indicated by the GPS satellite systems and the known fixed positions

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Many of the errors affecting the measurement of satellite range can be completely
eliminated or at least significantly reduced using differential measurement techniques.
DGPS allows the civilian user to increase position accuracy from 100m to 2-3m or less,
making it more useful for many civilian applications.

The Reference receiver antenna is mounted on a previously measured point with known
coordinates.

The receiver that is set at this point is known as the Reference Receiver or Base Station.
The receiver is switched on and begins to track satellites.

It can calculate an autonomous position using the techniques mentioned in section 3.1.
Because it is on a known point, the reference receiver can estimate very precisely what the
ranges to the various satellites should be.

The reference receiver can therefore work out the difference between the computed and
measured range values. These differences are known as corrections.

The reference receiver is usually attached to a radio data link which is used to broadcast
these corrections.

The rover receiver is on the other end of these corrections. The rover receiver has a radio
data link attached to it that enables it to receive the range corrections broadcast by the
Reference Receiver. The Rover Receiver also calculates ranges to the satellites as
described in section 3.1. It then applies the range corrections received from the Reference.