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Global Navigation Satellite System (GNSS)
GNSS is working based on the signals broadcasted by the satellites. GPS, GLONASS,
Galileo and BeiDou are independent global navigation systems, and each one of them
can be used independently to find out the coordinates of receiver globally anywhere
and at any time. The GNSS system functions on the basic principle of trilateration. If
the distance is known from three known points, we can find the location of unknown
point in the space [7]. GNSS will use the satellites for known positions. The satellites
are moving as per the Kepler’s laws. Thus we are able to find out the position of the
satellite at the given time, using Keplar’s parameters called ephemeris, transmitted
by all the satellites, which are in the view to receiver. Then the receiver will calculate
the time difference between the transmitted signal from the satellite and received
signal in the receiver. The time difference multiplied with the velocity of light will
give the distance between the respective satellites and receiver. If three satellites are
visible to the receiver, we can compute the positions of the receiver. The clocks at
the satellites are synchronized up to a Nano-second accuracy. However, the clock
at the receiver end is not synchronized with the clocks at satellites. This difference
in synchronization between the clocks at satellites and receiver is known as bias, an
unknown parameter. So, four visible satellites are required to find out four unknowns
including clock bias. The time difference can be calculated using principles of signal
processing.
Thus we have the following four equations for unknown variables.
(x −x1)2 + (y −y1)2 + (z −z1)2 = (d1 + b)2 = ρ2
1
(13.2)
(x −x2)2 + (y −y2)2 + (z −z2)2 = (d2 + b)2 = ρ2
2
(13.3)
(x −x3)2 + (y −y3)2 + (z −z3)2 = (d3 + d)2 = ρ2
3
(13.4)
(x −x4)2 + (y −y4)2 + (z −z4)2 = (d4 + d)2 = ρ2
4
(13.5)
Here (x, y, z) are the coordinates of the unknown receiver and b is bias.
These independent GNSS systems will consist of three segments as shown in
Fig. 13.7b which are called as
(a)
Space Segment
(b)
Control Segment and
(c)
User Segment
Space segment includes satellites, which are transmitting signals towards the
earth as shown in Fig. 13.8. Control segment consists of reference stations and
master control station to track and model the orbital parameters of the motion of the
satellites. Ground antennas at earth station will uplink these model parameters to the
satellites.