GPS is a satellite-based navigation system consisting of 24 satellites. It uses phase modulation and direct sequence spread spectrum modulation to transmit signals from satellites to receivers. By measuring distances to several satellites, trilateration can precisely determine a receiver's location and time. However, factors like atmospheric effects, multipath interference, and clock errors can introduce noise and reduce accuracy, which differential GPS aims to mitigate.

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GLOBAL POSITIONAL
SYSTEM
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What is GPS?
 Global Positioning System (GPS) is a satellites based navigation
system made up of a network of 24 satellites.

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GPS modulation
 GPS satellite transmissions utilize direct
sequence spread spectrum (DSSS)
modulation.
 DSSS provides the structure for the
transmission of ranging signal and essential
navigation data such as satellite
coordinates. The ranging signal are pseudo
random noise (PRN) codes that binary
phase shift key (BPSK) modulate the satellite
carrier frequencies.

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 Amplitude Modulation (AM) also known as
amplitude-shift keying. This method requires
changing the amplitude of the carrier phase
between 0 and 1 to encode the digital signal.
 Frequency Modulation (FM) also known as
frequency-shift keying. Must alter the frequency of
the carrier to correspond to 0 or 1.
 Phase Modulation (PM) also known as phase-shift
keying. At each phase shift, the bit is flipped from 0
to 1 or vice versa. This is the method used in GPS.

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Basic Liner and Angle modulation
Schematics for GPS

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Amplitude Modulation GPS

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Frequency Modulation GPS

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GPS SIGNAL STRUCTURE

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Binary phase shift keying
(BPSK) modulation
 Binary phase shift keying (BPSK) is a simple digital signalling
which an RF carrier is either transmitted or with a 180º phase
shift over successive intervals in time depending on whether
a digital 0 or 1 is being conveyed.
 The data waveform alone is considered a baseband signal,
meaning that its frequency content is concentrated around
0 Hz rather than the carrier frequency.
 Modulation by the RF carrier centres the frequency content
of the signal about the carrier frequency, creating what is
known as a band pass signal

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(DSSS) Direct sequence spread
spectrum modulation
 Direct sequence spread spectrum is an
extension of BPSK or other phase shift
keyed modulation used in GPS. Mixes a
slow signal with fast pseudo-random
signal (FPR). The Signal still contains
original information but occupies much
more bandwidth.

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GPS Demodulation
Typically, there are two things that make locking onto
a GPS signal difficult. One, the phase and position of
the incoming direct-sequence spread spectrum
(DSSS) code is unknown. The receiver must test many
shifted versions of the DSSS code until it is able to bring
the receive signal into alignment with its local copy.
Second, the GPS satellites are traveling fast which
can shift the carrier frequency GPS receiver must first
replicate the PRN code that is transmitted by the SV
being acquired by the receiver; then it must shift the
phase of the replica code until it correlates with the
SV PRN code.

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Basic Demodulation Schematics
for GPS

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Application of system
 Global Positioning System (GPS)
 Network of satellites that orbit the earth at certain
degrees and distance from the surface of the earth.
 Generally consists of a transmitter and a receiver
 Using the concept called trilateration to pinpoint a
location.
 Usually used to pinpoint locations and timekeeping
situations.

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 Consists of two radio frequencies or called carrier frequencies.
 1575.42MHz (referred as LINK 1 or L1)
 1227.60MHz (referred as LINK 2 or L2)
 High frequencies are used between the satellites and receiver
to reduce the effect of the propagation signals at ionosphere.
 The distance of the signal determines by the travel times of
the signal.
 Each satellite has its own PRN code – to ensure the receiver
knows from which satellite the data or signal came from.

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 Consists of two ranging codes:
 is C/A code or coarse/acquisition code
 precision (P) code
 GPS also has the navigation message – contains detailed
information about the satellites position and network.
 Three different areas of navigation message:
 1) time and date – atomic clock with continuous time
scale.
 2) the ephemeris – orbital information so the receiver
can calculate the position of the satellite.
 3) almanac data - contains information and status
concerning all the satellites.

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Exposure of noise in GPS
 The accuracy of the GPS is affected when there are any
disruption that interferes with the transmission.
 Accuracy of GPS can be effected by factors like:
 Atmospheric effect - varied conditions of earth.
 Ionospheric effect – free electron produces from ultraviolet
rays from the sun influence the propagation of the signal.
 Multipath effect - the signal passes through obstacles before it
reaches the receiver.
 Satellite clock error - clock correction fields in the navigation
data message are sized caused error from the residual clock
error.
 Ephemeris error - estimation of the location of each satellites
are computed and uplinked to all the satellites.
 Relativity error – theory of relativity causing the time ticking
faster at the satellite compared to the earth. (gravitational
effect)

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Solution to noise
 The improvement have been made in the GPS and is
called Differential Global Positioning System (DGPS).
 DGPS uses a network of fixed, ground-based reference
stations.
 Difference between the positions indicated by the GPS
satellite systems and the known fixed positions.
 Use the distance instead of time travel to determine the
position of receiver.
 Error are deducted by comparing the actual distance of
the reference point and the distance calculated by the
satellite.
 Downsides – the accuracy and precision degrade as the
target point distance to the reference point increased.

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conclusion
 GPS is one of the crucial daily thing related to us nowadays
 GPS is a network of satellites that continuously transmit coded
information
 Makes it possible to precisely identify locations on earth by
measuring distance from the satellites
 GPS is also used for accurate time measurements to ensure the
accuracy of time related field can utilize the accuracy to their best
use.