1

GLOBAL POSITIONAL SYSTEM

MUHAMMAD SYUHAIMI BIN ANAS51218114210

MUHAMMAD AKMAL ALIF BIN MD ALI51218114255

2

What is GPS?

Global Positioning System (GPS) is a satellites based navigation

system made up of a network of 24 satellites.

3

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.

4

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.

5

Basic Liner and Angle modulation Schematics for GPS

6

Amplitude Modulation GPS

7

Frequency Modulation GPS

8

GPS SIGNAL STRUCTURE

9

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

10

(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.

11

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.

12

Basic Demodulation Schematics for GPS

13

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.

14

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.

15

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.

16

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)

17

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.

18

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.