LINKÖPINGS UNIVERSITY

Global Positioning System

TSKS03 Wireless system Hemambuja Chenna, 811117-6387

hemch623 2016-05-01

Table of Contents

ABSTRACT ..................................................................................................................................................... 3

ACRONYMS ................................................................................................................................................... 4

INTRODUCTION ............................................................................................................................................ 4

OVERVIEW OF GPS ....................................................................................................................................... 5

GPS COMMUNICATION ................................................................................................................................ 6

MODULATION ............................................................................................................................................... 7

CODING TECHNIQUES ................................................................................................................................... 8

CHANNEL IMPAIREMENT ............................................................................................................................. 9

GPS ERRORS ................................................................................................................................................ 10

SUMMARY .................................................................................................................................................. 11

REFERENCES ................................................................................................................................................ 12

ABSTRACT This report is documented for the course Wireless System TSKS03. This document gives

an overview of GPS system, modulation technique, coding technique, Channel

Impairment and errors in GPS system.

ACRONYMS GPS - Global Positioning System

DOD - Department of Defense

PRC - Pseudorandom code

BPSK - Binary Phase Shift Keying

QPSK - Quadrature Phase Shift Keying

DSSS - Direct sequence spread spectrum

CDMA - Code Division Multiple Access

FER - Forward error correction

PRN - Pseudorandom noise

INTRODUCTION The Global Positioning System, known as GPS, is based on a system of satellites which

support navigation in any place of Earth. GPS was initially developed by United States

Department of Defense (DOD) to meet the requirements of military. Later it was also

used for civilian applications. As of today it is a dual system that is being accessed by

both civilians and military [4, sec-1].

GPS provides information on timing and continuous positioning around the globe in all

weather conditions. Currently there is no limit to the number of users the GPS serves [4,

sec-1].

Fig.1 Global Positioning System [2]

GPS is based on 24 orbiting satellites around the earth. The exact position of an object is

determined by GPS receiver, based on the distance reach from four or more satellites as

shown in Fig.1 [2].

OVERVIEW OF GPS GPS is composed of three segments. They are satellite/space segment, ground/control

segment, and user segment as shown in Fig. 2. Space segment is a set of satellites that

are continuously orbiting and transmitting information and messages to the user.

Control segment keeps track of proper functioning and maintaining the satellites.

Control segment also gives regular updates to the satellites at least once in a day. User

segment is the segment used by the receiver which is called as GPS receiver [1, sec-3].

Fig.2. GPS segments [2].

GPS receiver is a radio processor that is good enough to resolve certain navigation

equations after dealing with the signal. The signal analyses the position, velocity and

time (PVT) of the end user [8].

Space segment transmitting information sends the signal in the form of navigation

message and codes. The receiver compares the signals received from at least four

satellites and calculate the time delay between satellite and the receiver [9].

The satellites, ground and receiver segments carry clocks. The satellite and receiver

clocks are stable and synchronized with one another as well as with ground clocks. The

receiver monitors the satellite clocks and their deviation from true time. This helps in

calculating the exact geographical co-ordinates [3]. Pseudorange measurement or

Carrier phase measurements are required for computing the co-ordinates. Pseudorange

or carrier phase is the distance or measure of range between the satellite and the

receiver [4, sec-2.5 & 2.6].

GPS COMMUNICATION Signals transmitted by GPS satellites encodes information about position of satellite,

clocks internal state and the strength of network [3]

GPS satellites provide radio signals at two carrier frequencies: 1575.42MHZ (L1 signal)

and 1227.60MHZ (L2 signal). The two digital (ranging) codes and a navigation message

modulates L1 and L2 signal carrier frequencies (refer Fig.3). Wavelengths for these

frequencies are 19cm and 24.4cm respectively [4, Sec-2.1].

Fig.3 (a) sinusoidal wave (b) digital code [4].

The two ranging codes that are transmitted by the Satellites are course/acquisition (or

C/A) code and Precise (or P) code [4, sec-2.1]. The C/A code is a set of 1023 binary digits

which repeats every millisecond. The C/A code has less precision, less complexity and is

freely accessed by civilian users. Every satellite is given a unique C/A code that helps the

receiver in identifying the satellite. The P code is a long sequence of binary digits which

repeats every 266 days. The signals are modulated using Binary Phase Shift Keying

(BPSK). The C/A code is modulated onto L1 signal only when the P code is modulated

onto L1 and L2 signals. The P code is more complicated, much faster and mostly used for

military purposes [4, sec-2.1].

Navigation message transmitted from GPS satellites has a data stream rate of 50bps.

Navigation message consists of 25 frames of 1500 bits each, divided into five 300 bits

sub frames as shown in Fig.4. Almanac data is system orbit specification. Ephemeris data

is exact system and clock correction [9].

Fig.4 Structure of navigation message frame [1, sec-4.4]

Navigation message a binary (modulo 2) is added to the code to travel from satellite to

receiver as shown in Fig.5 [5]. This message is also coded using BPSK [6, pg-15].

Fig.5 GPS Broadcasting signal [5].

Signal transmitted from satellites are encoded using Code Division Multiple Access

(CDMA). This enables the receiver to identify satellites from other satellites [3].

The Receiver first aims to acquire the signal in the form of PRN code, second to track the

signal as long as satellite is in use. Acquisition and tracking of the signal is obtained for

every satellite that is being used by the receiver .This process is done is a digital domain

by the receiver and it is a two dimensional (carrier & code)[5],[1, sec-5.1].

Code phase as well as carrier phase of the PRN code is shifted. Later this signal is

correlated to the original signal. Three correlators are frequently used one for carrier

tracking and other two for code tracking [1, sec-5.1].

MODULATION In BPSK modulation the RF Carrier is transmitted either phase shifted by 180 degrees or

as it is. BPSK signal can be visualized as an outcome of two waveforms. The two

waveforms are unmodulated RF carrier and Data waveform. For every subsequent

interval of Tb = 1/ Rb seconds, the value of data waveform has the value of either +1 or

−1. Here Tb is the bit rate and Rb is the data rate as shown in Fig. 6 [1, sec-4.2].

Fig.6 BPSK modulation [1] Fig.7 DSSS modulation [1]

Direct sequence spread spectrum (DSSS) is an extension of BPSK in GPS. DSSS

modulation in Fig.7 has a third component called spreading or pseudorandom noise

(PRN) waveform. PRN waveform has high symbol rate and is frequently periodic. PRN

waveform is referred to as PRN sequence or PRN code. Chip period Tc is the time

interval between transitions in spreading waveform. Chipping rate, Rc is the reciprocal

of Chip period. Post modulation, DSSS signal occupies a wide bandwidth by the PRN

waveform (Spread Spectrum) [1, sec-4.2]

The spread spectrum used here has three basic reasons for its use are

1. Phase inversion of the signal allows exact range in the receiver.

2. Signals can be sent at a same time and at same frequency by a number of

satellites.

3. DSSS contribute an important elimination of a category of interference [1, sec-

4.2].

CODING TECHNIQUES A signal that transmits through a noisy channel, is impossible to remain uncorrupted.

This effect is raised due to increase in technology today. A number of techniques have

been developed by researchers that can correct the errors without retransmission [9].

Channel coding is a process that proposes redundancies in the transmitted signal.

Coding comprise of adding a new bit to the data, which later is corrected by decoder at

the receiver end. Adding new bits have detriment of decreasing the data rate and

increasing the bandwidth of encoded signal [9].

Coding can be either error correction codes or error detection codes. Two error

correction coding techniques that are useful in GPS are Block Codes such as Hamming

codes and Convolution codes. Hamming code are Forward error correction (FEC) code

that are capable of reducing the errors through detection and correction methods.

Hamming codes can correct one bit error and detects two bit errors. Codes of length n

need to be encoded with n parity to assure certainty of information [9].

Convolution codes- In this bits are encoded by sending them through a finite state shift

register and output is obtained by adding a fixed set of previous bits. This is an

important coding technique in communication system [9].

CHANNEL IMPAIREMENT The three classes of Channel impairment that corrupts the conduct of satellite system

are Interference, multipath and ionosphere scintillation [1, sec-6.1].

Interference is caused when the receiver receives RF signals through the sources not in

the picture. Interference is in general unintentional. Interference is termed as Jamming

if intentional. Fig 8 shows the different types and sources of Interference [1, sec-6.2].

Class—Type Potential Sources

Wideband—band-limited Gaussian Intentional matched bandwidth noise jammers

Wideband—phase/frequency modulation

Television transmitters’ harmonics or near-band microwave link transmitters overcoming the front end filter of a GNSS receiver

Wideband—matched spectrum Intentional matched-spectrum jammers, spoofers, or nearby pseudolites

Wideband—pulse Any type of burst transmitters such as radar or ultrawideband (UWB)

Narrowband—phase/frequency modulation

Intentional chirp jammers or harmonics from an amplitude modulation (AM) radio station, citizens band (CB) radio, or amateur radio transmitter

Narrowband—swept continuous wave Intentional swept CW jammers or frequency modulation (FM) stations transmitters’ harmonics

Narrowband—continuous wave Intentional CW jammers or near-band unmodulated transmitter’s carriers

Fig.8 Types and sources of Interference [1, sec-6.2.1]

When the transmitted signal that is received after reflection or diffraction is referred as

multipath. A signal can reach the receiver in one direct path or multiple indirect paths..

Multipath alters the phase of received signal, open glitches in psuedorange and carrier

phase measurement which intern open errors in PVT. Excess attenuation of direct path

is caused when signal passes through a form is referred as shadowing as shown in Fig.9.

The power of a direct path is greater that the shadowed direct path [1, sec-6.3].

Fig.9 Multipath situation [1, sec-6.3]

Ionosphere scintillation a signal fading phenomenon occurs in Ionosphere layer in the

earth’s atmosphere. A particular signal in the absence of scintillation is given by

Here P is the power of received signal in watts, ω is the carrier frequency in

radians/second, s(t) is the normalized transmitted signal, and n(t) is noise. Scintillation

causes disturbance in amplitude and phase of received signal. A signal in the presence of

scintillation is given by

Where represent the amplitude fading caused due to scintillation. δϕ represents

phase change [1, sec-6.4].

GPS ERRORS Many types of random errors has effect on GPS measurement which can be categorized

as errors at transmitter, at receiver and errors during propagation (refer Fig.10). In

addition the geometric location of GPS satellites as viewed by receiver also effects the

computation of GPS location [4, sec-4].

Fig.10 GPS errors and biases [4, sec-4]

The satellite errors consists of Ephemeris (orbital) errors, satellite clock errors, and the

effect of selective availability [4, sec-4].

The receiver errors consists of multipath error, receiver clock errors, antenna phase

center variations and receiver noise [4, sec-4].

Ionosphere delay occurs in the ionosphere layer which is the top most layer on earth’s

surface. Ionosphere is dispersive in nature. Therefore a signal bends and changes in its

speed as it propagates through this layer. This creates measurement errors [4, sec-4.7].

Ionospheric delay is a major GPS error is corrected with the availability of two carrier

frequencies [4, sec-2.1]. Combining the observables of L1 and L2 reduces to a high

degree of accuracy [4, sec-4].

Using appropriate combinations of the GPS observables some of these errors and biases

can be reduced or eliminated [4, sec-4].

SUMMARY The Global Positioning System, known as GPS provides information on timing and

continuous positioning consists of 24 orbiting satellites around the earth. The segments

in system transmit and receiving signals in form of messages and codes.

The synchronized clocks in each segment and their deviation determines the

geographical co-ordinates. The two ranging codes and navigation message contains

information of the two carrier frequencies almanac data and ephemeris data.

CDMA is used in encoding the satellite signal. Acquisition and tracking process is

performed by the receiver. GPS satellites transmit radio signals using BPSK modulation.

DSSS an extension of BPSK occupies a wide bandwidth of PRN waveform. DSSS improves

the GPS system.

To correct errors a number of techniques have been developed. Codes that are useful

are hamming codes and convolution codes. Interference, multipath and ionosphere

scintillations reduces the accuracy of GPS system.

Using appropriate combinations of the GPS observables some of these errors and biases

can be reduced or eliminated

REFERENCES

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Elliott D. Kaplan (Author, Editor), Christopher Hegarty (Editor)

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Ahmed El-Rabbany

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