GNU Radio based open source software for signal processing. The Radio ... two custom made Low Noise Amplifier (LNA) with a gain of 11dB and 26 dB. USRP-.
GLOBAL NAVIGATION SATELLITE SYSTEM SOFTWARE DEFINED RADIO
by M Hassan Sajjad Malik Muhammad Aamir Malik
Supervisor Internal Dr Umar Iqbal Bhatti Ms. Salma Zaineb Farooq External Dr Muhammad Iqbal Dr Saqib Ali
Department of Electrical Engineering Institute of Space Technology, Islamabad 2014
2014
GLOBAL NAVIGATION SATELLITE SYSTEM SOFTWARE DEFINED RADIO
ii
GLOBAL NAVIGATION SATELLITE SYSTEM SOFTWARE DEFINED RADIO
A thesis submitted to the Institute of Space Technology in partial fulfillment of the requirements for the degree of Bachelor of Science in Communication System Engineering
by M Hassan Sajjad Malik Muhammad Aamir Malik
Supervisor Internal Dr Umar Iqbal Bhatti Ms. Salma Zaineb Farooq External Dr Muhammad Iqbal Dr Saqib Ali
Department of Electrical Engineering Institute of Space Technology, Islamabad 2014 Institute of Space Technology Department of Electrical Engineering iii
GLOBAL NAVIGATION SATELLITE SYSTEM SOFTWARE DEFINED RADIO
by M Hassan Sajjad Malik Muhammad Aamir Malik APPROVAL BY BOARD OF EXAMINERS
------------------------------------
Dr Umar Iqbal Bhatti
------------------------------------
Ms Salma Zaineb Farooq
iv
Certificate This is to certify that research work described in this project is the original work of authors and has been carried out under my direct supervision. I have personally gone through all the data/results/materials reported in the manuscript and certify their correctness/authenticity. I further certify that material included in this thesis is not plagiarized and has not been used in part or full in a manuscript already submitted or in the process of submission in partial/complete fulfillment of the award of any other degree from any institution. I also certify that thesis has been prepared under my supervision according to the prescribed format and I endorse its evaluation for the award of Bachelor of Science in Communication Systems Engineering degree through the official procedures of the Institute.
------------------------------------
Dr Umar Iqbal Bhatti
------------------------------------
Ms. Salma Zaineb Farooq
v
Copyright © 2014 This document is jointly copyrighted by the author(s) and the Institute of Space Technology (IST). Both author(s) and IST can use, publish or reproduce this document in any form. Under the copyright law, no part of this document can be reproduced by anyone, except copyright holders, without the permission of author(s).
vi
Acknowledgement We want to thank Carles Fernandez and Javier Arribas for their help regarding the GNSS software; Dr Muhammad Iqbal and Dr Saqib Ali for their valuable support and professional guidance to pursue this project. We would also like to thank Dr Umar Iqbal Bhatti, Ms Salma Zaineb Farooq and Dr Moazam Maqsood for their guidance, encouragement and assistance in keeping our progress on schedule. We would extend our thanks to Tariq Bilal, Yasir Mahmood and Farrukh for their cooperation with us.
vii
DEDICATION
To our parents
viii
Abstract This report describes the implementation of a Global Navigation Satellite System (GNSS) receiver based on the Software Defined Radio (SDR) approach. In comparison to a conventional GNSS hardware receiver, using an SDR offers more flexibility for signal processing and analysis at intermediate stages. Moreover, a software receiver can also be configured as a multi-constellation GNSS receiver with slight modifications. The proposed implementation uses the USRP/ DVT-B USB Dongle as the front end and a GNU Radio based open source software for signal processing. The Radio Frequency (RF) front end has an active Global Positioning System (GPS) antenna with a gain of 30 dB, a custom made Bias-Tee with an insertion loss of 1.14dB (to power up the active antenna), two custom made Low Noise Amplifier (LNA) with a gain of 11dB and 26 dB. USRPN210 is paired with WBX daughterboard for GPS signal reception. The operating system used for signal processing is Ubuntu 12.10 (32 bit) on Corei5 System. GNSS satellites Acquisition, Tracking and Navigation Solution computation are performed on the PC using GNU-Radio based open source software. The Position fix for GNSS Satellites (GPS and Galileo) is obtained in real-time from GPS and off line from Galileo; the signal is analyzed at various stages of signal processing. The position fix obtained is analyzed by importing the KML file into Google Earth. The real time signal acquisition results are analyzed and presented. The receiver is tested to acquire six satellites simultaneously in real-time in order to get very precise position fix. This receiver can be re-configured to obtain any application specific parameters when needed.
ix
Table of Contents Certificate ............................................................................................................................ v Acknowledgement ............................................................................................................ vii Abstract .............................................................................................................................. ix Table of Contents ............................................................................................................. viii List of Figures .................................................................................................................. xiii List of Tables ................................................................................................................... xvi List of Abbreviations ...................................................................................................... xvii 1
2
INTRODUCTION ....................................................................................................... 1 1.1
Background .......................................................................................................... 2
1.2
Motivation ............................................................................................................ 3
1.3
Objectives ............................................................................................................. 4
1.4
Thesis Outline ...................................................................................................... 4
GLOBAL POSITIONING SYSTEM .......................................................................... 6 2.1
Introduction .......................................................................................................... 6
2.2
Signal Frequency and Signal Components........................................................... 7
2.3
GPS Signal Generation......................................................................................... 7
2.3.1
Coarse Acquisition Code Generation.......................................................... 10
2.3.2
Precision Code Generation ......................................................................... 11 viii
2.4
3
4
GPS Segments .................................................................................................... 11
2.4.1
Space Segment ............................................................................................ 11
2.4.2
Control Segment ......................................................................................... 12
2.4.3
User Segment .............................................................................................. 13
GALILEO .................................................................................................................. 14 3.1
Introduction ........................................................................................................ 14
3.2
Signal Frequency Band and Carrier Frequency ................................................. 15
3.3
GALILEO Signals .............................................................................................. 16
3.3.1
E1 Signal ..................................................................................................... 16
3.3.2
E6 Signal ..................................................................................................... 17
3.3.3
E5a Signal ................................................................................................... 17
3.3.4
E5b Signal ................................................................................................... 17
3.4
E1 Signal Generation and Signal Modulation Scheme ...................................... 17
3.5
Galileo Segment ................................................................................................. 19
3.5.1
Space Segment ............................................................................................ 19
3.5.2
Ground Segment ......................................................................................... 19
3.5.3
User Segment .............................................................................................. 20
ACQUISITION ......................................................................................................... 21 4.1
Introduction ........................................................................................................ 21
4.1.1
Serial Search Acquisition ............................................................................ 21 ix
4.1.2
Parallel Frequency Space Search Acquisition ............................................ 23
4.1.3
Parallel Code Phase Search Acquisition ..................................................... 24
4.2
5
4.2.1
GPS ............................................................................................................. 25
4.2.2
Galileo ......................................................................................................... 27
TRACKING............................................................................................................... 29 5.1
Introduction ........................................................................................................ 29
5.2
Carrier Tracking ................................................................................................. 29
5.2.1
Phase Lock Loop......................................................................................... 30
5.2.2
Delay Lock Loop ........................................................................................ 32
5.2.3
Complete Tracking of GPS ......................................................................... 34
5.3
6
Results ................................................................................................................ 25
Results ................................................................................................................ 36
5.3.1
GPS ............................................................................................................. 36
5.3.2
Galileo ......................................................................................................... 40
NAVIGATION .......................................................................................................... 44 6.1
GPS..................................................................................................................... 44
6.1.1
Introduction ................................................................................................. 44
6.1.2
Message Content ......................................................................................... 45
6.2
Galileo ................................................................................................................ 53
6.2.1
Introduction ................................................................................................. 53 x
7
6.2.2
Page Layout ................................................................................................ 53
6.2.3
Nominal Mode Page Layout and Word Information .................................. 57
6.3
Navigation Data Identification ........................................................................... 57
6.4
Navigation Data Decoding ................................................................................. 58
6.5
Navigation Data Extraction ................................................................................ 58
6.6
Results ................................................................................................................ 58
6.6.1
GPS ............................................................................................................. 58
6.6.2
Galileo ......................................................................................................... 62
Hardware ................................................................................................................... 63 7.1
Introduction ........................................................................................................ 63
7.2
Active GPS Antenna .......................................................................................... 63
7.3
Bias Tee .............................................................................................................. 64
7.3.1
Lumped Element Bias Tee Model .............................................................. 66
7.3.2
TCBT14+ IC Bias Tee Model .................................................................... 67
7.4
Low Noise Amplifier Design ............................................................................. 68
7.4.1
Custom Made Low Noise Amplifier First Model ....................................... 68
7.4.2
Commercial Low Noise Amplifier ............................................................. 71
7.4.3
Custom Made Low Noise Amplifier Second Model .................................. 73
7.5
DVB-T USB Dongle .......................................................................................... 76
7.5.1
Testing Result ............................................................................................. 77 xi
7.5.2
Data Reception ............................................................................................ 78
7.5.3
DVB-T USB Dongle Library Installation ................................................... 79
7.5.4
Calibration of Dongle without TCXO Crystal ............................................ 80
7.5.5
TCXO Modified DVB-T Dongle ................................................................ 81
7.6
8
USRP N210 and WBX Card .............................................................................. 83
7.6.1
Interface ...................................................................................................... 84
7.6.2
Testing Results ............................................................................................ 86
SOFTWARE .............................................................................................................. 87 8.1
Introduction ........................................................................................................ 87
8.2
Ubuntu Installation Alongside with Windows ................................................... 87
8.3
Software Environment Setup ............................................................................. 89
8.3.1
Installation Processes .................................................................................. 89
8.3.2
Script Fetching and running ........................................................................ 89
CONCLUSION AND FUTURE WORK ......................................................................... 91 CONCLUSION ............................................................................................................. 91 FUTURE WORK .......................................................................................................... 91 REFERENCES ................................................................................................................. 92
xii
List of Figures FIG. 1.1 SYSTEM FLOW DIAGRAM ........................................................................................ 2 FIG. 2.1 GPS SIGNAL GENERATION ..................................................................................... 9 FIG. 2.2 COARSE ACQUISITION CODE................................................................................. 10 FIG. 3.1 GALILEO SIGNAL FREQUENCY BAND ................................................................... 16 FIG. 3.2 GENERIC VIEW OF E1 SIGNAL GENERATION ......................................................... 18 FIG. 4.1 BLOCK DIAGRAM OF SERIAL SEARCH ACQUISITION ............................................. 22 FIG. 4.2 BLOCK DIAGRAM OF PARALLEL FREQUENCY SPACE SEARCH .............................. 24 FIG. 4.3 BLOCK DIAGRAM OF PARALLEL CODE PHASE SEARCH ACQUISITION .................. 25 FIG. 5.1 BASIC DEMODULATION SCHEME FOR NAVIGATION DATA .................................... 29 FIG. 5.2 BASIC GPS RECEIVER TRACKING LOOP BLOCK DIAGRAM ................................... 30 FIG. 5.3 BLOCK DIAGRAM OF COSTAS LOOP ...................................................................... 31 FIG. 5.4 BASIC CODE TRACKING LOOP BLOCK DIAGRAM OF GPS ..................................... 32 FIG. 5.5 BASIC CODE TRACKING LOOP BLOCK DIAGRAM OF GALILEO .............................. 33 FIG. 5.6 BLOCK DIAGRAM OF COMBINE PLL AND DLL FOR GPS...................................... 34 FIG. 5.7 BLOCK DIAGRAM OF COMBINE PLL AND DLL FOR GALILEO............................... 35 FIG. 5.8 COARSE ACQUISITION (C/A) CODE FREQUENCY OVER TIME FOR GPS .................. 36 FIG. 5.9 CARRIER FREQUENCY DOPPLER SHIFT FOR GPS ................................................... 37 FIG. 5.10 CORRELATION RESULTS OF EARLY, LATE AND PROMPT SIGNALS OF GPS .......... 38 FIG. 5.11 NAVIGATION MESSAGE BITS FOR GPS ................................................................ 39 FIG. 5.12 CARRIER TO NOISE RATION OF GPS ................................................................... 39 FIG. 5.13 COARSE ACQUISITION (C/A) CODE FREQUENCY OVER TIME FOR GALILEO ......... 40 FIG. 5.14 CARRIER FREQUENCY DOPPLER SHIFT FOR GALILEO .......................................... 41 xiii
FIG. 5.15 CORRELATION RESULTS OF EARLY, LATE AND PROMPT SIGNALS OF GALILEO ... 42 FIG. 5.16 NAVIGATION MESSAGE BITS FOR GALILEO ......................................................... 42 FIG. 5.17 CARRIER TO NOISE RATION OF GALILEO ............................................................ 43 FIG. 6.1 PSEUDORANGE OF PRN1 OF GPS ......................................................................... 59 FIG. 6.2 GPS RECEIVER POSITION IN GOOGLE EARTH ....................................................... 60 FIG. 6.3 NAVIGATION SOLUTION WITH GPS ANALYSIS FROM “U-CENTER V8.11™” .......... 61 FIG. 6.4 PSEUDORANGE OF PRN12 OF GALILEO ................................................................ 62 FIG. 7.1 BIAS TEE CIRCUIT DIAGRAM ................................................................................ 65 FIG. 7.2 LUMPED ELEMENT MODEL OF BIAS TEE .............................................................. 66 FIG. 7.3 TCBT14+ IC OF BIAS TEE MODEL ....................................................................... 67 FIG. 7.4 INPUT RETURN LOSS OF TCBT14+ IC OF BIAS TEE MODEL ................................ 67 FIG. 7.5 FORWARD LOSS OF TCBT14+ IC OF BIAS TEE MODEL ........................................ 68 FIG. 7.6 LNA FIRST DESIGN CIRCUIT DIAGRAM ................................................................ 69 FIG. 7.7 LNA FIRST MODEL .............................................................................................. 69 FIG. 7.8 INPUT RETURN LOSS OF FIRST LNA MODEL ........................................................ 70 FIG. 7.9 FORWARD GAIN OF FIRST LNA MODEL ............................................................... 70 FIG. 7.10 NEW DESIGN PROPOSED FOR CUSTOM MADE LNA OF FIRST MODEL ................ 71 FIG. 7.11 COMMERCIAL LNA ............................................................................................ 71 FIG. 7.12 INPUT RETURN LOSS OF COMMERCIAL LNA ...................................................... 72 FIG. 7.13 FORWARD GAIN OF COMMERCIAL LNA ............................................................. 72 FIG. 7.14 CUSTOM MADE LNA SECOND DESIGN CIRCUIT MODEL .................................... 73 FIG. 7.15 CUSTOM MADE LNA SECOND DESIGN IMPLEMENTED MODEL .......................... 73 FIG. 7.16 INPUT RETURN LOSS OF CUSTOM MADE LNA SECOND MODEL ......................... 74 xiv
FIG. 7.17 OUTPUT RETURN LOSS OF CUSTOM MADE LNA SECOND MODEL ..................... 74 FIG. 7.18 FORWARD GAIN OF CUSTOM MADE LNA SECOND MODEL ................................ 75 FIG. 7.19 BACKWARD GAIN OF CUSTOM MADE LNA SECOND MODEL ............................. 75 FIG. 7.20 DVB-T USB DONGLE ........................................................................................ 76 FIG. 7.21 DVB-T DONGLE INTERNAL MODEL ................................................................... 76 FIG. 7.22 FM SIGNAL TESTING RESULTS OF DVB-T DONGLE ........................................... 77 FIG. 7.23 DIRECT METHOD OF DATA RECEPTION IN DVB-T DONGLE ............................... 78 FIG. 7.24 TCP LINK ESTABLISHMENT OF DATA RECEPTION IN DVB-T DONGLE............... 78 FIG. 7.25 DATA RECEPTION IN MATLAB VIA TCP LINK .................................................. 79 FIG. 7.26 CALIBRATION RESULT OF DONGLE WITHOUT TCXO CRYSTAL .......................... 80 FIG. 7.27 CALIBRATION RESULT OF DONGLE WITH TCXO CRYSTAL ................................ 81 FIG. 7.28 TCXO MODIFIED DONGLE HEATING PROBLEM SOLUTION ................................ 82 FIG. 7.29 USRP N210 ........................................................................................................ 83 FIG. 7.30 WBX CARD ........................................................................................................ 83 FIG. 7.31 EDITING WIRED CONNECTION ............................................................................ 84 FIG. 7.32 ERROR IN USRP N210 WHEN IMAGE NOT BURN................................................. 85 FIG. 7.33 USRP IMAGE BURNER ........................................................................................ 85 FIG. 7.34 PSD OF FM SIGNAL ............................................................................................ 86 FIG. 8.1 OPERATING SYSTEM: WINDOWS PARTITION TABLE ............................................. 87 FIG. 8.2 OPERATING SYSTEM: UBUNTU PARTITION TABLE ................................................ 88 FIG. 8.3 BOOT SOURCE SELECTION PANEL......................................................................... 88 FIG. 8.4 GNU-RADIO ERROR IN INSTALLATION SCRIPT ...................................................... 89 FIG. 8.5 GNU-RADIO CORRECTED INSTALLATION SCRIPT .................................................. 90 xv
List of Tables TABLE 2.1 EXCLUSIVE OR OPERATION OUTPUT.................................................................. 8 TABLE 3.1 GALILEO FREQUENCIES .................................................................................... 15 TABLE 6.1 GPS STRUCTURE OF NAVIGATION DATA ......................................................... 45 TABLE 6.2 SUB FRAME ID CODE ........................................................................................ 46 TABLE 6.3 EPHEMERIS PARAMETERS ................................................................................. 48 TABLE 6.4 CLOCK DATA REFERENCE TIME ....................................................................... 50 TABLE 6.5 STEP BY STEP PROCESS OF POSITION COMPUTATION ........................................ 51 TABLE 6.6 NOMINAL PAGE WITH BITS (ODD PAGE) .......................................................... 54 TABLE 6.7 NOMINAL PAGE WITH BITS (EVEN PAGE) ......................................................... 54 TABLE 6.8 ALERT PAGE WITH BITS (ODD PAGE) ............................................................... 55
xvi
List of Abbreviations AltBOC
Alternate Binary Offset Carrier
AS
Anti-spoofing
BOC
Binary Offset Carrier
BPSK
Binary Phase Shift Keying
C/A
Coarse Acquisition
CDMA
Code Division Multiple Access
CRC
Cyclic Redundancy Check
CS
Control Segment
DLL
Delay Lock Loop
DFT
Discrete Fourier Transform
E
Early
FEC
Forward Error Correction
FLL
Frequency Lock Loop
GNSS
Global Navigation Satellite System
GPS
Global Positioning System
GST
Galileo System Time
HOW
Handover Word.
IF
Intermediate Frequency
L
Late
LNA
Low Noise Amplifier
NCO
Numerically Controlled Oscillator
P
Prompt xvii
PCPSA
Parallel Code Phase Search Acquisition
PFSSA
Parallel Frequency Space Search Acquisition
PLL
Phase Lock Loop
PRN
Pseudo Random Noise
PSD
Power Spectral Density
PVT
Position Velocity Time
PyBombs
Python Build overlay managed bundle system
RF
Radio Frequency
RLM
Return Link Message
SA
Selective Availability
SS
Space Segment
SSA
Serial Search Acquisition
SV
Space Vehicle
TLM
Telemetry word
TOW
Time of Week
UAV
Unmanned Aerial Vehicle
US
User Segment
USRP
Universal Software Radio Peripheral
UTC
Universal Time Coordinated
VE
Very Early
VL
Very Late
xviii
1
INTRODUCTION
Global Navigation Satellite System is defined as a satellite system that provides autonomous Geo-spatial position with global coverage. The specific electronic receiver is used to compute navigation solution. The receiver computes navigation solution, which is typically used in scientific experiments and everyday life. GPS, Galileo, GLONASS, Beidou, are globally operating GNSS. All satellite system consists of the following segments: a) Space Segment b) Control Segment c) User Segment These three segments are nearly alike in all global navigation satellite technology. GPS is the only of the operational GNSS available. However the remaining three GNSS systems might be operational in next few years. This report describes the implementation of GNSS SDR for GPS and Galileo at L1 band. The SDR is implemented on USRP/DVT-B USB Dongle. A USRP (Universal Software Radio Peripheral) is a general purpose SDR (Software Defined Radio), available in different variants for different application requirements. This implementation uses USRPN210 with WBX daughter board. It provides 40MHz bandwidth and can be modified in firmware to give 50MHz bandwidth to cover all GNSS signals at the L1 band. A DVT-B USB Dongle is a device used by Europeans for satellite T.V coverage, but it is also capable of the reception of GPS and Galileo signals. It has a R820-T RF tuner and has the
1
bandwidth of 22 to 1700 MHz to cover all GNSS signals at L1 band. The custom made Low Noise Amplifier (LNA), custom made Bias-Tee and an active GPS antenna is used for GPS signal reception. The System Flow Diagram is shown in Fig 1.1.
Fig. 1.1 System Flow Diagram 1.1
Background
Knowing the position and location has always remained a source of keen interest for human beings. In the past, humans explored star constellations to determine their position. With the advent of industrial revolution, compass and other hand held instruments were being used to determine the direction and position. With the further advancement of technology, humans utilized the ancient concept of stars constellations and developed their own navigation satellite constellation for timing and position information for the entire globe [1].
2
Up till now industry has made much advancement in communications and navigation fields, but all those solutions were provided on very large scale integrated chips. During the past decade Software Defined Radio (SDR) technology has emerged and gained widespread popularity due to its capabilities like re-configurability and flexibility. SDR has made it possible to design a single system to perform multiple jobs at multiple frequencies. Development of more than one Global Navigation Satellite Systems (GNSS) and inclusion of more advanced signals for existing navigation systems raised the need of a reconfigurable receiver capable of covering all current and future navigational signals [2]. SDR solved this problem and provided interoperability between different GNSS systems for more accurate and reliable Position, Velocity and Time (PVT) estimates. SDR enables to simulate different scenarios and environmental conditions for the development of GNSS receivers for a specific application. GNSS SDR‟s are also being used for education and research purposes for the development of new applications. 1.2
Motivation
Pakistan currently does not have its own Satellite Navigation System which is a must in modern warfare and civil technologies. India is developing its own regional satellite navigation system. To complete the neighbor in this aspect of technology, Pakistan should also have its own satellite navigation system. However due to economic conditions of the country it is not possible to develop Pakistan‟s own satellite navigation system. Pakistan‟s Space and Upper Atmosphere Research Commission (SUPARCO) has collaborated with China to get access to Beidou system. To fully utilize the potential of any navigation systems, its receiver development should be promoted in Academia and 3
Industry. We selected this project to develop a Software defined GNSS Receiver which can help in education and application development in Academia and Industry. We also planned to include all Navigation Satellite System‟s capability in this software to make it more capable and fully utilize the potential of the existing Navigation Satellite systems. 1.3
Objectives
The objectives of the thesis are as follows: a) Choose the best method for acquisition among the three acquisition method i.e. SSA, PFSSA and PCPSA. The implemented algorithm of the best chosen method must be tested in real time for both GPS and Galileo. b) In depth study of carrier and code tracking loop. The implemented algorithms must be tested in real time for both GPS and Galileo. c) In detail study of navigation message of GPS and Galileo. Then test the implemented algorithms of navigation message identification, decoding and position computation for both GPS and Galileo. d) Design and implementation of bias tee and LNA for L1 band. 1.4
Thesis Outline
This thesis is organized as follows: Section 2 gives an overview of GPS. This chapter includes the GPS signal frequency, components, signal generation of C/A and P (Y) code and three GPS system segments i.e. SS, CS and US.
4
Section 3 gives an overview of Galileo system. This chapter includes the Galileo signal frequency band, carrier frequency, overview of Galileo E1, E6, E5a and E5b signal, E1 signal generation and three Galileo system segments i.e. SS, CS and US. Section 4 gives comparison of the three acquisition methods i.e. SSA, PFSSA and PCPSA. The best method is chosen. The chosen method is tested in real time; its experimental results for both GPS and Galileo are given in the last section of the chapter. Section 5 proposes carrier and code tracking algorithms. This chapter includes as overview of PLL and DLL. The algorithms are tested in real time; its experimental results for both GPS and Galileo are given at the end of the chapter Section 6 discusses in detail navigation message of both GPS and Galileo. This chapter also includes the navigation data identification, decoding and extraction algorithms. The algorithms are tested in real time for GPS and off line for Galileo; its experimental results for both GPS and Galileo are given at the end of the chapter. Section 7 discusses the components of RF front end i.e. active GPS car antenna, custom made Bias Tee, custom made LNA, USRP N210, DVT-B USB Dongle. It also describes the problems in Bias Tee and custom made LNA design along with the solution. Section 8 discusses the software installation and problem faced in software installation along with the solution. Section 9 finally draws conclusion.
5
2 2.1
GLOBAL POSITIONING SYSTEM
Introduction
GPS has been established by the United States Department of Defense. Presently, GPS is fully operational. This system offers accurate, endless, global; position, velocity and time information to the customs with GPS receiver. At least, four GPS satellite signals are used to compute position, velocity and time information to the users. Their main components are as follows: a) Space Segment: It consists of the space vehicles (satellites). b) Control Segment: It consists of the control systems of monitoring, tracking and control stations (including antennae) located in different regions of the world. c) User Segment: It consists of GPS receivers and the user community. The generated signal has a fundamental frequency
=10.23 MHz There are two carrier
signal L1 (
1575.42MHz) and L2 (
1227.60MHz) that are generated by an integral
multiple of
. The navigation message and ranging code are also broadcast on these two
frequencies. These signals are modulated by bi-phase modulation. The Coarse Acquisition code and Precision code are used for satellite clock related measurements and are categorized by unique PRN sequences. It uses CDMA technology to distinguish between the signals from different satellites. The navigation message is 750 seconds long have 1500 bits, 30 seconds long 25 frames and every frame is subdivided into 5 subframes each sub-frame is 6 seconds long have 300 bits in each. First three sub-frames are required for the navigation solution.
6
2.2
Signal Frequency and Signal Components
GPS signals are transmitted at two frequencies called L1 ( (
=1575.42MHz) and L2
=1227.60MHz). Both frequencies are derived from a common frequency of =10.23MHz, and obtained from
=154× =1575.42MHz,
=120× =1227.60MHz.
The signal is composed of three parts a) Carrier: Carrier wave of frequencies
and
.
b) Navigation data: It contains information regarding satellite orbits with a bit rate of 50bps (T=20ms) c) Satellite codes: Each satellite has 2 unique codes I.
Coarse Acquisition (C/A) code: It consists of 1023 chips (chip=bit) have a chipping rate of 1.023MHz and is repeated every ms. It is only modulated on L1 signal. In the sub-section 2.3.1 a block diagram is presented for generating of C/A code generation.
II.
Precision (P (Y)) code: It consists of 2.35*
chips and repeats after
every GPS week with a chipping rate of 10.23MHz. GPS week starts at Saturday/Sunday midnight. It is modulated on both two frequency signals L1 and L2. In the next sub-section a block diagram is presented for generation of GPS signal. 2.3
GPS Signal Generation
Complete GPS signal generation is shown in Fig 2.1 [2]. The block diagram is read from left. The main clock signal frequency
=10.23MHz is on left-bottom corner, which is
then multiplied by 120 and 154 to generate
and 7
respectively. At the bottom left, a
limiter is used to stabilize the main clock
before supplying it to the Precision code
generator and the coarse acquisition code generator, before supplying it to coarse acquisition generator it is divided by 10. At a bottom data information is fed to the data generator which generates data at the rate of 50bps, which is synchronized with the Code generator by X1 signal coming from precision code generator. After generation codes are merged with the data by modulo two operations and fed to the BPSK modulators for and
. The exclusive OR property of multiplication is shown in Table 2.1 . Table 2.1 Exclusive OR Operation Output
Precision code plus data is fed to the both BPSK generators for L1 and L2 frequencies while coarse acquisition code plus data is only fed to the BPSK generator for L1 frequency. At
BPSK modulator for C/A plus data code is at 90˚ out of phase from
BPSK modulator for P(Y) code. These two signals from BPSK modulators are then added together after -3dB attenuation of P(Y) code modulator signal, to form the complete
signal. The Signal from BPSK modulator at L2 frequency is set at -6dB
level to form complete signal at
.
The GPS has code size of 1023 chips, chipping rate of 1.023MHz (1 milli-second time period), data rate of 50Hz. 8
Fig. 2.1 GPS Signal Generation 9
2.3.1
Coarse Acquisition Code Generation
Coarse acquisition codes are referred as to Gold codes and Precision codes or simply PRN sequences. This code is present only on L1 frequency signal. Coarse acquisition code is generated by two selected Linear Feedback Shift Registers with a sequence length of N=
1 (here n=10). The sequence is comprised of 512 ones and 511 zeros
distributed randomly but complete series of chips is deterministic. The two LFSR are named as G1 and G2 having 10 cells each and generating two series of length 1023, these two series are modulo two added to form one coarse acquisition code of length 1023. After every 1023rd chip the two LFSRs are rearranged with all ones to start the code again. Feedback polynomial of G1 is ( )
i.e. 3rd and 10th cell state is
modulo 2 added and feedback to the register. Output chip is taken from the last cell at each clock. Similarly G2 has the feedback polynomial ( )
; G2
register supplies its output by modulo2 adding two unique cell states for each satellite at each clock. C/A code signal generation system is shown in Fig 2.2 [2].
Fig. 2.2 Coarse Acquisition Code 10
As depicted in Fig 2.2, the delayed version of G2 sequence from phase selector is modulo two added to give C/A code after each ms, the shift registers are then reset in synchronization with x1 from P(Y) code after every 1023rd chip. Various unique tap pairs (delays) from G2 generate 36 distinctive PRN Coarse Acquisition codes and any two PRNs have low cross-correlation i.e. are almost orthogonal. In fact, there are 37 PRNs but PRN no. 34 and 37 are alike. From the set of the leading 32 codes are allotted to the satellites and reused on the destruction of former satellites and launch of new satellites. Codes 33-37 are reserved for other purposes i.e. Ground transmitters. 2.3.2
Precision Code Generation
Precision is generated in the same way as C/ A code but in this case four LFSRs are used each having length 12 cells, two LFSRs combine to give X1 signal of length 15345000 chips which repeats every 1.5 seconds. Other two LFSRs are used to give X2 sequence of length 15345037 chips. X1 and X2 are joined with 37 dissimilar delays on X2 to give 37 dissimilar 1-week sections of precision code. From first 32 sections, each one is assigned to a satellite. 2.4
GPS Segments
GPS consists of three major segments are as follows: 2.4.1
Space Segment
Space segment has the following two major characteristics.
First is the arrangement of satellites in terms of tracks and placement within the orbits.
Second is the features of satellites that occupy every orbital slot.
11
The GPS plan initially for twenty four satellites. Initially it was planned for three approximately circular planes each with eight satellites later on this was amended to six orbital planes each with four satellites. The six circular orbit planes have 55° inclination and divided by 60° right ascension of the ascending node [1]. The orbital period is half a sidereal day so that satellites pass over the same location every day. Currently there are 32 GPS satellites in the constellation further addition of the satellites increase the receiver based calculations. The satellites constellation helps the user in range measurements. A satellite transmits a PRN-coded signal from which the ranging measurements are completed. A satellite has vehicle control subsystems and payloads. The payloads have a navigation data used to compute navigation solution and Nuclear Detonation Detection Systems. The vehicle control subsystems do functions such as retaining satellite antenna attitude towards the Earth and solar panels pointing attitude towards the Sun. 2.4.2
Control Segment
The control segment has:
A master station control
A substitute master station control
Four ground antennas
Six monitor stations.
It is responsible for maintaining the proper functioning of satellites, positioning the satellites in accurate orbits, monitoring health and status. Control System updates the navigation data of each satellite once per day i.e. Satellite‟s clock, almanac and 12
ephemeris data. The satellite can keep the navigation message data for least 14 days to an extreme of 210-day, update the data once per two week in intervals of 4 or 6 hours but if the data cannot be updated more than two weeks so update in an intervals greater than 6 hours [1]. The almanac and ephemeris data both are used to compute navigation solution. The navigation solution will be got by ephemeris data, but almanac is used to increase the precision of the position computed because the almanac consists of seven of the 15 ephemeris orbital parameters. Almanac data is used to guess the estimated satellite position and assist in satellite signal acquisition. CS also decides satellite anomalies, controls SA and AS and collect a pseudo range and carrier phase measurements at distant monitor stations to control satellite clock corrections, ephemeris data and almanac. 2.4.3
User Segment
It comprises of the user equipped with GPS equipment. All set of apparatus is usually called GPS receiver, which is used to determine the position, velocity and time information of the user by processing the L band signals conveyed from the satellites.
13
3 3.1
GALILEO
Introduction
GALILEO is being developed by the European Union. It will offer independent navigation and positioning facilities when fully operational. It is designed to provide service under all conditions and will notify operators within seconds about the failure of the satellites. The three main components are: a) Space Segment: It will comprise of 30 Medium Earth Orbiting satellites distributed evenly and regularly over three orbital planes and altitude of 23,616 km and inclination is 56˚ east [1]. b) Ground Segment: The main function of GS is handling the constellation of satellites; monitoring main functions for instance determining the orbit of the satellites. c) User Segment: It comprises of a user receiver having capabilities to provide different GALILEO services. It provides several navigation signals in right-hand circular polarization form in frequency ranges of 1164- 1215 MHz (E5a and E5b), 1260-1300 MHz (E6) and 15591592 MHz (E1). All satellites of this system share the same nominal frequency. It uses Code Division Multiple Access technique to distinguish between signals from different satellites. The PRN length chip is 4092. It uses binary offset carrier as a modulation scheme [1]. The five planned services are as follows:
14
a) An Open Service (OS) which deals timing, positioning and navigation signal that can be retrieved free. b) A Commercial Service (CS) that will offer a high precision positioning facility on payment. c) A Safety of life (SOL) signal that contain generally the validity and Signal in Space Precision data. d) The Public Regulated Service (PRS) is used strictly by government authorized users. e) Support of Search and Rescue (SAR). 3.2
Signal Frequency Band and Carrier Frequency
Galileo signals are transmitted at six frequencies and are derived from fundamental frequency
=10.23MHz shown in Table 3.1. It has three carriers E5, E6 and E1 signal.
The Galileo Signals are transferred in four frequency bands (E5a, E5b, E6 and E1) shown in Fig 3.1. They offer a wide band for the communication of the Signals [5]. Table 3.1 Galileo Frequencies
15
Fig. 3.1 Galileo Signal Frequency Band 3.3
GALILEO Signals
The signals sent by the Galileo satellites share the same frequency band that is possible due to the efficiency of the CDMA technique. Spread Spectrum signals are transmitted including different ranging codes per signal component, per signal, per frequency and per Galileo satellites [5]. Galileo Navigation signal details are as follows. 3.3.1
E1 Signal
This signal is transmitted in L1 band comprising two channels E2-B and E3-C. The signal supports four services OS, CS, SOL and PRS. E1-B is open access signal, its navigation data and ranging codes are not encrypted with a data rate of 250 bps. E1-C is a pilot channel. E1 signal is generated with CBOC modulation.
16
3.3.2
E6 Signal
This signal is transmitted in E6 band comprising two channels a data E6-B channel and a pilot E6-C channel. It is commercial access signal dedicated for commercial service. Its navigation data and ranging code are encrypted with a data rate 500 bps. E6 signal is generated by BOC modulation. 3.3.3
E5a Signal
This signal is transferred in E5 band comprising two channels i.e. data and pilot. It is an open access signal dedicated to open service. Its navigation data and ranging code are not encrypted with a data rate 25 bps. E5a signal is generated by AltBOC modulation. 3.3.4
E5b Signal
The signal is transferred in E5 band comprising two channels i.e. data and pilot. It is an open access signal supported for services OS, CS and SOL. Its navigation data and ranging code are not encrypted and hence accessible to all users with a data rate 125 bps. It also has encrypted commercial data and unencrypted integrity message. E5b signal is generated by AltBOC modulation. 3.4
E1 Signal Generation and Signal Modulation Scheme
Basic signal generation of E1 signal is shown in Fig 3.2 [5]. Their signal components are generated as follows:
The
The data channel is obtained when the subcarrier
with ranging code stream
is modulated
then resultant modulated with navigation data
.
17
The
pilot channel is obtained when the subcarrier
with ranging code
is modulated
.
Fig. 3.2 Generic View of E1 Signal Generation E1 signal is generated by the BOC modulation scheme. It was provided spectral isolation from the other same carrier frequency signal. It moves signal power away from the center of the band so it offers better tracking accuracy and rejection of multipath. It offers two autonomous design parameters, spreading code rate
and subcarrier frequency , they
reduced interference from the reception of other signal by concentrating signal power to allocated band. The redundancy in the lower and upper sidebands offers benefits in the handling of receiver for signal acquisition, tracking and data demodulation.
18
There are some problems, if the spreading code rate is smaller than the sub carrier frequency, the closely spaced peaks in correlation function can lead to irregular effects in tracking and can also limit the spectral coherence of the expected signal. 3.5
Galileo Segment
Galileo consists of three major segments are as follows: 3.5.1
Space Segment
The SS will comprise 30 satellites 27 active and 3 redundant in a Walker constellation. The Walker constellation required fewer satellites for a given level of coverage [4]. Walker constellations use circular inclined orbits of equal altitude and inclination, where the orbital planes are equally spaced around the equatorial plane and satellites are equally spaced within orbital planes [1]. The space segment has three orbital planes which are equally spaced with 56˚ inclination and each plane has 40˚ apart nine satellites. 3.5.2
Ground Segment
It has two main ground segments, the ground mission segment and the ground control segment. They are as follows: a) Ground Control Segment: It will perform the function of control and command of satellite constellations. Satellite control, dealing with the controlling and monitoring of individual satellites, which include: satellite TT&C, ranging, platform maintenance, on-board software maintenance and payload commands. Constellation management, dealing with the populating, maintaining and replenishing of satellite arrangement, which comprise: continuity of signal in space, repair of satellite arrangement and application of satellite replacement plan. 19
b) Ground Mission Segment: It provides main services for worldwide distributed network of Galileo sensors network [1] that monitors signal in space. The data gained from these sensors will be used for integrating, time synchronizing, monitoring and orbit determining. 3.5.3
User Segment
It consists up of user receiver and the main function is the reception of Galileo Signal to determine
pseudorange,
compute
the
navigation
synchronization.
20
solution
and
provide
time
4 4.1
ACQUISITION
Introduction
The purpose of acquisition is to determine visible satellites and rough estimate of code phase and the carrier frequency of satellite signals. a) Different PRN sequences are used to differentiate the different satellites. b) Code phase, is time association of PRN code in the present block of data. To remove the incoming code from the signal, it is necessary to know the code phase to generate PRN code that is perfectly aligned with the incoming signal. c) The carrier frequency, in situation of down conversion relates to IF. The IF must be identified from L1 carrier frequency. The line of sight velocity between the user and the satellite causes a Doppler effect so frequency can diverge up to ± 10 kHz from its nominal value. To remove the carrier from the signal, the frequency of the signal must be known in order to generate local carrier. The frequency search band will be ≤ 500 Hz. The three standard methods of acquisition are as follows: 4.1.1
Serial Search Acquisition
In this algorithm, incoming signal is multiplied a by locally generated carrier signal and PRN code sequences. PRN generator creates PRN sequence corresponding to a particular satellite. The created sequence has a particular code phase, ranging from 0 to 1022 chips for GPS and 0 to 4091 for Galileo. After multiplying the PRN sequence, the signal is multiplied by locally generated carrier signal [2]. Multiplication generated the in phase signal (I) and 90° phase shift version (the quadrature signal (Q)). Then I and Q signals are integrated and squared and added. Ideally the signal power must be located in the I part 21
as C/A code is modulated on that part. The phase of the received signal is unknown so it is necessary to investigate both I and Q signals [2]. It performs two sweeps, first is frequency to all carrier frequency of ±10 KHz in stages of 250 Hz for GPS and ±15 KHz in stages of 125 Hz for Galileo; second code phase sweep of 1023 codes for GPS and 4092 for Galileo. The block diagram of serial search acquisition is shown in Fig 4.1 [2]. The combination result of GPS is shown in equation 4.1 and Galileo in equation 4.2. (
)
(
)
(
)
(
)
The combination is of very large number so it is not used in this software receiver.
Fig. 4.1 Block Diagram of Serial Search Acquisition
22
4.1.2
Parallel Frequency Space Search Acquisition
In this method, the search is parallelized in frequency so only 1023 and 4092 steps are performed instead of 82,863 and 986,172 for GPS and Galileo respectively as in the serial search acquisition. In this method, the Fourier transform is used to implement a transformation to frequency domain. Block diagram of this method is shown in Fig 4.2 [2]. The incoming signal is multiplied by a locally generated PRN code sequence, with a code corresponding to a specific satellite and a code phase between 0 to 1022 chips for GPS and 0 to 4092 for Galileo [2]. Then frequency transformation is performed by forming a Fourier transform. Fourier transform could be executed as DFT. DFT length (number of samples analyzed) determine the accuracy of frequency resolution. In this case fs = 10 MHz for GPS and fs = 10 MHz for Galileo; DTF length N=10,000. The frequency resolution of GPS is shown in equation 4.3 and Galileo in equation 4.4. is as follows: (
)
(
)
Accuracy of this method is 1 KHz as compared to 250 Hz for GPS and 1.5 KHz as compared to 125 Hz for Galileo in the serial search acquisition. This method only steps through 1023 or 4092 different code phases. This is because of the use of frequency domain transformation with each code phase. Because of the use of frequency transformation, it is faster than serial search acquisition method.
23
Fig. 4.2 Block Diagram of Parallel Frequency Space Search 4.1.3
Parallel Code Phase Search Acquisition
In this method, the search is parallelized in code phase so only 81 steps for GPS and 241 steps for Galileo are performed instead of 1023 for GPS and 4092 for Galileo as in parallel frequency space search. In this method, circular cross correlation is performed between the input and the receiver generated PRN code without shifted code phase [2]. When frequency domain representation of cross correlation is established then time domain representation can be established by inverse Fourier transform. Block diagram of this method is shown in Fig 4.3 [2]. The received signal is multiplied by the locally generated carrier signal. Multiplying by signal generates the in phase (I) signal and by 90˚ phase shift generates quadrature phase (Q) signal. Both are combined to form complex signal ( )
( )
( ) to the DFT
function. The generated PRN code is changed to frequency domain and the result is complex conjugated. Fourier transform of input is multiplied by the Fourier transform of the PRN code. Then the result is transformed into time domain through inverse Fourier transform. Absolute value of inverse Fourier transform gives correlation between the input and the received PRN code. In this case, the search space is cut down to 81 for GPS and 241 for Galileo different carrier frequencies. It searches with a step of 250 Hz to carrier frequency up to 10 KHz 24
for GPS and 125 Hz to carrier frequency up to 15 KHz for Galileo. Efficiency of this method depends on the implementation of Fourier transform functions. This method is more accurate as it gives correlation value for each sampled code phase.
Fig. 4.3 Block Diagram of Parallel Code Phase Search Acquisition 4.2
Results
4.2.1
GPS
Software works at a sampling rate of 4Msps; The Doppler frequency search band is set to ±10KHz and Doppler frequency search step size is 250Hz. Acquisition results at point „A‟ in Fig 4.3 for PRN1 and PRN32 are shown in Fig 4.4 and Fig 4.5 respectively. 25
Decision of presence of any satellite is made by comparing the highest peak with the second highest peak in Doppler frequency and Code phase search. The distinct peak of PRN1 can be seen in Fig 4.4 while there is no distinct peak present for PRN32.
Fig. 4.4 Acquisition results of PRN1 of GPS
26
Fig. 4.5 Acquisition results of PRN1 of GPS 4.2.2 Galileo Software works at a sampling rate of 4Msps; The Doppler frequency search band is set to ±15KHz and Doppler frequency search step size is 125Hz. Acquisition results at point „A‟ in Fig 4.3 for PRN11 and PRN12 are shown in Fig 4.6 and Fig 4.7. Decision of presence of any satellite is made by comparing the highest peak with the second highest peak in Doppler frequency and Code phase search. The distinct peak of PRN11 and PRN12 can be seen in Fig 4.6 and Fig 4.7.
27
Fig. 4.6 Acquisition results of PRN11 of Galileo
Fig. 4.7 Acquisition results of PRN12 of Galileo
28
5 5.1
TRACKING
Introduction
Tracking module gets rough estimates of Doppler frequency and Code Phase from the Acquisition module and then refines these parameters for complete removal of Carrier and Coarse Acquisition code to get Navigation Data at baseband. Tracking module keeps lock of the changing code and carrier Doppler shift. 5.2
Carrier Tracking
The purpose of tracking is to refine the coarse values of code phase and frequency, obtained by acquisition and to keep track and demodulate navigation data of the specific satellites. Basic block diagram of demodulation scheme is shown in Fig 5.1. In Fig 5.1, tracking module has to generate two replicas, carrier replica to wipe off the carrier wave from the signal and next the code replica to wipe off the code phase and gives navigation message.
Fig. 5.1 Basic Demodulation Scheme for Navigation Data
29
5.2.1
Phase Lock Loop
To demodulate navigation data on the received signal a precise carrier wave replica has been generated. For this purpose, PLL or FLL are used. Block diagram of the PLL is shown in Fig 5.2. PLL has to generate two replicas, carrier replica to wipe off the carrier wave from the signal and next code replica to wipe off the code phase. The carrier loop discriminator is used to find the phase error on the local carrier wave replica [2]. The output of discriminator is the phase error. It is filtered and then fedback to the NCO, so the frequency is adjusted by this process. The local carrier could be almost an exact replica of the input signal carrier. But there is a problem with a normal PLL it is affected by 180˚ phase shift. To solve this problem the Costas loop is used. One property of Costas loop is that it is unaffected to 180˚ phase shift.
Fig. 5.2 Basic GPS Receiver Tracking loop Block Diagram 30
Block diagram of Costas loop is shown in Fig 5.3.
Fig. 5.3 Block Diagram of Costas Loop It has two multiplications; first product is between the input signal and the local carrier and the second is between 90˚ phase carrier wave quadrature parts (Q) and the input signal. The Costas loop tries to have all energy in the I arm. To keep all energy in the inphase part some feedback to the oscillator is needed. To find such a feedback term, phase error of local carrier for GPS is shown in equation 5.1. ( )
(
)
Where ø is the phase error. Such a phase error is minimum when the Q part is zero and I part is maximized. In case of Galileo pilot channel alone is used for phase error estimation. The pilot channel has no data bit, it is used longer coherent integration so improved carrier tracking 31
sensitivity. There is a loss of 3 dB by ignoring data channel and noise sensitivity is also enhanced by 3 dB. Therefore, special arctangent discriminator has been used as shown in equation 5.2. ( )
(
)
The special arctangent discriminator has twice longer operational range as compare to ordinary arctangent discriminator. It has more noise resistance. It can easily track the pilot channel. 5.2.2
Delay Lock Loop
In a typical GPS receiver, code tracking loop is a DLL which is called an early-late tracking loop. The idea behind the DLL is to correlate the input signal with three replicas (Early (E), Prompt (P) and Late (L)). Block diagram of the GPS DLL is shown in Fig 5.4.
Fig. 5.4 Basic Code Tracking loop Block Diagram of GPS
32
In case of Galileo, input signal is correlated with five replicas (Very Early (VE), Early (E), Late (L), Prompt (P), and Very Late (VL)). Code tracking of Galileo is steadier than carrier tracking. Code tracking algorithm is a two-step process. The first step of the process is the coarse tracking is used to proper convergence near real lock point. This is accomplished using the combination of five correlators (VE,E, L, P, and VL). It took advantage from the thinness of CBOC autocorrelation where VE and VL are positioned on secondary peak and prompt located on main peak. Block diagram of the Galileo DLL is shown in Fig 5.5.
Fig. 5.5 Basic Code Tracking loop Block Diagram of Galileo The first step is to convert C/A code to baseband simply by multiplying the incoming signal with a perfectly aligned local replica of carrier waves [2]. Then the signal is 33
multiplied by three code replica (E, P, and L) for GPS and by five code replica (VE, E, P, L and VL) for Galileo, which are generally generated with a spacing of ± 1/2 chips for GPS and ± 0.15 chips (E,L, and P), ± 1/2 chips (VE and VL) for Galileo. After the second multiplication, the outputs are integrated and dumped. The output of this integration is a numerical value that shows how much the specific code replica correlates with the code present in the incoming signal. This method is optimal when the local carrier wave is locked in phase and frequency. 5.2.3
Complete Tracking of GPS
Fig 5.6 shows the combined code and carrier tracking loop complete of GPS.
Fig. 5.6 Block Diagram of Combine PLL and DLL for GPS 34
Fig 5.7 shows the combined code and carrier tracking loop complete of Galileo.
Fig. 5.7 Block Diagram of Combine PLL and DLL for Galileo
35
5.3
Results
5.3.1
GPS
The PLL band is set to 50Hz and DLL band is set to 2Hz. The chip spacing for Early, Late and Prompt signals is set to 0.5 chips. Results at various point of tracking algorithm are given below. Fig 5.8 shows the Coarse Acquisition (C/A) code frequency variations over time. This variation is due to the Doppler shift. This is the error plus initial bias signal of PRN code generator in code tracking loop (DLL).
Fig. 5.8 Coarse Acquisition (C/A) code frequency over time for GPS
36
Fig 5.9 shows the Doppler shift of carrier signal. This is the error signal of the carrier tracking loop (PLL). Numerical control oscillator adds initial bias to it and generates the exact replica of carrier signal.
Fig. 5.9 Carrier frequency Doppler shift for GPS The PLL plus DLL loop discriminators computes the above given code and carrier Doppler shifts from the output of Early, Late and Prompt correlators. The prompt correlator of in-phase arm gives the navigation data at base band; the overall procedure‟s goal is to maximize the energy in prompt correlator of in-phase arm.
37
The correlator results for Early, Late and Prompt signals of in-phase arm are shown in Fig 5.10.
Fig. 5.10 Correlation results of Early, Late and Prompt signals of GPS The large amplitude of prompt signal can be seen in Fig 5.10 as compared to early and late signals. Fig 5.11 shows the Navigation message bits. The Navigation message is taken out from Prompt signal of in-phase arm after correlation. 38
Fig. 5.11 Navigation Message bits for GPS Fig 5.12 shows the carrier to noise ratio of our GPS receiver.
Fig. 5.12 Carrier to Noise Ration of GPS 39
5.3.2
Galileo
The PLL band is set to 20Hz and DLL band is set to 2Hz. The chip spacing for Early, Late and Prompt signals is set to 0.15 chips, the chip spacing for very Early and very late is set to 0.6. Results at various point of tracking algorithm are given below. Fig 5.13 shows the Coarse Acquisition (C/A) code frequency variations over time for PRN12. This variation is due to the Doppler shift. This is the error plus initial bias signal of PRN code generator in code tracking loop (DLL).
Fig. 5.13 Coarse Acquisition (C/A) code frequency over time for Galileo
40
Fig 5.14 shows the Doppler shift of carrier signal of PRN 12. This is the error signal of the carrier tracking loop (PLL). Numerical control oscillator adds initial bias to it and generates the exact replica of carrier signal.
Fig. 5.14 Carrier frequency Doppler shift for Galileo The PLL plus DLL loop discriminators computes the above given code and carrier Doppler shifts from the output of VE, E, P, L and VL correlators. The prompt correlator of in-phase arm gives the navigation data at base band; the overall procedure‟s goal is to maximize the energy in prompt correlator of in-phase arm. The correlator results for VE, E, P, L and VL signals of in-phase arm are shown in Fig 5.15. The large amplitude of prompt signal can be seen in Fig 5.15 as compared to early and late signals. 41
Fig. 5.15 Correlation results of Early, Late and Prompt signals of Galileo Fig 5.16 shows the Navigation message bits. The Navigation message is taken out from Prompt signal of in-phase arm after correlation.
Fig. 5.16 Navigation Message bits for Galileo 42
Fig 5.17 shows the carrier to noise power of Galileo receiver
Fig. 5.17 Carrier to Noise Ration of Galileo
43
6 6.1
NAVIGATION
GPS
6.1.1
Introduction
The navigation data of GPS is 1500 bit long frame comprising 5 sub frames, each frame having length 300 bits. Each sub frame has 10 words; each word is 30 bits long. Sub frames 1, 2, and 3 are repeated in each frame [2]. Sub frames 4 and 5 have 25 versions (with the same structure, but different data) referred to as page 1 to 25 [2]. It has bit rate of 50 bps, one sub frame transmission lasts for 6 s, one frame transmission lasts for 30 sec, and one complete navigation message lasts for 12.5 minutes. a) Sub frame #1: It contains the clock data which determines the time at which the received signal was transmitted from the satellite and moreover it also contains the information regarding the health of the transmitting satellite i.e. whether to trust the data or not. b) Sub frame #2 &3: They contain the ephemeris data of satellite which is used to compute satellite position i.e. orbital position. c) Sub frame #4 & 5: They contain almanac data and it has 25 versions so in total 50 sub frames repeat after 12.5 minutes. Almanac data contains information about ephemeris and clock data of all satellites in the constellation with reduced precision. It also contains information of ionospheric parameters, health indicators and UTC parameters. Table 6.1 [2] illustrates the complete structure of navigation data. The left most blocks shows sub frame number and next blocks show the information in the sub frame.
44
Table 6.1 GPS Structure of Navigation Data
6.1.2
Message Content
First three sub frame message structure will be discussed in this thesis, they will be used to compute navigation solution. 6.1.2.1 Starting Words of sub-frames Each sub-frame consists of 10 words and always starts with TLM and HOW pair. TLM is the starting word of every sub frame, it is repeated every 6 sec. It has an 8 bit preamble (used for frame synchronization) and 16 bits reserved space with parity. HOW is the 2nd word of every sub frame, it contains 17 bits truncated TOW with two flags which contains the information of anti-spoofing (means to protect the user data) for the user. And the next three bits contain the sub frame ID i.e. in which sub frame out of the five
45
sub-frames this word is in at the current time. The ID code shall be as shown in Table 6.2 [5]: Table 6.2 Sub frame ID Code
6.1.2.2 Sub frame 1 Content The important details of third through ten words which will be used for clock correction and SV health their details are as follows: 6.1.2.2.1 Transmission Week Number (TWN) Bit 1 to 10 of the word three shall give TWN of the SV. It shows the week number at the start of the data set transmission [5]. The week number increments at the start/end of week epoch. 6.1.2.2.2 User Range Accuracy (URA) Index Bit 13 to 16 of the word three shall give URA index (N) of the SV [5]. The URA index is used to compute accuracy of user (X) in meters. Accuracy of user will be computed as follows: 46
(
If value of N ≤ 6, X=
If vale of 6
