Luận văn research and development of advanced signal processing algorithms for multi gnss software receivers

Atomic Frequency Standards Assisted Global Navigation Satellite System Additive White Gaussian Noise Binary Offset Carrier Code Division Muliple Access Central Processing Unit Delay L

MINISTRY OF EDUCATION AND TRAINING HANOL UNIVERSITY OF SCIENCE AND TECHNOLOGY

‘TRUONG MINH DUC

RESEARCH AND DEVELOPMENT OF ADVANCED SIGNAL PROCESSING ALGORITHMS

FOR MULTI-GNSS SOFTWARE RECEIVERS

MASTER OF SCIENCE THESIS:

COMPUTER AND COMMUNICATION ENGINELRING

ACADEMIC SUPERVISOR:

Dr Tạ Hải Tùng

Hana 2015

_BO GIAO DUC VA DAO TAQ | TRƯỜNG ĐẠI HỌC BACH KHOA HA NOL

TRUONG MINH DUC

NGHIÊN CỨU VÀ PHAT TRIEN GIAI THUAT

XỬ LÝ TÍN HIỆU TIÊN TIỀN CHO

BO TITU MEM DA IIE TIONG GNSS

Chuyên ngành : Kỹ thuật máy tỉnh và truyền thông

LUAN VAN THAC Si KHOA HOC

KỸ THUẬT MAY TINH VA TRUYEN THONG

NGƯỜI HƯỚNG DẪN KHOA HỌC :

TS Ta Hai Tùng

Ha NGi — Nam 2015

Acknowledgements

An the first words of this thesis, | am extremely grateful to those who in various ways

contributed Lo all the research activ ilies presonited in this thesis

Foremost, I would like to express my sincere gratitude to my advisor Dr Ta Tai Tung

for the continuous support, for his patience, motivation, enthusiasm, and immense

Imowledge His guidance helped me in all the time of res

very important

The list could not be complete without Growing NAVIS project, funded by the Luropean Commission under the IP7 Call Galileo.2011.4,3-1 International Activities (Grant Agreement No 287203), for supporting my internship at ISMB from March to June, 2014

Last but very not least, Ï cannot thank enough my parents and my sister for their belief,

encouragement and Tove that definitely arc my limitless power

Commitment

1 commit myself to be the person who was responsible for conducting this study All reference figures wore oxtracled with clear derivation, The presented resulis are truthful and have not published in any other person’s work

HaNoi, December 29% 2014

Truong Minh Đức

TOM TAT LUAN VAN

Ngày nay, định vị sử đựng vệ tỉnh đóng vai trò quan trong trong nhiễu lĩnh vue: giao thông, bản đổ, cứu hộ, giám sái mỗi budng, quân sự, Các hệ thống định vị sử dụng

vệ tỉnh hiện đại bao gồm cả các hệ thông cũ đang, được nâng cap nhu GPS, GLONASS hay các hệ thống mới đang được xây dựng như Galile, Beido đều øung cấp các tin Tưệu mới với các công nghệ tiên tiễn như: điều chế dịch sóng mang nhị phân (BOC), kỹ thuật ghép kênh điều chế sóng mang thích nghỉ tương quan (CASM) Một yếu câu xmới đặt ra cho các bộ thu tin hiệu là phải có khả năng hoạt động với các tin hiện mới, hoạt động đa hệ thống

Tuy nhiên, dễ đáp ứng yêu cầu này, các bộ thu củng truyền thống (ASIC) cân phải thiết

Xế và chế tạo lại Điều này yêu cẩu chỉ phí tương đối cao Do vậy, hướng phái triển các

bộ thu mềm hoạt động trên các vĩ xử lý có khả năng lập trình được đang được quan

tâm rộng rấi củng với sự phát triển mạnh mẽ vẻ năng lực tính toán của các vì xử lý nảy

Bộ thu mm có tru điểm là cầu trúc xử lý linh hoạt, mẻm đẻo đễ đàng thực hiện việc

nang cap, thay déi do vay hoan toan có thể đáp ứng dược yêu cầu trên

Ngoài ra, nhiêu ứng dụng định vị sử dụng về linh có yêu cầu cao về độ an loàn và tỉnh chỉnh xác như hàng hãi, hàng không, hay đường sắt Hiện nay, một trong những mối de dọa chính tới độ chính xác của dich vu định vị sử dựng về tỉnh là can nhiều như phá sóng hay giả mạo tin hiệu Tuy nhiên, tin hiệu GNSS phổ thông không được trang bị bất kỳ ruột phương pháp chống can nhiều rào bến Irong nó Vì vậy, các phương pháp

chống can nhiễu trong quá trình xử lý tin hiện lả một ván dé cần được xem xét trong,

r thực tế đó, luận văn tập trung vào nghiên cứu và phát trình bộ thụ

smém da hệ thống vả nghiên cứu về các phương pháp phát hiện giả mạo tỉn hiệu Đóng, góp chính của luận văn như sau

N cửu và phải tr giải pháp dịnh vị da hệ thủng so với giải pháp dơn hệ thống sử đụng dữ hiệu thực

«_ Nghiên cứu hai phương pháp phát hiện giả mạo tín hiệu, với tên gọi SigriTest và GoF, và đánh giá khả năng hoạt động của hai phương pháp này với bộ đữ liệu ghuấn TEXTBAT được sử dụng rộng rấi trơng dánh giá hiệu răng cửa các kỹ thuật phát hiện/ loại bỏ giá mạo tin hiệu

một bộ thu muểm đã hệ thống, đánh giá hiệu nắng của

Từ các nội dung như trên, luận văn dược tổ chức nhu sau:

« Chương 1: Fundamental Background: Chương nảy giới thiệu tổng quan vẻ môi trường định vị đa hệ thống — kiến trúc cơ bản, trạng thái hiện tại của các hệ

thống, định vị sử dụng vệ tỉnh hiện có, ưu điểm và thách thức của định vị đa hệ

thống Cơ số lý thuyết về bộ thu mềm cũng dược giới Hiệu ở chương tây.

«_ Chương 2: Design of Mulfi-GNSS receiver: Chương nảy tập trung váo thiế

gủa bộ thu mm Cách triển khai cae module của bộ thu mềm và phương thức

hoạt động của từng module cũng được trinh bảy trong chương này:

ing detection on software receiver: Chuong nay tong hap cic

« Chương 3: §p

thông tì cơ bán vẻ hai phương pháp phát hiện giả mạo sứ dụng trong bộ thu mềm và cách triển khai các module nay Ngoai ra chuong nay củng điểm qua xuột số đặc chiếm của bộ dữ liệu chuẩn TEXTBATT

« Chuong 4: Experiment Results: Chuong nay trình bảy về kết quả thứ nghiệm hoạt động của bộ ha nêm với cô 4 hệ thông định vị sử dụng vệ tĩnh loàn cầu Cac kết quả thực nghiệm cho thấy giải pháp định vị đa hệ thống là hoản toản có thế thục hiện được và giải pháp nảy cho hiệu năng tắt hơn giải pháp đi vị đơn hệ thống về cả độ chính xác, khả năng sẵn sàng và dộ tỉa cậy, Kết quả thử nghiệm của hai phương pháp phát hiện giá mạo tín hiệu cũng được trình bảy trong chương này Hai phương pháp này đêu cho thấy khả năng phát hiện được tín hiệu bị giả mạo khi áp dụng với bộ dữ liêu TEXTBAT,

«© Kết liận

TTém lại, luận văn đã thực hiện được các yêu cầu đặt ra ban đâu nhu nghiên cứu và phát

triển một bộ thu mễm đa hệ thông, thử nghiệm hoạt động của bộ thu mêm: nghiên cửu,

kiểm thử khả năng phải hiện giả mạo của hai phương pháp SignTest va GoF

Table of Contents Acknowledgements

Chapter 2; Design of Multi-GNSS receiver

1 Receiver general architecture

Chapter 3: Spoofing detection on software receiver

1 11 Sgoofing detectiontheory Hypothesis tosh - -

12 Application of hypothesis test to GNSS receivers 63

13 SianTest

1.4 Goodness of Fit Test

2 Spoofing detection method implementation

3 TEXTBAT datasets

Chapter 4: Experiment resulls

2 Signal synchronization and Demodulation modul m m

Atomic Frequency Standards

Assisted Global Navigation Satellite System

Additive White Gaussian Noise

Binary Offset Carrier

Code Division Muliple Access

Central Processing Unit Delay Lack Loop

Frequency Lock Loop

Ficld-programmable gate array

Geometric Dilution Of Precision

Geostationary Earth Orbit

Global Navigation Satellite System

Goodness of Fit

Global Positioning System

Graphics processing unit

Interface Control Document

Tnlernediate Frequency Inclined geosynchronous orbit

Tndiar Regional Navigation Satcllits System,

Navigation Data Unit Position Dilution Of Precision Phase Lock Loop

Positioning, Navigation and T'uming, Pseudo-Random Noise

Position, Velovity, and Time Quasi-Zenith Satellite System Radio Frequency

Regional Navigation Satellite System Software Defined Radio

Time Dilution Of Precision Texas Spoofing Test Battery Wide Area Augmentation System

List of Figures

Figure 1 GNSS architecture

Figure 2 Architecture of navigation payload 000000e AM

Figure 3 Control segment architecture and functions [12] - - 18 Figure 4, Augmentation Systems

Figure 5 Number of SVs in multiGNSS systems

Figure 6 Spectrum of GNSS signal

Figure 7, Satellite navigation principle

Figure 8 GNSS signal processing chain

igure 9, Acquisition’s search space -

Figure 10, Basic GNSS receiver tracking loop diagram

Figure 11 Code tracking loop block diagram "

Figure 12 Carrier tracking loop block diagram

Figure 13 Kepler parameters and satellite orbit

Figure 14 ‘The elliptic orbit with(E, coordinates

Figure 15 Time differences between system lime, satel

igure 16 Receiver GNSS Architecture

Ligure 17 Block diagram of the parallel code phase scarch algorithm

Figure 18 Flowchart of acquisition algorithm

Figure 19 Complete tracking loop diagram

Figure 2U Flowcharts of tracking algorithm -

Figure 21 Flowehart of GPS data demodulation function -

Figure 22 Flowchart of GLONASS data demodulation function

Figure 23 Flowchart of Gulilen data demodulation fanction

Figure 24 Flowchart of Beidou data demodulation function

Figure 25 Flowchart of finding preamble function

Figure 26 Flowchart of pseudorange calculation algorithm

igure 27 Flowchart of receiver position calculation algorithm oo) Figure 28 Correliturs profile in the ahsence (upper) and presence of disturbances

Tigure 29 Example of p-value k=10 - 7 Figure 30, Flowchart of SignTest function

Figure 31, Flowchart of Chi2GoF fanetion co nieroee

Figure 32 The antenna on the roof (the circled one) - - 73 Ligure 33 Graphic User Laferfacc of onr receiver

Figure 34 Choosing data file error in GUI

Figure 35 Search space of acquisition

Figure 37 Result af DLL and PLI of the tracking loops

Signal synchronization resutt

Figure 39 Data demodulation results

| Skyplot af all GNSS system GPS PVT result — Google Earth cá nà

„ Skyplot (eft) and Positioning accuracy (right) of all GNSSe Skyplot & Positioning accuracy of {GPS & Galileo} combination 34

| Skyplot & Positioning accuracy of {3 GPS & 2 Beidou} combination 85 Skyplot & Positioning accuracy of {four systems} combination 86 Time history oŸ C/NU for điiferent PRNs .- Doppler frequency fy of different PRNs 89 Results of Sign Test applied to a pair of correlators with đEL=1.5chips and imtegration period Tỉnt= Íms „90 Figure 49, Results of GoF test applied to a pair of correlators with dEL=1.Schips and integration period Tint = is

List of Tables

Table 1, Characteristics of GNSS civil signals

‘Table 2, Acquisition parameters of cach signal [21|

Table 3 GNSS navigation message characteristic

‘Table 4, Kepler parameters

Table 5, GNSS system’s preamble -

Table 6 Sign Test and GoF Test Parameters - -

‘Table 7, Intermediate frequency and bandwidth of GNSS khen 7 Table 8, Perfarmance øÍ stand-alone positioning solufipns - - 83

‘Table 9, Standard deviation rati

GPS or GLONASS And until now a let of new GNSSes have been developing, and

will provide services for users all over the world in the near future such as Galileo or

Beidou ‘Ihe modem GNSSes provide new signal with advanced technology for

instance: binary offset carer (BOC), coherent adaptive subearmer modulation

(CASM) Ln respond to this fact, GNSS receivers should be upgraded to work with new signals and new systems

However, most of traditional GNSS receiver 1s hardware receiver, whose signal

processing part is performed by application-specific integrated circuit (ASIC) ‘Those

receivers are able to pracess signal at high speed, but they have low flexibility and

upgradoabilily (in order lo do that, the whole receiver should be re-designed and to- manufactured and this require a pretty high cost) In order to salve this problem, we

can use the sofware receiver approach This allows us lo build a high flexible and

easy-to-upgrade receiver because any change to the receiver can be done through modifying the software code So, all the upgrade to satisfy the new requirements (new signal, upgrade signal processing algorithm, working in larsh cuvironmeut ) can be implemented easily

On the other hand, the existence of multi-GNSS also leads us to same questions Can

we use those systems together to get a multisystem solution or just use them

separately? If we can combine them to form a solution, how does its accuracy compare

to the single sahition? And what is the drawback?

Moreover, many GNSS applications are considered safcty critical For cxample, maritime, aviation, and rail transportations often require stringent performance and the

detection of signal degradation lo preserve accuracy Currently, omte of’ the main threals

13

to GNSS accuracy is interference such as jamming and spoofing Unfortunately, the open GNSS signal does not have any “built-in” anti-interference methad So, anti-

inlerference methods also need to be considered in a GNSS receiver In a fully SOR

receiver, all intermediate results can be accessed casily, allowing, us to implement anti- interference techniques base on those results

‘Understanding about, those problems, my colleagues al NAVIS eonter and TISMB and I

conduct research and development on a multi-GMhSS receiver and also the anti- interference methods Spoofing detection is focused im this thesis, while interference mutigation methods can be found on Ms Nguyen Thi Thanh Tu’s thesis [17] So, the main contributions of this thesis can be summarized as follows:

Research and develop a multi-GNSS receiver, evaluate the perfonnance of

miulti-GNSS solution over the single solution with real collected data

Research and implement two spoofing detection methods, namely Signtest and

GoF test, and vahdate them against TEXTBAT which is a widely used test

bench for performance assessment of spoofing detection/mitigation techniques

Thus, with these contents, this thesis is organized as follows:

Chapter 1: fundamental Background: ‘this chapter presents an overview of

mula-GNSS environment — general architecture, slalus of GNSS sysiems,

advantages and challenges of multi-GNSS environment and the theoretical basis

about software receiver

Chapter 2: Design of Multi-GNSS receiver: ‘I'his chapter focuses on how the

Teceiver is designed The implementation of modules mide the reeciver and the

ways they work are also presented in this chapter

Chapter 3: Spoofing detection on software receiver: This chapler summaries the basis information about the spoofing detection methods used in our receiver

and how they are implemented Some characteristies of the TEXTBAT datasets

are also shown here

14

Chapter 4: Experiment Results: This chapter shows the results of the receiver with real dala testing from all four available GNSS systems Those resulls are analyzed to make performance comparisons between single GNSS-solution versus multi-GNSS solution The testing results of the two spoofing detection methods against the TEXTBAT dataset are presented here too

‘Truong Minh Duc, Vinh ‘The La, Tung liai 'T'a, Gustavo Ldelforte (2013), Mult- GNSS Positioning Solutions with Real Dala Collecled in South-Fast Asia Region, SGNSS 2013, Istanbul, Turkey

Truong Minh Duc, Gamba, M T., Motella, B., Falletu, E., Ta Hai Tung (2014), Lmabling GNSS software receivers with spoofing detection techniques: a test against some TEXBAT datasets, COMNAFT 2014, Hanoi, Vietnam

Chapter 1: Fundamental Background

Tn any period of human history, navigation always has an important role for many purposes: exchanging positioning information, leading the ways, military

apphecalion, Since ancient imes, people have used a lot of methods to navigale such

as based on the known location, construction milestones (lighthouses for instance),

monitoring ocean currents, observing the pasition of the constellations combined with

the ust of a compass and nap However, these methods have limilations such as high computational complexity, imprecision and low reliability Along with the development of science and technology, satellite navigation was bom, initially for

‘using, in the military field and then be extended to other civil applications

16

Figure 1 GNSS architecture

Figure 2 shows the high-level architecture of the navigation payload which is

responsible for the generation and transmission of the ranging codes and navigation

data on the allocated radio frequencies to the user segment The Tracking, Telemetry and Control (TT&C) subsystem receives the predicted navigation data and other

control data from the control segment to control the payload The Atomic Frequency

Standards (AFSs) are used as the basis for generating the ranging codes and carrier frequencies The Navigation Data Unit (NDU) contains the ranging code generators and the navigation data uploaded from the control segments This subsystem also

interfaces to the cross-link receiver/transmitter for intersatellite communication, and

ranging The L-band subsystem is responsible for transmitting the navigation signals to users

1.1.2 Control segment

The control segment is responsible for maintaining the satellite constellation and their

proper functioning In principle, this segment consists of three subsystems, namely:

Master Control Station (MCS) Monitor Stations (MS) and Ground Antennas (GA)

The functions assigned for each sub-system can be summarized in Figure 3

17

‘Master Cantrat Station

» Resource alleaton and t=hedul%e += Mavigelion message generation + Saloifts health anc hourekeeping

* SV prosentont aad generation

* Conztlgten syrehreriz on seenng

"EDS system stats/pertormance evualo~

The user segment is made of a wide range of different receivers, with different

performance levels The receiver estimates the position, velocity and time of the user

on the basis of the signals transmitted by the satellites The common functionalities of

any receiver can be summarized as follows:

18

1.2

Identification of the satellites in view,

Estimation of the distance between the user and the satellites

Iriateration to estimate the user location

Status of all GNSSes

Tn this section a short averview of the stalus of the different analysed GNSS is

presented for reference

GPS: navigation satellite system of the U'S, as remarked GPS has been the first

GNSS and it has been continuously working for decades The 1* satellite of

GPS was launched in 1978 Indeed, since its setup, the system has undergone

maintenance and has been modernized with the launch of new types of satellites deserving new features The full constellation foresees a total of 24 medium clevation orbils (MEO) satellites on 6 umfonnly spaced orbits with an

inclination of 55° on the equatorial plane

Tn June 2011, three of the 24 slols were expanded, and six satellites were

repositioned As a result, the system currently operates a 27-slot constellation Beside those satellites, GPS constellation always has some backup satellites

which can work as normal satellites if needed In June 2013, the total number of

GPS satellites in the constellation is 32 [16]

Galileo: The European GNSS foresces 27 MEO working satcllites, nine for cach

one of 3 uniformly spaced orbits with an inclination of 56° on the equatorial

plane

Currently the Galileo system is under deployment and only its first 4 satellites

have been launched Such satellites are transmitting their open service signal

with the navigation message since last 12 March 2013 thus cnabling for the first

time a Galileo-only position fix New satellites should be launched over the next

few years to allow the system to become fully operative [6]

19

GLONASS: It is the second oldest GNSS developed by the old Soviet Union and then Russia since 1976 Afier a decline m capacity during the late 1990s, in

2001, the restoration of the system was made a top goverment priority and

fanding was substantially increased By 2010, GLONASS had achieved 100%

coverage of Russia's territory and in October 2011, the full orbital constellation

of 24 satellites was restored, enabling full global coverage Its constellation

foresees 24 MEO satellites on three orbits with 8 uniformly spaced satellites in

each orbit Orbits have an inclination of 64,8° on the equatorial plane Currently there are 2 working satellites in the system [9]

eidou: ‘This Chinese GNSS foresees the use of three different types of satellites: gcosynchronous cqustorinl orbit, (GRO), inclined geosynchronous orbit (GSO) with an inclination angle to the equatorial plane of 55°, and MEO

At present the Beidou System consists of a total of 14 satellites Five of them

are GEO salellites, other five are T4SO and finally there are 4 MEO satellites New launces should take place in the next future im order to extend the total mumber of satellites in the constellation [4]

QZSS: ‘This Japanese KNSS is constituted, for the time being, of only one IGSO

satelite Soon others salcliles should be launched and until the full

constellation is reached with 7 satellites Among others, this system transmits

some GPS like signals, so that its satellites can be used ad extra GPS satellites

thal have always a quite high clevation for users in Japan and in other Rast and

SIA countries This feature improves the availability and the quality of the GPS

service in critical conditions in which a consistent share of the sky is masked, as

in the case of urban canyons On top of this the syslom will acl as an augmentation system providing information enabling errer corrections in the

covered region and it will also offer new additional services [11]

20

«© We also have some others augmentation system (AGNSS) such as MSAS, EGNOS, WAAS but each of these systems only provides services for users in

a specific location

Figure 4 Augmentation Systems

So with all those GNSS, RNSS and AGNSS, there will be a lot of satellites for navigation purposes Figure 5 shows the number of satellites for navigation in the sky

all signals from current GNSSes

21

that GPS stand-alone gives users worldwide a mean horizontal positioning

accuracy (over 95% of time) of about 30 meters, while that of the combined

GPS and Galileo positioning is less than 5 meters

22

* Reliability increase: Intentional and un-intentional interference sources, inchiching jammers and spoofers, are major threats for GNSS services The redundaney provided by multi-systems and multi-frequency bands are really important to increase the robustness of the receivers, as well as the reliability of the positioning services

1.4 Challenges of multi-GNSS

Along with those advantages, muli-GNSS also brings us some challenges:

& Infer-system interference: GNSSes broadcast navigation signals in overlapped frequency bands This fact could be convenient Grom the paint of view of the receiver design, but on the other hand raises the issues of inter-system interference However, as for GPS and Galileo the signals were designed in the ways that reduce the interference while support interoperability (European Union, 2010)

© Complexity increase: new and upgraded GNSSes broadcast modem signals, which have advanced but complex structures, in multiple frequency bands The:

reliability to navigation services, but also challenge receivers to accommodate

signals give much improvement in terius of accuracy, availability, and

for such advantages Ta mulli-<GNSS solutions, the analog paris of a receiver must operate with multiple systems, multiple frequency bands at larger signal

bandwidths These requirements surely increase the complexity and

consequently the cost of the receiver As for the digital parts, the signal processing requires more advanced and complex algorithms to cope with multiple systems, multiple channels as well as to fully exploit the advantages of the modern signals Ihis fact increases the computational complexity, the resource capability requirements and eventually the cost of the receiver

Recently, logether wilh the rapid improvement in compulational capability of

programmable processors such as CPU, GPU, DSP, and FGPA , the software

23

receiver approach, which minimizes the hardware requirements, is favourable for the multiGNSS solution because of its flexibility, easy-touperade for

complex requirements,

However, the important advantages of multi-GNSS cnviroument together with the

development in the electronic industry give a promising future for multi-GNSS

solution

2 Software Receiver

2.1 Software receiver overview

Nowadays, most of traditional GNSS receivers are hardware receivers, in which the

signal processing part of the receiver is performed by application-specific integrated circuit, This allows the receiver having high compulalion speed but low Mlexibility and upgradeability

However, il is consolidated that the Software Defined Radio (SDR) technology apptied

to GNSS receivers produces significant benefits for prototyping new equipment and analysing signal quality and performance [13] ‘hanks to the replacement of some

hardware components with far more flexible and easier-to-test software-based signal

processing techniques, SDR technology allows the implementation of positioning

engines with a high flexibility level Today there is fervent activity in the design of novel architectures, each of which may be tailored to diverse envirornnents and new systems and services (such as GLONASS, LIGNOS, Galileo, BeiDou/ Compass, Q85, commercial high precision satellite services, local and regional differential services, ete ) The advantages of a fully SDR int this highly dymamic scenario, with respeet lo

an equivalent fully hardware device, can be synthesized in three key points [19]: configurability, updatability/ upgradeability and flexibility In a fully SDR receiver, all intermediate results such as the tracking correlations, Doppler fiequency code and carrier phase, navigation message, etc are fully accessible making easy ta

24

implement and test new advanced signal processing algorithms based on those

intermediate results

Furthennors, the demand in upgrading old receivers lo get a mulli-GNSS salution,

providing high availability, accuracy and reliability, is becoming, essential Just adding,

new functions rather than re-designing the whole circuit in hardware, makes the SDR a

fimdamental tool to develop a flexible and easily reconfigurable receiver, capable to

timely follow the evolution of the services and of the signals In addition, thanks to the

very [asl microprocessor development in computalional capability, the speed gap

‘between hardware and software becomes smaller For all these reasons, the SDR shows

to be a very promising approach Consequently, the last decade has seen a prominent proliferation of SDR solutions in the field of GNSS receivers (see [13] and references

therein),

2.2 Software receiver architectarc

Basically, GNSSes are based on trilatoration technique for positioning, With this

technique, a receiver needs to measure the distances from its location to at least 4 imown points (ie satellites) — 3 for its 3 coordinate and 1 for the time error between

yeceiver and GNSS system time ‘hese distances and points form 3 spheres whose intersection determines the receiver’s location (Figure 7) Moreover, along with

eceiver position, we can also determine receiver velocity and the accurate lime

So, in order to get the receiver position, velocity and time, the GNSS reociver has to

specify satellites position (center of spheres) and distance between satellites and

receiver (radius or spheres) The following section will shows how the receiver docs

these jobs through its signal processing chain (Figure 8)

' Figure 8 GNSS signal processing chain

2.2.1 Frontend

The radio frequency (RF) signal from the antenna goes through the front-end, which is

responsible for conditioning and converting the analog RF signals to the digital IF samples The sampling frequency and IF are specified by the frontend itself, for

example with MAX 2767 front-end, the sampling frequency is 16.368 MHz and IF is 4.092 MHz The general representation of a digital GNSS signal after the front-end is

shown on (1)

26

tịm|—xl2Ce[n- Ø]eo(2Z(ƒ„¡ 0.5625 5 fal, 1g)! aT] w

where C’is the carrier power (W), c[ii] is the spreading code of the navigation signal, as for the CDMA signals, 7 is the PRN number uniquely assigned for a satellite, however for the GLOMASS FDMA signal, i is common for all satellite; fis, fy denote the Intermediate Frequency (il) and Doppler shift (11z) respectively; & = 0 for the COMA signals, but as for the GLONASS FDMA signal, kis uniquely assigned for a satellite in

view, 4

= Vy stands for the sampling, period (s) (Fs is the sampling, frequency (Hz);

g is the initial carrier phase (rad); 0 is the initial code delay (samples), and my is the Additive While Gaussian Noise (AWGN) with vero mean (4 — 0) and variance on? Ứng:

~N (0, ci? )) It should be noted that PRN codes are used not only for the multiple access purpose, but also for the ranging purpose to measure the distance between the

receiver and the satelhics m view Table 1 also summarizes the characieristics of the

PRN codes of each system

Table 1 Characteristics of GNSS civil signals

The main task of the signal synchronization block is to create a local replica of r[n],

referred to as [vi], for code and carner wipe-off This block is usually divided inte

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also referred as the Cross Ambiguity Function between the received signal and the local replica over a dwell time T, which is often equivalent to one full PRN code length (see Table 1) The correlation value is compared with a pre-determined threshold to decide which hypothesis between Hy (unmatched), H, (matched) 1s truc [5] The uncertainty of the three parameters creates a 3-D search space for each signal as reported in Table 2 Note Umit: iu Table 2, for code phase search, the unverlainly is one full code, with the conventional step size of 0.5 chip (0.25 chip for H1 due to BOC(1,1) anodulation of the signal [6]; and for the Doppler search, the uncertainty is for a general

moving receiver, with the conventional step size being: Af; = = 112|

‘Table 2 Acquisition parameters of cach signal [22]

Sigual fork | 8 (chip) | A@(chip) | 7, Uz) 47, (KI)

These parameters define the sive of the search-space (N, x Np, where N; is (he number

of columns (ic code delays to be tested) and Ny is the number of rows (1c Doppler

shifts to be tested) igure 9 shows the acquisition’s search-space

The acquisition process correlates the incoming signal (r|nJ) will Ihe tentative signal

rịn| Lò measure the similanty belween the two signals Basically, there are two

acquisition approaches as follows:

* Serial search: the acquisition process tests one cell (ie.a tentative signal) at a

© Parallels in Doppler shift dimension: a whole column of the search-space

are tested at once

In fact, parallel acquisition approach can be done in both dimensions, ie code delay and Doppler shift However, the one performed in the Doppler shift dimension is less

efficient than the other [20] So in this thesis, only the parallel in code delay dimension

is used

b Tracking

Except the satellite id, the signal parameters estimated by the acquisition process are not accurate enough to be used for positioning and navigation, Moreover, these

parameters change over time due to the Doppler effects on code and carrier Therefore,

to completely remove the code and carrier from the received signal, the

synchronization block needs another process, so-called tracking, in order to produce fine estimates of the signal parameters as well as to dynamically follow their variations A standard tracking process consists of two concatenated loops, which are

code tracking and carrier tracking The two loops are strictly interrelated, and work ina

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concatenated way [12] Basic GNSS receiver tracking loop diagram is shown on Figure 10

ule Iale power”

paths and conelalcd with Gare versions, an carly aud a late of the local PRN code and also the local PRN code the prompt version ‘The two versions early and late are equally spaced, 0.5 chip for T1-CYA, BI, T1-OF, and 0.25 chip for Fl (due to BOC(,1) modulation |6|), about the prompt PRN code Based on the early and late correlation values, the “normalized early minute late power” discrimination function (Equation @)) [3] is formed to tune the code phase ostimate perfectly matching with the received one Figure 11 shows code tracking block diagram

PLL/FLL discriminator

Afo incr

Figure 12 Carrier tracking loop block diagram

The two first multiplications wipe off the carrier and the PRN code of the input signal

To wipe off the PRN code, the Tp output from the early-late code tracking loop described above is used The loop discriminator block is used to find the phase error on

the local carrier wave replica, The output of the discriminator, which is the phase error (or a function of the phase error), is then filtered and used as a feedback to the

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numerically controlled oscillator (NCO), which adjusts the frequency of the local carrier wave In this way the local carrier wave could be an almost precise replica of the inpul signal carrier wave The discrimination function used in this thesis is the phase error (equation (3))

Coordinate | wos-s4 | ps-9002 system GIRE CGU82000

a Frame synchronization

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‘The navigation message of each system consist of superframes, each superframe is

divided into frames, and each frame is made from subframes (Galileo 1 decentralize

the navigalion message inlo frame>sublrame>page>part [4]) The firsk step of the

demodulation process is determining the start of subframe a.k.a frame synchronization

Bach subframe (part with Galileo El) has a preamble which is a specific sequence of

bit, Actually, itame synchronization is Ggnre out this sequence of bil in the imput navigation bit stream Because of the Costas loop’s ability to track the signal with a

180° phase shifl, this preamble can occur in an inverled version Naturally, these bwu

Dit patterns can occur anywhere in the received data so an additional check must be carried out to authenticate the preamble ‘Ihe authentication procedure checks if the

same preamble is repeated every period of time corresponding to the time between

transmissions of two consecutive subframes

The preamble search is implemented through a correlation The first input to the

correlation function is the mooming sequence of navigalion dala bits This sequence is

represented with-l’s and 1’s The second input to the correlation function is the

preamble bits also represented with 1’s and 1’s The location of preambie in the bit

steam is Unc locauon that [he correlation result reaches maximum value (equal to the

length of preamble)

b Navigation mossage decode

After the location of preamble is determined, based on the structure of subframe of

each GNSS, the receiver can decode the subframe to recover the navigation messages Note that some signals are also applicd oncoding scheme such as meander

(GLONASS), FC (Galileo), BCL (Beidou) Then to decode the subframe, the

receiver must decode those encoding schemes first The decoding algorithm can be found on cach system TCD

The navigation message includes (i) ephemeris data, used to calculate the position of

each satellite in orbil, and (H1) almanac data, providing information about the lime and

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status of the entire satellite constellation The content of navigation messages is also

validated in this module based on the error detection and correction mechanism of the

signals, see Table 3

2.2.4, Satellite position computation

From the ephemeris information of the navigation message, the receiver can caloulate

the satcllile position GPS, Galileo and Beidou’s cphemeris consist o[ Keppler orbit parameters, and then the receiver can compute the satellite position based on those

parameters GLONASS transmits satellite ECFF position (with the update period of 15

nunutes) and some paramoters to calculate the differences betwean the satellite position

at the current time and the satellite position at epoch

Each satellite in GNSS systems has a specific orbit This orbit and satellite position in

the orbit can be determined through 6 Kepler parameters as in Figure 13

® The others symbols are Kepler parameters, see Table 4

Table 4, Kepler parameters

7 True anomaly Position in the plane

Tet consider a coordinate system in the orbital plane with origin al the Farlh’s certer Ở

‘The é-axis points to the perigee and the 1)-axis toward the descending node ‘Ihe C-axis

is perpendicular to the orbit plane as in Figure 14

n=7sinƒ =2 xasin E= bsin E = aV1 — £? sinE œ

Trom equation (6) and (7):

where M is mean anomaly Krom equation (8) and (9), we have the relationship

‘between M, E and f The value of M and 5 others Kepler parameters (except f, see

Table 4) are provided in ephemeris (along wilh some other adjustment parameter) The

algorithm used to calculate satellite CUI’ position from these parameters can be found

in [16]

Fach GNSS also uses its own ECEF courdinale system (sec Table 3), and there are

some minor differences between them (origin, axis, some constants .) However,

these differences are quite small and our current works only focus on the standard

positioning solution (with meter acouracy) So we can bypass this error But, if we aim for a high precision solution, we will need a coordinate synchronization scheme

2.2.5 Pseudorange calculation

GNSS systems use electromagnetic waves (which travel with the speed of light) to

transmit signal to receiver So, we can calculate distance between receiver and

satellites by multiply transit Lime with speed of hight However, the accurale transmit

time is not easy to get because of the differences between satellite's and receiver's

clock as in Figure 15

From Figure 15, the travel ie can be compute as:

traveltime = 7 + (ðt„ — dt*) (10) However, the differences between satellite time and system time can be calculated via

clock correction parameters from the navigation message So, we can consider 6t° 0,

then, equation (10) becomes

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6t,, is the same for all satellites of a GNSS system and will be considered as a variable

when the receiver calculates receiver position

Time of transmission Time of receiving

a

ote

cal System time i

Figure 15 Time differences between system time, satellite time and receiver time

On the other hand, it is known that the travel time from the satellites of each GNSS to

5+83 ms

the Earth is within a defined range based on the satellite orbit altitude (e.g

for GPS) This is used to set the initial pseudorange The satellite closest to the Earth is

the satellite with the earliest arriving subframe, then the travel time of this satellite will

be set into a predefined value Travel time of the remaining satellites is then computed

with respect to this channel [18]

Finally, from the travel time, pseudorange can be computed as:

pseudorange = traveltime -c = ct + cét, (12)

However, in addition to clock error, pseudorange is also affected by a number of other

errors In fact, pseudorange has the following values:

«17: Other error sources such as multipath, receiver hardware error ‘hese errors

also cannot be calculalted

From the analysis above, we also need to note that: in the multi-GN3s solution, each

system needs at least 2 satellites because pseudorange is calculated from the

differences between travel tines of satellites

2.2.6 PVT computation

In this seotion, we will bypass the error sources t,, E, 7 in equation (13) In fact,

ionospheric and tropospheric error will be considered to increase accuracy

With tis the accurate travel time, cz is the real distance from satellite to receiver or the

length of the vector connecting the satellite position and the receiver Revert the sign of

&t,, in equation (12) we have

Pg =A GEE a)? + rh — yy? + ah — 3? — ctl aa)

where,

© Py: pscudorange of satellite j [rom system k

2d, y&l, 2&4 coordinate of satellite 7 from syslem k

*® Xu Ya: x! Teceiver’s coordinate

« 6t%: clock error between receiver and system k

Pz, in (14) can be calculated with a receiver estimate position:

Bray = GET B® + OT BLP + GH — A)? — ci q5

where,

©) Bx.j Approximate pscudorange

© dt: Estimate clock error

« £,, 3, 4,: Estimate receiver position

According to the Taylor expansion, we have Lirst-order approximation:

pny = Pay — Peg = aki dm, + ak lay, + ab ax, — cht (16)

where

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© Ax, Ayy, Ady, Aty: Differences between true values and estimate values

© ak, a%), ak cam be determined as:

With 4, ; is the distance hetween satellite 7 and receiver estimate position Actually, we

can use pscudorange py; matead of ; Erom (16), with m systems, each system has

Mt satellites (with total N satellite), we will have a system of equations for each

equation like (12) The matrix form of the equations is:

We need to solve the equation system (18) to calculate AX Generally, A is not a square

matrix, then (18) cannot be solved by a cammon method (multiply Ap by the inversion

matrix of A), and have tu be solved by Isast mean square method as in cquation (19)

lI

where: AP is the transpose matrix of A Matrix (A?A)~1A? is also called pseudoinverse

matrix of A

Combining AX and the zeceiver estimate position, we can get receiver position This

position then is used as receiver estimate position for the next iteration ‘[he receiver

repeats this process until AX is small enough (lower than a predefined threshold)

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‘The condition to implement this solution are the number of variables lower than the number of equations, ie number of rows of A must be greater than or equal to the number of columns of A

Besides, trom matrix A, we can also compute matrix G which characterizes the geometric error of the position as follows:

Bis Bar Bar Ba Br B21 Ba: Bay Baz B72

«= eqtay-1 = [B21 B32 Baa Baz Br

Em B72 Bra Bes Bor

From this matrix, receiver can figure out the dilution of previsian values (DOP) For cxample:

© Geometric Dilution Of Precision (GDOP)

GDOP = 911 + G22 + Gas + Gat + Gss + Ges + G77 A0

® Position Dilution of Pravision (PROP):

> Time Dilution of Preesion (TROP)

‘The DOP values will be used to evaluate the results of positioning

So, through this section, we have an overview of how the receiver performs the positioning since the data is obtained from the antenna till the receiver position is

calculated.