Research and development of advanced signal processing algorithms for multi GNSS software receivers (nghiên cứu và phát triển giải thuật xử lý tín hiệu tiên tiến cho bộ thu mềm đa

MINISTRY OF EDUCATION AND TRAINING
HANOI 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 ENGINEERING

ACADEMIC SUPERVISOR:
Dr. Tạ Hải Tùng

Hanoi – 2015


BỘ GIÁO DỤC VÀ ĐÀO TẠO
TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI
---------------------------------------

TRƯƠNG MINH ĐỨC

NGHIÊN CỨU VÀ PHÁT TRIỂN GIẢI THUẬT
XỬ LÝ TÍN HIỆU TIÊN TIẾN CHO
BỘ THU MỀM ĐA HỆ THỐNG GNSS

Chuyên ngành : Kỹ thuật máy tính và truyền thông

LUẬN VĂN THẠC SĨ KHOA HỌC
KỸ THUẬT MÁY TÍNH VÀ TRUYỀN THÔNG

NGƯỜI HƯỚNG DẪN KHOA HỌC :
TS. Tạ Hải Tùng

Hà Nội – Năm 2015

2


Acknowledgements
In the first words of this thesis, I am extremely grateful to those who in various ways
contributed to all the research activities presented in this thesis.
Foremost, I would like to express my sincere gratitude to my advisor Dr. Ta Hai Tung
for the continuous support, for his patience, motivation, enthusiasm, and immense
knowledge. His guidance helped me in all the time of research and writing of this
thesis. I could not have imagined having a better advisor and mentor.
I deeply acknowledge the supports from all the NAVIS and ISMB members, who
everyday show me the intelligence, kindness and hospitality. Among them, I would
like to mention Micaela Troglia Gamba, Emanuela Falletti, Nguyen Dinh Thuan,
Nguyen Thi Thanh Tu whose contributions from the very beginning of my research are
very important.
The list could not be complete without Growing NAVIS project, funded by the
European Commission under the FP7 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, I cannot thank enough my parents and my sister for their belief,
encouragement and love that definitely are my limitless power.

3


Commitment
I commit myself to be the person who was responsible for conducting this study. All
reference figures were extracted with clear derivation. The presented results are
truthful and have not published in any other person‟s work.
HaNoi, December 29th, 2014

Trương Minh Đức

4


TÓM TẮT LUẬN VĂN
Ngày nay, định vị sử dụng vệ tinh đóng vai trò quan trọng trong nhiều lĩnh vực: giao
thông, bản đồ, cứu hộ, giám sát môi trường, quân sự,… Các hệ thống định vị sử dụng
vệ tinh hiện đại bao gồm cả các hệ thống cũ đang được nâng cấp như GPS, GLONASS
hay các hệ thống mới đang được xây dựng như Galileo, Beido đều cung cấp các tín
hiệu mới với các công nghệ tiên tiến như: điều chế dịch sóng mang nhị phân (BOC), kỹ
thuật ghép kênh điều chế sóng mang thích nghi tương quan (CASM)… Một yêu cầu
mới đặt ra cho các bộ thu tín hiệu là phải có khả năng hoạt động với các tín hiệu mới,
hoạt động đa hệ thống.
Tuy nhiên, để đáp ứng yêu cầu này, các bộ thu cứng truyền thống (ASIC) cần phải thiết
kế và chế tạo lại. Điều này yêu cầu chi phí tương đối cao. Do vậy, hướng phát triển các
bộ thu mềm hoạt động trên các vi xử lý có khả năng lập trình được đang được quan
tâm rộng rãi cùng với sự phát triển mạnh mẽ về năng lực tính toán của các vi xử lý này.
Bộ thu mềm có ưu điểm là cấu trúc xử lý linh hoạt, mềm dẻo dễ dàng thực hiện việc
nâng cấp, thay đổi do vậy hoàn toàn có thể đáp ứng được yêu cầu trên.
Ngoài ra, nhiều ứng dụng định vị sử dụng vệ tinh có yêu cầu cao về độ an toàn và tính

chính xác như hàng hải, hàng không, hay đường sắt. Hiện nay, một trong những mối đe
dọa chính tới độ chính xác của dịch vụ định vị sử dụng vệ tinh là can nhiễu như phá
sóng hay giả mạo tín hiệu. Tuy nhiên, tín hiệu GNSS phổ thông không được trang bị
bất kỳ một phương pháp chống can nhiễu nào bên trong nó. Vì vậy, các phương pháp
chống can nhiễu trong quá trình xử lý tín hiệu là một vấn đề cần được xem xét trong
các bộ thu.
Từ những yêu cầu thực tế đó, luận văn tập trung vào nghiên cứu và phát trình bộ thu
mềm đa hệ thống và nghiên cứu về các phương pháp phát hiện giả mạo tín hiệu. Đóng
góp chính của luận văn như sau:
 Nghiên cứu và phát triển một bộ thu mềm đa hệ thống, đánh giá hiệu năng của
giải pháp định vị đa hệ thống so với giải pháp đơn hệ thống sử dụng dữ liệu
thực.
 Nghiên cứu hai phương pháp phát hiện giả mạo tín hiệu, với tên gọi SignTest và
GoF, và đánh giá khả năng hoạt động của hai phương pháp này với bộ dữ liệu
chuẩn TEXTBAT được sử dụng rộng rãi trong đánh giá hiệu năng của các kỹ
thuật phát hiện/ loại bỏ giả mạo tín hiệu.
Từ các nội dung như trên, luận văn được tổ chức như sau:
 Chương 1: Fundamental Background: Chương này giới thiệu tổng quan về
môi trường định vị đa hệ thống – kiến trúc cơ bản, trạng thái hiện tại của các hệ
thống định vị sử dụng vệ tinh hiện có, ưu điểm và thách thức của định vị đa hệ
thống. Cơ sở lý thuyết về bộ thu mềm cũng được giới thiệu ở chương này.

5


 Chương 2: Design of Multi-GNSS receiver: Chương này tập trung vào thiết kế
của bộ thu mềm. Cách triển khai các module của bộ thu mềm và phương thức
hoạt động của từng module cũng được trình bày trong chương này.
 Chương 3: Spoofing detection on software receiver: Chương này tổng hợp các
thông tin cơ bản về hai phương pháp phát hiện giả mạo sử dụng trong bộ thu

mềm và cách triển khai các module này. Ngoài ra chương này cũng điểm qua
một số đặc điểm của bộ dữ liệu chuẩn TEXTBAT.
 Chương 4: Experiment Results: Chương này trình bày về kết quả thử nghiệm
hoạt động của bộ thu mềm với cả 4 hệ thống định vị sử dụng vệ tinh toàn cầu.
Các kết quả thực nghiệm cho thấy giải pháp định vị đa hệ thống là hoàn toàn có
thể thực hiện được và giải pháp này cho hiệu năng tốt hơn giải pháp đị vị đơn hệ
thống về cả độ chính xác, khả năng sẵn sàng và độ tin cậy. Kết quả thử nghiệm
của hai phương pháp phát hiện giả mạo tín hiệu cũng được trình bày trong
chương này. Hai phương pháp này đều cho thấy khả năng phát hiện được tín
hiệu bị giả mạo khi áp dụng với bộ dữ liệu TEXTBAT.
 Kết luận
Tóm lại, luận văn đã thực hiện được các yêu cầu đặt ra ban đầu như nghiên cứu và phát
triển một bộ thu mềm đa hệ thống, thử nghiệm hoạt động của bộ thu mềm; nghiên cứu,
kiểm thử khả năng phát hiện giả mạo của hai phương pháp SignTest và GoF.

6


Table of Contents
Acknowledgements .......................................................................................................... 2
Commitment ..................................................................................................................... 4
List of Acronyms.............................................................................................................. 9
List of Figures ................................................................................................................ 11
List of Tables.................................................................................................................. 12
Introduction .................................................................................................................... 13
Chapter 1: Fundamental Background .......................................................................... 16
1.

Multi-GNSS environment................................................................................. 16
1.1.

1.2.
1.3.
1.4.

2.

Architecture of GNSS ................................................................................ 16
Status of all GNSSes .................................................................................. 19
Advantages of multi-GNSS ....................................................................... 22
Challenges of multi-GNSS ........................................................................ 23

Software Receiver............................................................................................. 24
2.1.
2.2.

Software receiver overview ....................................................................... 24
Software receiver architecture ................................................................... 25

Chapter 2: Design of Multi-GNSS receiver ................................................................ 41
1.
2.

Receiver general architecture ........................................................................... 41
Module implementation .................................................................................... 42
2.1.
2.2.
2.3.
2.4.
2.5.

Signal synchronization............................................................................... 42
Data demodulation ..................................................................................... 47
Satellite position computation ................................................................... 54
Pseudorange computation .......................................................................... 56
PVT computation ....................................................................................... 57

Chapter 3: Spoofing detection on software receiver ................................................... 62
1.

Spoofing detection theory................................................................................. 62
1.1.
1.2.
1.3.
1.4.

2.
3.

Hypothesis test ........................................................................................... 62
Application of hypothesis test to GNSS receivers..................................... 63
Sign Test .................................................................................................... 65
Goodness of Fit Test .................................................................................. 65

Spoofing detection method implementation ..................................................... 66
TEXTBAT datasets .......................................................................................... 70

Chapter 4: Experiment results ..................................................................................... 73
1.
2.

Graphic User Interface...................................................................................... 74
Signal synchronization and Demodulation module.......................................... 76

7


2.1.
2.2.
3.

PVT Computation ............................................................................................. 79
3.1.
3.2.

4.

Signal synchronization............................................................................... 76
Data demodulation ..................................................................................... 79
Single GNSS solution ................................................................................ 81
Multi-GNSS solution ................................................................................. 84

Spoofing detection ............................................................................................ 87

Conclusion...................................................................................................................... 93
References ...................................................................................................................... 95

8


List of Acronyms

AFS

Atomic Frequency Standards

AGNSS

Assisted Global Navigation Satellite System

AWGN

Additive White Gaussian Noise

BOC

Binary Offset Carrier

CDMA

Code Division Multiple Access

CPU

Central Processing Unit

DLL

Delay Lock Loop

DOP

Dilution Of Precision

DSP

Digital signal processing

ECEF

Earth-Centered, Earth-Fixed

EGNOS

European Geostationary Navigation Overlay Service

FDMA

Frequency Division Multiple Access

FEC

Forward Error correction

FFT

Fast Fourier Transform

FLL

Frequency Lock Loop

FPGA

Field-programmable gate array

GDOP

Geometric Dilution Of Precision

GEO

Geostationary Earth Orbit

GNSS

Global Navigation Satellite System

GoF

Goodness of Fit

GPS

Global Positioning System

GPU

Graphics processing unit

ICD

Interface Control Document

IF

Intermediate Frequency

IGSO

Inclined geosynchronous orbit

IRNSS

Indian Regional Navigation Satellite System

9


MEO

Medium Earth orbit

MSAS

Multi-functional Satellite Augmentation System

NCO

Numerically controlled oscillator

NDU

Navigation Data Unit

PDOP

Position Dilution Of Precision

PLL

Phase Lock Loop

PNT

Positioning, Navigation and Timing

PRN

Pseudo-Random Noise

PVT

Position, Velocity, and Time

QZSS

Quasi-Zenith Satellite System

RF

Radio Frequency

RNSS

Regional Navigation Satellite System

SDR

Software Defined Radio

TDOP

Time Dilution Of Precision

TEXTBAT

Texas Spoofing Test Battery

WAAS

Wide Area Augmentation System

10


List of Figures
Figure 1. GNSS architecture ....................................................................................... 17
Figure 2. Architecture of navigation payload ............................................................ 18
Figure 3. Control segment architecture and functions [12] ..................................... 18
Figure 4. Augmentation Systems ................................................................................ 21
Figure 5. Number of SVs in multi-GNSS systems ..................................................... 21

Figure 6. Spectrum of GNSS signal ............................................................................ 22
Figure 7. Satellite navigation principle ...................................................................... 26
Figure 8. GNSS signal processing chain..................................................................... 26
Figure 9. Acquisition’s search space .......................................................................... 29
Figure 10. Basic GNSS receiver tracking loop diagram ........................................... 30
Figure 11. Code tracking loop block diagram ........................................................... 31
Figure 12. Carrier tracking loop block diagram ....................................................... 31
Figure 13. Kepler parameters and satellite orbit. ..................................................... 34
Figure 14. The elliptic orbit with(ξ, η)coordinates.................................................... 35
Figure 15. Time differences between system time, satellite time and receiver time
........................................................................................................................................ 37
Figure 16. Receiver GNSS Architecture .................................................................... 41
Figure 17. Block diagram of the parallel code phase search algorithm .................. 43
Figure 18. Flowchart of acquisition algorithm .......................................................... 44
Figure 19. Complete tracking loop diagram .............................................................. 45
Figure 20. Flowcharts of tracking algorithm............................................................. 46
Figure 21. Flowchart of GPS data demodulation function. ..................................... 48
Figure 22. Flowchart of GLONASS data demodulation function ........................... 49
Figure 23. Flowchart of Galileo data demodulation function .................................. 51
Figure 24. Flowchart of Beidou data demodulation function .................................. 52
Figure 25. Flowchart of finding preamble function. ................................................. 53
Figure 26. Flowchart of pseudorange calculation algorithm ................................... 58
Figure 27. Flowchart of receiver position calculation algorithm ............................ 59
Figure 28. Correlators profile in the absence (upper) and presence of disturbances
(bottom) [8] ................................................................................................................... 64
Figure 29. Example of p-value. k=10 .......................................................................... 67
Figure 30. Flowchart of SignTest function ................................................................ 68
Figure 31. Flowchart of Chi2GoF function ............................................................... 69
Figure 32. The antenna on the roof (the circled one) ................................................ 73
Figure 33. Graphic User Interface of our receiver ................................................... 74

Figure 34. Choosing data file error in GUI. .............................................................. 75
Figure 35. Search space of acquisition. ...................................................................... 76
Figure 36. Available PRN ............................................................................................ 77

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Figure 37. Result of DLL and PLL of the tracking loops......................................... 78
Figure 38. Signal synchronization result.................................................................... 79
Figure 39. Data demodulation results ........................................................................ 80
Figure 40. Skyplot of all GNSS system ....................................................................... 81
Figure 41. GPS PVT result – Google Earth ............................................................... 81
Figure 42. Skyplot (left) and Positioning accuracy (right) of all GNSSes............... 83
Figure 43. Skyplot & Positioning accuracy of {GPS & Galileo} combination. ...... 84
Figure 44. Skyplot & Positioning accuracy of {3 GPS & 2 Beidou} combination. 85
Figure 45. Skyplot & Positioning accuracy of {four systems} combination. .......... 86
Figure 46. Time history of C/N0 for different PRNs ................................................ 88
Figure 47. Doppler frequency fD of different PRNs. ................................................. 89
Figure 48. Results of Sign Test applied to a pair of correlators with dEL=1.5chips
and integration period Tint = 1ms .............................................................................. 90
Figure 49. Results of GoF test applied to a pair of correlators with dEL=1.5chips
and integration period Tint = 1ms .............................................................................. 91

List of Tables
Table 1. Characteristics of GNSS civil signals .......................................................... 27
Table 2. Acquisition parameters of each signal [21] ................................................. 28
Table 3. GNSS navigation message characteristic. ................................................... 32
Table 4. Kepler parameters ......................................................................................... 35
Table 5. GNSS system’s preamble parameters. ........................................................ 54
Table 6. Sign Test and GoF Test Parameters. ........................................................... 69

Table 7. Intermediate frequency and bandwidth of GNSS ...................................... 73
Table 8. Performance of stand-alone positioning solutions ..................................... 83
Table 9. Standard deviation ratio. .............................................................................. 87

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Introduction
Nowadays, satellite navigation plays an important role in many fields such as transport,
maps, find and rescue, environmental monitoring, military applications … Since the
80s of the last century, Global Navigation Satellite Systems (GNSS) were built like
GPS or GLONASS. And until now a lot 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. The modern GNSSes provide new signal with advanced technology for
instance: binary offset carrier (BOC), coherent adaptive subcarrier modulation
(CASM),… In respond to this fact, GNSS receivers should be upgraded to work with
new signals and new systems.
However, most of traditional GNSS receiver is hardware receiver, whose signal
processing part is performed by application-specific integrated circuit (ASIC). Those
receivers are able to process signal at high speed, but they have low flexibility and
upgradeability (in order to do that, the whole receiver should be re-designed and remanufactured and this require a pretty high cost). In order to solve this problem, we
can use the software receiver approach. This allows us to 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 harsh environment…) can be
implemented easily.
On the other hand, the existence of multi-GNSS also leads us to some questions. Can
we use those systems together to get a multi-system solution or just use them
separately? If we can combine them to form a solution, how does its accuracy compare
to the single solution? And what is the drawback?

Moreover, many GNSS applications are considered safety critical. For example,
maritime, aviation, and rail transportations often require stringent performance and the
detection of signal degradation to preserve accuracy. Currently, one of the main threats

13


to GNSS accuracy is interference such as jamming and spoofing. Unfortunately, the
open GNSS signal does not have any “built-in” anti-interference method. So, antiinterference methods also need to be considered in a GNSS receiver. In a fully SDR
receiver, all intermediate results can be accessed easily, allowing us to implement antiinterference techniques base on those results.
Understanding about those problems, my colleagues at NAVIS center and ISMB and I
conduct research and development on a multi-GNSS receiver and also the antiinterference methods. Spoofing detection is focused in this thesis, while interference
mitigation 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 performance of
multi-GNSS solution over the single solution with real collected data.
 Research and implement two spoofing detection methods, namely Signtest and
GoF test, and validate 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
multi-GNSS environment – general architecture, status of GNSS systems,
advantages and challenges of multi-GNSS environment and the theoretical basis
about software receiver.
 Chapter 2: Design of Multi-GNSS receiver: This chapter focuses on how the
receiver is designed. The implementation of modules inside the receiver and the
ways they work are also presented in this chapter.
 Chapter 3: Spoofing detection on software receiver: This chapter summarizes
the basis information about the spoofing detection methods used in our receiver
and how they are implemented. Some characteristics of the TEXTBAT datasets

are also shown here.

14


 Chapter 4: Experiment Results: This chapter shows the results of the receiver
with real data testing from all four available GNSS systems. These results 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.
 Conclusion.
The results of this work have been published in 1 national conference, 2 international
conferences:
 Truong Minh Duc, Ta Hai Tung (2013). Development of real multi-GNSS
positioning solutions and performance analyses. In Advanced Technologies for
Communications (ATC), 2013 International Conference on (pp. 158-163).
IEEE.
 Truong Minh Duc, Vinh The La, Tung Hai Ta, Gustavo Belforte (2013), MultiGNSS Positioning Solutions with Real Data Collected in South-East Asia
Region, ISGNSS 2013, Istanbul, Turkey.
 Truong Minh Duc, Gamba, M. T., Motella, B., Falletti, E., Ta Hai Tung (2014),
Enabling GNSS software receivers with spoofing detection techniques: a test
against some TEXBAT datasets, COMNAVI 2014, Hanoi, Vietnam.

15


Chapter 1:
1.

Fundamental Background

Multi-GNSS environment

In any period of human history, navigation always has an important role for many
purposes:

exchanging

positioning

information,

leading

the

ways,

military

application,… Since ancient times, people have used a lot of methods to navigate such
as based on the known location, construction milestones (lighthouses for instance),
monitoring ocean currents, observing the position of the constellations combined with
the use of a compass and map… However, these methods have limitations such as high
computational complexity, imprecision and low reliability. Along with the
development of science and technology, satellite navigation was born, initially for
using in the military field and then be extended to other civil applications.

1.1.

Architecture of GNSS

In principle, a GNSS is comprised of three main segments, namely:
 Space segment
 Ground segment
 User segment
The interactions among these segments are illustrated in Figure 1.
1.1.1. Space segment
The space segment is the satellite constellation of a GNSS. GNSS satellites transmit
PRN-coded signals from which the ranging measurements are made. The signals also
contain navigation data including the information about the position and status of the
satellites in the sky. Basically, a satellite includes payloads (e.g. navigation payload,
nuclear detonation detection system - NUDET...) and vehicle control subsystems.

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


Figure 2. Architecture of navigation payload

Figure 3. Control segment architecture and functions [12]
1.1.3. User segment
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


 Identification of the satellites in view,
 Estimation of the distance between the user and the satellites
 Trilateration to estimate the user location

1.2.

Status of all GNSSes

In this section a short overview of the status of the different analysed GNSS is
presented for reference.

 GPS: navigation satellite system of the US, as remarked GPS has been the first
GNSS and it has been continuously working for decades. The 1st 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
elevation orbits (MEO) satellites on 6 uniformly spaced orbits with an
inclination of 55° on the equatorial plane.
In June 2011, three of the 24 slots 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 foresees 27 MEO working satellites, nine for each
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 enabling 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].

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 GLONASS: It is the second oldest GNSS developed by the old Soviet Union
and then Russia since 1976. After a decline in capacity during the late 1990s, in
2001, the restoration of the system was made a top government priority and
funding 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 24 working satellites in the system [9].
 Beidou: This Chinese GNSS foresees the use of three different types of
satellites: geosynchronous equatorial orbit (GEO), inclined geosynchronous
orbit (IGSO) 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 satellites; other five are IGSO and finally there are 4 MEO satellites.
New launces should take place in the next future in order to extend the total
number of satellites in the constellation [4].
 QZSS: This Japanese RNSS is constituted, for the time being, of only one IGSO
satellite. Soon others satellites 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
that have always a quite high elevation for users in Japan and in other East and
SEA 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 system will act as an
augmentation system providing information enabling error corrections in the
covered region and it will also offer new additional services [11].

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 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
overtime.

Figure 5. Number of SVs in multi-GNSS systems
Moreover, each GNSS also broadcast different signals; Figure 6 shows the spectrum of
all signals from current GNSSes.

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Figure 6. Spectrum of GNSS signal

1.3.

Advantages of multi-GNSS

In the past few years, we only can use GPS to navigate, but right now and in the near
future, we will have much more systems to use. This fact gives us some benefits such
as:
 Availability increase: Obstacles like high buildings, trees… in urban canyons
prevent a receiver from receiving signals from at least 4 satellites of a GNSS
system for positioning purposes. However, if the receiver can work with other
GNSS systems (i.e. more navigation satellites), the problem of lacking satellites
is solved. This is the scenario in which the multi-GNSS environment shows its
most importance.
 Accuracy improvement: Multi-GNSS environment is significant for not only
availability but also accuracy improvements. Research in (Shega, 2013) showed
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.

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 Reliability increase: Intentional and un-intentional interference sources,
including jammers and spoofers, are major threats for GNSS services. The
redundancy 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, multi-GNSS also brings us some challenges:
 Inter-system interference: GNSSes broadcast navigation signals in overlapped
frequency bands. This fact could be convenient from the point 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 modern signals,
which have advanced but complex structures, in multiple frequency bands.
These signals give much improvement in terms of accuracy, availability, and
reliability to navigation services, but also challenge receivers to accommodate
for such advantages. In multi-GNSS solutions, the analog parts 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. This fact increases the computational complexity, the
resource capability requirements and eventually the cost of the receiver.
Recently, together with the rapid improvement in computational capability of
programmable processors such as CPU, GPU, DSP, and FGPA…, the software

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receiver approach, which minimizes the hardware requirements, is favourable
for the multi-GNSS solution because of its flexibility, easy-to-upgrade for
complex requirements.
However, the important advantages of multi-GNSS environment 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 computation speed but low flexibility and
upgradeability.
However, it is consolidated that the Software Defined Radio (SDR) technology applied
to GNSS receivers produces significant benefits for prototyping new equipment and

analysing signal quality and performance [13]. Thanks 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 environments and new
systems and services (such as GLONASS, EGNOS, Galileo, BeiDou/ Compass, QZSS,
commercial high precision satellite services, local and regional differential services,
etc...). The advantages of a fully SDR in this highly dynamic scenario, with respect to
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 frequency, code and
carrier phase, navigation message, etc…, are fully accessible, making easy to

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implement and test new advanced signal processing algorithms based on those
intermediate results.
Furthermore, the demand in upgrading old receivers to get a multi-GNSS solution,
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
fundamental 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 fast microprocessor development in computational 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 architecture

Basically, GNSSes are based on trilateration technique for positioning. With this
technique, a receiver needs to measure the distances from its location to at least 4
known points (i.e.satellites) – 3 for its 3 coordinate and 1 for the time error between
receiver and GNSS system time. These distances and points form 3 spheres whose
intersection determines the receiver‟s location (Figure 7). Moreover, along with
receiver position, we can also determine receiver velocity and the accurate time.
So, in order to get the receiver position, velocity and time, the GNSS receiver 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 does
these jobs through its signal processing chain (Figure 8).

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