Journal of Global Positioning Systems (2002)
Vol. 1, No. 2: 73-84
Architecture and Signal Design of the European Satellite Navigation
System Galileo - Status Dec. 2002
Guenter W. Hein and Thomas Pany
Institute of Geodesy and Navigation, University FAF Munich
Received: 23 December 2002 / Accepted: 24 December 2002
Abstract. This paper starts with a brief discussion of the
Galileo project status and with a description of the
present Galileo architecture (space segment, ground
segment, user segment). It focuses on explaining special
features compared to the American GPS system. The
presentation of the user segment comprises a discussion
of the actual Galileo signal structure. The Galileo carrier
frequency, modulation scheme and data rate of all 10
navigation signals are described as well as parameters of
the search and rescue service. The navigation signals are
used to realize three types of open services, the safety of
life service, two types of commercial services and the
public regulated service. The signal performance in terms
of the pseudorange code error due to thermal noise and
multipath is discussed as well as interference to and from
other radionavigation services broadcasting in the E5 and
E6 frequency band. The interoperability and
compatibility of Galileo and GPS is realized by a
properly chosen signal structures in E5a/L5 and E2-L1-
E1 and compatible geodetic and time reference frames.
Some new results on reciprocal GPS/Galileo signal
degradation due to signal overlay are presented as well as
basic requirements on the Galileo code sequences.
Key words: GPS, Galileo, Signal Design, European
1 Introduction
Based on the communication of the European
Commission of February 9, 1999 the satellite navigation
system Galileo is presently under development in Europe
(European Commission, 1999). The goal on the one hand
is to achieve independence of and also an effective
supplementation to the GPS. On the other hand the aim is
to considerably improve Europe's capability to gain and
preserve an important share of the world market for
satellite navigation and related applications and services.
The strategic and commercial importance of Galileo for
Europe is out of discussion. This has repeatedly been
confirmed at the European Council conferences in
Cologne, Feira, Nice, Stockholm and at last in Laaken on
December 14 and 15, 2001 (Belgian EU Presidency,
2001).
2 Status
The Galileo project is carried out in co-operation by
bodies of the European Union (EU) and the European
Space Agency (ESA). In principle it is to be realised in
three phases: project definition, development and
implementation. The fundamental decision for the
realization Galileo was made by the council of the
European ministers of transport at March 26, 2002.
According to the present planning the development and
validation phase should cover the period 2002-2005, the
implementation phase 2006-2007, and the operational
phase could start in 2008 (Fig. 1).
3 Architecture
The main characteristics of the Galileo system
architecture can be summarised as follows (Weber et al.,
2001):
• Independence of other satellite navigation systems
• Interoperability with GPS (GLONASS)
• Service concept (open, commercial, safety critical,
regulated)
• Implementation of an Integrity Service
(inside/outside Europe)
• Independence between Integrity Service and Galileo
control System (GCS)

74 Journal of Global Positioning Systems
Fig. 1 Galileo master schedule
USER SEGMENT
IULS
ICC
IMS
Network
TTC
L-band
S-
band
L -b
a
nd
S-band
NSCC
OSS
Network
GALILEO
GALILEO
GLOBAL COMPONENT
GLOBAL COMPONENT
NAVIGATION CONTROL &
CONSTELLATION MANAGEMENT
INTEGRITY DETERMINATION
& DISSEMINATION
L-band
NAV
UHF
S&R
….
REGIONAL
REGIONAL
COMPONENTS
COMPONENTS
IULS
ICC
IMS
Network
…
IULS
ICC
IMS
Network
GEO
NLES
MCC
RIMS
Network
EGNOS
LOCAL
LOCAL
COMPONENTS
COMPONENTS
…
Local
MS
DATA
LINK
Local
MS
DATA
LINK
COSPAS-SARSAT
GROUND SEGMENT
SERVICE
CENTRES
MEO CONSTELLATION
UMTS
UMTS
USER SEGMENT
IULS
ICC
IMS
Network
TTC
L-band
S-
band
L -b
a
nd
S-band
NSCC
OSS
Network
GALILEO
GALILEO
GLOBAL COMPONENT
GLOBAL COMPONENT
NAVIGATION CONTROL &
CONSTELLATION MANAGEMENT
INTEGRITY DETERMINATION
& DISSEMINATION
L-band
NAV
UHF
S&R
….
REGIONAL
REGIONAL
COMPONENTS
COMPONENTS
REGIONAL
REGIONAL
COMPONENTS
COMPONENTS
IULS
ICC
IMS
Network
…
IULS
ICC
IMS
Network
GEO
NLES
MCC
RIMS
Network
NLES
MCC
RIMS
Network
EGNOS
LOCAL
LOCAL
COMPONENTS
COMPONENTS
…
Local
MS
DATA
LINK
Local
MS
DATA
LINK
…
Local
MS
DATA
LINK
Local
MS
DATA
LINK
COSPAS-SARSAT
GROUND SEGMENT
SERVICE
CENTRES
MEO CONSTELLATION
UMTS
UMTS
Fig. 2 Overview of the Galileo system architecture
• Global services (SAR, and referred to navigation
data related services)
• Global location and time dissemination on the basis
of a global constellation
• Regional components (Monitor and uplink stations)
• Integration with regional systems (e.g. EGNOS)
• Integration with local (differential etc.) systems
• Compatibility with future mobile radio networks
(UMTS)

Hein & Pany: Architecture and Signal Design of Galileo - Status Dec. 2002 75
It can be seen from Fig. 2 that the main extension of
Galileo compared to GPS consists in the implementation
of a global / regional segment for integrity monitoring.
The goal is to assist the safety critical aircraft navigation
(landing approach CAT I) and to locate and guide railway
trains (Train control).
Space Segment
Fig. 3 Galileo space segment
The space segment of Galileo is intended to consist of a
total 30 Mean Earth Orbiting (MEO) satellites configured
as walker 27/3/1 (+ 3 replacement satellites) constellation
(Benedicto et al., 2000), i.e. distributed over three orbital
planes (Fig. 3). The altitude is 23616 km, and the
inclination is 56°. The satellite design (Fig. 4) is based on
already carried out precursor programs (e.g.
GLOBALSTAR) including critical payload technologies,
which are developed in accompanying ESA programs.
The Galileo satellite has a mass of 625 kg, generates a
primary power of 1500 W and belongs with dimensions
of 2.7 x 1.2 x 1.1 m3; to the category mini-satellites. The
satellite comprises all standard systems for orbit and
attitude control, thermal control, etc. Unlike GPS, also
Laser retro-reflectors will be integrated in order to assist
the orbit determination by satellite Laser ranging.
Attitude
Sensors
L-Band
Antenna
Laser
Retro –
Reflector
Attitude
Sensors
L-Band
Antenna
Laser
Retro –
Reflector
Fig. 4 Galileo satellite with retracted solar generators
The navigation payload is the heart of the Galileo
satellite. The payload is a regenerative transponder with
modern digital and semiconductor technology applied to
the essential subsystems. It consists of atomic clocks
(Clock Monitoring and Control Unit), the signal
generator (Navigation Signal Generation Unit) with CPU,
the frequency generator (FPGU), the output amplifier
(Solid State Power Amplifier) and the L-band antenna
sub-system. As atomic clocks two Rubidium standards (5
10 -13 over 100 s) and two space-borne H-Masers (5 10 -14
over 10000 s) are to be used.
Ground Segment
As already outlined, the Galileo ground segment
comprises the control segment for operation as well as
orbit and time determination (GCS or Ground Control
Segment) and the system for integrity monitoring (IDS or
Integrity Determination System).
The number of elements in the GCS and the IDS are
under further investigation in the present definition phase.
The GCS will consist of about 12-15 reference stations, 5
up-link stations and two control centres. The IDS for
Europe will include 16-20 monitor stations, three up-link
stations for integrity data and two central stations for
integrity computations. In the European area the
integration with the EGNOS ground segment plays an
important role.
User Segment
Like with GPS the Galileo user segment consists of all
users on land, on water, in the air and in space. Fig. 5
shows the shares of various application on the European
GNSS market as predicted for 2005 (volume 8 billion
EURO).
Car Navigation
23% Mobile Phones
73%
Aviation
1%
Fleet Mgt
1%
Augmentation
1%
Leisure
1% Surveying
1%
Car Navigation
23% Mobile Phones
73%
Aviation
1%
Fleet Mgt
1%
Augmentation
1%
Leisure
1% Surveying
1%
Car Navigation
23% Mobile Phones
73%
Aviation
1%
Fleet Mgt
1%
Augmentation
1%
Leisure
1% Surveying
1%
Fig. 5 Prediction of the European GNSS market for 2005
Tab. 1 displays the requirements on the Galileo
performance parameters (elevation mask, accuracy,
coverage, availability, integrity) as posed by two
basically different applications. The requirements for

76 Journal of Global Positioning Systems
Fig. 6 Galileo ground segment (example)
safety critical applications are identical with the aviation
specifications for the precise landing approach of CAT I.
In case of the requirements from the mass market it is
important that these apply for elevation masks above 25°.
This accounts for the specific conditions of land
navigation in urban areas (obstructions, multi-path).
Tab. 1 Selected requirements on Galileo
parameter mass market safety critical
elevation mask 25° 5°
accuracy (95 %) 10 m horizontal 4 m vertical
coverage global global
availability > 70 % > 99 %
integrity not required
mandatory
(6 s, 10 -7 )
4 The Galileo Frequency And Signal Baseline
A tentative Galileo frequency and signal plan was
presented at the ION GPS-2001 (Hein et al, 2001) which
became meanwhile the baseline for the development of
Europe´s satellite navigation system. Over the last
months several modifications took place leading to a
refined signal structure. The main changes and add-ons
are described in the following and after that the complete
signal structure will be outlined.
Recent Developments
In the lower L-band (i.e. E5a and E5b) the central
frequency for E5b was moved to 1207.140 MHz in order
to minimize possible interference from the Joint Tactical
Information Distribution System (JTIDS) and the
Multifunctional Information Distribution System
(MIDS). All signals on E5a and E5b are using chip rates
of 10 Mcps. The modulation for that band is still being
optimized with the possibility to process very wideband
signals by jointly using the E5a and E5b bands. This joint
use of the bands has the potential to offer enormous
accuracy for precise positioning with a low multipath.
Data rates have also been fixed.
In the middle (i.e. E6) and upper (i.e. E2-L1-E1) L-band
data and chip rates were also defined as well as Search
and Rescue (SAR) up- and downlink frequencies.
Extensive interference considerations took place in
E5a/E5b concerning Distance Measuring Equipment
(DME), the Tactical Air Navigation System (TACAN)
and the Galileo overlay on GPS L5; in E6 concerning the
mutual interference to/from radars and in E2-L1-E1
frequencies with regard to the Galileo overlay on GPS
L1.
The EC Signal Task Force and ESA have refined criteria
for the code selection and have as well formulated the
requirements on each frequency. Reference codes have
been selected allowing initial assessments. Parallel
investigations are on-going addressing alternate solutions
for the Galileo codes and targeting improved
performances, see e.g. (Pratt, 2002).
Frequencies and Signals
Galileo will provide 10 navigation signals in Right Hand
Circular Polarization (RHCP) in the frequency ranges
1164-1215 MHz (E5a and E5b), 1215-1300 MHz (E6)
and 1559-1592 MHz (E2-L1-E11), which are part of the
Radio Navigation Satellite Service (RNSS) allocation. An
overview is shown in Fig. 7, indicating the type of
modulation, the chip rate and the data rate for each signal.
The carrier frequencies, as well as the frequency bands
1 The frequency band E2-L1-E1 is sometimes denoted as L1 for
convenience.

Hein & Pany: Architecture and Signal Design of Galileo - Status Dec. 2002 77
Pilot
(*) 1207.140 MHz
1164 MHz 1164 MHz 1214 MHz 1214 MHz
1260 MHz 1260 MHz
1300 MHz 1300 MHz 1189 MHz 1189 MHz
(*) 1176.45 MHz
1278.75 MHz
1544 MHz
1544.2 MHz
1559 MHz
1587 MHz
1591 MHz
1563 MHz
1575.42 MHz
OS Data
10 Mcps
Data 50 sps
Pilot Channel
CS Data
BPSK(5 Mcps)
Data 1000 sps
Pilot Channel
OS/SOL/CS Data
10 Mcps
Data 250 sps
Pilot Channel
PRS Data
BOC(10,5)
Data 300 sps
OS/SOL/CS Data
BOC(2,2)
Data 250 sps
Pilot Channel
PRS Data
BOC(n,m)
Data 300 sps
OS Data
10 Mcps
Data 50 sps
Pilot Channel
CS Data
BPSK(5 Mcps)
Data 1000 sps
Pilot Channel
OS/SOL/CS Data
10 Mcps
Data 250 sps
Pilot Channel
PRS Data
BOC(10,5)
Data 300 sps
OS/SOL/CS Data
BOC(2,2)
Data 250 sps
Pilot Channel
PRS Data
BOC(n,m)
Data 300 sps
Pilot Pilot Pilot
Frequenc y
E5a E5b E6 L1E2 E1
PRS Data
BOC(10,5)
Data TBS sps
PRS Data
BOC(n,m)
Data TBS sps
Galileo Assigned Frequency Band
GPS L5 Band
Glonass L3 Band
Carrier Frequencies
(*) In case of separate modulation of E5a and E5b signals
GPS L1 Band
SAR DownlinkSignals Accessible to all Users, with data partly encrypted
Signals to which access is restricted through the use of
encryption for Ranging Codes and data
Signals to which access is controlled through the use of
encryption for Ranging Codes and data
Fig. 7 Galileo frequency spectrum
that are common to GPS or to GLONASS are also
highlighted.
All the Galileo satellites will share the same nominal
frequency, making use of Code Division Multiple Access
(CDMA) compatible with the GPS approach.
Six signals, including three data-less channels, so-called
pilot tones (ranging codes not modulated by data), are
accessible to all Galileo Users on the E5a, E5b and L1
carrier frequencies for Open Services (OS) and Safety-of-
life Services (SoL). Two signals on E6 with encrypted
ranging codes, including one data-less channel are
accessible only to some dedicated users that gain access
through a given Commercial Service (CS) provider.
Finally, two signals (one in E6 band and one in E2-L1-E1
band) with encrypted ranging codes and data are
accessible to authorized users of the Public Regulated
Service (PRS).
Tab. 2 Main Galileo navigation signal parameters
freq. Bands E5a E5b E6 E2-L1-E1
Channel I Q I Q A B C A B C
modulation type being optimized [AltBOC(15,10) or two
QPSK2]
A Æ BOC(10,5)
B Æ BPSK3(5)
C Æ BPSK(5)
A Æ flexible BOC(n,m)
B Æ BOC(2,2)
C Æ BOC(2,2)
chip rates 10
Mcps
10
Mcps
10
Mcps
10
Mcps
5.115
Mcps
5.115
Mcps
5.115
Mcps
m × 1.023
Mcps
2.046
Mcps
2.046
Mcps
symbol rates 50 sps N/A 250 sps N/A TBD sps 1000 sps N/A TBD sps 250 sps N/A
user min. received
power
at 10o elevation
-158
dBW
-158
dBW
-158
dBW
-158
dBW
-155
dBW
-158
dBW
-158
dBW -155 dBW -158 dBW
-158
dBW
2 Quadrature Phase Shift Keying
3 Binary Phase Shift Keying

78 Journal of Global Positioning Systems
A ½ rate Viterbi convolutional coding scheme is used for
all the transmitted signals.
Four different types of data are carried by the different
Galileo signals:
• OS data, which are transmitted on the E5a, E5b and
E2-L1-E1 carrier frequencies. OS data are accessible
to all users and include mainly navigation data and
SAR data.
• CS data transmitted on the E5b, E6 and E2-L1-E1
carriers. All CS data are encrypted and are provided
by some service providers that interface with the
Galileo Control Centre. Access to those commercial
data is provided directly to the users by the service
providers.
• SoL data that include mainly integrity and Signal in
Space Accuracy (SISA) data. Access to the integrity
data may be controlled.
• PRS data, transmitted on E6 and L1 carrier
frequencies.
A synthesis of the data mapping on Galileo signals is
provided in Tab. 2.
Modulation Schemes
Given the frequency plan defined earlier and the target
services based on the Galileo signals, the type of
modulation of the various Galileo carriers are resulting
from a compromise between the following criteria:
• Minimization of the implementation losses in the
Galileo satellites, making use of the current state of
the art of the related equipments.
• Maximization of the power efficiency in the Galileo
satellites.
• Minimization of the level of interference induced by
the Galileo signals in GPS receivers.
• Optimization of the performance and associated
complexity of future Galileo user receivers.
The modulation chosen for each of the Galileo carrier
frequency is presented in the following subsections. For
the E5 band in particular, the trade-off analysis is on
going between two alternate solutions that will be both
described.
The main modulation parameters for Galileo signals are
summarized on the Tab. 2. The following notation is
used:
- CX
Y(t) is the ranging code on the Y channel (“Y”
stands for I or Q for two channels signals, or A, B or
C for three channels signals) of the X carrier
frequency (“X” stands for E5a, E5b, E6 or L1).
- DX
Y(t) is the data signal on the Y channel in the X
frequency band.
- FX, is the carrier frequency in the X frequency band.
- ScX
Y(t) is the rectangular subcarrier on the Y channel
in the X frequency band.
- m is a modulation index, associated to the modified
Hexaphase modulation.
Modulation of the E5 Carrier
The modulation of E5 will be done according to one of
the following schemes:
A. Two QPSK(10) signals will be generated coherently
and transmitted through two separate wideband
channels on E5a and E5b respectively. The two
separate E5a and E5b signals will be amplified
separately and combined in RF through an output
multiplexer (OMUX) before transmission at the
1176.45 MHz and 1207.14 MHz respective carrier
frequencies.
B. One single wideband signal generated following a
modified BOC(15,10)4 modulation called
AltBOC(15,10) modulation. This signal is then
amplified through a very wideband amplifier before
transmission at the 1191.795 MHz carrier
frequencies.
In case A the E5 signal can be written as:
() ()()( )()()
(
)
()()()() () () tFtCtFtDtC
tFtCtFtDtC
tS
bE
Q
bEbE
I
bE
I
bE
aE
Q
aEaE
I
aE
I
aE
E..2sin..2cos.
..2sin..2cos.
55555
55555
5ππ
ππ
×−×+
×−×
=
The modulation in case B is a new modulation concept
which main interest is that it combines the two signals
(E5a and E5b) in a composite constant envelope signal
which can then be injected through a very wideband
channel. This wideband signal then can then be exploited
in the receivers.
A detailed description of the AltBOC modulation can be
found in (Ries et al., 2002b).
Implementation trade-offs and performance comparison
between the processing of the very wideband
BOC(15,10)-like signal and the joint processing of two
separate QPSK signals of 10 Mcps on E5a and E5b is on-
going.
Modulation of the E6 Carrier
The E6 signal contains three channels that are transmitted
at the same E6 carrier frequency. The multiplexing
scheme between the three carriers is a major point under
consideration today, which shall be carefully optimized.
This optimization process shall take into account payload
4 BOC( fs, fc), denotes a Binary Offset Carrier modulation with a
subcarrier frequency fs and a code rate fc.

Hein & Pany: Architecture and Signal Design of Galileo - Status Dec. 2002 79
and receivers implementation complexity and associated
performances (including compatibility aspects).
The investigated solutions are time multiplexing and a
modified Hexaphase modulation (so-called Interplex
modulation). The modified Hexaphase is taken as
baseline but the final selection process is on going
between those two potential solutions. A QPSK signal
resulting from the combination of two channels is phase
modulated with the third channel. The modulation index
m is used to set the relative power between the three
channels.
Using a Hexaphase modulation, the E6 signal can be
written:
() () ()()
[
]
()()() () ()
[
)..2sin())sin().(...)cos(..
)..2cos())sin(.)cos().(..
6666666
66666
6tFmtSctCtCtCmtDtC
tFmtCmtSctDtC
tS
EE
C
E
B
E
A
E
B
E
B
E
E
C
EE
A
E
A
E
Eπ
π
×+−
×−
=
]




×


To be consistent with the relative powers required
between the three channels, a value of m=0.6155 has
been chosen for the modulation index.
Modulation of the E2-L1-E1 Carrier
In the same way than the E6 signal, the L1 signal
contains three channels that are transmitted at the same
L1 carrier frequency using a modified Hexaphase
modulation. Time multiplexing is also being analyzed.
The E2-L1-E1 signal, using a Hexaphase modulation, can
be written:
() () ()()
()()() ()()
11 1
111
1
11 111
111
.. ().cos(). ()sin())cos(2..)
.. ()cos()...().sin())sin(2..)
AAa Cb
LL L
LLL
LBB bABCa
LL LLL
LLL
CtDtSctmCtSct mFt
St
CtDtSctmCtCtCtSct mFt
π
π

−×


=
−+


The same modulation index of m=0.6155 is used.
5 Galileo Spreading Codes
The pseudo random noise (PRN) code sequences used for
the Galileo navigation signals determine important
properties of the system. Therefore a careful selection of
Galileo code design parameters is necessary. These
parameters include the code length and its relation to the
data rate and the auto- and cross-correlation properties of
the code sequences. The performance of the Galileo
codes is also given by the cold start acquisition time.
A first set of reference codes is being retained that offer a
good compromise between acquisition time and
protection against interference. These codes are based on
shift-registered codes, which will be generated on-board.
The reference ranging codes are constructed tiered codes,
consisting in a short duration primary code modulated by
a long duration secondary code. The resulting code then
has an equivalent duration equal to the one of the long
duration secondary codes. The primary codes are based
on classical gold codes with register length up to 25. The
secondary codes are given by predefined sequences of
length up to a 100.
Further alternative codes are presently investigated
(Pratt, 2002) and flexibility in the on-board
implementation is being considered to foresee the
generation of other types of codes.
Code Length
The code length for Galileo channels carrying a
navigation data message shall fit within one symbol in
order to have no code ambiguity. The resulting code
lengths are shown in Tab. 3.
Tab. 3 Spreading codes main characteristics
channels types of
data
code
sequence
duration
primary code
length
secondary code
length
E5aI OS 20 ms 10230 20
E5aQ no data 100 ms 10230 100
E5bI OS/CS/SoL 4 ms 10230 4
E5bQ no data 100 ms 10230 100
E6A PRS TBD - -
E6B CS 1 ms 8184 -
E6C no data 100 ms 10230 50
L1A PRS TBD - -
L1B OS/CS/SoL 4 ms 8184 -
L1C OS/CS/SoL 8124 25
For the data-less channels, the basic approach is to
consider long codes of 20 ms length. Alternate solutions
are however being investigated. The first one is to follow
a GPS L5 approach consisting of a short code of 1 ms
length equally long to the code in quadrature. The second
one is to have a much longer code, which could have
duration of 0.7 s as in the case of the L2 civil signal.
Especially in the case of E5a and E5b it would be useful
to determine the data-less code length by analyzing the
susceptibility against local interference.
Auto- and Cross-Correlation Properties
The cross-correlation properties (interference) are partly
determined by the actual code sequences as will be
discussed below. Especially for E5a careful code
selection is necessary because at this frequency band
Galileo and GPS use the same modulation scheme and
code rate.

80 Journal of Global Positioning Systems
Acquisition Time
Acquisition time is highly dependent on the applied
receiver acquisition technique, but generally 30-50 s for
cold acquisition time is envisaged for simple receivers on
the E5 signals. For the CS on E6 a acquisition time of 30
s is planned if it is considered as a single frequency
product. If not, there will be no specific requirement of
the E6 acquisition time. Similar consideration applies for
the E2-L1-E1 signal. Again it should be stressed that
acquisition time performance is highly dependent on
affordable receiver complexity.
Encryption
Simple, inexpensive code encryption, which can be
removed on request from the ground, is foreseen for the
encrypted CS. Code encryption should be realized as a
technique controlling the access of code and data without
too much constraints and efforts on the user segment. The
removal of the encryption should not create a legacy
mantle in the user segment and the complexity of the
encryption should be a result of a trade-off of market
analysis and adequate protection needed for securing
those markets.
Service Mapping on Signals
The data carriers will be assigned to provide the
following service categories which are summarized in
Tab. 4.
Tab. 4 Galileo services mapped to signals
Id OS
SF
OS
DF
OS
IA
SoL CS
VA
CS
MC
PRS
E5aI,Q
E5bI,Q
E6A
E6B,C
L1A
L1B,C
CS Commercial Service DF Dual Frequency
IA Improved Accuracy MC Multiple Carrier
OS Open Service PRS Public Regulated Service
SoL Safety of Life Service SF Single Frequency
VA Value Added
The OS signals would use unencrypted ranging codes and
unencrypted navigation data messages on the E5 and E2-
L1-E1 carriers. A single frequency (SF) receiver uses
signals E2-L1-E1B and E2-L1-E1C and might receive the
GPS C/A code signal on L1. A dual frequency (DF)
receiver uses additionally signal E5aI and E5aQ and
potentially the GPS L5 signal. Improved accuracy (IA)
receivers result by using additionally signal E5bI and
E5bQ.
The SoL service would use the OS ranging codes and
navigation data messages on all E5 and E2-L1-E1
carriers.
The Value Added (VA) CS signals would use the OS
ranging codes and navigation data messages on the signal
E2-L1-E1B and E2-L1-E1C and additional CS encrypted
data messages and ranging codes on the signal E6B and
E6C. The Multi Carrier (MC) Differential Application CS
could use in addition the OS ranging codes and
navigation data messages on the signal E5a and E5b.
The PRS signals would use the encrypted PRS ranging
codes and navigation data messages on the E6 and E2-
L1-E1 carriers, represented by signals E6A and E2-L1-
E1A.
6 Search And Rescue
The SAR distress messages (from distress emitting
beacons to SAR operators), will be detected by the
Galileo satellites in the 406-406.1 MHz band and then
broadcasted to the dedicated receiving ground stations in
the 1544-1545 MHz band, called L6 (below the E2
navigation band and reserved for the emergency
services). The SAR data, from SAR operators to distress
emitting beacons, will be used for alert acknowledgement
and coordination of rescue teams and will be embedded
in the OS data of the signal transmitted in the E2-L1-E1
carrier frequency
7 SOME PERFORMANCE PARAMETERS
Overall performance evaluation of Galileo signals is
currently investigated. A major difference of Galileo
signals to the currently emitted GPS signals is the BOC
(resp. AltBOC) modulation scheme and the large
bandwidth employed for most of the signals.
An important parameter in this context is the pseudorange
code measurement error due to thermal noise. Error!
Reference source not found. shows the Cramer-Rao
lower bound (Spilker, 1996) for this value of all Galileo
signals and the GPS C/A and L5 signal. A receiver DLL
bandwidth of 1 Hz is assumed and a value of –205 dBWs
is used to convert the minimum received power to a
typical carrier to noise density value. The power of the of
the processed signals in one frequency and service (i.e.
data and pilot channels) are combined.
From Tab. 5 it is evident that BOC signals exhibit low
pseudorange code measurement errors because the power

Hein & Pany: Architecture and Signal Design of Galileo - Status Dec. 2002 81
spectral density is located at the lower and upper
boundary of the frequency spectrum and not at the center
as it is for BPSK or QPSK signals.
Tab. 5 Code accuracy due to thermal noise
processed signals modulation power
[dBW]
bandw.
[MHz]
code noise
[cm]
E5a or E5b BPSK(10) -155 24 4.6
E5a+E5b, non-coh. BPSK(10) -152 24 3.2
E5a+E5b, coh. BOC(15,10) -152 51 0.8
E6A BOC(10,5) -155 40 1.7
E6B+E6C BPSK(5) -155 24 6.2
L1A BOC(14,2) -155 32 1.2
L1B+L1C BOC(2,2) -155 24 5.5
GPS C/A BPSK(1) -160 24 23.9
GPS L5 BPSK(10) -154 24 4.1
This also implies that the autocorrelation function of
BOC signals shows several peaks and dedicated
algorithms must be implemented in the receiver to track
the correct (central) peak. Tracking of BOC signals is
discussed in (Betz, 1999 and Pany et al. 2002).
Large signal bandwidths allow the use of a very narrow
correlator spacing. Low thermal noise and low code
multipath are the resulting benefits. Code multipath
envelopes differ significantly if BOC and BPSK signals
are compared as shown in Fig. 8 and Fig. 9. For these
figures a coherent early minus late code discriminator is
used. A common discriminator spacing of d=1/14 is
chosen to allow for visual comparisons of all signals and
to track the central peak of the BOC(14,2) signal. The
multipath signal is -3 dB weaker than the direct signal.
Note that typical multipath amplitudes are in the range
between -7 and -10 dB.
Fig. 8 Multipath error envelope, green: BOC(15,10)5, black:
BOC(10,5), blue: BPSK(10), red: BPSK(5).
The figures show that multipath performances of BOC
signals is generally better than for BPSK signals but a
detailed investigations taking into account multipath
mitigation algorithms and dedicated multipath scenarios
will give more insight (Winkel, 2002).
5 A standard BOC modulation scheme was used.
Fig. 9 Multipath error envelope, black: BOC(2,2), red:
BOC(14,2), blue: BPSK(1).
If E5a and E5b are tracked coherently, this results in an
extremely low code tracking error due to thermal noise
(cf. 3rd line of Error! Reference source not found.) and
good multipath mitigation performance. If the E5a and
E5b are tracked separately (non-coherently) as QPSK(10)
signals and combined after correlation (i.e. averaging of
E5a and E5b pseudoranges) the performance gain is
much less (cf. 2nd line of Tab. 5).
8 Recent Results Of Interference Studies
The use of the frequency range 960-1215 MHz,
containing the lower L-band E5a and E5b, by
aeronautical radionavigation services is reserved on a
worldwide basis to airborne electronic aids to air
navigation and any directly associated ground-based
facilities and, on a primary basis, to radionavigation
satellite services. This multiple allocation causes
interference, which has to be assessed carefully to allow
the usage of GPS/Galileo navigation signals for safety
critical applications.
Discussion on interference assessment of DME/TACAN,
JTIDS/MIDS and radar out of band radiation over L5,
E5a and E5b have been conducted since several years.
Interference due to these ground-based sources increases
with altitude since more interfering signals are received.
The sensitive parameter in this context is the acquisition
threshold having limited margins to cope with
interference of 5.8 dB for GPS L5, 4.8 dB for E5a and 3.3
dB for E5b. Tracking threshold and data demodulation
threshold values are a few dB higher. A standard time
domain pulse blanking receiver and advanced signal
processing is assumed to be used (Hegarty et al., 2000).
It should be noted that in contrast to the US, Europe does
not plan at present to re-allocate certain DMEs to
circumvent this problem.

82 Journal of Global Positioning Systems
9 Compatibility/interoperability Of Galileo-GPS
Galileo shall be designed and developed using time,
geodesy and signal structure standards interoperable and
compatible with civil GPS and its augmentations.
Compatibility is in this context understood as the
assurance that Galileo or GPS will not degrade the stand-
alone service of the other system. Interoperability is the
ability for the combined use of both GNSS to improve
upon accuracy, integrity, availability and reliability
through the use of a single common receiver design.
Signal-in-Space
The Galileo/GPS interoperability is realized by a partial
frequency overlap with different signal structures and/or
different code sequences. At E5a (resp. L5) and E2-L1-
E1 (resp. L1) Galileo and GPS signals are broadcasted
using identical carrier frequencies. At L1 spectral
separation of GPS and Galileo signals is given by the
different modulation schemes. This allows jamming of
civil signals without affecting GPS M-code or the Galileo
PRS service.
Using the same center frequencies drastically simplifies
receiver frontend design at the cost of mutual interference
of both systems. This so-called inter-system interference
adds to the interference of navigation signals belonging
to the same system, called intra-system interference. Only
the sum of both types of interference is relevant for
determining the receiver performance.
Interference has been described in (Hein et al., 2001, de
Mateo et al., 2002 and Ries et al., 2002a) and a brief
overview plus update shall be given in the following. For
details we refer to (Godet et al., 2002), where satellite
orbital parameters, antenna diagrams, user locations,
signal characteristics are described. It can be shown that
the C/N0 degradation of GPS C/A code signals due to
Galileo BOC(2,2) signals is never above 0.2 dB over the
world at any time. For the International Space Station it is
0.22 dB. The maximum C/N0 degradation as a function of
geographical coordinates is shown in Fig. 10.
The maximum GPS C/A code intra-system interference
computed is below 2.7 dB. This represents the maximum
self-interference that GPS C/A codes are currently
suffering and explains that GPS C/A real power is about
3 dB above specifications.
Fig. 10 Maximum GPS C/A code C/N0 degradation in [dB] due to inter-system interference from a Galileo BOC(2,2) signal on E2-L1-E1.
The maximum inter-system interference (0.2 dB) cannot
occur at the same time nor at the same space than the
maximum intra-system interference. Conversely, the
maximum intra-system interference is reached when the
inter-system interference is minimal.
The maximum total (intra- plus inter-system interference)
is shown to be slightly above 2.7 dB, which yields a
degradation of current GPS C/A code worst case link
budget by only 0.05 dB6.
6 By modifying the GPS constellation (number of satellites and power),
this value can go up to 0.08 dB, cf. (Godet et al., 2002)
It should be noted that C/A degradation due to other
Galileo signals is much less than for the BOC(2,2) signal
(Hein et al., 2001). Therefore, there is a high confidence
that no GPS user will be affected by the Galileo signal
overlay on L1.
GPS L5 signal C/N0 degradation due to Galileo E5a as a
function of geographical coordinates is shown in Error!
Reference source not found.. Galileo signal degradation
due to GPS signals has also been investigated and a
summary is shown in Tab. 6.
From Tab. 6 it is evident that reciprocal interference
levels are very low on L1. They are more significant in
E5a/L5. We noted in the last section that DME

Hein & Pany: Architecture and Signal Design of Galileo - Status Dec. 2002 83
interference of E5a and L5 signal leaves only a small
margin to civil aviation users at high altitudes, especially
over Europe where no DME reallocation is planned.
Therefore GPS degradation on Galileo in E5a must be
carefully assessed in future work.
Tab. 6 Reciprocal level of interference (worst case link budget
degradation / inter-system C/N0 degradation)
frequency band GPS induced interference
on Galileo
Galileo induced
interference on GPS
L1 0.03 dB/0.09 dB 0.05 dB/0.2 dB
E5a/L5 0.5 dB/0.8 dB 0.2 dB/0.4 dB
Geodetic Coordinate Reference Frame
For the Galileo coordinate reference system international
civilian standards will be adopted. However, for various
reasons the realization of the Galileo coordinate and time
reference frame should be based on stations and clocks
different from those of GPS. These reasons include
independence and vulnerability of both systems, allowing
one system to act as a backup solution for the other.
The Galileo Terrestrial Reference Frame (GTRF) shall be
in practical terms an independent realization of the
International Terrestrial Reference System (ITRS)
established by the Central Bureau of the International
Earth Rotation Service (IERS).
The ITRF is based on a set of station coordinates and
velocities derived from observations of VLBI, LLR, SLR,
GPS and DORIS. A reduction of the individual
coordinates to a common reference epoch considering
their station velocity models is performed using fixed
plate motion models or estimated velocity fields.
GPS uses WGS84 as coordinate reference frame,
practically also a realization of the ITRS, realized by the
coordinates of the GPS control stations. The differences
between WGS84 and the GTRF are expected to be only a
few cm.
This implies for the interoperability of both GNSS
systems that the WGS84 and GTRF will be identical
within the accuracy of both realizations (i.e. coordinate
reference frames are compatible). This accuracy is
sufficient for navigation and most other user requirements
and the remaining discrepancies in the 2 cm level are
only of interest for research in geosciences.
Transformation parameters can be provided by a Galileo
external Geodetic Reference Service Provider – if needed
at all. At the moment it is not foreseen to put such
information in the navigation data message.
A coordinate reference frame has to be accomplished by
an Earth’s gravity model. For example, the WGS84 uses
a spherical harmonic expansion of the gravity potential
up to the order and degree 360. For Galileo a similar
model must be considered. In that context the European
satellite gravity missions GOCE and CHAMP as well as
the American mission GRACE are of importance.
Time Reference Frame
The Galileo System Time (GST) shall be a continuous
coordinate time scale steered towards the International
Atomic Time (TAI) with an offset of less then 33 ns. The
GST limits, expressed as a time offset relative to TAI,
95% of the time over any yearly time interval, should be
50 ns. The difference between GST and TAI and between
GST and UTC(Pred) shall be broadcasted to the users via
the signal-in-space of each service.
The offset of the GST with respect to the GPS system
time is monitored in the Galileo ground segment and the
offset is eventually broadcasted to the user.
The offset might also be estimated in the user receiver
with very high accuracy by spending just one satellite
observation – the accuracy is (probably) higher than that
one (eventually) broadcasted. Thus, broadcasting might
be not necessary for the general navigation user.
Interoperability Summary
The Galileo system follows international
recommendations for steering of its time and coordinate
references (UTC and ITRF). This itself enables a possible
high level of interoperability in case GPS follows the
same, very reasonable, rules.
Acknowledgements
The article is based on work of the European Commission
Signal Task Force (Hein et al., 2002). The underlying
investigations were supported by many European national space
agencies like e. g. Centre National d´Etudes Spatiales (France),
Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany)
and Defence Science and Technology Laboratory (United
Kingdom) Their support and contribution is acknowledged.
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