Journal of Global Positioning Systems (2005)
Vol. 4, No. 1-2: 176-183
Galileo Receiver Core Technologies
Pavel Kovář, František Vejražka, Libor Seidl, Petr Kačmařík
Czech Technical University in Prague, Faculty of Electrical Engineering, Technická 2, 1 66 27 Prague 6, Czech Republic
Tel: [phone] Fax: Email:
Received: 30 November 2004 / Accepted: 12 July 2005
Abstract. The modern satellite navigation system Galileo
is developed by European Union . Galileo is a completely
civil system that offers various levels of services
especially for civil users including service with safety
guarantee. Galileo system employs modern signal
structure and modern BOC (Binary Offset Carrier)
modulation. The Galileo Receiver is investigated in the
frame of the GARDA project solved by consortium under
leadership of Alenia Spacio – LABEN. The aim of
Galileo Receiver Core Technologies subtask is to
investigate the key problems of the Galileo receiver
development. The Galileo code and carrier tracking
subtask of the Galileo Receiver Core Technologies is
carried out at the Czech Technical University. The
problem was analysed and split to the particular tasks.
The aim of this paper is focused on BOC correlator
architecture. The correlation function of the BOC
modulation is more complex with a plenty of correlation
peaks. The delay discriminator characteristic of such
signal has several stable nodes, which cause stability
problem. The standard solutions of this problem like
BOC non-coherent processing, very early – very late
correlator and deconvolution correlator are analysed. The
new correlator architecture for BOC modulation
processing has been developed. The developed correlator
has two outputs, one for fine tracking and the second one
for correct node detection. The second output is based on
comparison of the correlation function envelopes. The
simplified method of correlation function envelope
calculation is described in this paper. The correlator is
planned to be tested in the GRANADA software
simulator including a sophisticated method of correlator
output combination.
Key words: Galileo, Galileo core technologies, Galileo
receiver, code tracking, carrier tracking.
1. INTRODUCTION
1.1 Galileo
The European GNSS system Galileo (that is currently
under development) operates on the same ranging
principle as the existing GPS and GLONASS systems do.
The big benefit of this system is that it is a completely
civil system, which offers to the user various types of
services, which are adjusted to the civil user
requirements. Besides Open Service, which is free of
charge, the system offers services with guarantee of the
service performance by the system provider, customer
driven local element services and Public Regulated
Service for governmental needs.
Galileo shares the same basic operating principle with the
GPS, but the system architecture and service model are
based on the latest knowledge.
1.2 GARDA project
The basic architecture of the Galileo user receiver is
similar to the GPS one, yet some approaches to the
receiver design are more complex. Galileo receiver
development is investigated within the GARDA (GAlileo
Receiver Development Activity) project, performed by a
consortium established under the leadership of Alenia
Spacio – LABEN. GARDA is funded by the GJU
(Galileo Joint Undertaking) in the frame of the Galileo
R&D activities under the EC 6th Framework Program.
The project consists of three tasks, which cover Galileo
user receiver development including development plan
consolidation, software Galileo receiver development,
receiver prototyping, and last, but not least, core
technology task.

Kovář et al.: Galileo Receiver Core Technologies 177
1.3 GRANADA
GRANADA (Galileo Receiver ANalysis And Design
Application) is the software simulator of the Galileo
developed in the frame of GARDA project by Deimos
Space company. Software simulator consists of Bit-True
GNSS SW Receiver Simulator and GNSS Environment
and Navigation Simulator.
Mono-channel Bit-True GNSS SW Receiver Simulator
serves for detail analyses of the Galileo signal processing,
signal propagation, multipath propagation, interference,
and other related problems. Bit-true simulator is based on
detail modelling of the signal processing inside the
Galileo receiver.
On the other hand, the Environment and Navigation
Simulator is determined for analyses of the position
determination algorithms, satellites constellation etc. The
macro model of the receiver behaviour, propagation
channel, noise, etc. are employed in this simulator.
The only basic most common features and algorithms of
the Galileo receiver are implemented to the simulator.
Some marginal problems of the Galileo are simplified or
not implemented.
1.4 Galileo Core Technologies
The aim of the core technologies subtask is to investigate
the critical principles and technologies of the Galileo
system. The technologies are to be tested with the
GRANADA software simulator. The other goal of the
core technology task is to implement the new features to
the GRANADA software.
The Galileo receiver core technology task was launched
in January, 2005, thus the current state of the task is the
preliminary phase and the problem is being analysed. The
analysis and preliminary experiments results of the
Galileo receiver core technology are concentrated in this
paper.
Two main Galileo core technologies have been assigned
to the Czech Technical University:
• Galileo code tracking,
• Galileo phase tracking.
The present simulation results with GRANADA tool
have mainly verification purpose. The fundamental
problems like performance of tracking loops in presence
of additive white Gaussian noise were analysed. The
performance parameter (variance of tracking error in this
case) was compared with theoretical assumptions with
good agreement. This simulation also showed some
weakens and inconveniences of GRANADA mainly
belong to impossibility to perform a multi frequency
signal tracking.
2. PROBLEM ANALYSIS
The code and carrier tracking are very complex problems,
which very closely relate to each other. The main
function of the Galileo receiver is an estimation of the
code delay and the carrier phase of the receiving signal.
The estimation is usually realized by use of correlation
reception principle, where the replica of the Galileo
signal is synchronized with the received signal. The
feedback tracking circuits are commonly used. The
tracking loops can be classified to the following main
categories:
1. Single frequency scalar tracking loops –
individual tracking loops for each satellite signal
2. Multicarrier scalar tracking loops – complex
tracking loops for all signals of individual
satellite
3. Multicarrier vector tracking loop (VDLL) – one
complex tracking loop for all signal components
of all satellites
The other classification criterion of the signal tracking
methods is according to interaction of the code and
carrier tracking:
1. Independent code tracking and carrier tracking
2. Independent carrier tracking and code tracking
with carrier aided
3. Integrated (joined) code and carrier tracking
The last classification approach to the code and phase
tracking is according to the design principle of the loop
low pass feedback filter:
a) Deterministic approach (classical control filter),
b) Stochastic approach (Wiener or Kalman filter).
The problem can be analyzed according to many other
criteria. Basically code and carrier tracking is very similar
to the GNSS signal tracking, but several problems arise in
consequence with higher Galileo signal complexity. This
problem has been identified and some of them will be
solved in the frame of core technology project. The
identified particular problems are listed below:
1. BOC and AltBOC discriminator
a. Delay discriminator
b. Phase/Frequency discriminator
c. Detection of the correct peak of the
correlation function

178 Journal of Global Positioning Systems
d. Sensitivity of the discriminator to
multipath
2. Cycle slip detection technique
3. Ambiguity resolution
4. Tracking strategy
a. Independent code tracking and carrier
tracking
b. Independent carrier tracking and code
tracking with carrier aided
c. Integrated (joined) code and currier
tracking
5. Tracking loops
a. Tracking loop de velopment method
b. Dynamic performances of the tracking
loops
c. Loop stability
6. Tracking strategy in environment with
shadowing
In this early phase of Galileo development, the research is
focused on the basic solution of most critical problems.
The one of the key problem of the Galileo receiver is the
processing of the ranging signal with BOC (Binary Offset
Carrier) modulation. This problem is analyzed in the rest
of this paper.
3. STANDARD GNSS CORRELATOR
The essential navigation receiver block for an estimation
of the pseudorange is called correlator. The standard
GNSS correlator is designed for BPSK modulated
ranging signal. The adoption of the standard GNSS
correlator for BOC modulated ranging signals is
discussed in this paragraph.
The architecture of adopted delay correlator is very
similar to the BPSK one, see Figure 1. The ranging code
(
)
0
cNft
⎢
⎥
⎣
⎦ and digital carrier
(
)
(
)
0
sgn sin 2
M
ft π can be
multiplied and the resulting code
(
)
,MN
ct can be used for
the despreading of the received signal.
(
)
(
)
(
)
(
)
,0 0
sgn sin2
MN
ctcNft Mft π =⋅
⎢⎥
⎣⎦ (1)
Figure 1. BOC correlator
The BOC delay discriminator characteristic of Early
minus Late amplitude discriminator and Early minus Late
power discriminator for BOC(1,1) modulation are
displayed on the Figure 2.
-2 -1.5 -1 -0.500.5 11.5 2
x 1 0
-6
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
E-L amplitude
d is c r im inator
E-L power
discriminator
Figure 2. BOC(1,1) delay discriminator characteristic

Kovář et al.: Galileo Receiver Core Technologies 179
The problem of the BOC correlator is in existence of
more than one stable node on the discriminator
characteristic, see Figure 3. The problem with multiple
stable nodes is even more complicated for higher order
BOC modulation, where a plenty of these nodes occur.
Range of st ability
Dτ
0
Stable node
Unstable nod
e
Dτ
0
Stable node
Unstable nod
e
R ange of stability
BOC(1 5 ,1 0)
BOC(1,1)
Figure 3. Stable and Unst ab l e no d es of the BOC discriminator
characteristics
The number of false stable nodes in coherent delay
discriminator characteristic for modulation BOC(N, M) is
given by
21
22
N
SM
−
⎢
⎥
=
⎢
⎥
⎣
⎦. (2)
This problem causes significant reduction of the r ange of
the delay lock loop (DLL) stability. The DLL can
potentially track false stable node without any indication.
Discussed problem is demonstrated by the following
simulation, see Figure 4. The several experiments of the
DLL hang-up stage are displayed on this figure. The
initial delay error of each experiment is set to zero value.
The DLL mostly tracks the correct node. Some of the
experiments diverge to the false node or totally diverge
due to the noise in loop.
False node tracking of BOC modulated signal is a very
serious problem, which must be solved.
Figure 4. Simulation results of the tracking errors of BOC(1,1) signal by Early minus Late power correlator

180 Journal of Global Positioning Systems
4. EXISTING BOC CORRELATORS
4.1 Non-coherent BOC processing
Since the both sidebands of BOC modulation contain the
same information the particular sideband can be
processed separately and result can be non-coherently
combined, see Figure 5. Of course, this method is non
optimal and does not use BOC modulation benefits. On
the other hand, the particular sidebands can be easily
processed in classical BPSK manner. The separate
sideband processing can also be useful when one of the
two sidebands is corrupted with interference.
USB Filter
LSB Filter
Correlator
Correlator
Early
Early
Late
Late
|x|
2
|x|
2
|x|
2
|x|
2
0
0
ω
ω
ω
Figure 5. BOC non-cohere nt processing.
4.2 Very early – very late correlator
The most obvious way to handle the problem with
tracking of correct peak of BOC modulation correlation
function is the technique denoted as very early – very late
(VEVL) correlator, also known as “bump-jump” method,
see Fine and Wilson (1999), Barker et al.(2002). In
comparison to classical early – late correlator structure,
VEVL has a further couple of early and late taps, see
Figure 6. This extra couple of taps are adjusted to track
the side-peaks of correlation function.
PRN generatorCod e NCO
Σ
Eary Late
discriminator
w r ong pe ak
detection
pseudorange
rece ived
signal
lo op filter
Early
Late
Prompt
Very-earl
y
Ve ry- late
Σ
Σ
Σ
Σ
Figure 6. Structure of Very Early Very Late correlator.

Kovář et al.: Galileo Receiver Core Technologies 181
The early and late taps together with prompt tap are
intended for tracking the correct (centre) peak of the
correlation function like in the classical early – late
correlator. The spacing (a correlator width) is adjusted to
enable tracking the narrow peak of particular type of
BOC correlation function. The additional very early –
very late taps are set to watch the side-peaks of the
correlation function. When the correlator tracks the
correct correlation function peak, the prompt tap output is
greater than from very-early and very-late ones. In case
of repetitively greater output from very-early or very-late
taps, the wrong peak tracking is declared. Then the phase
of a local signal replica is adjusted to restore the tracking
of the correct peak.
4.3 Deconvolution correlator
This method is based on the linearization of discriminator
characteristic (S-curve function) with using of multiple
taps in the correlator structure, see Fante (2003). The
discriminator characteristic of the classical no-coherent
two taps early and late correlator (NCEL) is given by
22
( )(/2)(/2)SRD RD ττ τ =+−− , (3)
where ()R
τ
is cross-correlation function,
τ
is tracking
error and D is the spacing between the early and late
taps. The two taps discriminator characteristic for BOC
modulation has multiple wrong stabile nodes (Figure 3).
To obtain the linear monotonic discriminator
characteristic in the entire range of tracking error
τ
, the
number of taps are incorporated into correlator structure.
The outputs of particular taps are then scaled by ()am
coefficients to meet this demand. The discriminator
characteristic is then given by
22
1
()( )((0.5))
N
m
SamRmND ττ
=
=+−+
∑, (4)
where the N is the number of couples of taps. In
comparison to early late structure, this correlator has
worse sensitivity.
5. PROPOSED BOC CORRELATOR
The aim of the development of the new correlator is to
find such a correlator that fully utilize the BOC
modulation benefits and is not sensitive to the false node
tracking. The developed correlator should have two
outputs; first output should be equal to the tracking error
of coherent processing of BOC modulated signal and the
second one should compare envelopes of correlation or
similar product which has only one stable tracking node.
The first section of the correlator is comprised of the
BOC delay correlator (Figure 1). The second section is a
sum of the both side-band early minus late discriminators
(
)
U
D
τ
and
(
)
L
D
τ
which is derived from side-band
correlators outputs
(
)
U
R
τ
and
(
)
L
R
τ
(Figure 7).
The upper-side-band correlator
(
)
U
R
τ
gives correlation
between received BOC modulated signal
(
)
,NM
ct
and
spectrally shifted PRN code
(
)
,NM
x
t,
(
)
(
)
0
j2
,0
M
ft
NM
xtcNfte
π
=⋅
⎢⎥
⎣⎦ . (5)
The BOC modulated signal can be decomposed to
Fourier series as follows
(
)
()
()
()
()
0
,
j2 2 1
0
,2 1
2jsgn(21)
21
2jsgn(21) .
21
MN
nMft
n
Nn M
n
ct
n
cNft e
n
nxt
n
π
π
π
∞+
=−∞
∞
+⋅
=−∞
=
−⋅ +
=
⋅=
⎢⎥
⎣⎦ +
−⋅ +
=+
∑
∑
(6)
We can resolve this situation in frequency domain
(
)
() ()
,
0
F
2jsgn(21)
221
21
MN
N
n
ct
nXMfn
n
π
π
∞
=−∞
= ⎡⎤
⎣⎦
−⋅ +
=+
+
∑ (7)
where
(
)
N
X
ω
is spectrum of the PRN code with chip-
rate 0 Nf .
The signal ,(2 1)
jsgn(2 1)
21 NnM
nx
n+
−
⋅+
+ is one of the PRN
components of the BOC modulated signal. Due to the
limited (however non-zero) cross-correlation between
,(2 1)NiM
x+ and ,(2 1)NjM x+, ij≠, the proposed upper
sideband correlator
(
)
U
R
τ
estimates cross-correlation
between spectrally shifted PRN code
(
)
,NM
x
t and related
component ,
2NM
x
π
of the received signal. The correlator
output
(
)
U
R
τ
is given by
() () ()
2
UN
RR
τ
τετ
π
=+, (8)
where the
(
)
U
R
τ
is an autocorrelation function of PRN
code with chip-rate 0
Nf and co mponent
(
)
ε
τ
covers the
cross-correlation remainder of other signal components
(
)
,NM
wt
()
()
()
()
,,2 1
0
,,
2jsgn(21)
21
2,
NM Nn M
n
n
MN NM
n
wtx t
n
ct x
π
π
∞
+⋅
=−∞
≠
−⋅ +
=
=
+
=−
∑
(9)

182 Journal of Global Positioning Systems
()( )()
1
*
,,
d
MN NM
T
ctwtt ετ τ =+
∫. (10)
Analogically, the lower-size-band correlation is given by
()() ()
*
2
LN
RR
τ
τετ
π
=+. (11)
The output of discriminator second section
(
)
2
D
τ
summarizes the sideband outputs
(
)
U
D
τ
and
(
)
L
D
τ
.
Suitability of discriminator characteristic is conditioned
by monotony of the
(
)
U
R
τ
and
(
)
L
R
τ
sides. It depends
mainly on the relationship of the wanted correlation
()
2N
R
τ
π
and the parasitic correlation
(
)
ε
τ . The
situation is much better for higher order BOC modulation
(
M
N>> ).
Thus, this correlator (Figure 7) has been designed and
simulated. The calculated discriminator characteristic of
the correlator for low order modulation BOC(1,1) is
shown on the Figure 8. T he di scri m i nat or ch aract eris ti c of
the proposed correlator has only one stable node, which is
convenient.
IF signal
Carr ier NCO
PRN generator
Code NCO
Correlator
Correlator
Correlator
Early
Early
Early
Late
Late
Late
|x|
2
|x|
2
|x|
2
|x|
2
|x|
2
|x|
2
cos( )sin( )
x
j
x
sin x
sgn x
cos x
D
U
D
1
D
2
D
L
−
π
2
−
π
2
Figure 7. Proposed BOC correlator.
-2 -1.5 -1 -0.5 00.5 11.5 2
x 10
-6
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2x 10
10
D1
D2
Figure 8. Discriminator characteristic for BOC(1,1) modulation
In the frame of GARDA project described BOC
correlator is planned to be investigated and tested in
GRANADA Galileo system simulator. For example, the
sophisticated method of combining information from
both correlator outputs should be developed and tested.
6. CONCLUSIONS
The Galileo receiver development is carried out in the
frame of GARDA project. The project is financed by the
GJU (Galileo Joint Undertaking) in the frame of the
Galileo R&D activities under the EC 6th Framework
Program. The key technologies concerning Galileo
receiver and Galileo correlators are developed. The
Czech Technical University is GARDA project
consortium member with responsibility for the Galileo
code and carrier tracking problems.
The Galileo system uses some modern sophisticated
modulation schemes based on the BOC modulation. The
correlation function of the BOC modulated signal has
several correlation peaks, which cause the problem of
detection of the correct one. In the frame of the project

Kovář: Galileo Receiver Core Technologies 183
the new correlator for processing the BOC modulated
signal has been developed. The developed correlator has
two delay discriminator outputs: the first for fine tracking
and the second based on comparison of the correlation
function envelope power. The discriminator characteristic
has only one stable node and serves for the detection of
incorrect tracking node. The correlator is planned to be
tested with the GRANADA tool.
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Barker B.; Betz J.; Clark J.; Correia J.; Gillis J.; Lazar S.;
Rehborn K.; Straton J. (2002): Overview of the GPS M
Code Signal [online], MITRE Technical Papers Archive,
[cit. 2004-11-04]
http://www.mitre.org/work/tech_papers/tech_pa
pers_00/betz_overview/betz_overview.pdf
Fante R. (2003): Unambiguous Tracker for GPS Binary-
Offset-Carrier Signals [online], MITRE Technical Papers
Archive, [cit. 2004-11-04]
http://www.mitre.org/work/tech_papers/tech_pa
pers_03/fante_tracker/fante.pdf
Fine P.; Wilson W. (1999): Tracking Algorithm for GPS Offset
Carrier Signals Proceedings of ION 1999 National
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