Journal of Global Positioning Systems (2007)
Vol.6, No.2: 119-125
Development of the EGNOS Pseudolite System
Ruizhi Chen, Antti Hyttinen, Yuwei Chen
Finnish Geodetic Institute, Finland, Geodeetinrinne 2, Pl-15, FIN-02431, Masala, Finland
Mårt en Strö m, Heikki Laitin en
Space Systems Finland, Kappelitie 6, FIN- 02 200, Espoo, Finland
Michel Tossaint
European Space Agency, ESTEC, the Netherlands
Sven Martin
EADS Astrium GmbH, Germany
Abstract. In order to access the Satellite Based
Augmentation System (SBAS) service, the end user
needs to have a direct line of sight to at least one of the
Geostationary Earth Orbit (GEO) satellites transmitting
the augmentation messages. This requirement is critical
for users in environments such as city canyons, valleys
and fjords since high buildings and mountains in the
vicinity of the end user can easily block the lines of sight
to the GEO satellites. The situation becomes worse at
high latitudes because of the low elevation angles to the
GEO satellites. Even a very low obstacle can block the
lines of sight to the GEO satellites. This limitation
reduces SBAS Signal in Space (SIS) availability
significantly at high latitude especially for land
applications. This paper presents a solution of
transmitting the European Geostationary Navigation
Overlay Service (EGNOS) SIS using a pseudolite. The
EGNOS pseudolite functions in a similar way as a GEO
satellite. It will provide not only a terrestrial-based
solution for transmitting the EGNOS SIS, but also a
ranging measurement for the navigation solution. The
EGNOS pseudolite system mainly consists of a Master
Control Station, an EGNOS Data Server, EGNOS
pseudolites and the user terminal. A preliminary test on a
surveyed site has been carried out to verify the
functionalities of the system. The data set collected from
the test has been processed with two scenarios: one with
four GPS satellites, while the other with three GPS
satellites plus an EGNOS pseudolite. Both data
processing scenarios have similar satellite geometries.
The test result shows that the positioning accuracies are
similar for both scenarios.
Keywords: EGNOS, Pseudolite, SBAS
1. Introduction
EGNOS (European Geostationary Navigation Overlay
Service) is a European Satellite Based Augmentation
System (SBAS). Consisting of three GEO (Geostationary
Earth Orbit) satellites and a network of Ranging and
Integrity Monitoring Stations (RIMS), the EGNOS
system provides the end users with differential
corrections, integrity information, and additional ranging
measurements from the GEO satellites to improve the
positioning accuracy, availability and integrity.
However, the low elevation angles to the GEO satellites
have caused a lot of difficulties for accessing the EGNOS
SIS at high latitudes because even a very low obstacle
can block the lines of sight to the GEO satellites (Chen,
2003). This limitation reduces the EGNOS SIS
availability significantly in Nordic countries especially
for land applications. Although the situation in central
Europe is better than that in Nordic countries, there are
still many locations, such as city canyons, valleys and
downhill ski centres that do not have good visibilities to
the EGNOS GEO satellites.
Non-GEO transmission of the EGNOS SIS has been
investigated in order to improve the EGNOS SIS
availability. These implementations include the EGNOS
TRAN (Terrestrial Regional Augmentation Network)
solution (Redeborn et al., 2003), ESA’s SISNeT (Signal
In Space over the Internet) solution (Toran et Al., 2002a)
and EDAS (EGNOS Data Access System) solution
(Toran an d Ventura-Traveset, 2005).
The EGNOS TRAN solution augments the GEO
transmission of the EGNOS SIS by using terrestrial

120 Journal of Global Positioning Systems
transmission technologies such as GPRS (General Packet
Radio Services), LORAN-C, and VHF (Very High
Frequency) (Redeborn et al., 2003).
The ESA’s SISNeT technology offers one of the
terrestrial dissemination means for dissemination the
EGNOS SIS (Toran et al., 2002a, 2002b; Chen et al.,
2003, 2004). In particular, it allows accessing the
EGNOS SIS through the Internet in real time. The
SISNeT server can be accessed free of charge, only
requiring setting up a free access account (which can be
requested through [e-mail] t ) (Toran et al., 2002a,
2002b). As the user can fetch EGNOS messages
whenever he or she need s without waiting for it from the
GEO satellites, therefore it is possible to reduce the
initiation time for providing the first EGNOS-corrected
coordinates from minutes to tens of seconds over a GPRS
connection.
The EDAS solution extends the SISNeT concept with a
wider sense including more services, more secure and
efficient data transmission links, more stable data server
and more flexible for adopting new serv ices to the syste m
(Toran and Ventura-Traveset, 2005).
All solutions mentioned above cover only the
dissemination or re-transmission of the EGNOS SIS. The
EGNOS pseudolite solution will provide not only a
terrestrial-based dissemination solution for the EGNOS
SIS, but also an additional ranging measurement. The
ranging capability is one of the significant advantages of
the EGNOS pseudolite solution comparing to the TRAN,
SISNeT and EDAS solutions. Furthermore, the EGNOS
pseudolite solution can serve an unlimited number of
users within the radio coverage area of the pseudolite
because it works in a broadcast mode. This is another
advantage over the GPRS solution, which can support
only a limited number of active data calls simultaneo usly
to one base station. Furthermore, the end user does not
need to pay any air-fee for receiving the EGNOS SIS by
using the EGNOS pseudolite solution. This advantage
makes it possible to utilize the solution for hot-spot like
applications that have a large number of users within a
small area. The last, but not least, advantage of the
EGNOS pseudolite solution is that end users do not need
any additional terminal or device to receive the EGNOS
signal because the signal will be received by an EGNOS
enable receiver. However, the navigation software in the
user terminal has to be modified in order to take the
advantages of the modified EGNOS SIS transmitted by
the EGNOS pseudolite.
The only disadvan tage of the EGNOS pseudolite solu tion
is its limited radio coverage. A network of pseudolites
will be required if a large service coverage area is
required. However, the ranging capability from such a
network of pseudolites will be beneficial for improving
the positioning geometry in the degraded environments
such as city canyons, fjords, and locations with difficult
terrains.
2. The EGNOS Pseudolite System
The EGNOS pseudolite system consists of the following
components: the EGNOS data server, the Master Control
Station (MCS), the pseudolites and the user terminal as
shown in Fig. 1.
The EGNOS Data Server is the component that provides
the EGNOS SIS of a particular GEO satellite. It can be
connected either to an EGNOS receiver or to the client
component of the EDAS system. For the first case, the
EGNOS SIS is obtained from the GEO satellite, while for
the second case, the EGNOS SIS can be directly obtained
from the EDAS over a terrestrial data communication
link.
EDAS
DataServer
Antenna UserReceiverand
Terminal
MasterControl
Station
Reference
Receiver
Pseudolite
Pseudolite
GPS
EGNOS
SBASMsg
EGNOS
GEO
Modified
SBASmsgs
Fig. 1. Overview of the EGNOS Pseudolite System
The EGNOS pseudolite transmits a SBAS compatible
signal that including augmentation contents for the
corresponding EGNOS pseudolite. In order to generate
this new SBAS signal, the Master Control Station needs
to:
• Estimate the clock offset of the pseudolite and
synchronize the clock of the pseudolite to
EGNOS network time;
• Encode the clock offsets and other parameters
in the SBAS format;
• Modify the corresponding SBAS messages;
and
• Send the modified SBAS messages to the
pseudolites from which the SBAS messages
will be broadcasted to the end users.
An EGNOS pseudolite is an RF (Radio Frequency)
transmitter that will transmit a SBAS compatible signal
that includes augmentation contents for the corresponding
EGNOS pseudolite.

Chen et al.: Development of the EGNOS Pseudolite System 121
The user terminal will receive the SBAS data streams and
the ranging measurements from pseudolites, GPS and
GEO satellites, and calculate the navigation solution s .
2.1. The EDAS Data Server
The main functions of the EDAS Data Server are: 1) to
obtain the EGONS SIS either from EDAS or from the
GEO satellites and 2) to send the SBAS messages in real
time to the MCS. The SBAS messages will be processed
in the MCS rather than in the EDAS Data Server.
2.2. The Master Control Station
An EGNOS pseudolite will be functioning in a similar
way as a GEO satellite. It transmits a SBAS compatible
signal and provides an additional ranging signal for
positioning. However, the EGNOS pseudolites will not
be monitored by the EGNOS ground segment, thus, the
EGNOS pseudolites have to be monitored and operated
independently. The Master Control Station is the
component in the system to carry out this task.
The MCS configures the system and monitors its
performance. The MCS contains a server to allow remote
connections. This serv ice enables clien ts to connect to the
MCS remotely over the Internet for configuring th e MCS
settings and monitoring the system performance
remotely. Fig. 2 shows the main functions of th e MCS.
Fig. 2. An overview of t he main tasks of the control station.
One of the main tasks of the MCS is to modify the SBAS
messages received from EDAS before forwarding them to
the EGNOS pseudolite for re-transmission. Most part of
the new EGNOS compatible SBAS signals for the
EGNOS pseudolites are based on the original EGNOS
signal. However, any information related to the EGNOS
GEO satellite in the original EGNOS signal has to be
replaced with the corresponding information of the
pseudolite e.g. clock corrections and integr ity info rmation
of the pseudolite and coordinates of the antenna of the
pseudolite. The message types that are needed to be
modified include: MT-1 for the PRN mask, MT 2-5 and
MT-24 for the fast corrections and integrity information,
MT-6 for integrity information, MT-7 for fast correction
degradation factors, and MT 9 messages for position of
the antenna of the pseudolite. The MT 9 messages from
the original EGNOS signal of a GEO satellite will be
replaced by the new MT 9 messages for the pseudolite.
The user terminal will decode the coordinates of the
antenna of the pseudolite from the MT-9 messages and
apply it for the navigation solutions. Therefore, this is a
plug-and-play solution without manually inputting the
coordinates of the pseudolites by the users. Table 1 sh ows
the messages types that might need to be modified.
Fig. 3 shows the modification process of the SBAS
signals to be transmitted by the EGNOS pseudolite.
In addition to the modification of the EGNOS messages,
the MCS is also responsible for monitoring the connected
pseudolite signals. This task is two-fold: 1) the MCS
needs to perform integrity checks, verifying whether the
system is performing as expected, e.g. the characteristics
and correctness of the signal; 2) the MCS also handles the
synchronisation of pseudolite transmission to EGNOS
Network Time. The synchronisation task is done based
on the input from the reference receiver. The clock of the
pseudolite is firstly synchronized to the GPS time using
the receiver as shown in Fig. 4, and then to EGNOS
network time by applying the correction information in
the MT-9 messages of the EGNOS signal.
Table 1. Message types required to b e modified
Content to
be modified Message Type to be
modified/added
Reason for the
modification
PRN mask MT1 Add PRN mask for
pseudolites
Fact
Corrections MT2-5, MT 24 Add fast clock corrections for
pseudolites.
UDRE MT 2-5, MT
24, MT 6 Add integrity information for
pseudolites
Fast
Correction
Degradation
Factor
MT 7 Add Fast Correction
Degradatio n Factor
for pseudolite
Ephemeris
parameters MT 9, MT 17 Add WGS-84 coordinates for the
EGNOS pseudolites.
clock offset
and drift MT 9 Corrections to EGNOS Network
Time for the
pseudolite.
1.1 Receive
SBAS messages
from EDAS
2.2 Receive
EGNOS
Network Time
information
1.2 Modify
SBAS
messages
1.3 Send SBAS
messages to
PLs
2.1 Receive PL
pseudoranges
2.3 Compute
synchronization
corrections for
PLs
2.4 Send sync
messages to PL

122 Journal of Global Positioning Systems
The clock offset p tΔ of the pseudolite from the GPS
time can be estimated as:
m
p
m
ptrtttr Δ+−=Δ⇒Δ−Δ+=
ρρ
where ρ is the pseudorange measurement to the
pseudolite, r is the known distance between the phase
centres of the antennae of the pseudolite and the reference
receiver, m tΔ is the estimate of the clock offset of the
reference receiver from the GPS time plus the corrections
for the cable lengths.
Fig. 3. General pr ocessing logic of modifying or adding an EGNOS
message.
Fig. 5 shows the synchronization accuracy in meters. The
pseudolite is synchronized to GPS time using a very low
cost GPS module (iTrax03) as the reference receiver. The
synchronization accuracy is ±2.3m for a 24h observation
session. The clock offset of the pseudolite is estimated
once per second, encoded in the SBAS messages and
transmitted to the end users via the pseudolite.
Fig.4. Synchronizing the clock of the pseudolite to the GPS time with a
reference receiver.
Clock offset in meters
Fig.5. Accuracy of the time synchronization betw een the pseudolite and
GPS
2.3. The EGNOS Pseudolite
A pseudolite is an RF transmitter installed on the ground.
It is called GPS pseudolite when it transmits a GPS-like
signal (Cobb, 1997). GPS pseudolites are used in many
positioning and naviga tion app lications (Wan g, 2002). As
our pseudolite transmits an EGNOS-like sign al, therefore
it is called EGNOS pseudolite (Chen et al., 2006) as
shown in Fig.6.
The EGNOS pseudolite is based on the GSG-L1E signal
generator from Space Systems Finland. It consists of two
separate modules: a digital module and an RF module.
The digital module contains a CPU and a digital circuit
for generating the PRN code. The embedded software
running on the CPU controls the digital circuit and
handles the communicatio ns towards the MCS. Using the
communication protocol the embedded software receives
and processes e.g. the navigation messages that will be
modulated onto the EGNOS signals.
Identify the corresponding EGNOS
m
ess
a
ges
t
o
be
m
od
ifi
ed
o
r t
o
be
a
dded.
Modify the original
messages with the
encoded augmentation
parameters
Return the modified
message to the same slot
of the origi nal message
trains.
Start
End
Extract the correspond-
ing EGNOS messages
from the original
EGNOS message train
Create a new message
e.g. MT 9 for EGNOS
pseudolites
Message
Exis
t
s ?
Re-schedule the
original message trains
and insert the new
message to the new
messa
g
e train.
No Yes
r
ρ
m
t
Δ
EGNOS Pseudolite Reference
Receive
r
p
tΔ
p
t
Δ

Chen et al.: Development of the EGNOS Pseudolite System 123
The RF module generates the L1 carrier and modulates it
with the C/A code and the navigation message generated
by the digital part. The pseudolite is optimized for high
resolution on the frequency, which is required for
synchronizing its clock to the EGNOS Network Time
with a high precision.
2.4. The User Terminal
The user terminal is a Pocket PC based device holding
the navigation software for exploring the functionalities
of the EGNOS pseudolite system. It works together with
a COST (Commercial of The Shelf) receiver unit with a
modified firmware that can track multiple pseudolites as
shown in Fig. 7.
Fig. 6. The EGNOS pseudolite developed by Space System Finland Ltd.
Fig. 7. User terminal of the EGNOS pseudolite system.
In order to explore the functionalities of the EGNOS
pseudolite, the receiver unit should be able to
• Track the GPS satellites, the GEO
satellites, and the EGNOS pseudolites,
and
• Send the following raw data to the user
terminal over a Bluetooth data
communication link for navigation
processing:
- GPS ephemeris and pseudoranges;
- The raw bit streams (500 bps)
containing the SBAS messages from
the EGNOS psedolites and/or the
GEO satellites (whenever visible);
and
- Ranging measurements to the
pseudolites.
As the GPS module is a low cost one that has limited
processing power. It can decode only one SBAS data
stream in real-time. In our case, it is required to decode
multiple SBAS data streams from the GEO satellites and
pseudolites. Therefore, the decoding process for multiple
SBAS data streams is implemented in the user terminal
side (Pocket PC).
Furthermore, the navigation software in the user terminal
should distinguish the EGNOS pseudolites and the GEO
satellites and apply no ionosphere corrections to the
ranging measurements of the EGNOS pseudolites as
these ranging signals do not go through the ionosphere .
Fig. 8. Components of the EGNOS pseudolite system.
The navigation software in the user terminal has the
functionalities of real-time positioning, data logging and
play-back function. While running in a real time mode,
every single bit across the serial cable can be logged to
the user terminal in binary format. By using the play-back
function of the terminal software, the user can re-process
the same data set with different positioning modes. This

124 Journal of Global Positioning Systems
function is convenient and important for system testing
and performance evaluation. Fig. 8 shows all the
components of the EGNOS pseudolite system.
3. Preliminary Test Results
A preliminary test has been carried ou t on a surveyed site
on the roof of the official building of the Finnish
Geodetic Institute. The objective of th e test was to verify
the functionality of the system. The data set collected
from the test was processed in two scenarios: one with
four GPS satellites while the other with three GPS
satellites plus one pseudo lite. The satellite geometries for
both data processing scenarios are similar as shown in
Fig 9. All EGNOS GEO satellites are masked out for the
test. The EGNOS pseudolite installed on the same roof
was employed in this test. It transmitted the modified
SBAS messages and provided a ranging measurement for
the positioning solutions. No SBAS corrections were
applied for both data processing scenarios.
Fig. 9. Sky plots for two data processing scenarios.
Fig. 10 shows the differences of the positioning
accuracies of these two data processing scenarios. It is
obvious that the positioning accuracies of both scenarios
are similar.
4. Conclusions
The EGNOS pseudolite system brings two benefits to the
GNSS users: additional ranging measurement(s) and the
SBAS corre c tions. A pr e l i minary test h a s be e n carried out
at a surveyed site for testing the functionalities of the
system. The data set collected from the test has been
processed with two scenarios: one with four GPS
satellites, while the other with three GPS satellites plus an
EGNOS pseudolite. Both scenarios have similar satellite
geometries. The test result shows that the positioning
accuracies are similar for both scenarios. To evaluate the
performance of the system and the benefits of the SBAS
corrections, further tests are still needed.
0500 10001500 2000 2500 3000 3500 4000
-5
0
5
east
0500 10001500 2000 2500 3000 3500 4000
-5
0
5
north
0500 10001500 2000 2500 3000 3500 4000
-5
0
5
height
Fig. 10. Positioning accuracy of two data processing scenarios. Solid
line: for the scenario with 4 GPS satellites, dotted line: for the scenario
with 3 GPS satellites + 1 pseudolite.
Acknowledgements
The EGNOS pseudolite system was developed under a
project funded by the European Space Agency. The
authors would like to take this opportunity to thank ESA
for the supports in the project.
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