International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 1
Number 2
June 2007
137
Software Navigation Receivers for GNSS and
DVB
F. Vejrazka, P. Kovar, M. Eska & P. Puricer
Czech Technical University in Prague, Prague, Czech Republic
ABSTRACT: We describe the software GNSS receiver, its schema, implementation into a computer, results
of tests and application for railway, municipal transportation and for shipping of dangerous matters. The
receiver, originally for the Galileo system, is on a printed board which is the size of a Euro Card (160?100
mm). Because the Galileo signal is not in the air, it was modified for the GPS and GLONASS systems.
Experimental GNSS receiver (EGR) was used as a tool for its development and it is also described. Even if
we use the receiver which is able to process signals of all three systems, it is impossible to ensure reception of
GNSS signals in adverse conditions (under leaves canopy, in urban canyons, in hollow tracks, etc.). Therefore
we have studied the possibilities of communication systems which will use modern signals known from
satellite navigation and we have obtained very interesting results when we used DVB-T transmitters as
beacons.
1 EXPERIMENTAL GNSS SOFTWARE
RECEIVER (EGR)
Requirements of the Czech Ministry of Transport
have led us to development of a Galileo navigation
receiver. After analysis of state of Galileo system
and other GNSS navigation systems we formulated
our own requirements on the receiver which have
been as follows:
1 The processing of all GPS, GLONASS, SBAS
and Galileo signals
2 High flexibility and rapid implementation of the
new signal processing
3 Enough performance for the very complex signal
processing
Those requirements can be satisfied by the
software defined radio (SDR) architecture. The
principal schema of the software defined radio is
shown on Fig. 1.
RF front
end
Analog
to
Digital
Convertor
Programable
logic
Computer
Fig. 1. Block diagram of the SDR receiver solutions
Requirements mentioned above have led to
receiver concept which will allow to process signals
with wide frequency bandwidth. The signal samples
are firstly processed in the programmable logic,
where the bandwidth of the signal is reduced and
then the signal is processed in a computer. Both
programmable logic and the computer can be parts
of a FPGA.
Demand of flexible development of hardware and
software of the receiver has tended to an experimental
hardware platform which would allow very simple
replacement of particular components and blocks
(radio frequency filters, etc.) and allow very easy
creation and realization of signal processing
algorithms.
138
The block diagram of the experimental GNSS
receiver is in Fig. 2. The receiver consists of three
parts: radiofrequency unit, DSP unit and PC
workstation.
The radio frequency unit consists of four
independent radio channels which can operate at any
frequency in range 1 – 2 GHz. The bandwidth of
each channel is 24 MHz; the RF unit supports active
and passive GNSS antennas. The intermediate
frequency is 140 MHz, gain of the receiver can be
controlled either by AGC (>40 dB) loop, either via
external input signal by DSP. The output signal of
radio frequency unit is digitalized by 8 bit A/D
converter with sampling frequency 80 MHz and the
remaining signal processing is performed in
programmable digital hardware. The advanced FPGA
Virtex II Pro by Xilinx with integrated PowerPC
processors is applied and placed in prototyping board
which has reduced technological demands for FPGA
board development and construction.
Main task of connected PC is to be user interface.
It also translates programs in Simulink language,
runs them, loads them into FPGA and serves
as display and control unit for their verification in
EGR.
LNA
Channel 1
Cannel 2
LNA
A/D
Virtex II Pro
Prototyping
Board
DSP Xilinx
DSP Unit
Radio Frequency Unit
GNSS antenna
Synthetiser
PC workstation
Cannel 3
LNA
Cannel 4
LNA
Fig. 2. Block diagram of the Experimental GNSS Receiver
(EGR)
The receiver is equipped with switched power
supplies and high precision frequency reference with
stability 0.03 ppm. The complete receiver mounted
into the 19-inch rack is in the Fig. 3. The receiver is
capable to process the all known navigation signals
except of the Galileo E5 signal. The concept of the
modernized version of the experimental receiver
with higher performance and capable to process
Galileo E5 signal was prepared; receiver is currently
in prototype realization state.
EGR has served as a development tool and has
been used for development of the GNSS receiver for
railway applications.
Fig. 3. Experimental GNSS Receiver in 19-inch rack
2 GNSS RECEIVER FOR RAILWAY
APPLICATIONS
The Czech Republic is characteristic by its dense
railway network because of the long tradition of this
kind of transport. Besides of the primary network
(railway corridors), which is equipped with the
railway signalling interoperable with the systems
used by surrounding countries and EU, the
secondary railway network is usually safeguarded
with the national non compatible systems. The use of
GNSS in safety railway system would bring many
savings.
Since the position information is to be used by
railway station equipment for traffic control and
guaranty of safety in appropriate area, the GNSS
receiver as a source of information has to meet
reliability, integrity and safety requirements based on
common standards for signalling systems (EN
50129).
The main problem of the use of commercial
GNSS receiver in railway operations is verification
of proper function of its software. Therefore the
Ministry of Transport decided to order design of the
receiver which algorithms could be documented and
proved by a simple way.
The hardware of the designed receiver supports
reception and processing of signals of all three
systems (GPS L1 C/A, GPS L2C, GLONASS,
Galileo E1 and WAAS/EGNOS). Receiver is built
on PCB of Euro Card size (160×100 mm, Fig. 4),
designed for industry temperature range -40 ÷
+80 ºC and the mechanical construction meets the
standards for rail signalling systems. The circuit
design utilizes solely modern 3.3V technology.
Consumption of the receiver is approximately 4W.
Receiver cooling is ensured by the passive cooler
with no rotating parts. The receiver was successfully
tested as the GPS receiver by the Spirent simulator.