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

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.

139

Fig. 4. PCB of the GNSS receiver for railway application

3 DVB – T AS A NAVIGATION MEAN

There are problems with the use of GNSS (signal

availability and signal power level) in so called hard

conditions: under leaves canopy, in urban

environment, in hollowed tracks and indoor. Even

various methods have been developed to process

weak GNSS signals the reliable reception is not

satisfied. One method - Assisted GPS (AGPS) – is

closely integrated with communication infrastructure.

It can imply an idea to use it for navigation; there are

a lot of mobile phones providers that use their

networks for user position determination. They use

Signal Strains (SS), Time of Arrival (ToA) or

Differential Time of Arrival (DToA) navigation

methods. But geometry of mobile phones base

stations and their signal properties don’t satisfy

required precision of position.

Properties of signals of DVB-T transmitters are

similar to ranging signals of GNSS systems. We will

deal with the exploitation of the DVB-T signal for

position determination using ToA or DToA

navigation methods. The similar system was

described in (Do 2006, Rabinowitz 2003) where

American ATSC Digital Television (DTV) signal

has been tested for ranging. This DTV system used

8-ary Vestigial Sideband Modulation (8VSB) with

symbol rate 10.76223 MHz. The signal delay was

derived from the symbol synchronization.

3.1 DVB-T signal characteristics

The European standard DVB-T (ETSI EN 2004) is

based on OFDM multiplex with OFDM symbol rate

of approximately 1 – 4 kHz depending on the mode

(2K, 4K or 8K - number of carriers) and signal

bandwidth (channel spacing). The OFDM

synchronization methods are described in (Keller et

al. 2006). The acceptable synchronization error for

successful data demodulation is equal to the fraction

of symbol duration T

i.e. tens or hundreds of

microseconds. This synchronization error

corresponds with the range error in the order of

kilometers or tens of kilometers, which is not

practically usable for navigation.

On the other hand, the minimum accessible

deviation (

)

MIN

of the range measurement in

additive white Gaussian noise (AWGN) channel

(Cook 1969) is determined by (1) and depends on

the signal effective bandwidth

, signal energy E,

and single side spectral power density of the noise

( )

MIN

(1)

where c is speed of light,

( )

f S f df

∞

−∞

∫



(2)

and |S(f)| is spectral energy density of the processed

signal complex envelope. The optimal signal

processing is based on correlation reception.

3.2 Correlation reception and simulation results

The correlation reception is based on the

computation of the cross correlation function of the

processed signal and its known replica. The signal

delay is determined by position of the maximum of

this function. Thanks to the good signal to noise

ratio, the DVB-T signal has not to be processed

continuously like GNSS signal to reach reasonable

measurement error. It is sufficient to work with

snapshots of the signal. The following analysis

focuses on processing of one OFDM symbol

duration snapshots in 8K mode, which is typical

mode for the DVB-T service in the Czech Republic.

The DVB-T signal (ETSI EN 2004) consists of

the transmitted data s

(t), scattered pilot cells s

(t),

continual pilot carriers s

(t), and TPS carriers s

TSP

(t)

( ) ( ) ( ) ( ) ( )

DVB d sp cp TSP

s t stststs t=+++

. (3)

The OFDM symbol with duration T

consists of

symbol part T

, which is extended by the guard

interval

∆

. All components of the OFDM symbol

are orthogonal for the symbol part duration T

The method of replica generation uses a part of

the DVB-T signal only, for example continual pilot

carriers, and the rest part of the signal is assumed as

a signal modulated by the random data. The benefit

of this procedure is the fact that such proper part of

signal is identical in every symbol and is orthogonal

140

to the rest of signal components in vicinity of the

correlation maximum. It means that those random

signals do not affect the range determination. The

weakness of this procedure can be seen in the fact

that only small part of the signal energy is used for

ranging.

The simulated range measurement errors obtained

by the application of the method described above are

in Fig. 5. The theoretical values of standards

deviation of range error (1) and simulated ones for

various SNR are compared in Table 1.

Fig. 5. PDF of range measurement error of the DVB-T signal

The ranging performance (see Table 1) of the

DVB-T signal is very good despite of the fact that

only continual carrier part of the DVB-T signal was

used for range estimation. This is thanks to wide

bandwidth of the DVB-T and good signal to noise

ratio in the service area.

Table 1. DVB-T 8K Signal Range Measurement Error

_________________________________________________

SNR [db] σ

[m] σ

[m]

theory simulation

_________________________________________________

5 0.487 0.422

10 0.274 0.224

15 0.144 0.134

20 0.086 0.079

25 0.048 0.046

30 0.028 0.031

_________________________________________________

The comparable range measurement error in

GNSS receiver can be reached by long averaging

process (Kaplan 2003). The computational

requirements to the DVB-T signal processing for

obtaining comparable error are therefore much lower

than for the case of the GNSS signal processing.

3.3 Multipath sensitivity

Multipath is the main part of errors of the navigation

systems based on ToA or DToA navigation method.

Error can be expressed by so called multipath error

envelope, which shows the worst positive and

negative measurement errors for the channel with

one straight and one reflected delayed signal (Misra

& Enge 2001). Power level of the reflected signal is

expressed by the Multipath-to-Direct Ratio (MDR).

The range error caused by the multipath is lower

for the DVB-T then for the GPS C/A because of the

wider signal bandwidth (Fig. 6). However the

multipath error in DVB-T signal ranging is

negatively influenced by the relatively high adjacent

correlation peaks.

0 100 200 300 400 500

-40

-30

-20

-10

Differential path length [m]

Range error [m]

0 100 200 300 400 500

-40

-30

-20

-10

Differential path length [m]

Range error [m]

DVB-T

GPS L1 C/A (d=1)

GPS L1 C/A, narrow correlator (d=0.1)

MDR -6dB

Fig. 6. Multipath error envelope of DVB-T and GPS

(d is GPS correlator spacing)

3.4 Software receiver for DVB-T ranging (Fig. 7)

Precision of the position determination of the ToA or

DToA systems depends on the signal arrival time

measurement error, transmitters–receiver geometry

and propagation effects, e.g. multipath. The software

DVB-T receiver was developed for further

investigation of ranging performance, propagation

phenomena and positioning tests. The receiver

captures snapshots of the DVB-T signals on various

frequencies and sends them via USB interface to the

PC workstation. Because of the low computational

complexity of the signal processing, the real time

signal processing has been programmed in Matlab

environment.

4 CONCLUSIONS

The Experimental Software GNSS Receiver is the

tool for experiments with real GNSS signals which

cannot be carried out with the commercial receivers.

141

Its main advantage is versatility. The utilization of

the modern development tools for FPGA, signal

processing and embedded computers supports rapid

implementation of the investigated algorithms to the

receiver.

Fig. 7. Experimental DVB-T receiver for navigation

Processing of the wideband Galileo E5 signals

requires more powerful algorithms and therefore

next generation of EGR will be based on Virtex 4

FPGA with higher performance. It will be able to

process all Galileo signals in space.

First analysis and experiments indicate that DVB-

T signal can be used for mobile navigation based on

ToA or DToA positioning principles. The range

measurement errors are comparable with the

measurement errors of the GPS. Moreover, the

DVB-T signal is available indoor at sufficient level.

The DVB-T ranging can be used in stand alone

navigation system or it can be integrated with other

navigation systems.

The next research will be focused on study of

errors caused by the propagation of the DVB-T

signal and transmitters’ geometry. The other

technical problem to be solved is determination of

the OFDM symbol timing or symbol synchronization

to the UTC or GPS time with precision of one

nanosecond and better.

ACKNOWLEDGEMENT

Research described in the paper was financially

supported by the research program MSM

6840770014 of the Czech Ministry of Education.

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