ul 2004 
REAL TIME MAPPING WITH DGPS-ENABLED NAVIGATION EQUIPMENT 
A. Pala?, G. Sanna*?, G. Vacca*? 
* University of Cagliari Department of Structural Engineering, 09123 Cagliari, Italy 
(antpala, topoca, vaccag)@unica.it 
KEY WORDS: Surveying, Internet, Cartography, Updating, GPS, Real-time, User 
ABSTRACT: 
Today, differential GPS techniques allow common GPS receivers to achieve the precision levels required for mapping purposes. The 
development of systems for mobile Internet access (mainly GPRS) provides a fast and reliable method for feeding differential 
corrections to a GPS receiver in any area covered by a cell telephone network. With the increase in the available bandwidth, and the 
example of data streaming applications like Internet-Radio or Internet-TV, the researchers are now trying to use the Internet as a 
medium for transmitting GNSS code and phase corrections for real-time surveys, and examining advantages and limitations of this 
approach. In this context the EUREF (EUropean REference Frame) started in June 2002 a project named EUREF-IP, with the 
purpose of developing a stable and robust infrastructure for broadcasting differential corrections via Internet. Inside this project the 
Topography Section of the University of Cagliari has settled up a permanent station, which since July 2002 sends differential 
corrections over the Internet for DGPS and RTK positioning. The permanent station, identified as “CAGZ”, is part of the IGLOS 
network and, since October 2003, of the EPN network. This paper describes the CAGZ permanent station, the servers software and 
the field tests performed during this two years of uninterrupted GNSS data transmission, to evaluate the performances of the two 
servers activated. Also different network connections (LAN, GSM-Internet, GPRS-Internet) were compared in order to assess the 
improvements achieved by transmission medium. We present also some DGPS ant RTK surveys, performed with geodetic and hand- 
held receivers for updating medium-scale and large-scale cartography. 
1. INTRODUCTION 
With the growing possibilities of the Internet and the increase in 
the available bandwidth, applications like Internet-Radio or 
Internet-TV data streaming are becoming mature and stable. 
This brought the researchers to usc the Internet as a medium for 
transmitting GNSS code and phase corrections for real-time 
surveys. Research in this field has put in evidence advantages 
and limitations of this approach. 
Among the many advantages, one is overcoming the single-user 
limitation, typical of transmission systems like the GSM 
modem. Corrections broadcast via Internet are in fact available 
from an assigned Web address and port, therefore several users 
can connect with any wireless system (GSM, GPRS, UMTS) 
and through any Internet provider to that address and download 
in real time the differential corrections. Another advantage over 
radio transmissions is not being bound to a limited range from 
the reference station, as long as the client has a connection to 
the Internet. 
On the other hand, this method has some drawbacks such as 
high network latency times and sudden disconnections {rom the 
server during the survey. Latency is surely one of the greatest 
problems, especially in RTK surveys, and is substantially tied to 
the data transmission rate, and thus the system used for 
connecting to the Internet. Currently we have a range of 9.6 
kbit/s (with a GSM modem) to 57.6 kbit/s (with a GPRS 
connection), up to a potential rate of 2 Mbit/s for the UMTS 
network (not yet completely operative in Italy). Also, the data 
rate can be influenced by network overload conditions. The 
scientific community tries to find solutions to these problem, in 
order to improve the reliability of the transmission of 
corrections from the server to the clients. In this context the 
EUREF (EUropean REference Frame) started in June 2002 a 
project named EUREF-IP, with the purpose of developing a 
stable and robust infrastructure for broadcasting differential 
corrections via Internet. 
^ 
Inside this project the Topography Section of the University of 
Cagliari has settled up a permanent station, which since July 
2002. sends differential corrections over the Internet for DGPS 
and RTK positioning. The permanent station, identified as 
“CAGZ”, is part of the IGLOS network and, since October 
2003, of the EPN network. 
The transmission of the differential corrections is performed by 
two server applications; one developed at the project start by the 
Cagliari research group named DGGI (Differential GPS and 
GLONASS via Internet); the other, active since June 2003, uses 
the NTRIP infrastructure (Networked Transmission of RTCM 
over IP) developed by the EUREF. The advantages of NTRIP 
over the simple TCP connection used (among others) by DGGI 
are described by its authors in (Weber, 2003) 
This paper describes the CAGZ permanent station, the servers 
software and the field tests performed during this two years of 
uninterrupted GNSS data transmission, to evaluate the 
performances of the two servers activated. Also different 
network connections (LAN, GSM-Internet, GPRS-Internet) 
were compared in order to assess the improvements achieved by 
transmission medium. 
2. THE CAGLIARI PERMANENT STATION 
2.] The Permanent Station 
The first step in joining the EUREF-IP project was building a 
server to broadcast the permanent station corrections. The 
station consists of a GPS-GLONASS Javad-Topcon Legacy/E 
receiver with external frequency source (cesium) and a Regant- 
2 choke-ring antenna. The receiver firmware is able to generate 
the RTCM messages for code-differential and RTK corrections. 
The Internet server runs on a PC with an AMD Duron 700MHz 
CPU, 128 MB RAM, 30 GB HD, and Linux SuSE operating 
system. The PC is connected to the receiver (via serial cable) 
and to the Internet.