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The most important aspect of the technical DGPS is the 
transmission of the corrections from the master to the rover 
receiver. 
From this point of view, there are some important initiatives in 
Europe, both in public field and in private one. 
In Germany, the SAPOS System offers four different service 
areas: 
- EPS: broadcasting of DGNSS correction data via audio 
radio transmitter on UHF/RDS and, in collaboration with 
Deuche Telekom AG, on LW/RDS. This is for real-time 
position with an accuracy of 1+3 m and a cycle rate of 3-5 
sec; the user interface is RTCM 2.0; 
-  HEPS: broadcasting of DGNSS correction data, via the 2m 
band radio transmitters of the AdV (Arbeitsgemeinschaft 
der  Vermessungsverwaltungen der Länder der 
Bundesrepublik Deutschland). In different states, this 
service will also be additionally provided using GSM 
mobile telephone. This is for real-time position with an 
accuracy of 1-45 cm and a cycle rate of 1 sec; the user 
interface is RTCM 2.1; 
- . GPPS: appropriation of original GNSS observation data of 
reference stations via Internet. This is for a near real-time 
post-processing with an accuracy less than 1 cm, a cycle 
rate of 1 sec and the user interface is RINEX 2.0; 
- . GHPS: appropriation of original GNSS observation data of 
reference stations via Internet. This is for post-processing 
with an accuracy of <I cm, a cycle rate of 1 sec and the 
user interface is RINEX 2.0. 
In the last two services the observation data is still mainly 
provided via mail boxes, which can be addressed with fixed 
network and radio telephone, or on data carrier. 
In Switzerland, the SWIPOS Service offers two positioning 
service: 
-  Swipos-NAV: the data corrections from the reference 
station are turned into a format RDS compatible and, then, 
transmitted via FM. This service implies that the user has a 
RDS special decoder that transmits to the GPS mobile 
receiver: the user interface is RTCM. The accuracy will 
result dependent from the type of GPS mobile receiver. 
-  Swipos-GIS/GEO: the correction is in real time with GSM 
modem. A telephone business number ( 0900-XXXXXX) 
is associated to a server that takes data from AGNES 
network (21 permanent GPS-stations | uniformly 
distributed in Switzerland). 
In Italy,it has been being operational for many years a network 
of Permanent Stations managed by the A.S.I. (Italian Spatial 
Agency) that coordinates both the reference stations of 
ownership both those displaced near Public Corporate 
(University, Local Corporate, Scholastic Institutes, etc.). 
The offered service refers to the possibility to get in post- 
processing the data of the stations through internet connection 
and the availability of password for the manager. 
Since the end of 2002 there is also a TIM (Telecom Italia 
Mobile) Service called "GeoData". 
It allows the correction of the topographical measurements 
using the GeoTIM network, constituted by 34 (23 certified) 
permanent GPS-stations, uniformly distributed on the whole 
national territory and inserted in the Italian GPS Reference 
Network (IGM95). 
The corrections, elaborated by the GeoTIM network, for 24 
hours a day, are available in RTCM and RINEX data formats. 
The 23 permanent GPS-stations are in the principal Italian 
towns (Ancona,Bari, Bologna, Milano,Roma,etc.). 
69 
Fig 2. Geo-Data (TIM) Network in Italy. 
They are possible two ways of access to the service: 
1. _ Post Processing: the data corrections are available on the 
Web. After the survey with a GPS mobile receiver, memorizing 
data on magnetic support, it’ s possible to acces to differential 
corrections on the Web. It s important to specify:the 
permanent GPS-station, the date and the hours of interest, the 
cycle-rate desired (1 second , 5 seconds, 15 seconds, 30 
seconds). The data will be furnished in the RINEX standard 
data format. At this point with the common elaboration 
softwares on PC, you can operate the corrections of the 
surveying data. 
It is possible to enter in the service through internet, using 
password available with opportune economic accords 
(subscription, payment in case of need, etc.). The connection 
results easy and the download is very quickly, following simple 
and well organized instructions. 
2. Real Time: the correction is directly effected during the 
survey. A telephone number of radiomobile network ( 335 8820 
YYY) is associated to each of the 23 permanent GPS-stations. 
In this way, while you are effecting the survey with the mobile 
GPS, you have to start the call in data transmission, through 
GSM modem. VAS (TIM Service Center) establishes the virtual 
connection with the selected permanent GPS-station, allowing 
this last to transmit a flow of D-GPS corrections. It's used the 
standard protocol RTCM for data transmission. This signal is 
elaborated in real time from the receiver's software, envolving 
the measureaments precision. 
2. EXPERIMENTAL TESTS 
Purpose of the present job it will be to verify the accuracy of 
use of GSM telephone system in the transmission of 
corrections from a master to a GPS rover. 
The system will be composed by following components: 
- . Master Station : permanent GPS-station in the Polytechnic 
of Bari (Caprioli 1997). The system is realized with a GPS 
receiver (Chocke Ring antenna) with GSM modem 
connected to mobile phone network and to PC, that 
acquires data in continuous with URS software. 
- Rover: GPS receiver LEICA system 500- SR530 (tracks 
the L1 C/A code and L2 P-code), dual fre-quency antenna 
AT502, terminal TR500 to set parameters in the receiver 
and to steer the GPS, PC Card of 16 MB like data storage 
medium (it's preferrred to the Internal Memory), Wavecom 
GSM module with‘ business SIM card for data 
transmission, Satelline 3AS/3ASd Radio modem.