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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
similar session performed in August 2002 using the DGGI
server, when the EUREF-IP system was just starting.
Comparing the values (Figure 5) puts in evidence the
remarkable improvement brought by the Nrrip system and by
the more stable GPRS network.
followed on foot the path to be surveyed. The first surveys
regarded elements already present in the 1:1000 maps, in order
to evaluate the precision of the system. Later, new elements
were surveyed and inserted in the map. The results confirmed
the expected precision for the real-time differential mode, which
was sub-meter.
60.00
50.00 ENTRIP ——4
m DGGI
40.00 +—
30.00 |—
Percent %
20.00 {-—
10.00 |— fl
1.2 3 4 50 7.8.9, 0. 11402 534 BB 171817 -
Latency s
Figure 5. NTRIP-DGGI server comparison
4.0 DGPS Tests
The initial DGPS mode test was performed using the Javad-
Topcon geodetic GPS+GLONASS receiver in the Cagliari city
area, scheduled in order to have the maximum GLONASS
visibility. The receiver, besides sending the NMEA messages to
the connected PC, recorded the raw data on its internal memory
for later post-processing, in order to have a comparison with the
real-time results. The raw data recorded in the survey were
processed in kinematic mode with those of the permanent
station CAGZ. For every epoch, we calculated the distance
between the point obtained from DGPS and the one from
kinematic post-processing (KIN-PP), and the distance between
stand-alone position (PPS) and KIN-PP (Figure 6). The
differences in height were calculated by the same way. The
r.m.s. were 0.89 and 1.27 m for DGPS, 5.5 m and 22.5 m for
SPP, respectively in horizontal and in height.
us
Q Kin-ppc (
@ DGPS+DGLONASS
Q SPP
Figure 6. Comparison between obtained positions
Figure 7 shows a GPS survey performed in order to update a
1:10000 cartography in the Cagliari area.
Other DGPS tests were performed with a Trimble GeoExplorer
hand-held GPS receiver.
The tests were performed in small urban areas in Cagliari.
DGPS mode tests have been performed both in motion and in
tixed positions; the instrument was carried by an operator which
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i
i
i
i
i
LA
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Figure 8. DGPS survey using hand-held receiver
5. CONCLUDING REMARKS
The tests have shown that the positioning precision level
obtained using the Internet as a transmission medium for
differential GPS code and phase corrections depends upon
different factors, such as the protocol used or the connection
system. It was evident that using a simple TCP-connection
protocol is not sufficient to ensure the necessary robustness to
the system, while using multicast-oriented systems like Icecast
the stability is substantially improved.
In this work we preferred the GPRS over the GSM as a
connection system, because of the competitive costs offered by
the telephone companies in Italy. In fact, the GPS connection
are priced on the volume of transmitted and downloaded data
(which is very low in the case of DGPS data), while the GSM
costs are time-based. Anyway, the latency times with the GPRS
connection have been found to be lower than the GSM ones,
