International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
4.1.1 Delayed mode test: The robustness of the infrastructure
and the improvement brought by the Ntrip protocol where
evaluated by using three figures of merit (FOM) (adapted from
Cannon, 2003) for the phase ambiguity fixing. The first one (I,)
consists of the percentage of ambiguities fixed within a given
number of epochs. The second (I,) is the percentage of correctly
fixed positions. Finally, the third FOM (1;) is the time required
to fix a given number of ambiguities (30); this accounts for the
many single-point positions due to the excessive latency of the
corrections. Tables from 2 to 7 show the results of the tests on
the distances of 10 m, 3 km, and 15 km.
The FOMs for the 10 m test allowed us to compare the GSM
and GPRS connections with the Ethernet LAN, whose latency
time was not influenced by network traffic (being the network
so small). The results put in evidence the good response of
GPRS in respect to GMS, and of both GPRS and GSM in
respect to the LAN. The DGGI server did not show differences
with the Nfrip either using the 18/19 messages, or the 20/21
ones.
The tests performed at distances of 3 and 5 km have shown that
Ntrip has a more stable behaviour compared with DGGI, due to
the automatic reconnection system typical of the streaming
protocols. In fact as the distance increases, only using Nerip the
time required to fix a given number of ambiguities (I) remains
almost constant. Also the GPRS network response is nowadays
equal if not better than the GSM one.
Corrections | Ntrip DGGI
Transmission |
Mode | Msg Msg
| 3/18/19 3/20/21
our) 1900 >} 1060
88 100 97
I, UO mee 7
100. oidocoeries due cu.
ics 55200. 1.198. sl d
Table 2. LAN Connection (10 m) a
Corrections Ntrip DGGI
Transmission
3/18/19 3/20/21
I, 100 100 100
SS 32 100 9]
I «7s 95 96
cess 100
>15s 100
I 072 453 196
Table 3. GPRS Connection (10 m)
Corrections | Ntri p TRECE. pou 7»
Transmission | E eee MM RAM
Mode | Msc | Msg
os kot dos on rea teen POND oo BAIR
emi | 390 |. 100. [|
«$5 | 32 0
«2s | 92 91
se Te TTR TTY |
Phe À | 0 1
5 01 10 MO
Table 4. GSM Connection dom =
54
Corrections | Ntrip DGGI |
Transmission | ———— a
Mode Msg Msg |
3/18/19 3/20/21 |
I, 100 100 ed
«Ss 0 45 |
«7s 94 SS |
bs Wl
3. : À
>15s. 100 1 + 100 a
ae 240 over 600s |
Table 5. GPRS Connection (3 km)
Corrections Ntrip DGGI ie
Transmission
Mode Msg Msg
s 318419 | 32021] |
I I, 100 100
<5s 78 81
I “Ts 89 86
2 j<iss 91 89
>15% 100 100
I; 480 over 600 s |
Table 6. GSM Connection (3 km)
| Corrections Ntrip DGGI
Transmission b d
Mode Msg Msg
3/18/19 3/20/21
I, 68 61
<55 0 39
I «s 59
P sls 83 91
ea 105 MO OO oos
[3 300 over 600 s |
Table 7. GPRS Connection (15 km)
4.1.2 Extrapolated mode test: As already said, in
"extrapolated" mode latency times are directly reported in the
NMEA GGA messages. This way, it is possible to plot a
diagram of the latency values relative to the fixed positions.
Figure 4 shows a comparison of the results for the LAN, GSM
and GPRS connections, on the distance of 10 m.
| 90.00
80.00 OLAN-NTRIP |
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
BIGPRS-NTRIP
OGSM-NTRIP
| NOTEN qs aer ofp
Latency s
Figure 4. Comparison of the latency times
The diagram shows that the latency is lower in the LAN
connection (1-2 s), higher in the GPRS (3-4 s), an even higher
in the GSM (over 7 s).
In order to evaluate the improvements brought by the Ntrip-
GPRS systems, the results of the RTK “extrapolated” session,
carried out on September 2003, were compared with those of a
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