International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
for a successful RTK operation is the ability to transmit timely
and reliably the reference station measurements to both rovers,
where the integer ambiguity resolution is performed.
2.2 Format of the reference station measurements
The differential GPS message format plays a significant role in
the reliability of any RTK operation since the induced data flow
can overload the communication link that conveys the base
station measurements to the rovers. The Radio Technical
Commission for Maritime services (RTCM) was the first
organisation to implement a standard structure for GPS
corrections. Each RTCM message contains a. variable number
of 30-bit words, of which the first two serve as the header. In
the frame of real-time positioning, messages 18 and 19 are of
primary interest, and the minimal amount of broadcasted data is
quantified by:
Flow [bps] = f x 2 x freq x (3 + 2 x N) x 5 (1)
where f = measurement rate
freq = 1 or 2, according to the mono or bi-frequency
characteristic of the receiver
N = number of satellites.
However, it is necessary to note that the coordinates of
reference station are also transmitted, yet at a slower rate than
that of the corrections. Nine 30-bit words are necessary to
describe the position of the reference station, which generates
the peak output of:
Flow [bps] 2 £x (2 x freqx (3. - 2x N) *9)x 5 (
No
—
Recently approved for public use, the CMR (Compact
Measurement Record) message was developed by Trimble to
deliver the corrections over communication lines of reduced
bandwidth. In its most recent implementation, the CMR+, the
position of the reference station is transmitted in separate
segments instead of a single block, as is done with the RTCM
message. The formula describing the peak output is:
Flow [bps] = f x (6 + N x (8 + (freqg — 1) x 7) + 16) (3)
À numerical example helps to illustrate these concepts. At the
time of the signal reception of 7 satellites, at 5 Hz a dual-
frequency receiver broadcasts at peak output:
€ Sx(2x2x(3*2x7)t*9)x 57 I925 bytes/s = 15400
bps of RTCM corrections, (4)
e 5x (6+7x(8+10%x7)+ 16) = 635 bytes/s = 5080 bps
of CMR+ corrections. (5)
In order to limit bandwidth and thereby avoid the saturation of
most of the communication lines, we will base our experiments
on the RTK-CMR+ corrections.
2.3 Transmission of GPS corrections via Internet
The choice of a suitable format of corrections is only one aspect
of the deployment of the GPS-RTK technique. Extreme
attention should be paid to the means of broadcasting data,
which must handle the high flow of GPS corrections required
for the accurate determination of the vehicle trajectory in real
time. Because of the increased capacity of the Internet, on-line
radios, which output continuous streams of Internet Protocol
(IP) packets, have become well-established services. Real-time
GPS data transfer requires relatively little bandwidth compared
to these applications. Consequently, the dissemination of RTK
corrections over the Internet constitutes an interesting
alternative to the use of point-to-point links inherent in the
radio and cellular networks.
2.4 NTRIP
In the context of the European reference frame, the Federal
Agency of Cartography and Geodesy of Frankfurt (Bundesamt
für Kartographie und Geodäsie) has developed a real-time
technique for the exchange of GPS data over the Internet
(Weber et al., 2003). The method, named NTRIP (Networked
Transport of RTCM via Internet Protocol) calls upon a
substantial array of servers that allows the simultaneous
connection of thousands of users (Figure 2). This feature, as
well as the difficulty to implement our own services on a
NTRIP server, has driven us to investigate the possibility of
using a single workstation as a server of CMR- corrections.
NTRIP client 1 ss NTRIP client n
HTTP stream
NTRIP broadcaster Telnet
HTTP stream
NTRIP server 1 NTRIP server x
i serial link !
GPS receiver 1 - GPS receiver x
Figure 2. The NTRIP architecture
As the http port (TCP' 80) is generally not filtered by a
firewall, such a server can be built with the assistance of
software that converts a serial flow of GPS data into TCP/IP
blocks. From the client side, the rovers can gain access to this
source of CMR- messages, provided that they are connected to
the Internet. The mobile PAorobus platform requires the use of
GPRS, a radio data transmission service that uses packet
switching on a cellular network. Data are structured in the form
of TCP/IP patterns and are able to reach a maximum flow of
171.2 kbps, which satisfies the bandwidth requirement
expressed in formula 5. Nevertheless, a GPRS-compatible cell
phone cannot transmit GPS corrections to a rover. In the case of
a manual introduction of the server IP address, the cell phone
web browser attempts to interpret the GPS corrections, which in
turn causes the session to time out, thereby losing the GPRS
attachment.
The solution lies in the integration of a GPRS module that
embeds several Internet communication protocols and converts
the TCP/IP data stream back to a serial link. Thanks to such a
peripheral, the rovers behave as if they were directly connected
to the server by a serial cable (Figure 3).
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