|
A Real-time System for Road Management 2. ENHANCING THE POSITIONING COMPONENT 2.1 Positioning using GPS-RTK A crucial element of any mobile mapping system is registering the frame pixels in a global coordinate system. This process, known as georeferencing, is partially limited by the accuracy of the positioning and orientation components of the image (Zhang et al., 2003). The decade-old RTK technology has gained a wide acceptance in geodetic engineering and in public works. This technology is an extension of relative positioning based on the interferometric principle of exploiting precise carrier-phase measurements in real-time. The attainable accuracy is at the centimetre level provided that the integer ambiguities can be resolved and fixed (Liu, 2003). In our context of a vehicle that is mapping the road centreline via a vertically-oriented camera, the positions of two GPS-RTK receivers and the derived azimuth provide efficient solutions under small banking angles (<4°). The underlying requirement 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 × 2 × freq × (3 + 2 × N) × 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] = f × (2 × freq × (3 + 2 × N) + 9) × 5 (2) 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 × (6 + N × (8 + (freq – 1) × 7) + 16) (3) A 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: • 5 × (2 × 2 × (3 + 2 × 7) + 9) × 5 = 1925 bytes/s = 15400 bps of RTCM corrections, (4) • 5 × (6 + 7 × (8 + 1 × 7) + 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.  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 Photobus 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). 2.5 Towards a mobile NTRIP The implementation of the broadcasted CMR+ messages via Internet is based on the condition that the server belongs to a local area network. To carry out an entirely mobile solution implies two simultaneous connections of GPRS modules that are equipping the base and rover stations with Internet access. Unfortunately, this is difficult to achieve as the cellular phone operators dynamically distribute private IP addresses to SIM cards that consequently do not accept any entering connections. To overcome this drawback, two solutions are foreseeable:
2.6 Field tests The cell phone operators allocate a portion of their infrastructure to GPRS, which decreases the voice capacity and thus challenges the service quality. As opposed to voice, data transmission is particularly sensitive to the network design since each of its bytes is equally meaningful. An interesting indicator of the availability of the cellular network is the Signal Quality Measure, as defined in GSM07.07 recommendation. Ranging from 0 (no signal) to 32 (excellent reception), such measures can be obtained by a periodic invocation of the Hayes command AT+CSQ. The results reflected in Figure 4 illustrate the constant coverage of Lausanne and its neighbourhood. Consequently, the GSM network in Switzerland is highly suitable for the broadcast of RTK solutions, since there is no significant degradation of performance compared to use of other transportation media (Figure 5). Nevertheless, particular attention should be paid to the conversion of serial GPS messages to TCP datagrams, as its misconfiguration enables only DGPS-code positioning accuracy.  Figure 3. Collecting GPS corrections via GPRS  Figure 4. GSM quality in poorly served areas in Lausanne  Figure 5. NTRIP results at EPFL (baseline < 1 km) |