Networked Transport of RTCM via Internet Protocol (NTRIP)

Transcription

1 Networked Transport of RTCM via Internet Protocol (NTRIP) G. Weber, D. Dettmering Bundesamt für Kartographie und Geodäsie (BKG), Richard-Strauss-Alle 11, Frankfurt am Main, Germany H. Gebhard, Lehrstuhl für Kommunikationstechnik, Universität Dortmund, Otto-Hahn-Str. 4, Dortmund, Germany Abstract. Within the framework of EUREF, the Federal Agency for Cartography and Geodesy (BKG) has developed a new technique using the open Internet for the real-time collection and exchange of GNSS data, as well as for broadcasting derived products. A major purpose of these activities is the dissemination of Differential GPS corrections (DGPS) for precise positioning and navigation. This paper describes the http-based technique for streaming GNSS data to mobile users connected to the Internet via Mobile IP-Networks like GSM, GPRS, EDGE, or UMTS. The technique establishes a format called Networked Transport of RTCM via Internet Protocol (Ntrip). desktop PC, while mobile applications might use a laptop or PDA with wireless Internet access through an integrated GPRS modem (see Fig. 1). Efforts to integrate the necessary client-software directly into DGPS/RTK receivers are under development. Keywords: Differential GPS, Internet, Real-Time, Positioning, Navigation 1 Introduction Due to the increased capacity of the Internet, applications which transfer continuous data-streams by IP-packages (IP for Internet Protocol), such as Internet Radio, have become well-established services. Compared to these applications, the bandwidth required for the transfer of real-time GPS positioning data is relatively small. As a consequence, the dissemination of DGPS corrections via Internet and mobile IP-Networks becomes an alternative to the usage of other terrestrial broadcasting techniques. Wireless Internet access is available nowadays in many regions. No significant degradation of positioning performance is expected when using it, but some remarkable advantages are possible. In addition, data streams from reference stations and (GIS-) data bases can be accessed simultaneously through one communication technique. Stationary applications may use a wired Internet connection through a Fig. 1: Simultaneous real-time access to DGPS and GIS data by means of Pocket PC (right) and plugged-in GPRS modem (left) 2 Basic Principle The generation of differential GPS correction data is usually done directly on the GPS receiver of a reference station, but can also be derived from networked observations obtained by a number of reference stations. The data stream thus generated is fed into a server which makes the data accessible on the Internet by means of an appropriate protocol (see Fig. 2). A mobile user might gain access to the Internet via mobile phone technology using a client program which forwards the correction data from

2 Fig. 2: RTCM data stream on the internet the server to a GPS receiver. The distance between the reference station and the client with the connected rover station is split into two parts. The larger part of the distance consists of a wired Internet connection, while the remaining part may be bridged using wireless mobile phone technology. It is up to the users of a DGPS or Real Time Kinematic (RTK) GPS service to choose a technique for retrieving correction data via the Internet. GSM, GPRS, and EDGE are among the choices. The introduction of the General Packet Radio Service (GPRS) as an IP packet-switching service for mobile data communication is of particular importance concerning DGPS applications. GPRS strengthens above all the mobile use of the Internet and is generally considered as the precursor of UTMS. In contrast to GSM, the data are divided into packets and transmitted separately, thus allowing optimum use of the bandwidth available within a mobile radio cell. Of decisive importance for DGPS applications is the fact that with GPRS, billing is performed on the basis of the received data volume. This means that the user will no longer pay for the duration of a connection, but for the quantity of data transferred. The available transmission capacity is shared between all participants of a radio cell, while occupying the radio line only when data packets are actually transmitted. Compared to the GSM technique, the costs for the GPRS transmission of DGPS data decreases by up to 90%. However, in the case of RTK applications the use of GPRS unfortunately allows today only a slight cost reduction owing to the tenfold higher data rate. The Internet (being conceived as an open net) is particularly well suited to transmit data between different providers over long distances. Uniform structures across national borders on a continental or global level can easily be created. However, the servers required must be tied to the Internet via interconnected broadcasters with sufficient bandwidth. Broadcaster operating expenses mainly depend on the number of simultaneously served clients. Ntrip The BKG, in cooperation with the University of Dortmund, has developed a http-based (Hyper Text Transfer Protocol) technique for streaming DGPS data, or other kinds of GNSS data, to mobile receivers via the Internet. This technique establishes a format called Networked Transport of RTCM via Internet Protocol (Ntrip), because its main application is the dissemination of corrections in the popular RTCM-104 streaming format. Sufficient positioning precision is obtained if correction data are not older than a few seconds. The RTCM standard is used worldwide, and most, if not all, DGPS receivers accept it. Ntrip s system components are - NtripSources, which generate DGPS datastreams at a specific location, - NtripServers, which transfer the data from one or multiple sources in Ntrip format, - NtripCaster, the major stream-splitting and broadcasting system component, NtripClients, receiving data of desired sources from the caster.

3 NtripClient 1 NtripServer 1 NtripSource 1 HTTP Streams NtripCaster HTTP Streams Fig. 3: Ntrip Components NtripClient N NtripServer M NtripSource M Administration HTTP/Telnet The data streaming is accomplished using the TCP/IP protocol stack. Several attempts, based on Serial-to-TCP conversion of streaming data on the reference-side (server) and TCP-to-Serial reconversion on the rover-side (client), have demonstrated the suitability of the TCP/IP protocol for streaming data to mobile IP clients. The new technique has two advantages: a) Ntrip is based on http as an application protocol layer on top of TCP. Consequently any data stream is transmitted exclusively through http s standard port 80. Thus, many problems with firewalls and proxyserver-protected LANs can be avoided. b) The caster, set up between server and clients as is usual in Internet Radio implementations, duplicates incoming source data, so that they can be received simultaneously by many users. While avoiding NtripServer NtripClient contacts, the caster furthermore acts as a protecting security element for data stream providers. GREF-IP (IP for Internet Protocol) has been created, which derives DGPS correction data for about 20 virtual reference stations spread over the territory of the Federal Republic of Germany. This service feeds its RTCM data into the Internet through an NtripCaster. Depending on the quality of the GPS receiver on the user side, potential accuracies of up to ± 0.5 m will be available countrywide. The central processing unit uses Trimble s GPSNet-Software for the generation of the correction data. For establishing DGPS services it is a necessity to offer correction data streams that span national borders. The European Permanent Network (EPN), comprising approx. 160 GPS permanent stations distributed over the whole continent and operated by European National Mapping Agencies (within the scope of the IAG Sub-commission EUREF), constitutes an appropriate datasource for the extension of the GREF-IP concept. In June 2002 EUREF adopted a resolution to provide real-time RTCM data via Internet to serve DGPS positioning and navigation applications. In this way a continental DGPS service is being set up, known as EUREF-IP. Those EPN stations considered for integration into EUREF-IP are operated at close proximity to Internet ports, and use GPS receivers that can generate RCTM correction data. In addition, a number of public or private institutions, have indicated an interest in providing DGPS or RTK data over the Internet. Some of them already make available test data streams. An overview on today s real-time GNSS data availability in Europe is given in Fig. 4. Reference stations As of July 2003, about 90 RTCM correction or raw data streams are available through Ntrip from the reference stations of various institutions. In Germany, a continuously operated network called GREF, comprising about 20 GPS/GLONASS reference stations monitored by BKG on a daily basis, is currently converted to real-time operation. The stations involved will be successively networked over leased lines or Internet connections with a central processing unit. Based on these networked reference stations a service called Fig. 4: Real-time data available in Europe through Ntrip, status July 2003

4 Accuracy and Latency With any kind of transmission technique the travel time of DGPS or RTK correction data is of decisive importance. The positioning software on the rover side requires the relevant data almost instantaneously (depending, to a certain degree, on the type of positioning). Considerably delayed, missing or irregularly arriving correction data will degrade accuracy, not such a problem when exclusive transmission paths (e.g. AM or FM broadcasters) are used. However, when using the Internet, a DGPS application shares resources of an Internet Service Provider (ISP) with other applications and hence the bandwidth cannot be controlled. The question arises in this context as to what extent the usual resources offered by professional ISPs meet the requirements of realtime positioning and navigation. Fig. 5 presents statistics on the latencies of DGPS and RTK correction data (formats RTCM 2.0 or RTCM 2.1) as derived from tests. The communication over Internet, and wired as well as wireless phone networks (GSM and GPRS), has been investigated. Latencies of the order of less than three to four seconds are typical for the situation in the Federal Republic of Germany. The tests performed in Europe (across national borders) show very few latencies exceeding four seconds. The latency of correction data caused by the inclusion of wireless Internet networks can therefore be considered so low that no noticeable influence on the achievable positioning accuracy exists. By means of Fig. 6, example-based typical files of 1s-recordings of a static position determination at a known site are illustrated. RTCM corrections were received via the Internet and mobile radio (GPRS). The figure shows differences to a known reference position over a period of one hour. Neither for DGPS nor for RTK applications were the achievable accuracies degraded by RTCM data transmission via mobile communication. Conclusions Ntrip is a new technology to transfer GNSS data (for example RTCM correction data) via the Internet and mobile IP networks. Practical tests do not show a significant degradation of performance compared to using other transportation media. Ntrip s main software components are developed within EUREF under the GNU General Public Licence. RTCM s Special Committee 104 has set up a Working Group Internet Protocol to further develop Ntrip towards an international standard. Due to its almost worldwide availability and the widespread hardware for wireless Internet access, Ntrip is the streaming method of the future. EUREF operates an NtripCaster. This service has reached Initial Operation Capability. Notice Advisories to Broadcaster Users (NABUs) inform users about temporary outages of the service. References EUREF-IP Real-Time Project euref_ip.html Distribution Maps of Real-Time GNSS Data Availability in Europe Networked Transport of RTCM via Internet Protocol (Ntrip), Documentation NtripDocumentation.zip Ntrip Software Downloads EUREF-IP Ntrip Broadcaster Notice Advisory to Broadcaster Users (NABU) Fig. 5: Latency of DGPS and RTK Corrections

5 Fig. 6: Accuracy of RTK over Internet and GPRS