Precise Positioning in Real-Time using Navigation Satellites and

Precise Positioning in Real-Time using Navigation Satellites and

Telecommunication

Anette RIETDORF1, Christopher DAUB2, and Peter LOEF3

1, 2Allsat GmbH network+services, Hannover, Germany, e-mail: [email protected] 3E.ON Ruhrgas AG, Essen, Germany, e-mail: [email protected]

Abstract - The paper gives an overview about the principles of DGNSS and RTK positioning

for navigation purpose. Special attention is given to the data transfer using GSM, GPRS and in

future UMTS. The need for special data transmission formats is described in more detail,

especially the benefits of using the new RTCM 3.0 protocol as a correction data format for

transmission via GPRS is explained.

1 Principles of DGNSS and RTK

1.1 Satellite positioning and navigation

Satellite positioning has been used for the past 20 years, but accuracy of positioning by satellites with a stand-alone receiver is still limited. Today, there are different possibilities to use satellites for positioning and navigation: not only the well-known American system GPS, but also combinations with the Russian system GLONASS or in future with the European system GALILEO. As a generic term we use GNSS (Global Navigation Satellite System) for these satellite systems. The calculation of a position is based on code pseudoranges and carrier phase measurements depending on the required accuracy.

As in all surveying procedures, inaccuracies and even errors also occur in GNSS positioning which impair the measured values. These effects can be eliminated by the formation of differences. Using Differential GNSS (DGNSS) techniques, a GNSS reference station is installed temporarily on a known position, which calculates correction parameters and sending them to a mobile GNSS rover. In this way the accuracy will be improved.

Fig. 1 Principle of Differential GNSS.

PROCEEDINGS OF THE 3rd WORKSHOP ON POSITIONING, NAVIGATION AND COMMUNICATION (WPNC’06)

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In the field of GNSS surveying, a distinction is made between station-dependent and distance-dependent errors. The term 'distance' refers here to the distance between the reference and mobile stations.The station-dependent errors include the signal propagation caused by reflection, also called multipath, the antenna phase centre variations or PCV and the user equipment errors such as receiver noise and PRN noise (Pseudo Random Noise). Within the receiver system, these errors occur in different ways on the reference and mobile stations. Distance-dependent errors consist of the orbital bias, i.e. insufficiently modelled orbit data (broadcast ephemeris), and errors induced by the diffraction of the GNSS signal in the different layers of the atmosphere.

1.1.1 DGNSS

In specified usage the term DGNSS has not the above explained general meaning. DGNSS stands for the positioning with C/A-code pseudoranges and corresponding correction data from a reference station. Using phase smoothed code ranges accuracies in the sub-metre level can be achieved. The correction data can be transmitted near real time via several manufacturer own formats or the receiver independent RTCM format. For DGPS applications the version RTCM 2.0 with only messages 1 and 3 is still usable as well as later versions.

1.1.2 RTK

For higher accuracy in cm-level the rover uses the real time kinematic (RTK) positioning mode with additional use of corrections of the carrier phase measurements. Similar to DGNSS, there is the possibility to use several own formats from each manufacturer as well as RTCM from version 2.1 on. Due to more messages than DGNSS-corrections the RTK correction data stream needs more bandwidth.

1.2 Reference networks

The effect of the physical correlation of the measurement values on the reference and mobile station can be effectively shown in the implementation of a local reference station (base station). Using Differential GNSS techniques, the correlation is ideally “1”, means the error influences on the measurement values obtained in both GNSS receivers, base and rover, impact with the same magnitude and with the same algebraic sign, that is to say, in the same direction. In the formation of differences, the error proportion is then eliminated to 100%. This case is depicted in the left half of the illustration in fig. 2. The ideal case occurs theoretically if the mobile receiver is operated very close to the reference station.With increasing distance between the reference and mobile receivers, the measured values decorrelate, which is represented here by means of an example. The error influences increase more and more.

Fig. 2 Physical Correlation in Differential GNSS measurements.


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In order to be able to operate over a more extensive area, the point is reached at some stage when a single reference station becomes inadequate. Generally one would calculate a range of about 20 km for each individual reference station and choose a distance between stations of about 30-35 km. Under unfavourable circumstances, as we have experienced some years ago with very strong ionospheric disturbances, the efficiency of individual reference stations is considerably restricted. It is at this point that the idea of networked reference stations comes into its own.

If data is collected from more than one reference station within a network, models for error-inducing effects can be calculated to use the correction data for longer distances between reference stations and hundreds of rover receivers simultaneously. Precise DGNSS positioning in a GNSS reference station network enables accuracies of a few centimetres in position and in height.

2 ascos, the satellite positioning service by E.ON Ruhrgas AG

E.ON Ruhrgas AG is one of the largest energy companies in Germany. Annual Gas send out is approximately 640 billion kWh. It is a private-sector international gas company with a 75-year history. Its main business is the supply of natural gas to national and international markets. E.ON Ruhrgas and its affiliates built and operate an extensive pipeline system for the procurement and marketing of natural gas. It extends from the North Sea to southern Germany and from Saxony to Franco German border and comprises over 11,200km of Pipelines. E.ON Ruhrgas applies satellite surveying and navigation to support a great number of work processes. GNSS methods are used to survey the approx 3,500 km long old network and the data is directly used for the E.ON Ruhrgas geographical information system (GIS). For this purpose, E.ON Ruhrgas has developed with the support of Allsat GmbH network+services a satellite reference service named ascos to make correction data available to the GPS and GLONASS satellite systems for real-time positioning. Reference station networking serves to enhance the integrity and the reliability of the correction data. Using a reference station network the user will be enabled to achieve higher positioning accuracies and initialization times in real-time applications are reduced considerably.

With ascos, E.ON Ruhrgas supplies standard real-time correction data for GPS and GLONASS positioning on a nationwide basis. Using the ascos service, a user can determine its position with two different degrees of accuracy easily and reliably with a GNSS receiver of any well-known type. No additional system components are needed. The precise real-time service of ascos called PED offers accuracies of 2 cm with a geodetic dual frequency receiver, while accuracies of 50 cm can be obtained using the ED real-time service on a single-frequency receiver.

Via the E.ON Ruhrgas communication network, the raw data acquired for each reference station are transmitted in real-time to a data centre for further processing. With reference stations from ascos and the BKG1 as well as the SAPOS® stations2 of the federal states, the ascos service can use approximately 300 reference stations (see fig. 3).

1 BKG: German Federal Cartography and Geodesy Office 2 SAPOS: Satellite Positioning Service of the German State Survey


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Fig. 3 ascos reference stations network.

3 Data communication in a reference network

Mobile phone systems (GSM, GPRS and UMTS) are used for data transmission between the mobile positioning systems and ascos. With the aid of reference station data, ascos calculates correction models and makes them available to the user. Initially, the user transmits an estimated position in the standardized NMEA format. Optionally, a virtual reference station (VRS) may be calculated or area correction parameters may be determined. Systematic errors are entirely eliminated by the correction models. While a VRS is calculated individually for each user, area correction parameters are tailored to the individual position by the receiver.

3.1 Formats

Several different data communication protocols exist for GNSS applications but two protocols have become standard, NMEA 0813 and RTCM. The abbreviations show the origin of these two protocols: NMEA is developed by the National Marine Electronics Association and RTCM by the Radio Technical Commission for Maritime Services.

NMEA-0183

The NMEA standard defines an electrical interface and data protocol for communication between marine instrumentation. This includes also GNSS equipment. Under the NMEA standard all characters used are printable ASCII text. Most processing programs providing real-time positioning information understand and expect data to be in NMEA format. This data includes the complete position, velocity and time solution computed by the GNSS receivers. The data is packed in form of so called sentences. Each sentence starts with a “$” and ends with a carriage return/line feed sequence and can not be longer than 80 characters.


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RTCM 3.0

RTCM 3.0 is designed to support real-time kinematic (RTK) operations. The reason for emphasizing RTK operations is that these operations involve broadcasting a lot of information and hence benefits from an efficient data format. The broadcasted information include data for GPS and GLONASS RTK operations with code and carrier phase observations, antenna parameters and additionally system parameters. The format is designed to make it possible to modify the specifications in order to include new signals e.g. GPS L2c and L5 signals and signals of GALILEO. The higher efficiency of RTCM 3.0 makes it possible to support RTK services with significantly reduced bandwidth. This is an advantage especially in wireless networks and mobile applications where the available bandwidth is considerably smaller than in wired networks. For clients using mobile communication which are charged by the amount of transferred data a reduced bandwidth means reduced operating costs. In February 2004, RTCM released the third version of their protocol of their recommended standards for differential GNSS. RTCM 3.0 has been developed as more efficient alternative to previous versions and is more efficient, easy to use, and more adoptable to new situations. The main complaint was that the parity scheme of Version 2 was wasteful of bandwidth and that the parity was not independent from word to word. Although, many bits were devoted to parity the actual integrity was not as high as it could be. RTCM 3.0 has corrected these weaknesses.

NTRIP

Networked Transport of RTCM via Internet Protocol (NTRIP) is the most recent protocol used in the world of GNSS and stands for an application-level protocol streaming GNSS data over the Internet. NTRIP is based on the popular HTTP streaming standard and is meant to be an open none-proprietary protocol. The application is not limited to a specific content of the data stream and offers the ability to distribute any kind of GNSS data. NTRIP enables streaming of GNSS data over any mobile IP network using the TCP/IP protocol. Most modern GNSS receivers understand and apply data that is transmitted via NTRIP.

3.2 Special details of RTCM 3.0 with GPRS

ascos transmits its correction data to the clients using modern telecommunication means such as GSM and GPRS. Using GPRS the client connects to the service via an IP-address and uses the internet protocol NTRIP. The connection via the internet offers the possibility to connect to another server at the same time. This allows a seamless data stream between the rover in the field and e.g. the GIS server in the main office for real-time data down- and upload. With this connection required data in the field e.g. maps and status information can be downloaded and updated information uploaded. Hence a faster data flow with colleagues can be realised. The main advantage of using GPRS instead of GSM is the package orientated billing and accounting. Especially, if the data format RTCM 3.0 is used, this leads to significantly reduced costs. Another advantage of using GPRS is the higher bandwidth of currently up to 115 kbits/s. The higher bandwidth ensures that the required data is transferred without delay.

3.3 Example of use in navigation

In various navigation applications special GNSS receiver are operating together with various multi-sensor systems. GNSS time, position and attitude information are needed to synchronise complex systems. All navigation information in a global coordinate system are sent to the other sensors or Geoinformation Systems. E.ON Ruhrgas AG uses a new helicopter- borne gas detection system for routine aerial inspections to check natural gas pipelines for tightness.


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At a cruising altitude of around 100 m, the measuring points of an infrared laser have a diameter of 1 m on the ground. With a helicopter traveling speed of about 20 m/s, the laser beam scans a corridor along the pipeline route, which can be as much as 18 m in width.

Fig. 4 Helicopter-born gas detection.

To direct that measuring beam onto the pipeline route, the position of the helicopter has to be known in real-time with an uncertainty of 0.5 m. Additionally the precise heading of the helicopter has to be determined, to guarantee the accurate flight path. Pipeline coordinates are used from geoinformation systems (GIS). The precise GNSS-measurement supports an inertial measurement platform (IMU) that is used for automatic beam orientation. The precise heading determination is done with a GNSS Two-Antenna System. Simultaneous GNSS-measurements of two L1/L2 GPS+GLONASS receivers enables an accurate heading determination better than 0.1 deg. Precise Positioning is done by using real-time correction data from the ascos satellite positioning services. Using positioning mode with carrier-phase smoothed code measurements, an accuracy of better than 0.5 m in real-time can be achieved. Correction data are transmitted to the GNSS-system via three GSM modems, they are equipped with mobile phone cards of different providers, to improve the continuous availability of correction data. The combination of RTCM 3.0 and GPRS improves the reliability and continuity of the reception of correction data and the costs are reduced significantly. Future developments of the helicopter-based solution will definitely include the modern communication means and the latest data protocol.

4 References

[1] http://igs.ifag.de/index_ntrip.htm

[2] T.S. Yan “Benefits of Telecommunications Technology to GPS Users”, “Conference Paper”, GNSS 2004 in Sydney, Dec. 2004


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Precise Positioning in Real-Time using Navigation Satellites and

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