GPS World, February 2009

Survey SURVEY CONSTRUCTION or even more giving users much improved positioning fl exibility and reducing infrastructure costs Currently several commercial NRTK services operate around the world and more comprehensive networks will be established to address some urgent environmental and geo hazard problems In Great Britain for instance Leica Geosystems in partnership with Ordnance Survey GB has off ered an NRTK service to its clients since 2006 the service is called SmartNet and subscribers can carry out positioning to centimetric accuracy with only one rover receiver Wireless communications using Global System for Mobile communications GSM GPRS links between a network control center and rover users play a vital role in ensuring high fl exibility mobility and availability when delivering the network corrections Although SmartNet is mainly designed to work under the master auxiliary concept MAC especially suitable for one way communication links broadcast like many other NRTK service providers and in order to guarantee full service coverage SmartNet takes advantage of mobile phone networks already operating across Britain to deliver its Radio Technical Commission for Maritime Services RTCM corrections ese network corrections are normally delivered via GSM GPRS mobile communications under the Networked Transport of RTCM via Internet Protocol NTRIP erefore SmartNet users can employ any mobile phone network with a good coverage of the geographic area where they are working Well developed mobile phone networks cover most of Great Britain off ering users a variety of voice and data services As in most parts of the world these networks were initially designed for the transmission of voice but due to the growing demand of services such as M2M communications and more recently mobile Internet they have also implemented data transmission support Even though the requirements of voice and data transmission diff er greatly both services are supported by the same platform and are generally included under the same mobile phone contracts erefore SIM cards that are normally used by the NRTK services subscribers in Britain are the same as those employed by common mobile phone users Past tests carried out by the authors reveal some availability problems in the SmartNet NRTK service mainly caused by instability in the wireless data link e availability of NRTK corrections was about 88 percent with the wireless connections being dropped after two or three hours of continuous activity e voice and data SIM cards used during these tests are believed to be the main cause of these instability problems when interrupting the GPRS connection after a long period of connection to the mobile phone network Since these are conventional SIM cards they use dynamic IP addresses and the interruption could therefore be caused when they renew their IP addresses or also when they prioritize voice traffi c across the mobile phone network Å FIGURE 1 SmartNet CORS as of August 15 2007 Leica Geosystems UK is trying to implement the use of a new kind of data only fi xed IP SIM card that should contribute to the solution of the instability problems in the wireless links currently used by its customers ese static IP SIM cards are the latest solution of mobile phone network providers to the increased demand on M2M wireless data transfer services under GSM GPRS protocols Leica provided the Institute of Engineering Surveying and Space Geodesy IESSG at the University of Nottingham with one of these SIM cards for a trial We here report the results of tests carried out with the new fi xed IP SIM cards Several static tests were designed to investigate the impact of the available voice and data and data only commercial GSM GPRS services on the availability and general performance of SmartNet SmartNet SmartNet comprises about 150 CORS distributed fairly evenly in the whole country as shown in FIGURE 1 Since July 2007 Leica Geosystems has also offered the Smart Net service to Ireland and Northern Ireland Although the majority of CORS are owned by OSGB known as OSNet 23 CORS are managed and owned by Ordnance Survey Ireland and Ordnance Survey Northern Ireland and 20 CORS are owned by Leica All sites in Ireland are GPSand GLONASS with OSGB currently upgrading OSNet to GPS GLONASS and Galileo ready receivers antennas around 40 so far Receivers at the CORS collect raw GNSS data from the satellites and pass them through dedicated communication lines or the Internet to a network control center CC At the CC a software suite called GPS Spider processses the observations www gpsworld com February 2009 GPS World 15