Wireless Delivery

February 1, 2009 By: Chris Hill, Jose Aponte, Mark Burbidge, Terry Moore, Xiaolin Meng GPS World

Assessing Network RTK

Wireless communications employed in a network real-time kinematic (NRTK) GNSS system to deliver network corrections to roving users play a vital role in achieving good positioning performance. The communication link must ensure high flexibility and availability to satisfy the requirement of centimetric positioning accuracy and mobility that NRTK can offer. Since the advent of General Packet Radio Service (GPRS) communication technology, it has been used as the best option to deliver the NRTK corrections to users and therefore it directly impacts the availability and general performance of the NRTK service.

In the past, commercial mobile phone network companies did not offer specialized "data only" transmission services through GPRS. Therefore, normal mobile phone Subscriber Identity Module (SIM) cards for voice and data had to be used for the transmission of the NRTK corrections. A SIM card is a component, usually in the form of a miniature smart-card that is theoretically tamper-proof and is used to associate a mobile subscriber with a mobile network subscription. The SIM holds the subscriber's unique MSISDN along with secret information such as a private encryption key and encryption/digital signature algorithms. Most SIMs also contain non-volatile storage for network and device management, contact lists, text messages sent and received, logos, and in some cases even small Java programs. The use of these cards was degrading the availability of the NRTK service by interrupting the communications between the network control center and users after long periods of connection, and also when priority was given to voice calls. The increase in the demand of services such as mobile Internet and machine-to-machine (M2M) data transmission using GPRS has seen the wide use of new SIM cards with fixed or dynamic IP addresses that are specially designed for data-transmission purposes through GPRS.

Network-based RTK GPS positioning can appropriately solve performance problems caused by distance-dependent errors such as satellites' orbit and atmospheric biases that were experienced with conventional single-reference-based RTK positioning. Such errors and biases can cause a phenomenon called spatial decorrelation of errors, which requires that a rover receiver performing RTK must be located at distances no greater than about 20 kilometers from a reference station in order to achieve centimetric positioning accuracy. However, NRTK uses the raw GPS observations gathered from a network of continuously operating reference stations (CORS) to create proper models that can mitigate most of the distance-dependent errors acting within an area covered by the CORS. NRTK accuracy is as good as or even better than that achieved with conventional RTK GPS positioning. Further, separations between the rovers from the nearest reference station can be extended to 50 kilometers or even more, giving users much improved positioning flexibility 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 offered 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 flexibility, 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. These network corrections are normally delivered via GSM/GPRS mobile communications under the Networked Transport of RTCM via Internet Protocol (NTRIP). Therefore, 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, offering 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 differ greatly, both services are supported by the same platform and are generally included under the same mobile-phone contracts. Therefore, SIM cards that are normally used by the NRTK services' subscribers in Britain are the same as those employed by common mobile-phone users.