A comparison of local and wide area GNSS differential corrections disseminated using the network transort of RTCM via internet protocol (NTRIP)
Psuedorange corrections (PRCs) have long been used to improve the accuracy of GNSS solutions in real time. Today, they continue to be useful for sub-metre level requirements, such as when setting ground control for satellite imagery and for en route navigation on land, in the air and at sea. The transmission of these corrections has traditionally been facilitated using either radio or satellite communications. The Networked Transport of RTCM via Internet Protocol (NTRIP) specification takes advantage of the availability of Internet over digital mobile phones to disseminate PRCs. In this report, NTRIP has been used to transmit both localized wide area and local PRC corrections over the Internet to a client receiver where they have been applied. The accuracy of different solutions is compared. In addition, the convergence of different solutions is analyzed. This analysis will enable potential users to determine the position and height accuracy that they can expect to achieve under various scenarios as well as the observation times which they should employ. Results for horizontal positions showed errors at a 95% confidence level to be at the 2-metre level for uncorrected GNSS, 30 cm for GNSS augmented with local corrections generated at UNB and 1.0 m for corrections generated 430 km away. The Canadawide Differential GPS (CDGPS) wide area system produced errors of 60 cm. Results for heights were of a similar order. However, we found that height solutions were significantly more correlated with observation time than were horizontal positions. Our work showed that NTRIP could be used easily to both disseminate and use localized wide area and local differential corrections. We believe that as costs for digital mobile service becomes cheaper and more widely available, NTRIP will become commonly used. In addition, we recommend that the CDGPS service consider supporting NTRIP. Currently, CDGPS has a limited user-base because it is accessible only with the use of receivers containing NovAtel®-based chipsets. We believe that NTRIP can potentially bring CDGPS to a much wider object. Finally, by far, the best position and height accuracies achieved were with the use of local differential corrections. Even when the reference receiver was 430 km from the user receiver, resulting solutions were better in both accuracy and precision than uncorrected solutions. Canada and New Brunswick each operate an Active Control Network, consisting of many continuously operating GNSS receivers that are already connected to the Internet. We believe that with very little effort, this network can be extended, using NTRIP, to disseminate DGNSS corrections.