Evaluation of DGPS and WADGPS Methods for Land and Marine Applications

Canadian and United States Coast Guard specify the DGPS horizontal tolerance to be 10 m (95%) and, under normal operating conditions, studies have shown that accuracies fall well within this bound. During high levels of ionospheric disturbance, however, considerable degradations in DGPS positioning accuracies can occur. Localized ionospheric events producing large gradients in total electron content (TEC) can cause significant positioning errors for a single reference station DGPS approach. Recent studies have shown that ionospheric events may occur in which DGPS positioning accuracies are degraded by factors of 10-30 throughout North America. This is a significant concern for DGPS services which must maintain reliable positioning accuracies. An alternative to single baseline DGPS is wide area DGPS (WADGPS). In WADGPS, a network of sparse (often dual frequency) GPS reference stations is used to spatially model correlated ranging error components. A commercial WADGPS system, developed by the Federal Aviation Administration (FAA), is the Wide Area Augmentation System (WAAS). In the WAAS, corrections are computed using a vector approach, and it achieves (typical) accuracies of 1-3 m. The WAAS service is considered to be a potential alternative for land and marine users, as compared with the single baseline DGPS capabilities offered by the Canadian Coast Guard, U.S. Coast Guard and DOT (NDGPS). The WAAS reference network is sparse, however, and the WAAS may not adequately model local ionospheric effects during increased ionospheric activity. In this paper, we conduct an extensive study of the effects of local ionospheric events and TEC gradients on DGPS users and analyse how these effects could be mitigated using different methods. We initially quantify the impact on single baseline DGPS and WAAS users in severe geomagnetic storm events using two different networks of varying sizes. Additionally, a broader analysis of spatial features is conducted for both single baseline DGPS and WAAS by computing “user” positions at different sites across North America. A total of 813 baselines (using sub-networks of the IGS and CORS reference stations) are processed to derive the DGPS solution set. WAAS position solutions are also computed at 40 “user” stations using archived WAAS correction messages obtained from the FAA. Results indicate that 1) for single baseline DGPS (baselines of 100-200 km in length), horizontal position accuracies are improved versus using WAAS, but horizontal DGPS errors may still be > 10 m (95thpercentile), and 2) for single baseline DGPS (baselines > 400 km), WAAS may provide improved position accuracies (but position errors are still increased significantly versus quiet conditions).

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