Differential and Wide-Area GPS (DGPS/WAGPS)
DGPS and WAGPS services are designed to support safety of life applications. Therefore, GNSS signal availability and reliability is of prime importance. As the GPS L1 C/A code is currently the only GPS signal formally recognized for public access, it is the fundamental measurement used in augmentation systems such as the Coast Guard DGPS service and the FAA Wide Area Augmentation System (WAAS). L1-only augmentation systems are less precise as first-order ionospheric errors cannot be fully corrected. Therefore, the precision of augmented positioning using DGPS or WAAS remains at the metre level. On the other hand, the design of the augmentation systems is such that they perform at higher levels of reliability and integrity, compared to existing RTK or PPP services.
DGPS base stations observe L1 pseudoranges and compute corrections for transmission to maritime users from high-frequency radio-beacons (see Figure 3). As a GNSS satellite signal footprint covers an area of several hundred kilometres on the Earth’s surface, common view is generally achieved over baselines of up to a couple of hundred kilometres. While error cancellation from observation differencing becomes less effective as the distance between the base and rover increase, metre-level precision is routinely achieved with good satellite geometry.
In Canada, the Canadian Coast Guard (CCG) http://www.ccg-gcc.gc.ca/CCG-DGPS/Marine-Differential-Global-Positioning-System provides DGPS as a public service. The corrections are broadcast from 19 base stations (CDGPS network) along the Atlantic and Pacific coasts and St. Lawrence waterway. CDGPS supports safe and efficient navigation by guiding vessels through prescribed corridors, harbors entries and docking lanes. With base stations performing autonomous integrity monitoring functions, users can be notified when system performance does not meet service specifications. In Canada, the Department of Fisheries and Oceans (DFO) has also been involved in planning service extension for navigation in arctic waters. In addition to the CCG, the private sector also provides some DGPS services for activities such as GIS data collection.
For air navigation in North America, a separate wide-area augmentation system known as WAAS has been conceived and deployed. While DGPS base stations are independent from each other and autonomous, WAAS references are part of a network of tracking stations that all report to a master station where wide-area corrections are computed and relayed to satellite uplink stations. The uplinks ensure that communication satellites with footprints covering different regions of North America reliably broadcast augmentation messages to all WAAS users. Unlike site specific DGPS corrections, WAAS corrections are estimated using measurements made from several reference stations and designed to be localized for any user in North America. For such purpose, state-space messages with satellite orbit, clock and ionospheric corrections are encoded separately and broadcast at different rates. The structure and content of WAAS messages are described in detail in the RTCA-159 standard.
In Canada, 4 international reference stations contributing to WAAS and hosted by Nav Canada are found in Gander, Goose Bay, Iqualuit and Winnipeg. As WAAS serves correction messages tailored to single-frequency users, the quality of the ionospheric grids they broadcast is dependent on the density of reference stations tracing in the different regions. With the sparsity of WAAS reference stations in Canada, the quality of ionospheric corrections cannot be as good as what is available in the continental US. Nevertheless, it is sufficient to meet specifications for en-route navigation and offers reliable integrity information as it is derived from a network solution.
Figure 3: Concept of DGPS/WAGPS