GPS Corrections Via The Internet
The Internet allows connectivity of real-time data streams across vast distances. Multiple servers at a control centre in Calgary continuously gathers observation data from all Can-Net GNSS reference stations, and creates a live database of regional and single base corrections. The connectivity from the individual GNSS references stations in crucial. Can-Net uses a wide variety Internet service providers (ISPs)types; dial-up, wireless, cable and phone lines. ISPs are national, regional and local such as Bell, Telus, Rogers, Shaw, eXplorenet, Aliant, etc.

Additionally, the Internet and, more over mobile Internet connectivity, is crucial in delivering the real-time correction to field users via the cell phone network. Here too the service providers are varied; Rogers, Telus, Bell, Fido and Mike. Real-time data streaming via the cell network is a unique service. Most cell phone data services are not contingent upon the time it takes for the service to be delivered. This time factor, also called latency, must be less than 1 second for the RTK correction to be relevant. Email, web browsing and other service can operate with a sizable latency. On occasion, latency within the cell network can increase. This usually occurs during peak times. It has also occurred when service providers constrict cell traffic when upgrading other network services.

The corrections can be in variety of flavors to fit the client’s needs; CMR+, RTK-RTCM,DGPS-RTCM. The Can-Net correction services also supply static (RINEX) data for post processing. Here too, they have been customized for clients needs: 1 second L1/L1 geodetic and 15 second L1 GIS. Other customized data sets can also be created upon request.

The computer at the control centre runs Trimble's GPSNet. While connected to all the receivers in the network, it performs several major tasks including:

• - raw data decoding of all major receiver brands (for example, Trimble), data import and quality check
• - RINEX (+ Compact RINEX) and DAT file data storage
• - antenna phase center corrections (relative or absolute)
• - modeling and estimation of systematic errors
• - generation of the VRS correction data
• - Generation of an RTCM data stream for the virtual position for each roving user in the field
• - transmission of RTCM data to the rover in the field.