Canadian Active Control System station map

The Geodetic Survey Division of Geomatics Canada, in partnership with Geological Survey of Canada, is presently operating the Canadian Active Control System (CACS) to provide improved GPS positioning capability for the Canadian surveying and geophysical community as well as for other spatial referencing needs. The system consists of unattended tracking stations, referred to as Active Control Points (ACPs), which continuously record carrier phase and pseudo-range measurements for all satellites of the Global Positioning System (GPS) within station view. Each ACP is equipped with a high precision dual frequency GPS receiver and an atomic frequency standard. Temperature, pressure and humidity data are also collected at selected ACP sites. The data collected at each ACP is retrieved on a daily basis by a central processing facility in Ottawa.

The CACS provides efficient access to modern spatial reference frames (NAD83CSRS, ITRF, etc.) and improves effectiveness and accuracy of GPS applications. This is accomplished by monitoring GPS integrity and performance from the analysis of data acquired through continuous tracking; by computing and making available precise satellite ephemerides (GPS orbits) and precise satellite clock corrections; by supporting Wide Area Differential GPS (WADGPS) development and other applications (geodynamics, precise time transfer, etc.)

The availability of precise ephemerides, precise satellite clock corrections and observational data from the ACPs offers significant benefits for Canadian users carrying out GPS surveys. These Active Network make it possible to position any point in Canada with a precision ranging from a centimetre to a few metres in relation to the national spatial reference frame without actually occupying an existing control monument or base station.

Positioning at the metre level from pseudorange (code) observations without the use of a base station is made possible by using CACS precise satellite clock corrections.  They can be applied anywhere in the world to correct the users observed ranges and, when used along with CACS precise ephemerides, provide positioning accuracies in the 1 metre range depending on the user's receiver measurement noise and multipath. The advantage of using satellite clock corrections is that some of the station specific errors can be accounted for directly, as opposed to assuming commonality of errors between base station and remote site as with local DGPS. Since the satellite corrections are based on a network of accurately known reference points, some of the uncertainty associated with using control data from a single base station is effectively removed. The distributed stations of the CACS and the International GPS Service (IGS) networks also ensures common satellite visibility for any  user at any time of day, eliminating the problem of 'matching' observations between remote and reference sites sometimes associated with Local DGPS (LDGPS) operations.

For survey projects using GPS phase measurements and requiring the highest precision, introducing CACS precise ephemerides in the data processing will reduce all orbit related errors in GPS baseline determinations to less than 0.1 ppm. These errors can reach 3 parts per million or more when ephemerides broadcast by the satellites are used. Furthermore, by including observational data from the ACPs in the data processing, a direct tie to the national spatial reference frame is established. Scale and orientation are provided by the precise ephemerides without occupation of any control points thereby increasing the efficiency of field operations and data processing. Depending on the GPS software, further advantages may be realized by using precise ephemerides, such as improved cycle slip detection and fixing capability, enhanced carrier phase ambiguity resolution, better and more consistent aposteriori error estimates, etc. Recent tests, combining CACS data and precise ephemerides, has shown static positioning precision at the centimetre level in each of the three-dimensional components for distances up to 600 km when appropriate software and adequate procedures are applied.

The Geodetic Survey Division also contributes CACS data to the International GPS Service (IGS) and participates as an analysis centre, thus having access to data from globally distributed fiducial sites for use in the computation of precise satellite ephemerides. Through the IGS, CACS data and related products are made available to international organizations such as the International Earth Rotation Service (IERS), the NASA Crustal Dynamics Data Information System (CDDIS), the US National Geodetic Survey (USNGS), the US Naval Observatory (USNO) and other organizations interested in Earth dynamics. The precise observations of the satellites made from the fiducial stations are used to establish the Earth Orientation Parameters (EOP) and derive inter-station baseline lengths and orientation for regional monitoring stations. Changes in baseline components over time provide quantitative data for studies of geodynamics, natural hazards and global change.