The Canadian Active Control System (CACS) was created to improve the precision and efficiency of GPS positioning in Canada and to provide easy access to the Canadian Spatial Reference System (CSRS). The CSRS is the national (common) reference system used throughout Canada.
In Canada, the CSRS provides the basic or "foundation" reference layer for use with data from GPS, GIS, and other positioning technologies. Regardless of where the data comes from, we need a common reference system to ensure that we can exchange data that may be collected at different times by different methods and from different places, without complication. If all position data were related to the CSRS then all the data would be compatible and could be shared very easily. If you are positioning with GPS, then using the Canadian Active Control System (CACS) is a key link in that it provides a 'tie' between modern positioning systems (such as GPS) and a common reference system (Canadian Spatial Reference System).
The Canadian Active Control System (CACS) consists of Active Control Points (ACP), distributed across Canada.