Understanding CORS: Continuously Operating Reference Stations
Introduction to CORS
The National Geodetic Survey (NGS) operates a set of stations that supply correction data to Global Navigation Satellite System (GNSS) Data. The GNSS correction data apply to the GPS as well as the GLONASS positioning systems. These are the so called Continuously Operating Reference Stations or CORS. With the carrier phase and code range data supplied by these stations, one can get 3 dimensional positioning data that is very accurate compared to the standard 15 meter accuracy of the GPS data. Surveyors, GIS/LIS professionals, engineers, scientists, and others can apply CORS data to position points at which GNSS data have been collected and get positioning accuracies that approach a few centimeters relative to the National Spatial Reference System, both horizontally and vertically.
These stations are set up through cooperative efforts of various government departments, academic as well as private and commercial organizations. New sites are included if they meet specified criteria set by NGS. The basis of any such correctional stations is that the location of such sites is known to a great accuracy.
For most GPS applications in surveying, mapping, and related disciplines, very high accuracies in the range of 10 centimeters and thereabouts are required. In order to simplify the process of relative GPS positioning, many organizations are establishing automated reference station facilities. These stations are, typically, facilities that continuously collect and record GPS data. Quite often these are unmanned and permanently configured. The CORS data are then referred to by users and compared with their own readings taken. A user could collect data in the field with a single GPS receiver, later retrieve the data collected during the same time span by a nearby CORS. The readings then can be combined to create relative positioning. One could easily manage to generate mapping grade data that is accurate to a few meters. The data then can be easily used in GIS maps. For survey grade data, you may have to use more than one CORS.
The process described is the so-called post process mode as you get the data from the CORS after the data collection phase. You can adopt a real-time processing method too. Except that the reference data will need to be transmitted to the user measurement units in real-time. The accuracy will get determined by the capabilities of the equipment used, of course.
There are a range of extra functions that can be implemented at the CORS. These can be utilized to generate satellite ephemerides and clock correction data. Several other studies can be carried out related to GNSS reception. The CORS are often described as base stations, active control stations, tracking stations, etc. Since errors that happen are dependent of the atmospheric conditions in the vicinity, it’s desirable that the reference site you use be close enough to the area you are mapping or surveying.