June 2, 2011
GPS: (Global Positioning System). The name of the satellite-navigation network maintained by the U.S. Department of Defense. Also, the term "GPS" is often treated more generically to refer to any device that depends on navigation satellites for functionality.
RTK: (Real Time Kinematic). Highly-accurate, highly-repeatable positioning. With RTK, a base station receiver is placed on a stable mount, allowing multiple GPS rover receivers to utilize this type of correction within a limited range of the base station.
DGPS: (Differential GPS). Techniques used to enhance accuracy, integrity, reliability and availability of GPS data. The following are all examples of DGPS:
WAAS (Wide Area Augmentation System): Free service offered through Federal Aviation Administration (FAA). Ground-based reference stations plus two geostationary satellites.
Point accuracy: 9 to 15 feet; Pass-to-pass accuracy: 6 to 12 inches.
RTK array/cluster (John Deere, Trimble, etc.): Annual subscription.Cost and point accuracy varies by the service and technologies being used.
Satellite correction (OmniSTAR XP, StarFire 2, etc.): Service offered by several companies using a correction. Some services are free while others require a subscription and the receiver in the tractor to be specific to the company offering the service.
Personal RTK base station (fixed or portable). Line of sight correction. Grower positions stationary base station in the best location to cover his acreage, or moves a portable base around with from field to field to get the best signal. Can be more expensive than using a service but better positioned for an individual's needs.
RTN (Real Time Network). Generic term for a correction service offering more reliability than a single-station RTK. Several CORS or RTK base stations are connected in a "mesh" so correction data can be used from multiple locations to increase accuracy, reliability, and the distance covered. Offered by several companies, however often associated with a subscription fee.
CORS (Continually Operating Reference Station). Coordinated by National Geodetic Survey of National Oceanic and Atmospheric Administration. Survey-grade GPS receiver is positioned in a fixed position providing continuous RTK-correction for receivers with Internet-accessible capabilities. Internet-capable cell phone or cell modem (available from various cell phone data providers) is used to transmit correction signals from a server to the tractor so no line-of-sight requirements. Requires cellular phone service and a data plan. No personal base station is required so some cost is lower.
(Information via the Website of John Fulton, precision ag specialist at Auburn University)