Making Sense of Precision Ag’s Alphabet Soup

Melissa SandfortAg Leader, Insights Weekly

Insights WeeklyPrecision farming tools are designed to simplify things for growers, and yet the terminology used in precision farming isn’t always so simple. But have no fear. The following definitions will guide you through the mysterious twists and terms found in the world of GPS (Psst … that stands for Global Positioning System).

GNSS: Global Navigation Satellite System is a navigation system with global coverage. GNSS is a method of improving the navigation system’s attributes, such as accuracy, reliability and availability through the integration of external information into the calculation. GNSS 1 consists of GPS, GLONASS, WAAS, SBAS and EGNOS.

GPS: The Global Positioning System is a U.S.-owned utility that provides position, navigation and timing. There are three segments – space, control and user. The space segment consists of the satellites in space, the control segment consists of the people who manage and maintain the space segment and the user segment are those of us who use GPS. GPS is what allows us to map fields and auto-steer equipment.

GLONASS: Global Navigation Satellite System is the Russian-owned utility equivalent to GPS. This system can be used in the U.S. if your receiver is set up to receive GLONASS signal.

WAAS: Wide Area Augmentation System is a supplement to GPS created with the goal of improving accuracy, integrity and availability of GPS correction in the U.S.

EGNOS: European Geostationary Navigation Overlay Service is the European version of WAAS.
SBAS: Satellite Based Augmentation System is a system that supports wide-area or regional supplementation through the use of additional satellites broadcast messages.

Base Station: The base station is a receiver and transmitter that is stationary. It receives GPS information, corrects and then transmits the corrected information to the rover.

Rover: We refer to the moving equipment as the rover. For example a tractor with receiver would be referred to as the rover.

Differential Correction: Uses two GPS receivers to determine position. One is typically on a tractor in the field known as the rover and the other is in a fixed location referred to as the base. The base is in a known surveyed location. It calculates GPS correction using the almanac to determine timing errors, compares actual timing to what it should be and then broadcasts a timing correction for each satellite. There are different levels of differential correction including Coast Guard Beacon, WAAS, OmniSTAR and RTK.

OmniSTAR: A type of differential correction that is privately offered and can be obtained by paying a subscription fee. There are three levels; HP with 2-4 inch accuracy, XP with 3-5 inch accuracy and VBS with 6-8 inch accuracy.

RTK: Real Time Kinematic navigation provides real time correction. RTK can achieve sub-inch accuracy through the use of two GPS receivers. Using a short baseline distance allows the RTK base station to correct for local atmospheric conditions, as well as GPS timing errors to achieve higher accuracy and better reliability.

NTRIP/CORS: Network Transport of RTCM via Internet Protocol and CORS – Continuously Operating Reference Stations – are forms of RTK differential correction that are done through the use of a cellular modem and base station network. This means that instead of using the traditional base station and radio to send correction data to a rover, data is sent using the internet to a cellular modem with a data plan.

VRS: Virtual Reference Solution requires the rover to send a latitude and longitude position to the VRS network. The network then creates a computer generated base close to the rover that is used for correction.

PRN: Pseudo Random Number is a numerical identifier used by the GPS receiver to determine which satellite it is looking at.

PRL: Preferred Roaming List refers to the cellular provider or set of providers that you will use to get a cellular signal for NTRIP/CORS correction.

Convergence: The act of converging is when the satellites used for differential correction or base satellites are matching up with those being used by the rover. This process determines if there are enough satellites in common for the best possible solution.

NMEA: One-way GPS communication. The receiver sends signal to the monitor. Messages cannot be changed and contain information about latitude/longitude, fix quality, number of satellites, HDOP, altitude, speed and other information.

Baud Rate: Bandwidth of message or the number of characters it contains.

Hertz Rate: How often the message is sent. One Hz equals one message per second.

Pass-to-Pass Accuracy: Indicates the relative accuracy of a GPS receiver over a 15-minute period.

Become a fan of Ag Leader on Facebook today, and get the latest precision ag videos on the YouTube channel. For more information about Ag Leader products and services, or to visit the blog site, go to