Precision Point Blog

Ag Leader GPS Terms Dictionary

by Kerri Birdwell on October, 09 2012 in Guidance & Steering

We know that many times when we start talking in technical terms, your eyes glaze over. But, that is coming to an end with the introduction of a mini blog series we're doing. We will take time over the next couple months to define a handful of the most commonly used and confused vocabulary. This week is GPS, so take a look and surprise your dealer, neighbors and coffee shop cronies with your infinite GPS knowledge.

GNSS: Global Navigation Satellite System is a navigation system with global coverage.  GNSS is a method of improving the navigation systems 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 US owned utility that provides position, navigation and timing.  There are three segments:  the space segment, control segment and user segment.  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 US 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 United States.

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 it 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 us with 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 achieving 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- A numerical identifier used by the GPS receiver to determine which satellite it is looking at.

PRL: Preferred Roaming List (PRL) 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.   Messages 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.  1 Hz equals 1 message per second.

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

Drift: GPS receiver accuracy over time.  This can be measured absolute or pass to pass.  Drift is caused by changes in satellite configuration, Earth’s atmosphere, operating near trees or other obstructions and satellite data errors.

Nudge: Allows you to adjust the swaths by a specified distance while leaving the AB line in its original location.

Shift: Allows you to adjust the original AB line location and save the changes.

Assisted Steering: A motor is attached to the steering wheel and is automatically adjusted by the steering controller to stay on the guidance line.

Integrated Steering: Steering is controlled by a hydraulic steering valve that is automatically adjusted by the steering controller to stay on the guidance line.

Manual Guidance: Operator still steers the vehicle but uses a lightbar as visual guidance to stay on the guidance line.

Elevation mask: There is an actual physical elevation mask similar to that of the horizon line that blocks our view of GPS satellites after they go below the line.  There is also an electronic filter that acts as an elevation mask.  This means that any satellites below a certain degree will not be used in the final position.

Kerri Birdwell

Kerri Birdwell came on board with the Machine Guidance Support team in October 2011. She is the Pacific Coast Field Support Technician. She provides technical support in the field and on the phone. Kerri graduated from Cal Poly, San Luis Obispo in 2009 with a degree in Agriculture Systems Management. After college she spent two years in the irrigation industry designing and making scheduling recommendations. Kerri was raised on a cattle ranch in Coalinga CA where she still spends the majority of her free time. She enjoys riding horses, hiking and being at the beach.

Read More by Kerri Birdwell

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