Summary
Accuracy varies depending on need and cost for Global Positioning Systems (GPS). A positioning system is a method of identifying and recording the location of an object or person. Positioning systems benefit farmers by helping to accurately identify and record the location and field functions of a combine, tractor, or sprayer. GPS uses satellite signals to define positions on earth.

Solution Steps

Static Accuracy Static accuracy is how the GPS industry refers to accuracy, and is what most companies use to advertise their accuracy levels because it can be proven. It is measured by using the standard deviation of error of a point set by a position receiver in a fixed position for 24 hours.

Pass-to-Pass Accuracy Pass-to-pass accuracy, also known as "repeatability," is much more difficult to measure. Pass-to-pass accuracy is how far off you are from one pass to the next, regardless of which pass you are on. Most often, pass-to-pass accuracy is approximately 2 to 3 times more precise than static accuracy. This measurement is most important in guidance applications, such as Parallel Tracking and AutoTrac. This accuracy is measured in 15 minute intervals.

Levels of Accuracy John Deere Ag Management Solutions advertises three levels of accuracy with the StarFire™ GPS system: WAAS, SF1 and SF2 WAAS is a government-sponsored differential correction signal with a static accuracy of approximately 75 cm (30 in.). This signal has no guarantee as to how long it will be available or when planned outages for maintenance are scheduled. WAAS is a low-level accuracy that is a free service for John Deere customers. WAAS pass to pass accuracy is +\- 18 inches NOTE: Not all markets in North America are covered by this signal. Therefore, John Deere cannot guarantee the performance, reliability, or availability of WAAS. SF1, is a single frequency, differential correction signal subscription available exclusively through John Deere. SF1 has a static accuracy of approximately 75 cm (30 in.). While the static accuracy of SF1 is similar to WAAS, there are some significant advantages of SF1 over WAAS. The pass to pass accuracy of SF1 is +\- 13 inches. SF1 has a guaranteed coverage area in all markets we serve, and because John Deere owns SF1 there is less risk of losing the signal. Since the government sponsors WAAS, John Deere can not guarantee the signal. WAAS operates using towers and satellites to distribute signal. When any of the towers need maintenance, the government shuts down the entire system. John Deere uses two layers of towers and satellites to distribute signal. When maintenance is necessary on one layer of towers, the other set of towers acts as a backup; therefore signal is always available. SF2, is a dual frequency, differential correction signal subscription available exclusively through John Deere. SF2 has a static accuracy of approximately 25 cm (10 in.) and is most useful for more precise guidance applications. SF2 subscription is a requirement to run programs such as AutoTrac. The pass to pass accuracy of SF2 is +\- 4 inches.

Update Rate Another term used in differentiating accuracy is "update rate." The update rate, measured in Hertz (Hz), is the number of times per second that a GPS receiver reports its location to the vehicle system. The two most common update rates are 1 Hz (1 time per second) and 5 Hz (5 times per second). Update rates are important in pass-to-pass accuracy because the receiver is moving. The more frequently the receiver receives an update, the chance of error is reduced. Update rates are especially important in high-precision guidance applications since the system constantly compares actual location with desired location. John Deere uses a 5 Hz update with all levels of signal. Some competitors use a 5 Hz update which does improve the pass-to-pass accuracy. However, SF2 is still 3 to 4 times more accurate than WAAS, even with the 5 Hz update rate. Therefore, in order to achieve the pass-to-pass accuracy desired with a guidance systems, it is recommended to use SF2 with a 5 Hz update rate.

Additional Information When using GPS, remember that no system is "error-free." Limitations and errors do exist in GPS systems. However, the measures of accuracy give an estimate of the reliability of a particular piece of hardware. John Deere Ag Management Solutions is committed to providing the best signal options and GPS hardware at the best price possible.

Affected Equipment

StarFire Position Receiver    S/N:  All StarFire Position Receivers