StarFire (navigation system)

StarFire is a wide-area differential GPS developed by John Deere's NavCom and precision farming groups. StarFire broadcasts additional "correction information" over satellite L-band frequencies around the world, allowing a StarFire-equipped receiver to produce position measurements accurate to well under one meter, with typical accuracy over a 24-hour period being under 4.5 cm. StarFire is similar to the FAA's differential GPS Wide Area Augmentation System (WAAS), but considerably more accurate due to a number of techniques that improve its receiver-end processing.

StarFire came about after a meeting in 1994 among John Deere engineers who were attempting to chart a course for future developments. At the time, a number of smaller companies were attempting to introduce yield-mapping systems combining a GPS receiver with a grain counter, which produced maps of a field showing its yield. The engineers felt this was one of the most interesting developments in the industry, but the accuracy of GPS, then still using Selective Availability, was simply too low to produce a useful map. The various providers went bankrupt over the next few years.

In 1997, a team was formed to solve the problem of providing a more accurate GPS fix. Along with members of John Deere's engineering team, a small project at Stanford University also took part, along with NASA engineers at the Jet Propulsion Laboratory. They decided to produce a dGPS system that differed fairly dramatically from similar systems like WAAS.

In theory the GPS signal with Selective Availability turned off offers accuracy on the order of 3 m. In practice, typical accuracy is about 15 m.

Of this 12 m, about 5 m is due to distortion from "billows" in the ionosphere, which introduce propagation delays that makes the satellite appear farther away than it really is. Another 3 to 4 m is accounted for by errors in the satellite ephemeris data, which is used to calculate the positions of the GPS satellites, and by clock drift in the satellite's internal atomic clocks.

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