StarFire (navigation system)

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.

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.

dGPS correct for these errors by comparing the position measured using GPS with a known highly accurate ground reference, and then calculating the difference and broadcasting it to users.

To make the corrections accurate over a large area, one would need to deploy many ground reference stations and broadcast a considerable amount of data for finely divided locations.

Using this information, the ionospheric effects can be calculated to a very high degree of accuracy, meaning the StarFire dGPS can compensate for variations in propagation delay.

In comparison, "normal" GPS receivers generally offer about 15 m accuracy, and ones using WAAS improve this to about 3 m. When initially deployed, StarFire used seven reference stations in the continental US.

Additional StarFire networks were later set up in South America, Australia and Europe, each run from their own reference stations and sending data to their own satellites.

RTK consists of a small tripod-mounted GPS receiver that uses StarFire signals to perform its own dGPS calculations relative to a point, normally the corner of a field.