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06-Sep-2017 03:18

Star Fire instead uses an advanced receiver to correct for ionospheric effects internally.

To do this, it captures the P(Y) signal that is broadcast on two frequencies, L1 and L2, and compares the effects of the ionosphere on the propagation time of the two.

Star Fire receivers also receive WAAS signals, ignoring their ionospheric data and using their (less detailed) ephemeris and clock adjustment data to provide about 50 cm accuracy.

In comparison, "normal" GPS receivers generally offer about 15 m accuracy, and ones using WAAS improve this to about 3 m.

The relative accuracy is likewise improved, to about 2.5 cm.

Even if the Star Fire correction signal is lost for more than 20 minutes, the internal ionospheric corrections alone result in accuracy of about 3 m.

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.

d GPS 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.

The various providers went bankrupt over the next few years.The corrections are generally valid for about 20 minutes.In addition to ephemeris and clock corrections, the signal also contains information on the health of each satellite, offering quality-of-service data in near real-time, with about a 3-second delay in updating the signals from the ground station. The first, retroactively known as SF1, offered 1-sigma accuracy of about 1 m.Using this information, the ionospheric effects can be calculated to a very high degree of accuracy, meaning the Star Fire d GPS can compensate for variations in propagation delay.

The second P(Y) signal is encrypted and cannot be used by civilian receivers directly, but Star Fire doesn't use the data contained in the signal; it only compares the phase of the two signals instead.

Star Fire broadcasts additional "correction information" over satellite L-band frequencies around the world, allowing a Star Fire-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.