In addition to these, the accuracy of the GPS position is also affected by the geometric
locations of the GPS satellites as seen by the receiver. In fact, it is only in a vacuum (free space)
that the GPS signal travels, or propagates, at the speed of light. Typical error is 10+ meters. All
GPS are 12 channel: can receive up to 12 satellites

1. GPS Ephemeris Errors

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Modeling the forces acting on the GPS satellites will not in general be perfect, which
causes some errors in the estimated satellite positions, known as ephemeris errors.
Forces on the GPS satellite Earth is not a perfect sphere and hence uneven gravitational
potential distribution

Other heavenly bodies attract the satellite, but these are very well modeled

Not a perfect vacuum hence drag but it is negligible at GPS orbits

Solar radiation effects which depends on the surface reflectivity, luminosity of the sun
distance of to the sun. This error is the largest unknown errors source.

2. Selective Availability

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To ensure national security, the U.S. DoD implemented the so-called selective availability
(SA) on Block 11 GPS satellites to deny accurate real-time autonomous positioning to
unauthorized users. SA was officially activated on March 25, 1990

SA introduces two types of errors. (1) delta error, results from dithering the satellite clock,
and is common to all users worldwide. (2) epsilon error, is an additional slowly varying
orbital error.

horizontal position of a stationary GPS receiver varies over time, mainly as a result of the
effect of SA.

The range error due to epsilon error is almost identical between users of short separations.
Therefore, using differential GPS (DGPS) would overcome the effect of the epsilon error.
DGPS provides better accuracy than the standalone P-code receiver due to the elimination
or the reduction of the common errors, including SA

The U.S. government discontinued SA on May 1, 2000, resulting in a much-improved
autonomous GPS accuracy.

3. Satellite and Receiver Clock Errors
Each GPS Block 11 and Block 11۸ satellite contains four atomic clocks, two cesium and two

rub

idium.

The newer generation Block IIR satellites carry rubidium clocks only. One of the onboard
clocks, primarily a cesium for Block 11 and IIA, is selected to provide the frequency and the

tim

ing requirements for generating the GPS signals.

The GPS satellite clocks, although highly accurate, are not perfect.

Satellite clock error is about 8.64 to 17.28 ns per day.

The corresponding range error is 2.59m to 5.18m, which can be easily calculated by
multiplying the clock error by the speed of light.