1.

Selective Availability Errors

.  Intentional SA degradations are applied to the

GPS navigation signals to create the SPS level of accuracy.  Two methods

are used.  The first method, called epsilon, alters the ephemeris (location)

parameters of the satellite to give an apparent shift of satellite position.  The

second method, called dither, alters the satellite clock frequency, thereby

introducing range errors in the C/A code, P(Y) code, and carrier signals. 

These errors resemble the naturally occurring ephemeris and clock errors. 

2.

Ionospheric Delay

.  Ionospheric signal propagation delay can vary from

40 60 metres 95% by day to 6 12 metres 95% at night.  This is a particular

problem with a single frequency user (i.e., a single frequency C/A code

set).  Dual frequency receivers can correct for ionospheric delays with a

residual error of some 4.5 metres  95%.  The satellite navigation message

contains correction coefficients for  the single frequency user to reduce the

ionospheric delay by appropriate algorithm.

3.

Tropospheric Delay

.  This signal propagation delay is caused by

moisture in the lower atmosphere.  Tropospheric delay may be up to 6

metres 95% in magnitude. Many receivers employ algorithms to minimize

this tropospheric delay error.

4.

Ephemeris Error

.  This error is the difference between the actual satellite

location and the position predicted by satellite orbital data.  Normally,

errors will be less than 8.2 metres 95%.

5.

Satellite Clock Error

.  This error is the difference between actual satellite

GPS time and that predicted by satellite data.  This error is normally less

than 6.5 metres 95%.

DGPS can correct for the errors and induced biases listed above in the following

manner:

1.

Selective Availability Errors

.  These errors are only of concern to the

SPS user. They resemble the naturally occurring ephemeris and clock

errors, except that they can be larger in magnitude and can change more

rapidly.  The epsilon error can be  a three dimensional error.  Therefore,

part of the error will appear as a common range error and part will remain

a residual ephemeris error.  The residual portion is normally small and its

impact remains small for similar look angles to the satellite. The dither

error can appear as a time and frequency bias.  This will be an error

common to all receivers and will not be affected by signal propagation or

distance from the satellite.  However, since it is rapidly changing, any

delay between the time of measurement at the reference station and time

of use at the user receiver will result in a residual clock error.  SPS DGPS

systems are normally designed with a rate of change term in the

corrections and rapid update rates to minimize this effect.

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