Page 60 - Electronic Navigation Cyber Book

Fortunately, ionospheric error is inversely proportional to the square of
the carrier frequency. GPS SVs transmit on two frequencies so that the
delay may be quantified in the receiver, an error correction figure
calculated and applied to the final fix solution. After all corrective data has
been applied to the solution in a single frequency GPS receiver system,
fix error due to the ionosphere is unlikely to exceed 10 m.

Tropospheric delay error

Extending from the earth's surface to an altitude of 70 km, the
troposphere also introduces a delay into the pseudo-range calculation.
Unfortunately the error is independent of frequency, but it is predictable.
GPS receivers hold a software solution in the form of a mathematical
model to eliminate the effect of this delay. Figures for relative humidity,
pressure and temperature are interfaced with the processor computer to
produce corrective data which is then applied to fix calculation. Error from
this source is unlikely to exceed 1 m.

Both ionospheric and tropospheric errors are reduced if ranges are
measured from SVs showing a high elevation from the receiver. Modern
receivers are capable of automatically selecting SVs with the highest
elevation or those exceeding pre-set limits.

Multipath error

This results from the reception of the same SV signal from more than one
source. A major contributor to this error is the reflected wave from an
object close to the receiving antenna. Each receiver position is unique
and therefore the error is not consistent. Final fix errors in the region of 1
metre can be produced by this effect. Careful positioning of the antenna
will eliminate this error.

© 2018 Digital Galaxy Index 60