Английский язык: Методические указания и задания к контрольным работам для студентов специальности «Техническая эксплуатация транспортного оборудования», страница 33

- Short term and long term frequency drift of satellite and receiver reference oscillators.

-  Refraction of satellite signal in ionospheric layers.

-  Error of ship’s speed input.

-  Error of geoidal and antenna height input.

1. Short Term and Long Term Frequency Drift of Satellite and Receiver Reference Oscillators.

When a satellite passes over a receiver on the earth, the distance between the satellite and the receiver decreases first and then increases. Because of the doppler shift, the receiving frequency is higher than the transmission frequency when the satellite is approaching the receiver, and lower, when going away, that is, that receiving frequency varies.

The shipborn receiver of the satellite navigation system incorporates a reference signal of 400 MHz. The difference between this reference frequency and the receiving frequency is the “doppler frequency” which draws a curve as the satellite passes. Since the doppler count is integral of the doppler frequency for a given period, instability of the satellite or receiver reference oscillator introduces error in the measured doppler count.

If the long term stability of oscillator is poor, the measured doppler count is shifted downward or upward in parallel from the normal curve. In this case, the number of interactions on which the receiver computes the satellite fix is satisfactory small but a constant amount of error is always involved in the obtained fix, in proportion to the deviation of the doppler count.

Refraction of Satellites Signal in Ionospheric Layer.

Since the satellites are placed in orbit at an altitude of approximately 1100 km, their signals must transverse the ionosphere on the way to earth. The ionosphere modifies the propagation path of satellite signals through refraction. Since the dielectric factors are in the  ionosphere  and in the troposphere, the electric wave arrives at the earth through the increased path. This increase in apparent range between the receiver and the satellite introduces an error in the doppler count and therefore in the computed satellite fix. The position fixing error caused by this factor ranges from 200 m to 500 m but it is comparatively small in night time when density of free electrons in the ionosphere decreases. In addition, this error is a function of elevation angle of the satellite; the smaller the elevation angle, the larger the error.

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