Rapid fading effects on airborne satellite communication receivers employing Granlund pre-detection diversity combiners

A two ray fading model is used to illustrate the effects of over water flight on satellite communications when a Granlund (1956) pre-detection diversity combiner is used to extend the receive antenna coverage. An airplane with multiple antennas covering non-overlapping arrival angles is investigated. A linearized model developed for the combiner is applied to the task of estimating phase tracking error, regeneration frequency, and signal to noise ratio degradation. The cases of the direct and reflected (specular) component arriving in the same or different antennas is investigated. It is shown that when the direct and reflected waves arrive in the same antenna the combiner is nothing more than a gain controlled amplifier with a control transfer function determined by the chosen filter functions. It is shown that when the direct and reflected wave arrive in different antennas, the regeneration frequency becomes a weighted average related to the arrival frequencies of the direct and reflected waves (where the frequency difference is a function of aircraft velocity). An expression for the phase tracking error in the case of the signals arriving in the different antennas is derived. The effects of phase tracking error on the received signal to noise ratio are analyzed. The channel distortion effects are analyzed for a typical application of the airplane communicating with a geostationary satellite.