One of the techniques used to increase the capacity of communications satellite systems operating under bandwidth limited conditions is frequency reuse through spatial and/or polarization isolation. In systems using dual polarization the isolation between the signals in each polarization state is limited by depolarizing effects of the earth station antennas, the satellite antennas and the propagation medium. The depolarizing effects of the propagation medium, which are minely due to rain, can significantly reduce the isolation between the cross-polarized signals. There is a need for collecting propagation data to provide better understanding and statistics of rain depolarization effects, and to monitor the quality of frequency reuse satellite links. Some of the INTELSAT IV-A satellites are equipped with a dual polarization experiment which can be used to gather propagation information on the satellite links.
The usual method of collecting data on the satellite link is to send a special carrier (usually C.W. signal) at the transmit end of the link, then receive that carrier (in both polarizations) at the receive end of the link by means of conventional receivers. This method requires the allocation of certain frequency bands for the carriers to be used in collecting the data, which reduces the available frequency spectrum of the link, hence a reduction in the capacity. Furthermore, if data from more than one carrier is desired, additional receivers are needed, which increases the cost of the data gathering.
A system which tracks an undesired cross-polarization signal using a phase locked loop is described in Fletcher et al., U.S. Pat. No. 3,883,871. The object of the Fletcher et al. system is to provide automatic nulling out of any undesirable received signals having approximately orthogonal polarizations. The received cross-polarization signals are applied as inputs to a hybrid junction which produces sum and difference signals. The sum signal includes components of the undesirable received signals and is used to coherently detect and null these unwanted components from the difference signal. The carriers of the desired and the undesired signals are slightly different and the phase locked loop tracks the undesirable signal. The Fletcher et al. system, however, does not measure the relative signal levels of each polarization to determine the depolarizing effects of present atmospheric conditions.
McAuliffe, U.S. Pat. No. 3,518,680, also discusses the use of a phase locked loop circuit in conjunction with a receiver of polarized signals. The McAuliffe apparatus eliminates cross-channel interference in a quadrature transmission system and describes a correlation means which detects the correlation between the in-phase and quadrature signals received. The correlation produces an error signal which indicates the cross-channel interference. The phase locked loop, in combination with the error signal, provides a control means such that cross-channel interference can be eliminated. McAuliffe does not, however, teach the correlation between two orthogonally polarized signals to measure the depolarization effects of atmospheric conditions.
The Patent to Voght, U.S. Pat. No. 3,943,517, discusses an adaptive polarization receiver that makes the system phase angle track the orientation angle of linear polarization of a radiated carrier. The Voght system compensates for distortion caused by undesirable polarization modulation of the carrier. The receiver taught by Voght incorporates a closed loop feedback network, in which an error signal is generated whenever the system phase angle is different from the polarization angle of the received signal. The Voght system does not function as that of the subject disclosure but is cited because of the closed loop feedback servo system.
U.S. Pat. Nos. to Ruthroff (3,500,207); Amitay (3,735,266); Ohm (3,914,764); Tirro et al. (3,986,123); Soma et al. (4,090,137); Beguin et al. (4,106,015); and Powell (4,107,678) teach other types of correlation receivers for cross-polarization signal reception, but do not disclose the use of a phase locked loop demodulator.