A system for detecting the rotational speed of a rotating body comprising the steps of generating two kinds of signals A and B having alternating wave forms and corresponding to the rotational speed of the rotating body by means of position-detecting devices (5a, 5b), the signals A and B having the same amplitude and having phases deviating by 90.degree., producing switching signals S(A), S(A), S(B), and S(B) from the signals A and B by means of zero-cross circuits (711, 712), producing inverted signals A and B from the signals A and B by means of inverter circuits (72, 73), producing differentiation signals dA, dA, dB, and dB by supplying the signals A, A, B and B to differentiation circuits (74, 75, 76, 77), and obtaining output signals corresponding to the rotational speed of the rotating body by selectively switching the differentiation signals dA, dA, dB, and dB in a switching circuit 78 by means of the switching signals S(A), S(A), S(B), and S(B), thereby detecting the rotational speed of the rotating body.
This invention relates to a satellite signalling system, such as a satellite paging system, having signal coverage over a very wide area. The satellite signalling system comprises a satellite to which is coupled an antenna for providing a signal beam having a variable beam area. The satellite can control the position of the antenna such that the signal beam can be directed in a predetermined sequence to a plurality of locations on earth. The antenna is arranged to vary the signal beam's area in dependence on the location to which the signal beam is directed. The signal beam comprises a data signal having a data rate which can be varied by the antenna in dependence on the beam area.
This invention relates to a satellite signalling system. A ground station receives paging signals from an earth-bound terminal, and an up-link radio transmitter/antenna transmits paging signals and control signals to a satellite in earth orbit. An up-link receiver coupled to the satellite receives the paging and control signals. A down-link transmitter is coupled to an antenna whose beam may be directed in a predetermined sequence to a plurality of locations on earth as paging signals are transmitted to earth.
