Abstract: A performance monitor for use with marine radars which produces a visual indication upon the radar display screen only when the radar transmitter power, and the radar receiver sensitivity and tuning are within predetermined limits. An RF sample of the radar transmitted signal is mixed with a local oscillator signal and the resulting IF signal is coupled to an AFC loop which controls the frequency of the local oscillator. If the amplitude of the radar transmitted signal is above a threshold level, a single frequency response signal of fixed power is generated by the performance monitor and transmitted back to the radar antenna. A distinctive test pattern is produced upon the radar display if the radar receiver tuning is accurate and its sensitivity is sufficiently high.

Abstract: An improved reliability microwave communications system combining discrete state modulation with diversity transmission and reception. Signals are transmitted and received in both horizontal and vertical antenna polarization using QPSK modulation techniques in the preferred embodiment. At the receiving terminal the received signal with the best quality as determined by predetermined receiver parameters is selected as the output data stream. Switching between received signals is accomplished using a hitless switch such that no data is lost when switching between received data streams. Redundant switching and transmitting circuitry may also be provided to further increase the reliability of the system.

Abstract: A digital data communication system employing quaternary phase-shift keying for transmission by a microwave radio link. The digital modulation of the quadrature channel is delayed from the in-phase channel by one-half the time duration of a bit so that the transmitted phase does not experience a quantum shift in excess of 90.degree.. This permits the use of an injection locked oscillator as an amplifier of the microwave transmission.

Abstract: A digital communications system with increased resistance to frequency selective fading. Digital signals to be transmitted are phase shift modulated upon a spectrum spreaded carrier. The carrier is generated by mixing the output of a band spread oscillator with a locally generated IF oscillator signal. The signal thus obtained is divided into transmission periods, each of which is longer than the maximum spreading time of the nonlinear transmission medium. The increased bandwidth occupancy thus obtained increases the immunity of the transmitted signals to frequency selective fades moving across the allocated bandwidth. The signals are received by a differentially adaptive receiver which automatically adjusts its response to the envelope of received signals.

Abstract: A system for communicating a data channel composed of a multiplicity of telephone-type messages wherein the invention incorporates circuitry for modulating service channel data onto a carrier of the telephone channel data and for demodulating the service data without interfering with the telephone data. An inverse modulator driven by detected telephone data is utilized to recover a carrier which is then tracked by a tracking filter to recover the service data.