Abstract: A navigation system including at least one pulse radar and a number of transponders which when receiving radar pulses transmit high frequency pulse modulated signals. When receiving an interrogating signal from the pulse radar the transponder transmits a response signal with a delay. The delay is selected in such a manner that the response signal arrives at the pulse radar within a time space lying between the moment when all signal echos, lying within a definite range, have been received and the moment when the next radar pulse begins. The display scan in the radar is delayed by the same value as the response signal. In this way the response signal competes with considerably weaker echo signals than in the case of a non delayed response.

Abstract: A method for preventing a radar beacon's response to received pulses from a vessel which originate from the side lobes of the vessel antenna. The carrier frequency of the radar signal pulses to and received by the beacon is measured and a value of this frequency is stored when the amplitudes of the signal pulses exceed a certain high threshold level (Th). If signals after that are received which exceed a low threshold level (Tl) (but not the high) the carrier frequency of such signals is measured and a corresponding value is stored. Measuring two subsequent and equal carrier frequencies is considered as a criterion that a response signal from the beacon responding to the second received signal should not be transmitted. An arrangement to carry out the method is described.

Abstract: A device for measuring the frequency of a pulsed carrier signal in which the carrier frequency is situated within the frequency range and to give a continuous signal whose frequency is equal to the measured carrier frequency. A frequency discriminator (3) gives an analog voltage via a track-and-hold circuit (4) and a DC-amplifier (15) to one input of a comparator (6) whose second input receives the analog voltage from the discriminator (3). The difference value from the comparator (6) is supplied to a voltage controlled oscillator (8). This is via a directional coupler or duplexer (9) and a controllable switch (1) connected to the frequency discriminator (3) which alternatively receives the input signal to the device and the output signal from the voltage controlled oscillator (8).