During communication in a system for radio communication where stations are called with selective calls the participating stations watch for priority calls on a number of other radio channels. The watching is performed by listening in to the other channels at each switching from transmission to reception and vice versa and also at time intervals if the switching is overly late.

To program a scanning receiver for selected priority, a counter in the scanning control circuit is manually stepped to the channel to be selected for priority and that channel is indicated in a memory by depressing a program switch in the program mode while the memory is being addressed by the scanning circuitry. When the scanning circuit thereafter begins scanning, it returns to the established channel. A special weather and alert channel may be energized on the frequency set aside for weather warnings and emergencies by depressing a switch. This channel is continually sampled when the switch is in the energized position so that all such warning signals will be received.

A telecommunication arrangement for transmitting a first communicating signal and receiving a second communicating signal, particularly useful for sattelite communications. A transmitter generates a first transmitting signal at a first time and generates a second transmitting signal at a second time different from the first specified time. The first transmitting signal is mixed with an oscillating signal by a first mixer for producing the first communicating signal. A second mixer mixes the received second communicating signal with an oscillating signal to produce a mixed signal having a frequency corresponding to a function of the frequencies of the second communicating signal and the oscillating signal. In response to the second transmitting signal, the mixed signal is converted into a fixed signal with a constant frequency by a converter. A receiver receives the fixed signal.

A frequency synthesized transceiver capable of tuning to a plurality of communication channels is disclosed. The transceiver includes a receiver section and a transmitter section which are coupled to the synthesizer which generates the appropriate injection signals to achieve tuning. The frequency synthesizer includes a multiposition switch which accesses various addressable memory locations in a programmable read-only memory where the appropriate divisors are stored to cause tuning of the synthesizer to the appropriate communication channel. A zone selector switch enables grouping and easy retrievability of channels. The synthesizer includes a priority channel monitoring system utilizing a channel element for rapid sampling. The divisors are supplied to a single synchronous binary swallow counter which works in conjunction with a dual modulus prescaler to monitor the frequency output of the voltage controlled oscillator. A programmable divider coupled to a reference oscillator source is compared with the output of the synchronous counter in a digital and analog phase detector. The phase detector supplies signals through a loop filter to apply the appropriate voltage to the voltage controlled oscillator. The phase detector includes means to rapid advance the voltage controlled oscillator to cause frequency tuning.

A navigational unit having a time-shared mode of operation for handling a plurality of different navigational signals is provided wherein IF components within the unit may have a dual dedication, i.e., used in common for handling all signals while normally present separate front end components and navigational display components may be dedicated exclusively to each separate signal, a central timing and control component coordinating the total operation.