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Background:
Radio receivers that are tuned to the same frequency are deployed at multiple locations in many air traffic control sectors to ensure seamless airspace coverage. The size of an air control sector and the terrain dictate the number and location of these receivers. |
This allows pilots to communicate with controllers without having to continually switch radio frequencies. The distance a plane is from a receiver and the “line of sight” between the plane and receiver will significantly impact the strength of the signal at each receiver.
Signals from all the receivers, regardless of the strength, are brought into an air traffic control switching system. In most air traffic control centers the controllers must select the receiver that provides the best signal. This requires the controller to know which receiver is best, based on the aircraft location, or to rotate through the receivers until a good signal is found. This is both cumbersome and time consuming. Digital Signal Products, Inc. has developed a Best Signal Selection (BSS) system that automatically selects the best signal from any set or pool of receivers.
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| Operation
The DSP Inc.'s BSS performs a real time signal analysis on all incoming audio. The DSP calculates Idle Channel Noise for each radio channel. When incoming radio audio crosses a preset threshold, audio samples are collected and stored for 3 frames of 20 msec each. A signal-to-noise ratio, average power, standard deviation, and spectral density analysis is performed on samples collected during each frame. This prevents any false selection due to an early arrival of a weaker signal.
Based upon the above analysis a Quality Factor (QF) for each individual audio channel is calculated, and the channel with the best QF is selected for output to the switching system. DSP Inc.'s unique design starts voice transmission to the switching system from the very beginning without any loss of pilot audio voice clipping.
To ensure that no part of a message is lost while the best signal is selected, the BSS stores the first 250 msecs of all received messages. Once the best signal is selected the processor starts to release the message from its beginning. It looks for natural pauses in the speech and starts removing the delay in a controlled and intelligent manner. The remaining channels in the pool of receivers send either an idle code or no audio to the switching system. The BSS is totally transparent to the switching system and the air traffic controllers. It ensures the Best Signal Selection without any voice clipping or degradation of audio.
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