Caveats

Even though the classifier has been carefully designed, the extreme complexity of classification requires the user to be aware of a few caveats. Common sense and visual inspection, as well as prior monitoring experience, are helpful when dealing with these issues.

Half-Duplex Signals

The classifier will only classify half-duplex or burst signals with difficulty. This is due to the nature of the signal, which is only present for a relatively small fraction of the total sampling time.

Multi-channel Systems

The current version of the classifier will only classify a 12-channel PSK CIS-12 signal as a single, aggregate signal – for all other multi-channel systems the classifier will classify each sub-channel as an individual signal.

MFSK

The current version of the classifier may indicate an incorrect number of tones for MFSK signals.

PSK A and B Signal Confusion

Under certain conditions the classifier cannot distinguish between the A and B variants of PSK modulation. A PSK signal of one variant may be mistaken for the other variant if the signal is offset from the real center frequency with a certain fraction of its baud rate; i.e., for PSK-2 ¼, for PSK-4 1/8 and PSK-16 1/16 of the baud rate. This behavior also includes multiples of the fractions.

OQPSK

The current version of the classifier has difficulties in correctly evaluating the baud rate of OQPSK signals, e.g., Inmarsat-Aero. Additionally, the phase constellation diagram may be incorrect.

Handling of Synchronous and Asynchronous Speeds

The handling of asynchronous Baudot signals with 1.5 stop bits is a challenge. If the classifier does not succeed in recognizing the stop bits, it will classify the signal as a synchronous signal having the double speed of the actual signal.

CW Recognition

The classifier may confuse a low rate FSK signal with a fast CW signal, particularly at the edges of the classification bandwidth. To lower the probability of such behavior, a classification model may be selected which only allows CW signals below 30 Bd to be classified at the edges of the classification bandwidth.