I have been using for some time an RTL-SDR and SDR # to receive APT from the NOAA satellites.
As we all know, they transmit on 137 MHz, in FM with a bandwidth of 35 KHz, a subcarrier at 2.5 KHz, modulated in amplitude. It is this amplitude modulation that contains the information.
On the waterfall display, we see a strong Doppler effect, and also that the information is contained only in a dozen of very thin (S-shaped because of Doppler) 2.5KHz-spaced lines that correspond to the subcarrier.
During the passage of a satellite, the lines are visible on the waterfall well before the FM demodulator of the program manages to extract the sub-carrier from the noise and after some more time, the satellite is close enough so all noise disappears and only then, we can decode the image with the decoding program.
Clearly, it would be very desirable to extract information from the signal as soon as it is displayed on the waterfall.
My question is:
The current FM demodulator in SDR# is originally intended to decode speech, that is to take notice of all the events occurring within the whole bandwidth.
But to demodulate a NOAA signal, we don't mind about what happens in all those gaps between the lines. We know in advance where the lines will be since we know that the sucbcarrier is at 2.5 KHz.
Of course, we must find a way to compensate for the Doppler shift.
Would it be possible to design a digital demodulator that would do this?
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