Re :Re: [sdrangel] An enhanced demodulator for the NOAA APT satellites?


Dear Édouard,
I tried to ask this question on a math forum, but so far, I got no precise answer. It is on this forum, and I included a photo of the signal as it looks.
If you look at an FM transmission modulated with a fixed 2400 sub carrier, you have several fixed peaks spreaded 2400 Hz apart. 
With an APT signal, since the subcarrier is amplitude modulated, you see the amplitude of all the peaks vary, all in the same way.

The NOAA used this kind of modulation in the 1960s, probably because the Doppler effect is less of a nuisance than for an SSB signal. The APT signal could thus be received with the simple analog receivers available at the time, at the cost of more bandwidth and a s/n ratio not as good as for SSB.
But nowadays, maybe a digital decoder specially designed for the APT could be more effective than a classical FM demodulator?

Thanks for you interest anyway and kudos for SDR angel!



Le mer., oct. 9, 2019 à 17:45, Edouard Griffiths
<f4exb06@...> a écrit :

are you sure it is the same transmission repeated along in frequency? This seems to be a waste in a fairly constrained band. I am a total newbie in APT so I just went to Wikipedia ( and read "The signal itself is a 256-level amplitude modulated 2400Hz subcarrier, which is then frequency modulated onto the 137 MHz-band RF carrier. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34 kHz."  so it does not seem to be exactly as you describe. But maybe some specialists could give a better answer.


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