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NOAA APT with Gnu Radio: UPDATED

Gary Schafer, July 2022

Gnu Radio has changed a fair amount since my post on demodulating and displaying NOAA APT signals. I've decided to update my Gnu Radio flowgraph due to the changes between Gnu Radio versions 3.7 / 3.8 and the current 3.10.

Updated Gnu Radio Flowgraph

The biggest change is with the RTL-SDR source block. The new one is the "Soapy RTLSDR Source" block. This new block restricts the number of available sample rates to a small number. The first flowgraph used a 1 MSa/sec sample rate, but the new Soapy block doesn't allow that rate. I've changed the rate to 1.024 MHz, the closest one. This required changing the interpolation and decimation values of the "Rational Resampler" block, as well as adjusting the values in the "QT GUI Time Raster Sink" and "QT GUI Time Sink".

I've also removed one of the lowpass filters before the demodulator. The bandwidth of the remaining lowpass filter might need to be adjusted downward. For example, you might try to lower the "Cutoff Freq" to 25 kHz.

Updated Gnu Radio flowgraph for demodulating and displaying NOAA APT images.

Here are the properties for some of the most important blocks. Not shown is the "Rational Resampler", but the interpolation and decimation values are shown in the main flowgraph diagram above. All of the other blocks are the same as the first post.

General properties for the "Soapy RTLSDR Source" block.
RF properties for the "Soapy RTLSDR Source" block. The "cf-200e3" means that the center frequency is set to 200 kHz below the desired frequency. The multiply and signal source block after this source block will shift the desired signal back to the proper frequency. The whole point of this is to deal with any possible LO feedthrough the RTL-SDR (or whatever SDR you're using) might have.
The properties for the time raster sink. This is what actually displays the image. Rather than hard setting the number of columns, I've simply made it the sample rate divided by 2. This is due to the fact that the APT standard draws 2 lines / second.
Properties for the time sink. The number of points is similar to the number of points in the time raster sink.

While there are minor changes in the flowgraph, the general steps remain the same. First step is to collect the NOAA APT signal itself. This is the job of the "Soapy RTLSDR Source" block combined with the "Multiply" and "Signal Source" blocks. The lowpass filter isolates the signal itself. The "Quadrature Demod" block extracts the information from the primary RF carrier. The following AM demodulator extracts the video signal itself from the AM subcarrier. The time raster sink creates the image.

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