The satellites
Satellites transmitt imagery using APT (Automatic Picture Transmission), an analogue transmission format. These satellites belong to the American scientific agency NOAA (National Oceanic and Atmospheric Administration).
| Mode | Bandidth | Polarisation |
| Narrow FM | 50 kHz | RHCP |
The modulation is FM (like normal broadcast FM radio), and once demodulated the amplitude of the 2.4kHz carrier tone (recognizable as a continuous "beeping") is used to draw the image line by line, like SSTV.
| Satellite | Frequency |
|---|---|
| NOAA 15 | 137.62 MHz |
| NOAA 18 | 137.9125 MHz |
| NOAA 19 | 137.1 MHz |
Procedure
- Never use AGC or tuner AGC. The reason is that signals are quite narrow and AGC is designed for more wide signals as for example DVB.
- Any SDR hardware will do the job. I have used generic RTL-SDR device with success.Beware of fake dongles, always buy on official stores only.
- The most important parameter is not the antenna, neither the SDR. It's the line of sight! It goes without saying that the best spot to receive APT is a mountain top, followed by any sufficiently flat and sparsely populated location.
- Software for SDR. With this software you record de signal audio. Later you will decodeir. I recomend CubisSDR for Macos. You can also try SDR++
- Decoding software. SatDump is an all in one solution for everything satellite reception related.
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