The AFC feature in HDSDR helps keep the SSTV signal lined up in whatever program you use to decode the pictures. Some SSTV programs/apps, like the RX-SSTV program I use on my Windows PCs, have their own AFC function to help it track the signal. I use AFC in both places when I decode SSTV pictures. Even though many amateur transceivers and receivers can receive ISS SSTV pictures on 145.800 MHz without any frequency adjustments, you can clearly see the small amount of Doppler on the received SSTV signals when using an SDR receiver. Here are screenshots taken at different points in the reception of an SSTV image. Here's one screenshot at approximately 1 minute into the transmission, showing both HDSDR and RX-SSTV on the screen: The zig-zag at the top of the HDSDR waterfall was due to moving the waterfall so I could see all of the received signal on my screen. I wanted to have the relevant parts of each program's window visible in the screenshots. At 2 minutes into the transmission... This was the view, just after the 3-minute transmission concluded: Note the TUNE frequency showing in the HDSDR window of each screenshot. The frequency was around 145.802 MHz when the 3-minute transmission started, and I pressed the AFC button in HDSDR as soon as I started to see and hear the transmission. You can also see the HDSDR waterfall in each screenshot, where the received signal was showing up at an angle - another indication that the Doppler effect was at work. I had RX-SSTV connected to HDSDR through a virtual audio cable, and you can see how much of the picture I received at each point in the transmission when I made these screenshots. From that transmission on Wednesday (13 April) afternoon came this picture: I posted my HDSDR RF recording (big WAV file), a smaller MP3 file with the received audio, these pictures/screenshots, and other pictures/screenshots in my Dropbox space at http://dropbox.wd9ewk.net/ (look for the folder "20160413-ISS_SSTV-DM43"). Great to hear! 73!