In 2017 ISS SSTV switched from transmitting images in PD180 to PD120. This is from the “What to expect during a pass” section of my ISS SSTV Reception Hints page. I wanted to save it somewhere in case they go back to using it at some point. For now I am updating the Reception Hints page to cover PD120 instead.
With PD180 it takes about 3 minutes to send an image. With PD120 it takes about 2 minutes to send an image. Since images transmitted with PD120 take less time to send than with PD180, more images can be received during a single ISS pass.
PD180 takes about 3 minutes to send an image.
An ISS pass that goes right overhead (90 degrees elevation), lasts about 10 minutes. ISS SSTV transmit time and off time are usually setup to provide the radio with a 50% duty cycle (only transmit half the time so the radio doesn’t overheat). With image transmission taking three minutes, off time will probably be three minutes as well.
With a 10 minute pass you’ll have the possibility to receive up to two complete images if timing, conditions, and setup are ideal.
When the ISS comes into view/has line of sight with you, this is known as Acquisition of Signal, or AOS. The ideal situation for a high elevation 10 minute pass would be if the first image started transmitting right at your AOS, and you had a directional antenna so you could receive the signal even while the ISS was very low in the beginning of the pass.
In this case you would be able to receive two complete images like this:
minute, image TX/off
0-3, image 1 (complete)
3-6, off
6-9, image 2 (complete)
9-10, off
Or if transmission started exactly one minute after AOS you’d have the opportunity to receive one complete image then and one through the last three minutes of the pass up until the Loss of Signal (LOS), like this:
minute, image TX/off
0-1, off
1-4, image 1 (complete)
4-7, off
7-10, image 2 (complete)
The more common situation will be that the first image transmission will start either before AOS or more than a minute after AOS. In this case you will only have the opportunity to receive one complete image. Those two situations might look like this:
minute, image TX/off
0-2, image 1 (incomplete)
2-5, off
5-8, image 2 (complete)
9-10, off
minute, image TX/off
0-2, off
2-5, image 1 (complete)
5-8, off
8-10, image 2 (incomplete)
A third possibility is to get the tail end of image one, a complete second image, and then the beginning of a third image, though this is uncommon.
minute, image TX/off
0-0:30, image 1 (incomplete)
0:30-3:30, off
3:30-6:30, image 2 (complete)
6:30-9:30, off
9:30-10:00, image 3 (incomplete)
Even though you’ll have the opportunity to receive two complete images, don’t expect to. It may take practice and it will certainly take the right setup and conditions, to get just one complete image. With that said, here are some tips that may help you get more images and/or better images.
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