Yesterday I spoke about the Światowid image downlink protocol. Today, Piotr Kuligowski SQ4NOW has published an image that he has been able to decode from Światowid. The image was taken at around 3:29 UTC and downlinked at 6:38 UTC over Warsaw.
Looking at SatNOGS recordings of this event, I have noticed that the image data is sent with sequence numbers, contrary to what I stated in the description of the protocol in my previous post. This is something that SatRevolution must have added down the road, since it wasn’t present when I worked with them in June.
The protocol is as I described, but the first two bytes of each Reed-Solomon block are used as a little-endian block counter. The remaining 46 bytes are used to send the JPEG file data. The block counter is reset to zero at the start of a new file, and is increased for each Reed-Solomon block.
This block counter allows for automatic detection of lost blocks and start of new images, so I have added an image decoder to the Światowid decoder in gr-satellites. The decoder is based on the 1KUNS-PF image decoder. If there are missing blocks, gaps full of zeros are inserted in the JPEG file in their position. This allows easily merging files decoded from different groundstations just by ORing the files.
As an example showing the image decoder, I have processed this SatNOGS recording, which was made by the station of Cees Bassa in the Netherlands. To process a SatNOGS recording with the gr-satellites decoder, the OGG audio must be converted to WAV (using
oggdec, for example), and the gain of the “Multiply Const” block in
swiatowid.grc must be changed from 10 to 1, since SatNOGS recordings usually have too much gain.
The recording only contains the beginning of the transmission. The pass was west to east and the transmission was done when the satellite was in view of Warsaw, so by the middle of the transmission the satellite is already below the horizon in the Netherlands. Still, 1128 blocks could be decoded correctly. This amounts to 51888 bytes. The complete file is 204796 bytes long.
The partial image decoded from the SatNOGS recording is shown below.
This image matches the one that Piotr has shown on Twitter. I find it interesting that the SatRevolution logo is already added on-board the satellite to the top left corner of the image.