That Happy Moment When

the demodulator you’ve been working on starts outputting meaningful data instead of gibberish (T7RPUR WIDE1 WIDE2 are valid packet radio strings).

Screen Shot 2015-08-24 at 8.45.48 AM

I’m writing the demodulator in C++ and it uses the Goertzel algorithm.  Need to continue to flesh out the framer, and hook up STDIN to accept sound from rtl_fm and my new SDR dongle.

The goal: automated reception of Space Station packet radio, powered by Raspberry Pi.

First WX Image from NOAA-18

I received my first WX image from NOAA-18 as it passed overhead just a few moments ago.  I was alerted to the pass by the Raspberry Pi based ground station-in-a-box I’ve been working on (far left in the picture) and used the WXtoImg software (center) to decode the signal from my radio, piped through the sound card box I like to use.

FullSizeRender 9

This was with a simple 2m J Pole antenna, but I plan to build an eggbeater or maybe a helical antenna soon.  I need to build a pre-amp too I think.

Here’s a video of what it all sounded like

Here’s my final image:


and an image from about the same time from the NOAA website for comparison:


Pictures from Space, Part 2

You can read part 1 here.

It worked really well, and I got to hear Cosmonaut Gennady Padalka’s (props to John Brier for recognizing the voice) answer student’s questions on Internet access while on the Space Station, what it is like looking at Earth from space, having ice cream in space, the risks of fire on the station, what kinds of cuisine they have, and what happens if you get sick in space, before the contact faded out.

Over 3 minutes at an extremely low (20 degrees or less) elevation. It was quite the treat. Here is the complete audio I recorded:

The next morning there would be two passes and SSTV was planned. I was hoping they’d sort out the snafu that was causing the carrier only (no picture) problems on Saturday, and I was not disappointed.

The first pass had its closest approach to our home at 1429 UTC, and started at 1424 UTC. I was delighted to hear the chirping sounds of the image data as the space station rose above the horizon to the East. They stopped after less then a minute, and then I was in the “3 minute” no signal period. (Each SSTV image has 3 minutes of radio silence between them.) And, since a pass is only about 10-11 minutes long that was a good chunk of the time right there. This radio silence period in particular seemed to go on for longer.

But then, the chirping began again and I caught it all on the recording I was making. The pass eventually faded out to static, with the image still downloading. I replayed the recording into the SSTV app (SSTV Slow Scan TV) on my iPhone and was treated to my first image:


Neat fact: It turns out that that artifact you see where the image “jogs” was received by everyone listening.

The next pass began at 1601 UTC, but this time I managed to capture a complete image, and with few artifacts:


It was really a treat, and I can’t wait to try again when the station broadcasts SSTV again (as they do a few times each year.)

Curious what the SSTV signal sounds like?  Take a listen (in fact, if you want, you can decode this into an image using that app or a similar app yourself – the mode is called “PD 180”)