Horus 39 – Fantastic High Speed SSDV Images

A great result from our high speed SSDV image (Wenet) system, which we flew as part of Horus 38 on Saturday Dec 3. A great write up and many images on the AREG web site.

One of my favorite images below, just before impact with the ground. You can see the parachute and the tangled remains of the balloon in the background, the yellow fuzzy line is the nylon rope close to the lens.

Well done to the AREG club members (in particular Mark) for all your hard work in preparing the payloads and ground stations.

High Altitude Balloons is a fun hobby. It’s a really nice day out driving in the country with nice people in a car packed full of technology. South Australia has some really nice bakeries that we stop at for meat pies and donuts on the way. Yum. It was very satisfying to see High Definition (HD) images immediately after take off as the balloon soared above us. Several ground stations were collecting packets that were re-assembled by a central server – we crowd sourced the image reception.

Open Source FSK modem

Surprisingly we were receiving images while mobile for much of the flight. I could see the Eb/No move up and down about 6dB over 3 second cycles, which we guess is due to rotation or swinging of the payload under the balloon. The antennas used are not omnidirectional so the change in orientation of tx and rx antennas would account for this signal variation. Perhaps this can be improved using different antennas or interleaving/FEC.

Our little modem is as good as the Universe will let us make it (near perfect performance against theory) and it lived up to the results predicted by our calculations and tested on the ground. Bill, VK5DSP, developed a rate 0.8 LDPC code that provides 6dB coding gain. We were receiving 115 kbit/s data on just 50mW of tx power at ranges of over 100km. Our secret is good engineering, open source software, $20 SDRs, and a LNA. We are outperforming commercial chipsets with open source.

The same modem has been used for low bit rate RTTY telemetry and even innovative new VHF/UHF Digital Voice modes.

The work on our wonderful little FSK modem continues. Brady O’Brien, KC9TPA has been refactoring the code for the past few weeks. It is now more compact, has a better command line interface, and most importantly runs faster so we getting close to running high speed telemetry on a Raspberry Pi and fully embedded platforms.

I think we can get another 4dB out of the system, bringing the MDS down to -116dBm – if we use 4FSK and lose the RS232 start/stop bits. What we really need next is custom tx hardware for open source telemetry. None of the chipsets out there are quite right, and our demod outperforms them all so why should we compromise?

Recycled Laptops

The project has had some interesting spin offs. The members of AREG are getting really interested in SDR on Linux resulting in a run on recycled laptops from ASPItech, a local electronics recycler!

Links

Balloon meets Gum Tree
Horus 37 – High Speed SSTV Images
High Speed Balloon Data Link
All Your Modem are Belong To Us
FreeDV 2400A and 2400B Demos
Wenet Source Code
Nov 2016 Wenet Presentation