HABbing, as it is known, is a growing hobby where enthusiasts use standard weather balloons to put small payloads into 'near space' at altitudes of around 30km.
The balloons usually carry a tracking device to check the position during the flight and often include sensors for temperature and pressure, as well as a video or stills camera to save media to an SD card for later retrieval.
However, enthusiast Dave Akerman realised he could use the Raspberry Pi computer to send live images back to Earth during the flight.
"In early May I received my first Raspberry Pi computer, and having flown several high altitude balloons before I thought about using one as a flight computer. In almost all of my previous flights I used Arduino Mini Pro boards, and these are ideal - tiny, weigh almost nothing, simple and need very little power. I looked at the Pi and saw none of these desirable features!" he reveals on his blog.
"What I did see though was a USB port offering quick, easy and inexpensive access to a webcam, meaning that for the first time I could have live images (SSDV) sent down by my payload - something that hasn't been done very often."
Akerman said one of the biggest challenges was the fairly hostile environment of near space, which has less than one percent atmosphere and temperatures down to -50C or so. The descent and landing can also both be quite violent.
If anything does go wrong with the technology the one-way radio link means the device can't be rebooted.
The Raspberry Pi also added some challenges thanks to its complexity and functions such as an SD card, USB port and operating system.
"Extra power and capability does come at a price and the first one is an increase in the power requirement from around 60mA to over 500mA, and that of course means much higher power dissipation," Akerman said.
"People often worry about the low temperatures in near space, but when your payload is generating a few watts of power that is not likely to be a problem! I was much more concerned with how hot it was going to get inside the payload, so I added some heatsinks to the Pi."
How to capture the live images also had to be addressed and Akerman applied a patch to the Debian OS to set up the webcam, choosing fswebcam as the program to take the photos.
"I tried a few webcams and settled on the Logitech C270 which is reasonable quality, light and cheap (in case the payload goes missing!)," he advised.
"Remember that the radio system has low bandwidth and with a typical flight lasting two hours or so we don't have time to send large images, so there's no point using the very best webcam and the highest resolution."
A simple shell script was used to take a photo every 30 seconds, saving them on the SD card so that the tracker program could choose the largest jpegs - deemed to be the best images - for transmission.
"Each chosen image is then converted to the form for download (split into blocks each with FEC) before being sent one block at a time. I interspersed the image data with telemetry - four image packets for each telemetry packet)," he said.
A launch video can be seen below. The results of the flight and the images from the Pi in the Sky project can also be seen by viewing this Flickr gallery.
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