An experiment to transmit JPEG images live during the flight over a low power radio link, by Philip Heron.

To transmit small (320×240, ~5kB) JPEG images over an 8-bit RTTY signal at 300 baud, with a transfer time of between 2 and 5 minutes depending on the complexity of the image.

The current module consists of:

• ATmega644P clocked at 7.3728MHz
• NTX2 radio module @ 434.075MHz
• C328-7640 JPEG Camera Module (SparkFun)
  • Takes fairly good images up to 640×480, but because of the slow data link it will only be used at 320×240. All of the JPEG compression is done on-board the camera so is ideal for using with a lower power microcontroller, which only needs to package and transmit the data.

• Reed-solomon encoder / decoder. See: rs8encode
• Modified version of dl-fldigi http://github.com/jamescoxon/dl-fldigi
• The flight software is at http://github.com/fsphil/hadie

• JPEG is not a good choice of format as missing blocks will likely cause unrecoverable errors in the rest of the image.
• RTTY is not the best mode for transmitting the data, but is incredibly easy to implement with the NTX2 radio module.
• A method of re-transmitting lost blocks - requires an uplink of some kind and the blocks to be stored.
• Distributed listener - each client uploading received blocks to a server.

The HADIE module flew on the 16th April 2010 as part of ProjectCirrus, reaching over 30km altitude. The module and camera worked throughout the flight, transmitting the position and image data until the harder than expected landing destroyed the board. The 10mw signal from the NTX2 was picked up as far away as Yarm in England, 366km to the east.

The payload was spinning for most of the flight causing the signal to fade in and out. Because of this about 5% of image blocks where lost which unfortunately resulted in most of the images being incomplete. Once a block is lost, any subsequent data received for that image is also lost.

Images from the main and uart cameras can be seen here: http://www.flickr.com/photos/fsphil/sets/72157623870036944/

A second flight was made on the 9th January 2011. Using much of the same hardware as the previous flight, its main job was to test the improved SSDV error recovery code. Most of the components survived the last flight and where reused for this one. The main differences where:

• Interrupt driven RTTY code with a single pin and two resistors. (3.3v → 330 ohm → NTX2 tx pin ← 2.2k ohm ← rtty output pin)
• The new robust image format, a lost packet will not ruin the rest of the image. See the SSDV page for more information.
• Quarter-wave dipole antenna, mounted so that the module doesn't take the full force of the landing this time.
• Canon A560 camera with CHDK and script to take regular images.
• A new version of the C328 UART camera with a wide-angle lens.
• A thicker polystyrene box with compartments for each of the components, wrapped in foil backed polystyrene sheets.

The balloon was released at 12:17 UTC and began heading east almost immediately. Reception on the small vertical became difficult within about five minutes of release, so we switched to using the stacked yagi array. Even with this the signal was fading fast and the last string to be fully decoded was at 13:09 UTC. There was enough time to receive three full images and fragments of a few more. Contact was re-established by G4FVP and M0DTS at 14:58, almost two hours later! By now the payload was descending and had stopped sending images. The last known position, taken from incomplete telemetry by G4FVP was at 54.383, -2.03966 from about 1300 metres up. The predicted landing spot was 54.3629, -1.88296:

M0DTS/Rob went to the area but no signal was received. The payload may have been damaged on landing or the hills could have blocking the already weakened signal. I don't know yet what happened to make the signal so weak, but there's a good chance the antenna was at fault. It was either made wrong or damaged in flight.

The highest altitude was received by M0DTS from incomplete telemetry, and was 30217 metres at 14:36:09. M0DTS also reported a likely burst at ~14:51 and with a 3.5m/s ascent rate puts the burst altitude at roughly 33.3km.

The flight code for Hadie-2 is available here: http://github.com/fsphil/hadie

A third flight is planned for early-March 2011. The plan this time is:

• A proper PCB instead of strip-board. [Maybe]
• Add a 16-bit checksum to the SSDV packet structure. [Done in github, being tested]
• Use 7-bit Reed-Solomon error correction on the telemetry strings.
• Have dl-fldigi configured to use a narrower filter width.
• Fixed a trivial bug that caused the baud rate to be slower than expected. [Done]

I plan to use a 1/4 wave vertical again, but made in the style of the bazooka vertical used on the first flight. The end of the coax will be stripped back 1/4 wavelength, with the outer shield folded back and split into the four radials. To help keep the antenna from flexing as the payload spins, the ends of the radials will be fixed to the payload box and a straw placed over the vertical element. Without any solder joints it should be much more reliable than my previous attempt at this style of antenna.