PiBalloon II – Design Ideas

In August, I built and flew a Raspberry Pi powered high altitude weather balloon. The Pi was ideal, as it provided a low power Linux platform in a credit-card sized package. Using Python, BASH, and caffeine, I was able to produce 1200 baud APRS packets to be sent through the audio jack to a Baofeng UV-3R. The Pi also took pictures using a webcam, and sent them out using Slow Scan Television (SSTV). This was sent through a Wouxun KG-UVD1 using VOX. This system worked very well, but there are plenty of design improvements to be made, so I need to start from scratch.

PiBalloon II it shall be called.

I came across a new balloon box as I was walking through a biology centric building on the UTD campus. 2014-09-24[1]I didn’t have a banana for reference, but I did have a Pi B+. The box (externally) is 7x7x5.5 inches, giving me the perfect amount of room for a GPS module, Raspberry Pi, 2 D cell lithium batteries, a webcam, and these Baofeng-on-a-chip modules [DRA818V is the search term, if the link dies].

The Baofeng-on-a-chip modules are programmable via UART and can transmit at 27 or 30dBm (500mW/1W). I haven’t received mine yet, but I expect China Air Mail to provide in a week or two. Because they’re programmable via UART, it is theoretically possible to send SSTV on 144.5 and APRS packets on 144.39 using the same module. Admittedly, you can’t get APRS while SSTV is running, but it still has a lot of potential. My favorite idea is to program in the top 15 repeater pairs and the top 5 PL tones, and go down the list kerchunking repeaters from the edge of space. One could also program the Pi to announce its status on their favorite repeater using a speech synthesizer. “This is your Pi speaking. Our current altitude is 82,348 feet, and ascending. It’s a little chilly outside at -37 degrees celcius. Batteries are looking good. Go ahead and put your seatbacks and tray tables in the upright and locked positions, as we expect burst at any minute now.” The possibilities are endless with a radio that can be programmed in real time by a Raspberry Pi.

Similar to the last launch, I’d like to monitor several different parameters and send them out using APRS. I plan on building another 1-wire thermal sensor network using the Maxim DS18B20’s or similar. This time I’ll be using the real sensors, not the unreliable parasitic power units. The TI ADC didn’t perform well either on the previous launch, so I’ll most likely load up an ATmega chip to handle battery voltage monitoring. Ideally I can just feed data back to the Pi using a UART or other GPIO interface.

TI offers this chip called the TMP513. Check it out. It interfaces using I2C, and can measure battery voltage, three temperature sensors (and a fourth internal sensor), and even has a GPIO pin. Mouser has them in stock for <$5. If I can figure out how to use this chip, I’ll swap out the 1-wire network and the ATmega in favor of this. It has potential.

There is a possibility of a December launch. This group requests that the payloads be <2lbs and <12″ on any face. Size isn’t a problem, but weight might be. Hopefully the PiBalloon II will come out under that constraint.

Stay tuned for more updates on this project.

Flight of the Leftovers – A Success!

On Sunday, August 17th, 2014 we launched a 600g Totex balloon with a homebrew APRS tracker, and recovered the payload just 5 miles from the launch site.

Around 8:30AM on Sunday August 17th, it was pouring outside of my apartment in Richardson. Weather radar showed a decent amount of storm cells moving in a west-to-east fashion across the entire metroplex. By 11:30 the majority of the cells were to be out of the area, but we expected heavy cloud cover and a light sprinkle. As you can see in the photos above, heavy cloud cover and a light sprinkle were provided.

At the launch site, we plugged in the batteries, tested the various systems, and deemed the payload OK to launch. SSTV images were streaming in, and APRS packets were being relayed by the TinyTrak4 in the chase Jeep. The balloon-filling team used an entire size-K tank of helium (200cu ft.) to gain ~11.87 lbs of lift (~13.2lbs – 600 grams of balloon). Our box only weighs 6lbs, so we had roughly 5.9lbs of free lift with the payload attached. Using the APRS data, I was able to determine that our average rate of ascent was ~1417feet/min (7.2m/s).

[Note on calculations: Using the Slope function in Excel, it told me the average rate of change between packets was 1063 feet. Because we were beaconing at a 45 sec interval (.75min), 1063/.75 = 1417 feet/min. This is hella fast.]

Once we had liftoff, we decided it was time to grab a quick lunch, and prepare for the chase. A local Sonic was able to grab our business. When the balloon reached around 40,000 feet, we noticed that the position packets stopped updating, but the telemetry indicated that things kept getting cooler. After some quick research, we determined that our GPS was soft-limited to 40,000 feet, despite the spec sheet stating 50km. [More research and discussion with the South Texas Balloon Launch Team indicated that a certain serial string must be sent to the GPS to enable high altitude mode; furthering the educational factor of this launch].

After about twenty minutes of munching, the telemetry data indicated a quick drop in temperature. This can only mean one thing – burst. Admittedly, we didn’t know which way to head, as the APRS data was not yet providing accurate position packets. As we were driving towards the predicted landing site, the balloon dropped below the 40k foot GPS ceiling, and started providing us with accurate position data. The packets kept streaming in, altitude getting lower each time. Finally, we stopped getting packets. The last packet indicated the balloon was a quarter mile from 121, not far from where we performed several “legal” U-turns. Lo and behold, one of the chase Jeep passengers spotted an orange box in a muddy field. It was exactly a quarter mile from highway 121. The landing site was approximately 5 miles from the launch site.

There were several learning experiences with this launch.

  1. Don’t fly DB9 connectors. They will come disconnected, and you will have a bad day.
  2. Unlock the GPS. Just do it.
  3. Buy the right batteries. Lithium Thionyl Chloride cells are BAD. They have a high internal impedance, yielding a much lower current draw. Lithium Sulfur Dioxide (LiSO2) are the correct choice. I compensated for the 200ma max draw on the batteries we had by doubling up, giving me the 400ma required by the Pi.
  4. Wide angle lenses take cooler photos of landscapes. The Canon SX110IS does not have a wide angle lens. There was potential for cooler photos.
  5. Use switching voltage regulators. Linear regulators will dump a ton of energy in the form of heat. Tons of weight can be saved by flying less batteries when the right regulators are flown.

That sums it up pretty well. Enjoy some photos of our activities: