Flight of the Leftovers?

As part of the Intro to Space Camp that I assisted with at UT Dallas, four balloons were purchased: two 1200g and two 600g Totex balloons. With the space camp, we launched one of the 1200g balloons, and “used” the other in a “learning experience.” So, presently, we have two 600g balloons and a size-K tank of helium.

Due to the complexity of the last launch, we’ve agreed to slim this one down. A lot. Here are our new goals:

  • Imaging. We want photos from the edge of space.
  • Telemetry. Temperature, altitude, and battery voltages are parameters we want to see.
  • Tracking. Recovery is imperative.

How do we go about doing this? With a Raspberry Pi of course! Back in June, I demonstrated SSTV transmissions using a webcam and GPIO pin 4 on the Pi. It was quite simple: take the picture, feed the png file to a program that pumps out a wav file, and then transmit the wave file using a modified version of the PiFM binary. The transmission is >10mW, possibly up to 20mW. I was able to key our on-campus repeater using a piece of RG-58 soldered onto the pin and a rubber duck antenna.

Imaging

Although we’re still in the design phase, I’d like to fly two cameras: SSTV and Hi-Res. As mentioned above, it’s not difficult to run SSTV from the Pi. The higher resolution images would be taken from a “Point-and-Shoot” camera, probably a Canon with CHDK. I personally own a Canon SX110, and with our current budget, we’ll likely be flying a certain SX110.

At the highest resolution (9MP), the SX110 will hold ~1000 photos on a 4GB card. Why fly only 4GB? CHDK is limited to 4GB cards due to the nature of the bootflag that tells the camera to boot into the modified firmware. Cards larger than that require multiple partitions, and then partition swapping to take advantage of the higher capacities. 4GB is simplest. Although we’d like the flight to be rather short, planning on a 4 hour flight would be wise. When you do the math, it yields 15 seconds as being the ideal interval for getting 4 hours of photos.

Telemetry

Education has been a goal of this launch, and being able to get something out of this launch will justify the expense of helium, balloons, and payload. Pictures are typically the most interesting and flashy byproduct of these events, but telemetry provides useful data that can influence future designs. What are things we can measure using Commercial-off-the-Shelf parts (COTS) and send through the APRS network? Several things came to mind: pressure, temperature, battery voltages and altitude. I’m sure SparkFun sells sensors that can measure many more things, but for the purpose of this launch we should probably stick to just those.

What parameters exactly should we measure? Internal temperature, external temperature, and altitude are most important, in my opinion. Behind those come battery voltages, and finally pressure. In all honesty, I don’t really care about pressure. Sure, it’s interesting, but I’m not going to bend over backwards (or pay more than a couple of dollars) to fly a pressure sensor.

Tracking

Oh, tracking. Tracking is easily the most complex part of any launch. There are many different ways to approach tracking, but for ease of reception, we’re going to narrow it down to “things that use the APRS network.” Although it would be fun to DF our way to the landing site, I’m going to opt to use the systems that will easily plot the balloons position on a GPS or aprs.fi.

On the last balloon launch, we flew a TinyTrak4 and a MicroTrak400. The TinyTrak performed flawlessly, and ultimately offered everything we needed to transmit position and telemetry. The only drawback to the TT4 is the price (yes, I am a broke Engineering student). $65 isn’t too much, but we’ve already determined that a Pi will fly. Maybe we can connect the GPS to the UART? What if we modulated the AFSK with the Pi, and pumped it out through the GPIO pin, like the SSTV?

 

Anyway, there are many things to consider. Budget is the main constraint. We’re not being funded by any group (aside from launch mechanics), and most of these things aren’t particularly cheap. Stay tuned for more info!