Payload - The Edge of Space

Introduction The space bug bit me when I was about six. Like a lot of kids, I spent lots of my summer earnings from mowing yards on rocket kits and telescopes. That interest never went away, and guided me into engineering. Soon after college, that itch came back even stronger, and since I was no longer living off Raman noodles, I decided that I needed a project. After several weeks of daydreaming, I hit upon a good idea for a project: design something that would go to the edge of space and that would take some pictures along the way. After some careful consideration, I finally settled on using a high altitude balloon. This project became my hobby for the next year and a half.

In order to get pictures from a high altitude balloon, several things have to happen. First, the camera has to take pictures automatically. Second, the balloon has to know where it is and report its location, otherwise you’ll never know where it landed. Finally, everything has to work under harsh environmental conditions, since the balloon will pass through areas of the stratosphere with temperatures as low as –70˚C, and atmospheric pressure as low as .1 psi.

A system has to be in place in order to coordinate all these tasks. Some high altitude balloons use expensive, complex, and often heavy flight hardware. Others use a simple position reporting device with a camera on a timer circuit. I didn’t want either of these extremes, so I began looking at other options.

I had first encountered the Basic Stamp during my undergraduate years at Georgia Tech, where we had used them for design competitions similar to Robot Wars. I thought they were pretty nifty little computers, but I didn’t fully understand their capabilities. With this design challenge, I gave them a closer look.

Based on the project objectives, I decided that the balloon payload needed to meet all the following design goals:

1. Accept and process GPS data (latitude, longitude, altitude, and time).
2. Operate multiple cameras.
3. Transmit activities performed, location, altitude, and temperature data.
4. Perform temperature measurements.
5. Record data.
6. Withstand the harsh environment at the edge of space.
7. Weight less than 6 lbs.
8. Cost under $1,000.

After ordering a BS2, a Board of Education, a GPS unit, a radio, I started tinkering with ideas. After some trial and error, I had figured out how the BS2 could perform all these functions.

One of the critical criteria used to select components was temperature range, since the balloon would encounter air at temperatures as low as -70C. Insulating the payload with 1” polystyrene would keep the components warmer than the outside temperature, but would not keep them above freezing. I ended up selecting the industrial version of the BS2, which can withstand temperatures down to -40C. Some of the other equipment (such as the GPS unit and radio transmitter) were only rated to -10C or -15C, but had enough thermal capacity that the actual components would remain warmer than the internal payload temperature. In addition to surviving cold temperatures, the components also needed to be able to operate in a vacuum.

Interfacing the GPS with the BS2 proved to be easier than anticipated. Most GPS units have configurable serial outputs that are extremely easy to integrate with the BS2. I had selected the Garmin eTrex, and it was no exception. Once connected, a single line of code is all that is needed to import current GPS position coordinates into the BS2’s program.

Solid state relays were used to allow the BS2 to operate the cameras. The cameras used on the flight were a film 35mm automatic camera (a Kodak ke60), and a digital HP 215, both of which have electronic shutters. The cases of these cameras were opened and leads were carefully soldered across the terminals to allow the BS2 to operate the cameras.

Temperature measurements and data logging were additional capabilities added to the system. For temperature measurements, the DS1620 (provided by Parallax) was used. The DS1620 is inexpensive, light, and can measure temperatures down to -55C. An 8k EEPROM was used for the data logging.

One of the more challenging aspects of the balloon was the communication system. Most other high altitude balloons use packet radio (or APRS). This can require additional hardware (which can be expensive and increase weight). So instead of using a digital communication protocol, I decided to go with audio communication, much like an automated weather report. To give the balloon a voice, I purchased the Quadravox QV306M4P playback module, which can hold over 4 minutes of pre-recorded sounds. Using numbers, letters, and the piloting alphabet, the balloon was able to relay back its latitude, longitude, altitude, and temperature, as well its status. The playback module was easily incorporated with the BS2 and a 2m handheld radio, and was cheaper and much lighter than some packet radio equipment.

The flight program was the most challenging part. In addition to coordinating the cameras, temperature measurements, GPS data processing, radio transmissions, and data-logging, the BS2 had to be programmed to react to any problems that might occur. This meant having a program that can react to problems like a power interruption (and BS2 reset), loss of GPS signal lock, GPS malfunction, and incorrect GPS data. It also needs to note whether it’s going up or down (which dictated how frequently it transmitted data), as well as the start of the descent (which can exceed 100mph). The final version of the flight program was robust and used 83% of the BS2’s memory and all but 1 bit of RAM.

In order to simulate a full-scale test, another BS2 was programmed to emulate the GPS input from the eTrex. In addition to ideal flights simulating various ascent and descent rates, GPS signal loss, GPS malfunction, and errant GPS data scenarios were also simulated. Several tests and code modifications resulted in an optimized final flight program.

All the critical components (including the BS2) were tested in a vacuum chamber to less than .1 PSI in order to verify they would work in the near vacuum environment. The BS2 had no problems, although the cameras experienced some coronal discharge failures (due to the flash) that forced some camera modifications.

On May 3, 2005, I filed a notice with the FAA and launched the balloon in Birmingham, Alabama with help from my wife and family. The cameras worked flawlessly, and the balloon transmitted its location, altitude, temperature, and status all through the flight. It achieved an altitude of 103,973 feet and endured temperatures as low as -8C. Strong upper winds carried it over 155 miles, and it landed about 30 miles west of Macon, Georgia. The pictures were fantastic: a black sky above, a blue sliver of atmosphere all around, and the earth far below.

I had managed to scratch my itch. At least for a while.

An early flight had reached 92,639 ft. That flight was launched on August 28th, 2004.

You can email the author at dave.harper@gmail.com.