High altitude balloon projects are an amazing way to learn about avionics and sensors. You'll need a way to track your balloon so that you successfully recover all the great pictures, videos, and sensor data you collected. If you are so inclined, you can spend quite a bit of time focused on this area.
On the other hand, if you are not that geeky or don't have the time to build out the avionics, you can purchase a SPOT Satellite Messenger to track down your balloon (http://amzn.to/12bQ5ac). The SPOT Messenger is a small device which will communicate it's GPS coordinates through a satellite network and display it's location on a web page every 10 minutes. One big downside of this approach is that the Spot Messenger only works for altitudes below 21,000 feet. Since a balloon filled to the proper lift weight will rise about 1,000 feet per minute, you'll only be able to track it's ascent for the first 20 minutes of a likely 2-3 hour flight. Of course, after the balloon bursts and your capsule falls back below 21,000 feet, you'll pick up the signal again.
We used the SPOT Messenger as a back-up and it worked perfectly.
Key aspects of our approach:
- Microcontroller for processing GPS and sensor data, logging that data to a local flash drive and sending the data to the radio modem.
- Sensors for GPS, pressure, temperature both inside and outside the capsule.
- A modem to transmit GPS and sensor data to the ground station.
- High-gain antennae
- A high-performance GPS module which can be configured to work at high altitudes
- Motor driver for science experiments
- Battery to power the system
- Battery eliminator circuit useful for experimenting with different power configurations.
- Independent GPS/Satellite messenger as a back-up for tracking
After much research we chose the ArduPilot Mega microcontroller from DIY Drones. This proved to be an excellent choice as the ArduPilot is Arduino-compatible (powerful and easy to program) includes a gyroscope, compass, accelerometer, a high-performance barometer, 4MB dataflash, and UBlox GPS. The UBlox can be configured to work up to 160K feet. We also added an I2C sensor for measuring temperature outside the capsule. The ArduPilot code is open-source and includes a lot of useful libraries. It was quite easy to set-up and was totally reliable throughout the flight. We have attached below the Arduino code we wrote.
We used the XTend 1W 900Hz RF Modem which has a 40-mile line of sight range and worked perfectly. We purchased two, one of for the capsule and the other for the ground station. The two modems came pre-configured to communicate with each other. We changed the RF rate to 9600 baud and used a high-gain antennae to extend the modem range.
We used a motor driver from Cana Kit to power the experiments. One nice thing about the motor driver is that it has a regulated 5V output which we used to power the ArduPilot. The ArduPilot documentation recommends a 5.2V power source on the output rail, but we had no problem running with 5V.
The battery eliminator circuit (BEC) was useful when we were experimenting with different power schemes. Ultimately we connected the 11V LiPo battery to the BEC, then used the BEC to drop the voltage to 9V which we then connected to the Cana motor driver. We used 9V as this was the voltage required to run the vacuum pump for the bacteria experiment and fire the igniter for the rocket experiment.
The list of items we purchased for the avionics and sensors are here: Purchase List