The term payload generally refers to everything the balloon has to lift, but we’ve decided to separate our documentation into the enclosure and the stuff we’re bringing up.
The very basic requirements for a payload include:
- A camera
- A beacon for recovery
Two common tools for doing recovery are beacons used by ham radio enthusiasts and cell phones that send text messages or data. Since ham radio requires a large initial cost for equipment that is generally not trivial to acquire, we opted for the cell phone + text/data route. This does have some downsides, however. Once the cell phone is out of range, it is completely inaccessible; we only know when it returns to cell range and can only hope it lands somewhere within range of a cell tower and can re-establish communications for long enough to tell us where it is.
When designing the system, we wanted to be able to:
- Take photos
- Take videos
- Trigger release of the balloon
- Upload location of the payload
- Play loud noise to aid in recovery
- Record GPS location
- Have ample battery life
- Compact and light packaging
With the declining cost of smartphones, and the prevalence of older used models, these devices are within an acceptable cost for accomplishing the entire package. We can develop software that controls every aspect, and even solves some problems that would otherwise require additional hardware. For example, by carefully monitoring the battery level, we can choose which aspects are most important. If the battery level is high, then we can take more video and photos and data collection. If the phone temperature is dropping too fast, we can use the phone to perform complex calculations that generate heat and warm up the phone. If the battery level is decreasing, we can take data less frequently and save more for the recovery effort. We can use the signal strength to determine when we can upload data about the journey. And we can use the ringer to play audio to help find the payload if it lands in a tree or back yard.
There are a few resources for finding an appropriate cell phone that will work. We currently only have software for Android phones. The website http://swappa.com allows people to sell their used Android devices, and cheap ones can be had for under $70 easily. Amazon and eBay are other great sources for cheap used Android phones. Make sure that you get a phone that is unlocked so that you can use a different cell phone provider with the phone.
Depending on your location, you may choose a different cell network company to accomplish this. You may decide to use your own SIM card from your phone (if your cell phone has one and the networks are compatible). Hopefully the recovery will be successful.
Some companies have prepaid plans that allow you to get a limited duration of service with no contract. This is ideal as in the event of loss you won’t be responsible for charges if someone finds the phone and continues to use it. T-Mobile has a prepaid suite of options, and ATT has a GoPhone, which is only $10 for the phone and SIM card. We purchased and activated a GoPhone, then removed the SIM and put it in our Android phone.
You must make sure your phone is SIM unlocked if you are putting a SIM into a phone from a different provider.