When I first thought about building GAGA-1 I imagined building a single flight computer that would control everything: it would read GPS, transmit telemetry, send SMS messages and operate the camera. But that would have created an enormous risk of complete failure.
If the main computer failed (and there are so many ways to fail: bad software, a hardware fault, loss of electrical power, loss due to low temperature/pressure) the entire flight would have been a failure: no pictures, no telemetry to aid recovery. In fact, the only thing that would have worked is the dumb latex balloon happily exploding at altitude.
So I've opted for a design with three independent systems with no electrical connections between them and independent power.
1. Flight computer
This main computer will receive data from the primary GPS, read internal and external temperatures, and transmit telemetry over the RTTY link. This is the Arduino-based system for which I am building a custom shield. The flight computer will transmit telemetry via an antenna on the bottom of GAGA-1 (since its primary use will be in flight we'll have a clear view of the capsule's underside). The GPS antenna will be on the top so that it 'points' towards the GPS satellites.
2. Recovery computer
This secondary computer will have an independent GPS and a GSM modem for low altitude telemetry. While the balloon is below 3km it will transmit its position and altitude using SMS messaging via an external antenna mounted on the top of GAGA-1 (since its intended to be used on and near the ground). This separate board will use it's own 3.7v battery from an old cell phone.
I've previously described the camera which will operate automatically throughout the flight.
The flight goals are as follows:
1. Safe recovery: hence the two independent computers for telemetry and GPS using independent GPS modules and different transmission methods.
2. Photography: the camera could fail, but I can't afford the weight budget of two cameras for this flight so I've done extensive testing on the camera itself.
3. Constant tracking: the flight computer will transmit RTTY as has been used by many others to track their flights. I hope to have constant RTTY data available so that the flight can be tracked continuously (since that's part of the fun).
The flight can survive a loss of either computer, and if both computers failed I'd still have balloon flight prediction and a mobile number stencilled on the box to aid recovery.