Three seperate processes:

• Keeping a list of waypoints. Waypoints consist of a position and an optional required altitude.
• Calculating “sub waypoints” for circling around to get to required altitudes etc.
• Calculating when a waypoint is reached based on GPS position and thresholds (we don't want it turning back on itself if it just misses it!)
• Calculating headings to go in based on the current position and the next waypoint (continuous so it will not drift off course)

• Maintaining straight flight.
• Calculate current heading
• Bank and turn based on heading to next waypoint and current heading

• Maintain constant airspeed, maybe based on stall speed at current altitude or fixed pitch.

To display:

• Readings from sensors.
• Flight plan and waypoints
• Attitude/artificial horizon
• Altitude.
• GPS position.
• Speed.
• Rate of descent/ascent.
• Payload status (ascending, descending, landed etc).
• Raw output from log.
• Battery voltages.
• Space remaining on disk.
• Number of photos taken and, depending on bandwidth, a compressed version of the last image.

To send:

• Cut down.
• Take photo.
• Change/add to flight plans.
• Edit/add to waypoints.
• Manual control for emergencies.
• Release parachute.