Picoatlas III is a continuation of the previous 2 flights, I have recently been working on the use of solar panels and low power modes withe arduino on a separate project and felt that this could be re-applied to picoatlas. The aim will still be to use cheap foil balloons and attempt to get them to float for an extended period of time.

To get the balloon to float we need to setup a situation where an increase in pressure results in a reduction in the lift from the helium to a point where the lift is neutral and the balloon floats. The pressure required to get to this point however must not exceed the bursting pressure of the balloon itself. Therefore we should fill the balloon to neutral and then add a little bit more - enough to get it to climb but not enough that the pressure required to float results in burst.

The actual fill of the balloon, whether it is 50% or 75% full, won't affect this (as long there is enough helium for neutral + a bit more) but will dictate the float altitude. To get a reasonable float altitude we need to use as small as possible payload. Also it would be sensible to use as few balloons as each balloon will add complexity to the model, if one balloon was to burst then it would change the dynamic of the setup considerably.

The key element of this payload is the use of a very small LiPo (85mAh) and a solar panel - this is to minimise the payload mass. To be able to use such a small LiPo it is necessary to implement aggressive power savings.

The current setup is:

• Arduino Mini Pro running at 3.3v and 8MHz
• Radiometrix NTX2 434.075Mhz
• Ublox NEO-5W GPS + small patch antenna
• 85mAh LiPo
• Solar panel
• Max1555 lipo charger

The Arduino is able to wake up from low power mode using the watchdog - this is setup to trigger after 8 seconds waking up the avr. Each of this 8 second sleeps is regarded as a 'loop' (rather then use the phrase cycle which refers to the background processing), depending on the current situation the Arduino may go back to sleep, turn on the GPS or transmit some RTTY. Much of the logic is based upon the current battery voltage.

1. Low power mode - this occurs when the battV < 725 - the payload keeps the GPS turned off and only transmits the beacon string every 20 loops ( ~2.5 mins). The aim of this mode is to maintain the payload overnight as well as to allow it to rapidly charge when starting from an exhausted battery
2. Normal power mode - this occurs when the battV > 725 - the beacon string is transmitted every 5 loops (~40 seconds)
   1. GPS is only turned on when the battV > 745 (the difference in modes is that the GPS can pull the voltage down quite a bit when powered up). It is turned on every 75 loops (~10mins) and once it has got a lock is turned off. During this time it'll transmit full strings every 5 loops, when it has a lock it'll transmit the same string 5 times to make it easier for listeners to get the key string. The GPS is then turned off.
3. When the battV > 800 it'll turn the GPS on every 40 loops - this is to take advantage of the full battery and good solar cell power in an attempt to get as much data transmitted as possible

• Construct antenna
• Construct payload
• Test with smaller solar panel

* Get balloons - ordered