Everybody wants to make a space trip, so my son Joost and I joined as well in 2008 (Joost was 14 years old then), that is, a trip to near space (31 kms high), through the "Pongsat" service offered by JP Aerospace.
To this end we built our own small (nano?) satellite called Pongsat-18X, named after the microcontroller in it. It was made to fit into a pingpong ball (which was a requirement for taking part in JP Areospace's journey). The satellite is filled with a Picaxe-18X microcontroller (a simple 8-bit controller from Microchip with a BASIC interpreter and program bootloader added), EEPROM memory, a rich set of digital and analog sensors and of course a small but special battery. It made recordings of temperature and other parameters each twenty seconds, stroring them in memory. Joost wrote the core of the program, I added some details to it and designed the electronics, as shown below:
A large number of sensors were included:
- A digital temperature sensor (DS18B20),
- An analog temperature sensor (as a double check, particularly for very low temperatures in the troposphere area),
- A simple tilt and an equally simple mechanical vibration sensor (relating tilt and vibration levels to wind speed),
- Two leds that can act as optical sensors (by reverse charging them and then measure the charge decay rate) and ... as leds,
- A voltage reference that can be used to monitor the battery voltage (which is particularly important for low temperatures - in which case the satellite can be put into sleep mode im order to keep the battery voltage above the critical level).
The programs (in Picaxe Basic) are available here for recording the sensor data during the flight and for reading the data from the PongSat's EEPROM after the flight.
Here are some views of its actual implementation (including top and bottom views). Note that the battery is mounted under the circuit board (it still fitted!) and therefore not visible in the fully assembled state:
A balloon flight by JP Aarospace with a module containing our satellite (amongst many others) brought it to near-space traversing the tropopause at approximately 14 kms. At the highest altitude (in our case ca. 31km) the balloon was made to burst and then the module returned in a free fall almost up to the very end where a parachute unfolded and a (quite hard) landing took place. A movie of this particular mission (Away 35) is available. Despite the rougher than expected landing, for us it all went extraordinarily well, as shown by the graphs on temperature, vibration/tilt and battery voltage level:
The combination of the graphs for the temperature and tilt/vibration (as measures of atmosphere stability) show clearly that the vehicle first traversed a turbulent troposphere, then entered a very stable stratosphere after passing the tropopause, until the balloon burst ... . This is also clearly visible from another flight's recorded video. Also the chilling of the battery is clearly visible through the sagging of its voltage, but it never came really close to the lower threshold value .
For the full story on how we built this tiny satellite, how it was flown upward, which measurement results we got and to follow current developments, see the following link towards our PICAXE-18X based NLSE-1 Pongsat project. It has most probably been the first electronics-based nano-satelite in a pingpong ball that has functioned the whole trip towards near-space and back to Earth, with registered temperatures as low as -60 C, thanks to a special LiPo battery rated to exactly this temperature.