Spacebits

This is the heart of the project and a true work of art.

There are many challenges to overcome with the payload and some of them are quite complex. Here’s an idea:

• The whole thing, box, computer, batteries, sensors, camera, etc, can’t weight more than 1.5Kg (~3.3lb). If it does, it can compromise the 30.000m goal.
• The box has to be sealed properly to keep the electronics working under near-space conditions. At around 10.000m we’ll get outside temperatures of about -50ºC (-58ºF). There’s also condensation and very low levels of pressure to deal with.
• It has to be tough. There’s a lot of turbulence during the flight and it will fall on the ground at approximately 30km/h. We don’t want it to tear apart.

The Box

The box is made of polyurethane, a rigid foam that is very easy to cut and handle and is covered with fiberglass for resistance and durability. Foam is great because it’s easy to work with and it’s light, which is very important.

• List of materials used to build the payload box:
• Polyurethane foam board, 20mm.
• Polyurethane Liquid Glue.
• Polyester resin and appropriate Catalyst.
• Fiberglass tissue 80g/m2.
• Foil thermal blanket.
• Contact glue.
• 3mm threaded rod.
• 3mm screw nuts.
• L shaped aluminium strips.
• 5mm aluminium tubes.

The box is divided in two pieces; the bottom one holds the computer and related electronics and the one on top contains the power supply and the autonomous GPS/GSM unit.

The outside of the box is covered with a special foil thermal blanket which reflects over 90% of the heat created inside and keeps the cold from entering. Remember that along the trip it will experience extremely low temperatures (-50ºC).

The Main Board

The main computer is based on an Arduino MEGA (Atmega1280) that read

s data from several instruments and sensors, logs the information to an on-board SD card logger and sends radio signals back to Earth with vital information, including the box coordinates. Here’s the list of sensors planned for Spacebits 1:

• IMU 5DOF (2-axis gyro IDG300 & 3-axis accelerometer ADXL330)
• Barometic sensor: SCP1000
• Humidity and temperature sensor: SHT15
• Compass module: CMPS03
• Optical Dust Sensor – Sharp GP2Y1010AU0F
• GPS Lassen IQ
• Phidgets Precision Voltage Sensor
• ACS712 Low Current Sensor Breakout
• RTC Module: DS1307

This information is sent to a logger using a uDrive-uSD-G1 Embedded Disk Drive.

For the radio communications we’ll be using:

• Xbee-PRO® 868 OEM RF Modules
• Yagi antenna bipole 1/2 wave 868MHz RP-SMA
• Yagi antenna direccional 868-914MHz RP-SMA
• XBee Explorer USB

About the GPS modules

Be careful on which GPS modules you choose for your HAB project. Most chipset have software limitations for altitude and speed imposed by the US government due to security reasons. Quoting from the Wikipedia:

“The U.S. Government controls the export of some civilian receivers. All GPS receivers capable of functioning above 18 km (60,000 ft) altitude and 515 m/s (1,000 knots) [96] are classified as munitions (weapons) for which U.S. State Department export licenses are required. These parameters are clearly chosen to prevent use of a receiver in a ballistic missile. It would not prevent use in a cruise missile since their altitudes and speeds are similar to those of ordinary aircraft.”.

So make sure you choose one that has no limitations or one that can be hacked to remove them.

The GPS/GSM backup

We spoke about Spacebits to Bre Pettis in the past about this. Make magazine was involved with at least one HAB project. He told us something valuable: getting it up there is easy, recovering it is not.

We know about this and we do want to recover the payload so we’ve spent loads of time testing everything out, making sure nothing fails. But something will, Murphy has been a friend of engineers for ages and we know him well.

Therefore, besides the main radio and computer, we’ve decided to build a completely autonomous backup unit to track the probe when it falls using a public GSM network and SMS messages. It has its own circuitry and its own power supply, so if anything bad happens to the main board, this may save our precious box.

The GPS/GSM unit consists of:

• One cheap Beq M23 GPRS USB Wireless PC Modem Network Card, which we teared apart and managed to isolate the RX/TX lines of the modem. FYI you can get these for 9€ on eBay.
• One EM406, a 20 channel GPS module.
• One Arduino pro mini 5v/16Mhz. It’s really small and fits great to this use-case. The Arduino runs the code that reads the coordinates from GPS receiver and writes the SMS to GSM modem.
• A pack of batteries and a 7805 based voltage regulator circuit.

Testing this unit was fun, to say the least. We wanted this to be fail proof so we spent ages refactoring the Arduino code and experimenting it with real scenarios. We took the car and went for a drive several times. One of the most important things to test was the eventuality of lack of GSM network. It had to recover when the signal strength came back. You can see a rather strange photo on the right side, that’s us placing the unit inside a microwave trying to induce loss of network (it didn’t work, BTW; the SMSes kept coming out of the oven).

Bottom line, we now have what we consider to be a very nice robust GPS/GSM unit, sending an SMS with its lat/long coordinates every 30 seconds to one of our cell phones and recovering successfully from network loss. #win