The MiniHAB project's goal is to build a minimal High Altitude Balloon platform.
By "minimal" I mean as simple and cheap as possible, in order to reduce costs, engineering efforts and (where applicable) paperwork.
The MiniHAB must anyway be able to:
- reach the stratosphere
- take photos
- be recovered upon return
The final project should be reasonably cheap, and the assembly should not require technical skills beyond the capabilities of a (sort-of advannced) computer user.
This could help making High Altitude Ballooning a much more widespread hobby, such as - for example - RC modeling
Payload: tracking system
The main cut I am thinking about is in the tracking system. Most HAB missions have a real-time system to read GPS co-ordinates and send them back to surface via a radio system.
This is often the most complicate part of a HAB project, as it requires a a GPS interfaced to a small computer, again interfaced to a radio that transmits data, be it in digital (e.g. APRS) form or analog (synthetized voice).
This has a few consequences, as it requires rather advanced engineering/programming skills, a rather important cost for the on-board platform, and some other gear at surface (receiving&decoding the signal).
First of all, this means that to assemble such a project you need a wide range of know-hows.
Second, in several countries there are strong limitations on automatic (i.e. unmanned) systems transmitting over the most common radio frequencies. It's sometimes forbidden, for example, to use a computer to transmit over the standard CB range, and this means the the radio system gets more and more complex (or illegal)
What are the alternatives to such a radio system?
In the last few years a new category of rather small and inexpensive GPS trackers have emerged, that are able to get GPS co-ordinates and transmit them via SMS to any mobile number.
The advantages of these devices are evident:
- monolithic build (no engineering/soldering/programming required)
- no paperwork needed to use the GSM system
The main problem with these devices is that the GSM signal won't be present over a certain altitude, so tracking during the flight won't be possible.
The real question is "up to what altitude can we have signal enough to send an SMS?". There is no "official" answer to this, this maximum altitude is often quoted as "around 1.000 meters" but this is just guesswork. In other occasions we have reports of mobile calls being made on-flight (the 9/11 calls being the most sadly famous ones) but there is no clear answer to this. I plan to send a tracker up with some skydivers to check the reliability of the tracker at 3-4.000 meters of altitude.
Then, we will have to check this information with the descent speed of the capsule. These are the times it took for the ICBNN capsule to get to surface from each altitude:
- 4.000 mt 37 min
- 3.000 mt 29 min
- 2.000 mt 20 min
- 1.000 mt 11 min
This means that if the tracker gets the GSM signal at an altitude of 2.000 meters we will have 20 minutes of tracking before the touchdown.
This is important for two reasons:
1) it allows the recovery team some time to approach the landing location and be faster in recovering the capsule, with less risks of losses and damages
2) in a hilly area, the capsule might land in a little, remote valley that has no GSM signal. Hopefully, the signal will be present in the same spot but at a higher altitude (for example, when the capsule is at 500 meters above ground), and the device will send us its co-ordinates just before touching
down and entering the "shadow zone".
Another consequence of this altitude problem is that we won't be able to know the altitude the missione has reached (these devices do not have a logger, they only transmit the co-ordinates). There is no solution for this, apart from purchasing a separate GPS logger, these are often even smaller and cheaper.
The cost advantages of this solution are evident. A GPS tracker costs around $200, while a complete GPS+Computer+Radio system goes around $400-500.
From the weight point of view, the advantage is also very important, even more if we consider the battery that a real radio system requires. A GPS tracker weighs well under 100 grams, compared to the abut 150 grams that the battery requires on its own.
The lightest and cheapest cameras suitable to run CHDK and thus be fully authomatized is the Ixus 80 (aka SD1100) or a similar model.
The weight of the camera (including its battery) is 185 grams.
Lighter cameras from other brands cannot be authomatized and require an eternal triggering system, again increasing weight and complexity.
Also, I have an Ixus 80 readily available (the one Iused to program the ICBNN flight) so that's handy...