When you send a balloon to the edge of space, tied to a bunch of equipment you'd quite like to see again - not to mention the data stored there - it's probably a good idea to know where it's gone.
GPS is a given. It's about the only (reasonably) reliable way of knowing exactly where something is. The problem is, how to get the GPS information down to the ground.
So connect it to a cellphone, and send updates over SMS, right? Sorry, no. For one thing, it's not legal to use a mobile phone in the air; for another, the phone would be having to transmit at maximum power just to keep in contact with the cell towers; finally, if the package drifts over waster ... well, there aren't too many cell towers in the North Sea.
APRS is a definite possibility, but it does have its problems. Firstly, the equipment tends to be weighty; secondly, it's power-hungry. A typical APRS/GPS unit will pull about 1.4W, and needs 7-28V DC, with an ideal of 13.8V (typical car voltage). If we allow a mission time of (say) 8 hours, to include location and retrieval, it would need its own power supply (the rest of the kit will run at 5V or 3V) providing around 8400mWH. Don't forget that high-altitude operation will chill the batteries, temporarily reducing their capacity substantially, so we would actually need rather more than that. The price is attractive: they're being sold on eBay for £85.
[LATER NOTE: oops - our INCO tell us we can't use APRS, but there are better options. More later!]
A more expensive candidate, but lighter and neater, is the SPOT Satellite GPS Tracker. There are no power problems - the device has its batteries - it's lightweight, at just under 150g all in, and it relays its tracking information to satellites, so there are no problems with drifting out of GPS range or beyond APRS relay stations. It's also waterproof (handy when it's passing through cloud), and reports from a number of other near-space balloon missions suggests that it continues working reliably in even in the temperatures and pressures at terminal altitude. The downside is that there's a service contract - US$99/yr for the basic service, plus another $39 for the remote tracking add-on. We've ordered one, and we'll keep you updated!
There's one significant problem with the SPOT, though - it doesn't record altitude. Obviously, that's rather important for us - knowing the map position of the instrument package is great, but it would be rather nice to know if it's at 30km height, or hanging in a tree! So it looks as if we may well need some secondary GPS+SMS management if we're to know when it's time to retrieve the package. It would also be handy to record the altitude profile throughout the mission: it will help to plan future lifts.
Last Updated (Tuesday, 30 August 2011 00:39)