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Hello everyone!
I'm making one of these:

I could get the SSID of any nearby Wifi from a small keychain Wifi finder. It wouldn't need to be so sensitive. It could miss a few and take 5 minutes until it gets the next one from high up. Then send it via 60mw Xbee. Without the UNO devel board this could be very lightweight. About the same weight as the ballast I need to keep the balloon upright anyway. The value of the hardware in case it were lost would be $50 instead of $150 for GPS. What other data could I send?

I would know the approximate location of the Balloon! Enough to find it anyway.
What do you think? Feasible?
Love it! What's the lightest weight and cheapest still camera? Maybe just store them. But I might not get the unit back if I can't track it until the end. I'm designing it with a control signal to pop the balloon like in the movie...
In my original design I'd only have to be less than a mile away. It's communicating to me via 60mw Xbee, not wifi. It is much easier for cheap hardware to detect a network SSID than to use it, even when it's Open. It is also possible to buy the 10mi Xbee unit. But you've given me an idea. I don't need the RF module. Instead I can just use wifi for communication! Duh! The problem is it would be rare to get a good connection when it's high. Many networks are closed. But that's OK. I could print a label on it that says "Take me to Open Wifi". When they get home, it would tell me where it is! If it can't get a strong signal from the air. Or "Take me to Starbucks for coffee!", but that would be harder to connect to the internet? Much more interesting and funny because they wouldn't know why it wants to go to Starbucks.
can't use WiFi with normal antennas for more than about 1,000 ft.
Mainly limited by its 20MHz wide signal. Whereas 802.15.4 (XBee et al) is 2MHz, lower modulation order.
But for airborne telemetry, the best choice is 902-928MHz.

Location and elevation: use a GPS chipset, powered up every x amount of time to save battery. RF signal strength is far too ambiguous due to the inverse square law in RF propagation.
I'm going to design it to be failsafe, so that I can bring it down at any time using Xbee when it's over a big field. If I lose radio contact for 10 minutes then it will come down rapidly on it's own. This way it can't get very far from it's last known location on a calm day. This would only happen if there's no road where it's going.

Any idea how to control its altitude with Uno?
sbright33 wrote:How can I release hot air on command to lower the balloon?
with a "dump valve"

How you build a dump valve for a trashbag balloon and how you control it with the Arduino are less obvious. The book on sale on that site may very well answer at least the first of those questions. As for the latter, an RC servo comes to mind.

Have Fun,