"73" in amateur radio shorthand means "Best Wishes". And it’s a weather balloon…a 6ft wide, 1,200g weather balloon, not a blimp. :P
There’s no real retrieving the balloon. When it reaches a certain altitude it bursts. Think of a sub…when it reaches a certain depth it gets crushed because of the pressure pushing in. When a balloon goes up, there is less pressure pushing in, allowing the balloon to expand. It will double in size every 18,000 feet. It can only stretch so far and at a certain altitude, a little over 100,000 ft, it bursts and the payload floats down via a parachute.
The upper limit of GPS devices depends on the GPS. The government limits GPS by speed and altitude. A GPS cannot work if it's moving faster than 999 mph AND is over 60,000 ft. Basically, they don't want anyone using a GPS to guide a missile. To comply most manufactures cap their GPS's at 60,000 ft.
I've got two devices for tracking. The first is a SPOT satellite transmitter. It gets a position from a GPS then transmits it to satellite every 10 minutes. You can read the location from a website. The site can be public or it can be protected with a password. Unfortunately the SPOT will not tell altitude. It's also only rated to 30,000 ft. On the upside, it will give you position on the ground.
The second device is an amateur radio (ham) transmitter. Every 30 seconds it sends a position report. It tells me longitude, latitude, altitude, speed and direction. I can download the flight data from http://aprs.fi after the flight. The GPS attached to the ham transmitter (a Garmin 18x) will work above 60,000 feet.
I use the buzzer because even though you get VERY accurate position reports, it’s hard to find a capsule that’s sometimes 50 feet up a tree. Not with the buzzer. You can hear it 500 yards away and close in easily. The only problem is, this time I’m also sending up a HD video camera and I don’t want to hear the buzzer the whole time.
I just ordered the starter kit from adafruit. I think the arduino combined with a ham radio license will definitely put me near the top of the nerd scale. A slide rule can’t be far off. ;)