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There are some rules about unmanned balloons in the FAA regulations. I can't give you the answer for your particular situation, but typically a balloon under 8 pounds with a tether that breaks at 50 pounds pull does not need FAA approval. Be sure you do not launch or land within 5 miles of an airport, and you should file a NOTAM report. You can google search for the NOTAM hotline number in your region
Since this design only needs to transmit the GPS, there is no need for an extra processor. If you want to add on sensors and transmit that data, then you will need to read all sensors into a processor and compile a serial data stream to send over the radio. Launching and recovering a balloon is much more difficult than you expect. Be sure you take your time and do your research. It's very deflating when you lose a $500 payload because of a silly mistake.
You don't need a HAM license. As to why...it just isn't required. Has nothing to do with spread spectrum.
You probably guessed by the title that you don't. 900 MHz is in the ISM (industrial, scientific, and medical) band. This is an unregulated frequency. The down side is that someone else with a higher powered radio may interfere with your transmissions. The XTend 900 uses spread spectrum, so you should be able to receive no matter what.
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