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The High Altitude Balloon

by Bryan A. Thompson

Last Updated  9/23/2007



How It Works

The balloon (or "Envelope", as they like to be called) is a 1200 gram Latex weather sounding balloon from Kaymont.  It’ll rise at about 1000ft/minute.  As the outside air pressure decreases, the balloon gets larger until it bursts at an altitude of 33.2 Kilometers = 20.63 miles = 108,923 ft.  Technically this isn't space, it’s the “Stratosphere”.  Space itself starts at about 100km in the “Thermosphere”.  The camera should be able to see the curvature of the Earth and bright blue glow of the atmosphere. 
The balloon is connected to an FAA-required radar reflector.  The radar reflector is connected to an inline parachute or “Recovery System” purchased from Rocketman Enterprises.  It’ll slow the descent to about 15MPH by the time the capsule reenters the atmosphere. 
The parachute is connected to a payload, or “Capsule” via a custom-made harness crafted by Jennifer Song.  The capsule is a styrofoam hot/cold shipping container about 11x9x7” in size made by ULine.  It looks like a small Styrofoam cooler, but the walls are about 1.5” thick.  This is to insulate the tracking electronics inside from the approximately -70C outside temperatures.
The Styrofoam cooler contains a Garmin GPS18 GPS puck that has been tested and is known to work at altitudes above 60,000ft.  It provides an update once per second via an RS232 serial port to a modem called an “OpenTracker+”.  The OpenTracker+ decodes the information in the serial data stream about time, position, altitude, course, speed and more from the GPS and creates an audio signal encoded in “AX25” packet format.  AX25 is a 1200 baud encoding scheme based on Bell 202, the same thing as CallerID uses to transmit data about the caller to a CallerID receiver/phone.  From there the audio stream is transmitted to the world via a Yaesu VX-2R amateur HAM radio at a power of about 3W.  The whole thing is powered by a 11V 7A Lithium Ion battery.  Also on board is a Canon SD40 4MPix camera capturing a side-looking image once per minute.  The camera is fired every time the OpenTracker+ transmits a data packet, so the images will be tied to a specific place and time.  The images are too large to transmit through the 1200 baud datalink, so they’re stored on a flash card that will be retrieved when the mission ends.  The whole payload capsule weighs a little under 5lbs.
On the ground, we have many ways to track the flight.  The transmitted packets are a part of a position tracking system set up by amateur radio folks.  It’s called “APRS”, or Amateur Position Reporting System.  The packets are received by a radio repeater tower and retransmitted on the same frequency (to increase the range of reception).  At the same time, it’s sent to the internet via a device called an “iGate”.  It’s possible to see this data through sites like the ones listed below.  In addition to the Internet, we have the capability of receiving the data packets directly using another amateur radio set to the same frequency.  They are decoded by an OpenTracker 2 hardware radio modem and sent to the laptop serial port.  Ryan Lantzer wrote some custom scripts that query the internet periodically, listen to the serial port for direct-received packet data, monitor a GPS connected to the laptop for the current receiver position.  The scripts will plot all data in real time in GoogleEarth and create driving directions from the tracking vehicle to the balloon.
In the air, we’ll have Nathan N’s aircraft monitoring the transmissions and relaying that data to the ground.  The aircraft will be in a much better position to receive the packet data after the capsule is back on the ground.  Nathan N has written some software to decode the received audio stream packet data using the soundcard in his laptop, and to speak navigation directions from his location to the balloon.  As of last night there were technical difficulties with the audio card in his laptop and we don’t really know what the status of the aircraft will be.

Here's a pic of the capsule interior before it went up.



Here's a pic of the composite "Double Bazooka" antenna.  It's a piece of coax between two wooden dowel rods, and covered in heat shrink tubing.



We’ll have amateur radio operators (Mark and Nathan Bookout) on the ground and in the air to assist with communications. 



The camera hack.  The wire and jumper header tucks up into the battery compartment, so it still functions as a normal camera.


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