The Flight of HackPittsburgh's C1 Balloon
The balloon was launched at 13:15 EST on November 11, 2012 from Gerstenslager Park near Wooster, OH Google Maps
Flight Path and Tracking
During the flight the position of the balloon was tracked by a ham radio system called APRS. (TODO link) A device on the balloon received the balloon's position using GPS and transmitted the position via data packets transmitted on 144.39 MHz.
When the balloon chase vehicles are close to the balloon the tracking signals can be received directly by equipment in the vehicles. However, the transmitter on the balloon has an output power of only 300 milliwatts (to reduce the size of the battery needed). This limits the range at which the balloon's tracking signals can be directly received.
Shortly after launch, as the crew took a lunch break at a Wendy's near the launch site, the balloon encountered jet stream winds and started traveling at 90 MPH. Because of this speed and the time it took for the crew to eat, the balloon quickly travelled beyond the range at which the chase vehicles could directly receive the tracking signals.
Fortunately the ham radio community provides an infrastructure that extends the range of APRS transmitters. This infrastructure consists of two types of stations:
1. Digipeaters A digipeater ("digital repeater") is an APRS station that receives transmissions from other APRS stations and repeats them. Digipeaters are typically located in high locations such as radio towers, and can significantly extend the range over which the position reports from an APRS station can be received.
2. iGates An iGate receives transmission reports from APRS station and forwards them via the internet to various internet servers, which store the transmissions and allow the data to be displayed by any internet connected computer.
As the crew finished lunch and resumed chasing the balloon we were about 80 miles behind the balloon, which was heading east along a path that roughly followed Interstate 80. Thanks to the APRS digipeaters and iGates provided by fellow ham radio operators, we were able to follow the track of the balloon on our cell phones and tablets via the internet. This worked well during the ascent portion of the flight. At 14:27 EST the balloon reached its maximum altitude of 111,759 feet and burst, beginning it's descent. As the descent began we received two APRS position reports from 102,264 feet and 95,486 feet.
For reasons that are still unknown we then stopped receiving APRS data from the balloon for 25 minutes. This made the chase crew apprehensive, since without accurate position information we would not be able to find the landing site of the balloon. Except for the position data transmitted by APRS, all of the experiment payloads on this flight recorded their data on board. If we did not recover the balloon we would not even have any interesting data to show for all of our efforts. (For future balloon flights we plan to send experiment results via ham radio during the flight, so we will have data even if the balloon is not recovered).
The chase vehicles continued east on Interstate 80 along the last known path of the balloon. We were getting quite dejected, since 25 minutes is close to the expected descent time. If the balloon were already on the ground without having transmitted it's position to the APRS internet servers we might never find it.
Suddenly we received two final APRS position reports via the internet; one from approximately 2000 feet above ground level, and one from 1000 feet. Since the balloon was now near the ground this gave us knowledge of where the balloon would land within a few hundred feet.
We received no further position reports via the internet. This was expected; when the balloon is below about 2000 feet above ground level it is unlikely the APRS signals will reach an iGate do to the low power of the onboard transmitter.
This is a screen shot from an APRS internet server showing the position reports that were received during the balloon flight. The green line east of Younstown is the 25 minute period during which no APRS data from the balloon was received by the internet servers.
The approximate landing location was in northwest Pennsylvania very close to the intersections of Interstate 80 and Interstate 376. Our next fear was we might receive position reports traveling east toward New Jersey at 70 MPH on the top of some truck!
Fortunately that did not happen. As we approached the intersection near the position reports we began to receive APRS data from the balloon payload directly on 144.39 MHz via receivers in the chase vehicles. The received position was transferred to a handheld GPS unit, which lead us to 60 feet from the tree the balloon payload had landed in.
More to come !!!