Finding the Payload
1The weather club at Ridgeview Middle School is getting ready to launch a high-altitude balloon. A small foam box called the payload hangs below the balloon and carries a camera and a GPS tracker. The balloon will rise about 30 km, burst in the thin air and drop the payload back to the ground on a parachute. It may land far from the launch site, so a chase team in two cars has to find it. The team plans to talk between cars with walkie-talkies, while the tracker in the payload radios its GPS position every 30 seconds.
2The two radios send information in different ways. The walkie-talkies use an analog signal: the radio wave changes smoothly to copy the shape of the speaker's sound wave. Any noise the signal picks up along the way changes that shape, and the listener hears it as hiss. The tracker uses a digital signal. Each position is sent as a short message of 0s and 1s, and every message ends with a check value worked out from the rest of the message. The receiving laptop works out the check value again. If the two do not match, the laptop throws the message away and waits for the next one.
3Club member Amara Oyelaran ran a range test along a long, straight reservoir a week before launch. She stayed at the boat ramp with a walkie-talkie and the laptop. Her teammate Joaquín rode with the club advisor to marked stops 1, 3, 5, 7 and 9 km down the shore road. At each stop, Joaquín read the same list of 40 words over the walkie-talkie, and Amara wrote down how many she understood. At the same stop, a spare tracker in the car sent 40 position messages. The laptop counted how many passed the check and whether it ever showed a wrong position. The team used the same radios, fresh batteries and the same antenna height at every stop.
4On launch day the payload may land behind a ridge, where the radios cannot reach the chase cars directly. The advisor plans to park a relay radio on the hilltop. A relay receives a signal and sends it on. An analog relay makes the signal it hears stronger and sends it again, hiss and all. A digital relay first decides whether each part of the message is a 0 or a 1, then sends out a fresh, clean copy of those 0s and 1s.