For our High Altitude Balloon flights, we use several different custom built systems to relay and/or store data and video from the balloon to the ground. These consist of the primary telemetry system which handles sampling the data from the student payloads and the low speed voice data downlink, the high speed digital data downlink which pulls data from the primary system, and the video system which operates independent of the two other telemetry systems and the student payloads.
The primary telemetry uses an FM transmitter in the 900MHz band attached to a voice synthesis chip. Data is sampled from analog to digital converters or pulse counters. This data is then fed in ascii serial format to the voice synthesis chip for transmission. Depending on the data itself (different words have different lengths), and the number of student teams each year, the rate at which a given team would receive a data sample is about once every couple minutes.
The secondary telemetry has in the past used an AM transmitter in the 434MHz band to downlink RTTY data at 80 baud. This generally allowed for a data sample once every 20-30 seconds. The secondary telemetry has now been outfitted with a 144MHz band transmitter and is now capable of 1200 baud amateur radio packet transmissions. Data samples could be as frequent as every 5 seconds, though the actual rate will be set in the configuration and will be chosen based on the amount of data needed to be downlinked.
This is the main board for the secondary telemetry system. Visible in this photo are the microcontroller, GPS receiver, barometric pressure sensor, and other support electronics. Yet to be installed are the temperature sensors, flash storage chip, and the radio module.
The video downlink uses an AM amateur television transmitter in the 434MHz band. Two cameras are in the payload. One points downward, and the other points to the side. A relay switches the video signal to the transmitter between the two on a timed interval. The video downlink also makes use of a video overlay board to display required callsign information as well as GPS data on the received video.