CHACAL (Communication System)
CHACAL was a high frequency communication system developed by the balloon division of the Centre National de E'tudes Spatiales (CNES) from France. It was introduced in 1976 to carry the telemetry (TM) during long duration flights balloon for the MIR (Montgolfier Infrarouge a hot air balloon that used telluric Infra Red radiation) and to improve the rate of scientific data collected by MIR gondolas.
The data was received from the balloon to three ground stations located in different continents. Generally, the first station was installed in the balloon launch base (Pretoria, Southafrica, for instance) while the others were located in points as far as Noumea, New Caledonia and Ecuador or Brazil in southamerica.
The CHACAL transmitter emitted a high frequency signal with a power of 50 watts alternately in two frequencies: 6 mhz and 18 mhz . These signals bounced on the E or F ionospheric layers, as do the commercial short wave international stations. This allowed the covering of great distances using a relative low emmiting power, and with a very good efficiency: as an average 90 % of the transmitted frames were received by the three stations. The system had a rate of 100 bits per second and used two antennas hanging from the flight train bellow the balloon: a dipole 8 meters long for 18 mhz band and a vertical "J" of 30 meters long for 6 mhz band.
CHACAL has it's setbacks too: it had no telecommand capabilities and the ground stations were distinct from each others. However it was used in a regular basis under MIR balloons until 1996.
Nowadays, CNES is studying the implementation of a similar ionospheric bouncing system called Numerical Short Wave (NSW) with some additions like the use of half duplex modems (allowing to send telecommands to the balloon), dynamic choose of transmission frequencies and real time distribution of data between stations.