Six channel ground to air communication is accomplished through a standard
RC transmitter, using pulse-code modulation. One channel, defined by a
binary switch on the transmitter, signals either manual or automatic mode.
In manual mode, the longitudinal and lateral cyclic commands as well as the
collective, tail rotor, and throttle commands are defined directly by the
transmitter controls and communicated to the vehicle.
The output of the ground computer, three commanded attitude angles and a
commanded altitude, are relayed to the transmitter over a serial line.
In automatic mode, these four ground computer commands are transmitted to the
vehicle, leaving the throttle channel idle.
Commands are transmitted to the vehicle at 
Hz.
The flight control computer interprets the signals from the six receiver
channels and drives six servos (cyclic, collective, tail rotor, throttle,
and grabber).
By maintaining so much commonality with the original RC equipment, we have
preserved the proven reliability in a simple communication architecture.
The emergency ``kill'' switch is implemented through a single channel on a second RC transmitter. An independent receiver on the vehicle directly controls a servo which restricts fuel flow to the engine. As required by the competition rules, this servo is powered by an independent battery. In the current configuration, the flight control system is not affected by the ``kill'' switch, allowing the vehicle to autorotate to a controlled landing.
Although not necessary to complete the mission objectives, an RF modem has been added for air to ground communication. This communication link will relay diagnostic information as well as the vehicle's state estimate, which will be used in future enhancements to the guidance and mission planning packages.