The RT-300 Radio Altimeter Receiver Transmitter is a pulsed radar that uses separate external transmit and receive antennas. The RT-300 is a self-contained microwave transmitter receiver and range computation system. It operates on short pulse, leading edge track principles to measure aircraft absolute altitude. Clearance altitude is displayed on a companion analog altitude display or on an ADI. Auxiliary outputs are provided to supply both analog and discrete information for various aircraft user equipment. The RT-300 contains power supplies, radio frequency transmitting, receiving circuitry, and timing circuits to determine aircraft AGL altitude. The RT-300 transmits radio frequency pulses and measures the time until the reflected signal is received, then determines aircraft AGL altitude above the terrain.  The dc output voltage of the RT-300 represents the absolute altitude. The output voltage is linearly related to the aircraft height above the closest terrain directly below the aircraft. The gradient is -4 mVs/feet with 0 mVs representing zero feet.  The precision altitude output is used to drive the primary radio altitude display in up to five RA-XXX indicators or Honeywell EFIS displays. The auxiliary altitude output is an ARINC 552 signal that can be used to drive the rising runway bar in any Honeywell Altitude Director Indicator, as well as interface with many other manufacturers display systems.The RT-300 can be hard mounted to the aircraft or mounted in a shock isolated tray as required. The RT-300 is 4.09 in. high, 4.56 in. wide, 11.07 in. long and has a maximum weight of 4.6 lb.

 The RF ground return signal from the receive antenna is input to the receiver subsystem where it is amplified and converted to produce an i-f signal. The i-f signal is amplified and detected to produce a video pulse, which is further amplified and applied to the processor. Within the processor, the detected video pulse is compared to the system sync pulse and the time difference between these two pulses is determined. This time difference is converted to a dc analog voltage (internal range voltage) proportional to the shortest range to the ground. The internal range voltage is fed to the output assembly for further processing. An Automatic Gain Control (AGC) voltage is derived in the range tracker and fed back to the receiver assembly to maintain the amplitude of the video signal at a constant level. The processor also contains the search and acquisition circuits that enable the system to initially acquire the ground return signal or reacquire the signal should tracking be interrupted. The search mode is initiated upon loss of signal. A sweep signal is generated that causes the system to search for the earliest return signal. This process continues until track is reestablished. Once track has been properly established, the search mode is inhibited. A search valid is output networks to cause the indicator needle to disappear from view during the search phase. A Sensitivity Time Control (STC) voltage is fed back to the receiver assembly to gain-program the receiver as a function of altitude. Gain programming prevents acquisition of the direct antenna leakage during the interval when the transmitter is on. The internal range voltage is processed within the output networks to produce dc outputs for driving the indicator needle and other aircraft systems required radio altitude information. Four altitude trip outputs are provided, which supply a ground at or below the preset altitude. The flag warn signal controls the indicator flag to provide warning of system malfunction. In addition, a track invalid signal is available that indicates if the RT-300 is not in the track mode. Two test inhibit inputs are provided to permit defeat of the pilot activated self test when the flight control system is engaged.