A system for determining a bearing to a radio signal source includes four antennas and a computer. The computer accepts signals from the antennas as inputs and identifies which antenna is providing the largest amplitude signal and which antenna is providing the next-largest amplitude signal. These amplitudes are subtracted to determine an amplitude difference. The bearing is then determined in accordance with a linear function of the amplitude difference. The linear function is a model of the piece-wise linear approximation of antenna signal amplitudes accepted by the computer. The slope used in the linear function depends on the identity of the antenna that provides the largest amplitude signal. When the antenna that provides the largest amplitude signal is mounted on a fuselage oriented toward a curvature of the fuselage, as when facing port or starboard, then a different slope is used than when the antenna that provides the largest amplitude signal is mounted on the fuselage and oriented toward for or aft. More accurate bearing calculations result from use of different slopes accounting for fuselage shape and elevation angle resulting from a difference between the altitude of the system and the altitude of the radio signal source. Bearing may be used for aircraft traffic alert and collision avoidance systems.