. Airborne radar. Airplanes; Guided missiles. 14-2] BASIC PRINCIPLES OF DOPPLER RADAR NAVIGATION 729 doppler shift and if we know the frequency of transmission, the velocity of light, and the direction of radiation, we can determine the only unknown quantity, the ground speed of the aircraft. The velocity can then be integrated to measure the miles traveled along the ground and combined with heading information to compute present position. A doppler navigation radar^ then, is an airborne radar which transmits electromagnetic energy toward the earth's surface and utilizes the doppler shift of t
. Airborne radar. Airplanes; Guided missiles. 14-2] BASIC PRINCIPLES OF DOPPLER RADAR NAVIGATION 729 doppler shift and if we know the frequency of transmission, the velocity of light, and the direction of radiation, we can determine the only unknown quantity, the ground speed of the aircraft. The velocity can then be integrated to measure the miles traveled along the ground and combined with heading information to compute present position. A doppler navigation radar^ then, is an airborne radar which transmits electromagnetic energy toward the earth's surface and utilizes the doppler shift of the received energy to determine two or three of the velocity components of the aircraft. This is illustrated schematically in Fig. Fig. 14-4 Basic Doppler Beam Configuration. The basic output of a doppler radar is a frequency, which is the observed doppler shift, given by the basic doppler equation IVf IV fd = —^ COST = ^ COST (14-1) C A where/d is the doppler shift, ^is the velocity of the aircraft, c is the velocity of light, T is the angle between the velocity vector and the direction of propagation, and X is the wavelength of transmission. Since the antenna beam has a finite width and since the scattering from the earth is randomlike, the information received from the ground is not a single frequency, but rather is in the form of a noiselike frequency spectrum as shown in Fig. 14-5. A certain amount of smoothing time is therefore required to determine the quasi-instantaneous velocity to a given accuracy. A velocity smoothing time constant of about 1 second is usually chosen; this value is limited by the need for compatibility with the dynamics of typical aircraft. However, the effective smoothing time for navigational distance measurement is the total time flown and, for typical systems, it turns out that the so-called JIuctuation error is completely overshadowed by certain other instrumentation errors after approximately 10 miles of Please note that these
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