Radars generally employ monopulse direction-finding to determine the bearing angle to a selected target. In a typical monopulse direction-finding scheme, a single transmitted beam from a transmitting antenna (illuminator) radiates the target and is reflected by the target. From the reflected beam that is received at the receive antenna of the radar, four reference beams are generated; this may be the result of using one antenna for both the transmit and receive functions or a separate antenna exclusively for the receive function.
From the four separate reference beams, the bearing angle to the target is derived in both the azimuth and elevation planes. This is illustrated in FIG. 1. As shown in the figure, the reference beams A, B, C and D from the four quadrants of receive antenna 101 are fed into comparator 103 where the sum and delta components are generated. The sum and delta components, in both the azimuth and elevation planes, are then input to their respective, separate channels 105, 107 and 109 where they are typically amplified and Doppler-filtered. Thereafter, the sum and delta components are input to signal processor 111 which performs the digitization and Fast Fourier Transform of them to produce the ratios of the delta over sum voltage, in both azimuth and elevation, that are indicative of the particular angle of the target's position with respect to the radar at the moment the radiating beam was reflected from the target.
A critical requirement in the proper operation of such a direction-finding radar to obtain accurate angular measurements of the target's position is that channels 105, 107 and 109 be balanced. The amplitude and phase errors between the channels that may result from component tolerances, temperature drift, etc. must be measured and removed. If such calibration is not performed, the amplitude and phase errors that are due to the channels themselves will falsely be attributed to the wave angle of the received beam and result in inaccuracies in the angular positional measurements of the target.
Currently, calibration of a monopulse radar is typically achieved by generating from sideband generator 201 a single sideband signal which is offset from the transmitting antenna frequency but within the Doppler passband of the receive antenna, dividing the signal with power divider 203 into three portions which are then injected to the three channels via couplers 205, 207 and 209 as shown in FIG. 2. While this is adequate to perform the calibration, the injection circuitry takes up volume since it is often built from bulky waveguide. Waveguide is used because coax-based transmission lines frequently experience phase delay changes with respect to temperature due to thermal expansion. Additionally, the couplers suffer insertion loss, thereby resulting in a decrease in the minimum signal sensitivity of the radar system. Further, since only a single frequency is injected into the Doppler filter passband, the calibration process must be repeated numerous times using different frequencies in order to calibrate the entire radar receiver passband.