Dual Antenna Configuration for MMCRs: A Method for Validating Antenna Characteristics at CART Sites

Atmospheric Radiation Measurement (ARM) Millimeter Wave Cloud Radars (MMCRs) rely on routine calibrations to provide accurate estimates of cloud reflectivities. Radar systems with scanning antennas routinely make use of calibrated corner reflectors to provide an external measurement of calibrated reflectivity (Sekelsky and McIntosh 1996). However, the MMCRs (Moran et al. 1998) are equipped with fixed beam vertically pointing antennas and must make use of an internal calibration. Instead of pointing the antenna at a calibrated target, the MMCR relies on combining measurements of the transmitted power, receiver gain, and antenna gain and beamwidth to compute an indirect measure of the calibrated reflectivity. Although the field calibrations of the transmitter and receiver are done frequently using reliable techniques, the antenna calibrations have been less definite. The antenna calibrations have only been performed at the time of manufacture at an antenna test range, and issues such as subsequent transportation stresses and weathering may have affected the antenna characteristics. Comparison with other well-calibrated radar systems at remote cloud and radiation testbed (CART) sites is often the only way to verify the performance of the MMCRs. The remoteness of these sites often limits the number of visiting instruments that can be used to compare with and calibrate the ARM instruments. It is useful then, to search for simple tools and procedures to assist with the calibration validation.