Fog remote sensing with a Ka-band Doppler Radar

A Ka-band Doppler scan radar for fog observation, equipped with a magnetron and digital phase correction technique, was designed and assembled. The radar is mounted on the flatbed vehicle. The antenna controller, transmitter, receiver, signal processor, and data acquisition and display system are contained in the cube enclosure on the front flatbed portion. The radar operates at the frequency of 34.75 GHz, and is equipped with a 2 m parabolic antenna of a gain of 54 dB and the -3 dB beam width of 0.3‹. The antenna scans according to the programmable sequence at the angle coverage of the azimuth of full circle or sector, and the elevation of -2 to +92‹. Maximum rotation rate is 36‹/s for azimuth, and 6‹/s for elevation. Transmitted signals of the peak power of 100 kW, pulse width of 0.5 micro sec, with random initial phase output from magnetron is radiated from the antenna

The 35 GHz of Dopplerized radars for vertical pointing and observation of cloud structure with the radars have been discussed before. However, horizontal scanning Doppler radar for this frequency band, and the three dimensional observation of fogs by the radar had not been addressed before. We discuss the development of the Ka-band Doppler scan radar, and the preliminary three dimensional observation of sea fog.

The radar exhibits the following properties: Stable Doppler velocity measurement is available, with magnetron transmitter, by memorizing the initial phase of transmitted signal, and by implementing phase correction to the phase data of received signal in digital processing. Nyquist velocity is expanded to approximately }9.7 m/s, by the double pulse method. Moreover, it is expanded to more than three times by correcting the aliaging of the velocity by software technique. The values of the minimum detectable radar reflectivity at the range of 2.5 km, 5 km, and 10 km are expected to be around -38, -30, and -20 dBZ, respectively on the horizontal scanning mode of 1 rpm.

The preliminary observations of sea fog, carried out for the evaluation of the radar, shows any new knowledge obtained by the function of Doppler scan radar as follows: Three dimensional shape of fogs, vertical structure, horizontal distribution, and the advection are grasped by the radar. The RHI Doppler observation shows the dual-layer structure of horizontal wind, where the wind speed of the upper layer is several m/s, and that of the lower one is almost 0 m/s. The sea fog seemes to keep the dual-layer of wind during the advection from ocean to inland. According to the echo intensity, the dual-layer structure in the vertical direction is not noticed. Observations of RHI and PPI scan show that the fogs are composed of small fog mass and/or parallel arrays. The difference of maximum and minimum intensity exceeds more than 20 dB. These observations show the usefulness of this radar measurement technique to detect features of fogs and its structue.