Velocity Ambiguity Mitigation in NASA D3R

The NASA dual-frequency, dual-polarization, Doppler radar (D3R) is a fully polarimetric, matched beam, scanning weather radar system which operates at the frequencies of 13.9 GHz (Ku-band) and 35.5 GHz (Ka-band). The maximum operational range of the radar is fixed at approximately 40 km for both the bands. The unambiguous range (r_a) and unambiguous velocity (v_a) both depend on the pulse repetition interval (PRT) at which the radar is operating, the relation between them is given by r_av_a=cλ/8, where, c is the velocity of light and λ is the wavelength of the radar. This is a basic dilemma associated with the Doppler radar (Zrinc and Mahapatra 1985). As mentioned above the range of D3R is fixed which leads to different unambiguous velocity at Ku- and Ka-band. Also the D3R operating frequencies are high which sets a limit to the maximum unambiguous velocity. The unambiguous velocity for Ku-and Ka-band when D3R is operating in uniform PRT mode are 10.8 m/s and 4.2m/s respectively. When an event takes place with strong winds the velocities get aliased significantly, especially in the Ka frequency band.

Prior works for velocity dealiasing have considered using techniques like staggered PRT and dual PRF techniques (Bharadwaj et al. 2010). D3R operates in staggered PRT mode, with a staggered PRT ratio of 2:3 which extends the unambiguous velocity of Ku band to 27 m/s and Ka band to 10.6 m/s. These extended unambiguous velocities are also not sufficient to represent the full range of velocities when strong winds are present in an event and the velocity components which are present outside the unambiguous velocity interval will alias back.

In D3R since both the frequency band radars are located in the same radar system, they share the same resolution volume when scanning. Using this information and prior information of the unambiguous velocities of both the frequency bands, a method is discussed in this work which will dealias the Doppler velocity. Simulated Doppler velocity data is used to characterize the method for dealiasing. The method will be tested, analyzed and demonstrated for the D3R Doppler velocity data taken for a variety of observations.

References:

Bharadwaj, N., Chandrasekar, V. and Junyent, F., 2010. Signal processing system for the CASA Integrated Project I radars. Journal of Atmospheric and Oceanic Technology, 27(9), pp.1440-1460.

Zrinc, D. and Mahapatra, P., 1985. Two methods of ambiguity resolution in pulse Doppler weather radars. IEEE Transactions on Aerospace and Electronic Systems, (4).