Waveform design considerations for CASA radar network

The Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) is embarking on a paradigm of network of small radars to sample the atmosphere at high resolution. A fundamental physical limit imposed by transmission from single radar is the problem of decreasing resolution as a function of range. In addition the lowest coverage altitude increases with range due to earth curvature. As an alternate solution, a networked radar environment concept has been proposed by Chandrasekar and Jayasumana, 2001 and McLaughlin, 2001. The basic principle of the networked radar environment is to provide good coverage, in terms of accuracy and resolution to a large area through a network of radars. In order to provide a more economically feasible solution to this approach, meteorological radar operation must change from S-band operation to higher frequencies (X-band). Chandrasekar et al (2004) describes the various challenges that arise for radars operating at X-band. CASA will deploy its first generation network of four low-powers, short-range, X-band, dual-polarized Doppler weather radars known as NETRAD. Doppler weather radars transmitting pulses with uniform pulse repetition frequency (PRF) have a fundamental limitation on maximum unambiguous range (rmax) and maximum unambiguous velocity (vmax) governed by rmaxvmax = cλ/8, where λ is radar wavelength and c is the velocity of light. The rmaxvmax limit reduces by a factor of three when the wavelength is changed from S-band to X-band. There is always a trade off between rmax and vmax (Rangevelocity ambiguity). Precipitation particles can be distributed over a large area and the dynamic range of the radar reflectivity can be as high as 80 dB which results in range overlay. Velocity measurements can span 100 m/s in severe storms resulting in velocity folding. NETRAD is primarily for”targeted applications” such as tornado detection, flash flood monitoring, and hydrological applications. Such applications will have range overlay and velocity folding problems with conventional pulse pair processing. A testbed of smaller X-band radar systems is being developed within CASA (Junyent et al, 2005). X-band radars have a low unambiguous velocity due to their short wavelength, and increasing the PRF will result in multiple trip overlays since storms can extend over a large distance. It can be observed that range-velocity ambiguity is more severe for X-band radars compared to the conventional S-band. Several rangevelocities ambiguity mitigation schemes have been proposed in the past. Staggered pulse-repetition-time (PRT) and multiPRF waveforms can be used to increase the unambiguous. Random phase coding of the transmitted pulse was proposed by Siggia (1983) to mitigate range overlay, and a systematic phase code and associated processing was suggested in Sachidananda and Zrnic (1999). A systematic phase code has been known to give better performance than random phase codes but requires a coherent transmitter such as a klystron or solid-state transmitter. This paper describes the adaptive waveforms for the individual radar nodes based on NETRAD operational requirements such as scan speeds, volume coverage pattern and system/hardware limitations to resolve range and velocity ambiguities.