Waveforms are used in different combinations in each VCP, to make the best use of the different PRFs. The distribution of the waveforms within a given VCP is related to the intended use of that VCP. Figures of each VCP with the distributions of wavforms used are available (see Figures 16-24).
Contiguous Surveillance (CS)
CS is a constant low PRF (long Rmax and low Vmax) employed to determine proper target location and returned power.
Contiguous Doppler (CD)
CD is a constant high PRF (short Rmax and high Vmax) employed to more accurately determine velocity and spectrum width. A result of the short Rmax (high PRF) is that multiple trip echoes frequently occur, making it necessary to use a range unfolding technique.
Split Cut (CS/CD)
The Split Cut mode is implemented on the two lowest elevation angles for VCPs 11, 21, and 32, and on the three lowest elevation angles for VCPs 12 and 31. At the same elevation angle, there is an entire 360o rotation using the CS waveform, then a second entire 360o rotation using the CD waveform.
NOTE: For the Split Cut elevations, dual-polarization base data are processed from the Contiguous Surveillance rotation.
: Volume Coverage Patterns (VCP) 11 (and 12) sample fourteen (twelve) elevation angles in about five (four) minutes. The lowest two (three) angles apply Split Cut with legacy range unfolding, middle angles use Batch (B), and higher angles use Contiguous Doppler without range unfolding (CD/WO) mode.
The Split Cut mode is used on the lowest elevation angles on all VCPs, to address multiple trip echoes and other radar data quality challenges. By using a low PRF in the CS mode first, the radar can properly identify target range. The CS and CD information is used together to "unfold" the velocity estimates to their appropriate range. Also, it's only on these elevation cuts that super-resolution data processing occurs.
Sachinananda-Zrnic (SZ-2) Split Cut (SZCS/SZCD)
SZ-2 is a different range unfolding technique applied to the Split Cuts of VCPs 121, 211, 212, and 221. SZ-2 is able to reveal far more velocity data compared to the other range unfolding technique. Details on both of these techniques are presented in later sections of Topic 3.
NOTE: As with the standard Split Cut mode, dual-polarization base data are processed from the SZ-2 Contiguous Surveillance rotation.
In the middle angles, the radar uses the Batch (B) Mode technique for one full rotation at each elevation angle. This technique uses alternating low and high PRFs on each radial, such that each radial is sampled with a number of low PRF pulses, then a number of high PRF pulses.
NOTE: For the Batch elevations, the dual-polarization base data are processed from the Continuous Doppler pulses for each radial.
Contiguous Doppler X (CDX, Contiguous Doppler with no range unfolding)
This is a high PRF model used to obtain all base data (including dual-polarization data) in the higher elevation slices (>7°). Since range folded echoes are unlikely, no range unfolding algorithm is applied to the data.
Multiple PRF Dealiasing Algorithm (MPDA)
The MPDA is implemented solely by invoking VCP 121. Though MPDA is not a waveform, it uses a unique combination of the SZCS, SZCD, CS, CD, and B waveforms. In the lower elevations of VCP 121, there are multiple Doppler rotations at each angle, each of which uses a different PRF.
: Volume Coverage Pattern (VCP) 121 samples nine elevation angles in about 5.75 minutes. The lowest two angles apply the SZ-2 technique as part of MPDA, the lowest five angles apply the MPDA technique, while 6.0 deg uses Batch (B), and the higher angles use Contiguous Doppler without range unfolding (CD/WO) mode.