CS is a constant low PRF (long R_max and low V_max) employed for the entire 360° sweep to determine proper target location and returned power. A CS rotation is part of the "Split Cut" mode. A CS rotation is also part of the Multiple PRF Dealiasing Algorithm (MPDA) processing, used exclusively in VCP 121.

CD is a constant high PRF (short R_max and high V_max) employed for the entire 360° sweep to accurately determine "1st guess" velocity and spectrum width information. A CD rotation is part of the "Split Cut" mode. One or more CD rotations are used as part of the MPDA processing on the lowest five slices in VCP 121. A result of the short R_max (high PRF) is that multiple trip echoes frequently occur, making the Range Unfolding algorithm necessary.

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. While staying at a particular elevation angle, there is one full rotation using the CS waveform, followed by one full rotation using the CD waveform.

The Split Cut mode is used on the lowest elevation angles, where multiple trip echoes are most likely to occur. By using a low PRF in the CS mode first, the radar can properly place targets. The CS and CD information is used together to "unfold" the velocity estimates to their appropriate range.

SZ-2 Split Cut (SZCS/SZCD)

As introduced in Section 6-4 Data Collection Operations, SZ-2 is range folding mitigation technique applied to the split cuts of VCPs 121, 211, 212, and 221. The technique examines returned energy from up to four trips using a sequence of eight modulation phases rotated 90 degrees relative to one another. Thus, the resulting pulse train repeats every 32 pulses (8 pulses per 90 degree sector = 32 pulses). 64 total pulses are used to reduce the variance of the estimate, thus the PRFs used for the SZCD cuts are hard coded at 64 pulses and cannot be edited. A narrow ring of range folded data is typical at the end of the first trip. With the SZ-2 elevations, this ring will not move since the PRF and the associated R_max do not change. Refer to tables 12-14 to see which PRF is hard coded for the SZ-2 split cuts. This section provides only a brief overview of the SZ-2 technique. A far more detailed discussion of SZ-2 can be found HERE and is listed in the references tab of this lesson.

Batch (B)

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, first transmitting pulses using a low PRF (CS) to obtain a long R_max then transmitting pulses using a high PRF (CD) to obtain more accurate velocity information. This alternating back and forth between a low and a high PRF is done for each radial until a full 360° sweep has been completed.

This is a high PRF used to obtain all base data in the higher elevation slices (>7°). However, no range unfolding algorithm is applied to the data. For example, at 7.5°, the radar beam is already at ~ 50,000 feet at 62 nm range (the shortest CD R_max ) and range folded echoes are unlikely (Figure 16).

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. For example, VCP 121 has a total of four antenna rotations at 0.5°. The first rotation is in CS mode, while the other three are in SZCD, CD, and CD modes, using PRFs 8, 6, and 4, respectively (Table 11).