Pulse Compression Weather Radar with Improved Sensitivity, Range Resolution, and Range Sidelobe

In recent years, solid-state weather radar (SSWR) using microwave semiconductor devices, such as GaAs or GaN HEMT, has largely replaced weather radar that uses an electron tube, such as a klystron or a magnetron, for a transmitter, and has become mainstream.
The many advantages of SSWRs compared with klystron or magnetron radar include high efficiency, small size, easy maintenance, low lifecycle cost and low spurious emission.
Pulse compression, a signal processing technique commonly used by radar, sonar and so on, is a key technology for SSWR from the viewpoint of securing the desired transmission energy and range resolution.
In pulse compression with frequency modulation, two principal methods are used, both of which are well known: Linear Frequency Modulation (LFM) in which the instantaneous frequency varies linearly with time and Non-Linear Frequency Modulation (NLFM) in which the instantaneous frequency varies non-linearly.
LFM, widely used in SSWR systems, is advantageous in that a range sidelobe can be suppressed to a low level by applying a lossy window function in correlation processing.
 However, LFM is disadvantageous in that radar sensitivity is sacrificed for the loss of the window function.
On the other hand, NLFM can improve the radar sensitivity compared with LFM, because a lossy window function is not required.
Therefore, for higher sensitivity, application of NLFM to SSWR has been investigated in recent weather radar development.
However, compared with LFM, with NLFM it is more difficult to reduce the range sidelobe level. In addition, when the NLFM waveform is implemented in a weather radar system, degradation of range sidelobe performance by the distortion properties in the transmitter is greater.
The suppression of the range sidelobe in NLFM is an issue that needs to be resolved, not only in the ideal simulation, but also in real implementation.

In this paper, it is presented quantitatively that the sensitivity, range resolution, and range sidelobe in SSWR can be improved by using our developed NLFM instead of the present LFM.
First, it is shown that in the ideal simulation it is possible to suppress the range sidelobe to a lower level than in the case of LFM with Blackman-Harris window function, by applying the original amplitude waveform to NLFM signal.
 Second, it is shown that in a loop-back test using the weather radar prototype, it is possible to suppress the range sidelobe to a lower level than in the case of LFM, by making high-precision corrections for the distortion properties in the transmitter, and it is also shown quantitatively that SNR and range resolution in this NLFM are improved compared with the LFM case.
Moreover, weather observations are performed by the developed NLFM, and it is confirmed that the improvement of the reception sensitivity in weather observations is comparable to the improvement of SNR in the loopback test.

Toshiba is conducting further evaluation of the proposed NLFM in weather observations and plans to make practical use of NLFM in SSWR within the next few years.