Development of 1.3-GHz range-imaging wind profiler radar

Wind profiler radar (hereafter WPR) is a useful means to measure vertical profiles of vertical and horizontal wind in the clear air. WPR uses Doppler shifts of clear-air echoes to measure wind profiles. A major source of clear-air echoes is atmospheric turbulence that produces perturbations of radio refractive index at the Bragg scale. Therefore Doppler spectra collected by WPRs have been used for retrieving turbulence parameters (e.g., dissipation rate, diffusivity). However, because range resolution determined by transmitted pulse width is typically greater than 100 m, range resolution of WPR is often not sufficient for quantifying turbulence parameters. Range imaging (hereafter RIM) is a technique that improves range resolution down to several ten meters by using frequency diversity and adaptive signal processing. Measurements using 50-MHz WPRs have demonstrated that RIM is useful for resolving turbulence and wind perturbations triggered by Kelvin-Helmholtz instability. However, 50-MHz WPRs cannot cover heights below 1 km due to system limitations.

WPRs using UHF band (300-3000 MHz) are able to measure wind and turbulence below 1 km height and hence are useful for measuring dynamical processes in the boundary layer. In order to provide a means to measure wind and turbulence in the boundary layer with high height resolution, we developed a 1.3-GHz range-imaging wind profiler referred to as RIM-LQ7. RIM-LQ7 was developed using the antenna, transmitter, and RF unit of LQ-7, which is a 1.3-GHz WPR produced by Sumitomo Electric Industries, Ltd. In order to implement RIM capabilities, a local oscillator which can change the transmitted frequency every transmission is used. Five frequencies (1227.0, 1227.25, 1227.5, 1227.75, and 1228.0 MHz) are able to be produced from the local oscillator. For real-time digital signal processing of RIM-LQ7, we developed a low-cost IF digital receiver. The digital receiver is designed to decode phase-modulated oversample (OS) signals collected with a rate of 10 MS/s. RIM-LQ7 is the first that is able to carry out RIM and OS at the same time.

The RIM and OS capability of RIM-LQ7 provide the opportunity to resolve fine-scale vertical structure of turbulence. Because other RIM WPRs collect signals using a sampling interval which matches their transmitted pulse width, the range weighting effect significantly deteriorates sensitivity at subgates near the edges of range gates. Therefore it was difficult for RIM to attain unambiguous measurement in height. On the other hand, because RIM-LQ7 is able to collect oversampled signals which overlap sampling volumes in the range direction, the range weighting effect in RIM is negligible for RIM-LQ7.

Field measurements to assess the measurement capability of RIM-LQ7 were carried out from June to July 2012. In the field experiment, 1-microsecond transmitted pulse width and five frequencies with 1-MHz span (i.e., 2-MHz frequency bandwidth) were used. Highest time resolution was about 3 seconds. From the field measurement, it is proved that RIM plus OS (RIM-OS), which was realized for the first time by RIM-LQ7, is useful to attain range resolution down to several ten meters even with the 2-MHz frequency bandwidth. First measurement results of instability and thermals in the boundary layer are also presented.