Proceedings with Extended Abstracts (single PDF file) - Radio ...

RANGE, RESOLUTION, AND SAMPLINGPaul E. Johnston 1,2 , Leslie M. Hartten 1,2 , David A. Carter 2 , and Kenneth S. Gage 21. CIRES/University of Colorado, 216 UCB, Boulder, CO, 80309-0216, USA2. NOAA Aeronomy Laboratory, 325 Broadway, Boulder, CO, 80305-3328, USAFigure 1 shows a nagging problem. This plot of wind speed profiles from ChristmasIsland, Kiribati, shows the results from wind profilers operating with three different pulse lengths.These curves are the average of all available February winds for the years 1994-1996. Twocurves are from a 915-MHz radar, using pulse lengths of 100 m and 500 m. The final curve isfrom a 1000 m long pulse from a 50-MHz radar. There are many measurements represented bythese profiles, and the estimated measurement errors in these profiles are very small. However,the low-level jet has a different magnitude and occurs at a different height in each curve.Figure 1. Mean February wind speed for 1994-1996 at Christmas Island, Kiribati.The 1000-m data are from the 50-MHz radar. The 100-m and 500-m data are fromthe 915-MHz radar. (Adapted from Johnston et al., 2002)262Johnston et al. (2002) discuss why the maxima don’t occur at the same heights whenobserved with different pulse lengths. This paper discusses why different pulse lengths don’tobserve the same peak velocity. Many in the profiler community would say that the answer isobvious; the longer pulse lengths average over larger volumes and average out peak velocities.This is a true and qualitative explanation of the observations, but some of the details need to beexamined to present a more complete picture. This is especially true since the data plotted ascontinuous curves in Figure 1 are really discrete points, each representing some volume in space.One classic approach to exploring the range behavior of a radar is to define a rangeweightingfunction, |W(r)| 2 . This is a description of the ranges that contribute to the observedsignal at a given range. It is derived from the time response of the receiver to the transmittedpulse, which is the convolution of the filter response to the transmitted waveform. One commonparameterization is to specify the B 6 - product of the system, where B 6 is the -6 dB (half-voltage)bandwidth of the receiver and - is the pulse length of the transmitted pulse. For a rectangulartransmit pulse and a Gaussian filter response the maximum Signal-to-Noise Ratio, SNR, occursat B 6 -=1.04, called the matched filter condition (Doviak and ZrniA, 1993). Several factorscontribute to the determination of the B 6 - product in a radar. These include maximizing SNR,setting the range over which the samples are independent, hardware implementation, and datastorage and processing limitations. Some Aeronomy Laboratory (AL) radars have B 6 - products