AN ALTERNATIVE METHOD FOR INFERRING WINDS FROM SPACED-ANTENNA RADAR MEASUREMENTS

One variation of the spaced-antenna technique for measuring atmospheri c winds using a pulse Doppler radar is based on a calculation of the c omplex temporal cross-correlation function for the backscattered field s detected by a pair of spaced receiving antennas. The delay to the pe ak of the cross-correlation amplitude, adjusted for the temporal evolu tion of the backscattered field pattern through an extended analysis, is related to the so-called trace velocity along the antenna pair base line, with two nonparallel baselines yielding the full horizontal wind vector. We suggest an alternative method for inferring the horizontal wind from the temporal cross correlation. We demonstrate that the slo pe of the cross-correlation amplitude at zero lag, normalized by the l evel of the cross-correlation amplitude at zero lag, is directly propo rtional to the component of the wind velocity along the antenna pair b aseline. We illustrate that this measure of the horizontal wind is ins ensitive to the temporal evolution of the backscattered field pattern. Therefore no extended analysis is needed to estimate the horizontal w ind vector. The advantage of this approach is that it is a very simple and direct method of retrieving the horizontal wind. Our theory indic ates that for many cases of interest the proportionality constant rela ting the normalized slope to the wind component along the antenna pair baseline depends only on the transmitting and receiving antenna diame ters and on the separation between the receiving antennas. This consta nt can be calculated, or the system can be calibrated against a rawins onde retrieval. A disadvantage to the slope technique is that it is so mewhat sensitive to the degree of vertical anisotropy of the scatterin g medium. We loosely define the degree of vertical anisotropy r as the ratio of the vertical to horizontal correlation length of refractive index irregularities having a spatial scale of the order of the Bragg wavelength lambda/2, where lambda is the radar wavelength. In the neut ral atmosphere, r typically satisfies 0 < r less-than-or-equal-to 1. W e find that for a horizontally isotropic, Kolmogorov-type refractive i ndex spectrum with lambda/2 < L0z, where L0z is the vertical outer sca le, the slope method is useful provided r is greater than the effectiv e system beam width lambda/D(e), where D(e) is an effective system ant enna diameter (approximately equal to the transmitter antenna diameter ). We note that this same restriction ensures that the correlation sca le of the backscattered field (i.e., the pattern scale) is solely a fu nction of the antenna diameters. The slope method is therefore limited to scattering from isotropic to moderately anisotropic turbulence (i. e., lambda/D(e) < r less-than-or-equal-to 1) provided lambda/2 < L0z a nd is perhaps best suited for wind profiling in the atmospheric bounda ry layer. A comparison of horizontal winds obtained by applying the sl ope technique to data from a 915-MHz spaced-antenna radar and winds fr om a rawinsonde retrieval showed good agreement.