RADAR BACKSCATTER FROM

eteorological phenomena can seriously limit mated by the sum of the variances of the shear and M radar performance. The principal effects are turbulent components by of signals by clouds, rain, , and the atmosphere and production of unwanted u; = u;hear + ufurbulmu· echoes by reflection from raindrops, , and Wind shear describes the almost universal and snowflakes. Attenuation effects become quite year-round tendency of the wind velocity to change significant above X-band (9300 MHz), but, back­ with altitude, usually increasing with height. An scatter from snow and rainfall generally dominate altitude of maximum wind velocity is always the detection and tracking problem at frequencies present, although it experiences seasonal changes down to L-band (1300 MHz). To compound the in height and intensity. The change in wind problem, the backscatter spectrum from precipita­ velocity with height can often be approximated by tion and chaff is broadened, mainly because of a constant gradient. The effect of wind shear is wind shear and air turbulence, limiting the ability illustrated in Fig. 1 where Vw is the wind velocity of doppler processors to separate targets from and Vr is the component of the wind velocity along clutter on the basis of their relative velocities. the radar beam direction. If we assume that the During 1966 and 1967 the Radar Techniques wind velocity gradient within the beam is constant, Group of the Applied Physics Laboratory per­ then for a Gaussian antenna pattern the standard formed a series of experiments to describe the deviation of the velocity spectrum due to wind characteristics of rain clutter. The investigation shear, is given by was conducted principally by F. E. Nathanson, l"shear = 0.42 k R¢2' Assistant Supervisor of this Group, and J. P. Reilly, assisted by J. Whybrew and A. Chwastyk, where k is the shear gradient in m/ sec/ km, R is who designed much of the radar equipment and the slant range in km, and ¢2 is the two-way, programmed the control and data-processing half-power antenna elevation beam width in equipment. * Efforts were concentrated on three aspects of the rain clutter backscatter phenomena: doppler spectrum spread, spatial uniformity, and Vw (wind velocity) frequency correlation. A coherent radar at 0 5800 MHz having a 1.4 , two-way beamwidth was used for these experiments. Doppler Spectrum Spread The doppler spectrum spread of echoes at low elevation angles is usually dominated by wind shear and turbulence. The doppler ANTENNA velocity spectrum variance, u;, can be approxi- Vr2 Vro Vrl PHYSICAL PICTURE DOPPLER VELOCITY ~ (SIDE VIEW) VELOCITY SPECTRUM *This investigation is reported in more detail in " Radar Precipitation Echoes : Experiments on Temporal, Spatial, and Frequency Correla­ tion," by F. E. Nathanson and J. P. Reilly, IEEE Trans. Aerospace and ElectronicSystems AES-4, No. 4, july 1968, 505-514. Fig. I-Effects of wind shear on the doppler spectrum.

16 APL Technical Digest radians. This formula indicates that wind shear and fixing a range gate at a selected minImUm effects should dominate at long ranges. range. The magnitudes of the amplitude of the The turbulence component, which arises from echoes from 100 to 200 pulses were added and the turbulent eddy currents in the wind, dominates at sum stored in a digital computer. The range gate short ranges. This component is nearly inde­ was then moved 1/ 2 to 1 pulse length, and the pendent of height and is not very sensitive to the procedure repeated. This process, which took from dimensions of the illuminated volume. The average 10 to 60 seconds, was continued until the desired turbulence component lies in the range of from 0.7 profile was obtained. to 1.0 m/sec, with the larger value applying to Figure 3 illustrates the nonuniformity in the altitudes below 3 to 4 km. backscatter from a heavy "uniform" rain as a Figure 2 summarizes measurements of u v taken function of slant range. Similar graphs from other on several days. One theoretical curve is drawn using the shear gradient k = 5.7 m/ sec/ km which applies when the radar points along the wind direction. Another curve is drawn using the shear w_ g~ ~~--~------+------4------~----~ to- :;, :::i ... ~ ~ ~---+:--+'-. ~-"--+-=""""-""""'+j <1::;:' 5~----~------~------r-----~------~ ELEVATION ANGLE ~ 45° 4.0 21 .5 SLANT RANGE (n. miles)

Fig. 3.-Heavy uniform rain backscatter coefficient with bright band.

rainstorms show even less uniformity. The data in the figure were adj.usted to compensate for the known range dependence of backscatter power and volume. A noteworthy feature is the "bright-band" effect which produces a marked increase in backscatter in a narrow altitude range near the freezing level, in this case about 11 ,000 ft. The "bright-band" arises from backscatter from snowflakes which acquire a coating of water as °0~------1~0~----~20------3~0------4~0------~50 they fall through the level at which melting occurs. RANGE (n . miles) The coating of water increases the dielectric con­ stant and results in a greater reflectivity over that Fig. 2-Spectrum standard deviation for rain echoes from either rain or snow. Aside from the " bright­ - composite graph from several tests. band" effect, the variations in the backscatter coefficient show that the assumption of uniform gradient 4.0 m/ sec/ km, which serves as a useful backscatter is usually unwarranted. approximation for an arbitrary radar azimuth. Because of the range dependence of the clutter Frequency CorreIa tion of doppler spectra, techniques that distinguish tar­ gets from clutter by virtue of differences in their Precipitation Echoes doppler spectra are also range dependent. One In many radar applications it is desirable to get effect is that the clutter rejection ability of a a good estimate of the mean value of rain or snow moving target indicator generally degrades as backscatter in as short a time as possible in order range is increased. to establish a detection threshold. This can be accomplished if a large number of statistically Spatial Uniformity of Precipitation independent echoes from successive pulses can be Echoes detected and summed. However, the difficulty in doing this rapidly at a fixed frequency is that the Many techniques proposed for detection of tar­ precipitation echoes remain correlated for a num­ gets in clutter depend on the clutter being spatially ber of milliseconds. Thus, a long time would be uniform. To measure spatial uniformity, "rain required to obtain statistically independent or profiles" were taken by transmitting a pulse train uncorrelated echoes. By shifting the transmission

July - A ugust 1968 17 frequency it is possible to obtain the results in a single frequency for a long time. Frequency step­ much shorter time. The extent to which a set of ping may enhance the performance of some tech­ echoes resulting from a certain transmitted fre­ niques, such as post-detection integration, in quency is correlated with another set whose which numerous statistically independent clutter frequency has been shifted by 6./ is described by samples are desired. the frequency correlation function, p (6./) , which can be written as peN) BEAM DIAMETER 650 FEET AT GATE ~ ('i:::/f)', EACH POINT = 1000 SAMPLES 0.8 • = 3.2 ~sec . where T is the pulse duration. The transmissions are o = ' . 6~sec . assumed to be rectangular pulses occurring very 0.6 D. = 0.8 Ilsec. closely in time with the echoes arising from a c;::-

ADDRESSES Principal recent addresses made by APL staff members to groups and organizations outside the Laboratory.

A.A Westenberg, "Applications of ESR D .M. Howard, "A Torsion Wire Boston, Mass., June 2-5, 1968. to Gas Phase Kinetics, " Chemistry Damper for the DODGE Satellite," R.E. Hicks, " Substrates for Large Scale Colloquia at Harvard University, Jan. Third Aerospace Mechanisms Symposium, Arrays," N ational Electronic Packaging 4, Catholic University, Jan. 16, and Jet Propulsion Laboratory, Pasadena, Conference ( N EPCON ), New York, N.Y. Princeton University, Apr. 16, 1968. Calif., May 24, 1968. June 4-6, 1968. N. Rubinstein, V.G . Sigillito, and J.T. P.M. Bainum, W. Stuiver, and R .E. D .D. Zimmerman, " Trends in Tech­ Stadter, " Upper and Lower Bounds Harkness, "Stability and Deployment niques for Thin Film Large· Scale to Bending Frequencies of Non­ Analysis of a Tethered Orbiting Hybrid Arrays," National Electronic Uniform Shafts and Applications to Interferometer Satellite System," Packaging Conference ( N EPCON), New Missiles," 38th Symposium on Shock and Eighth European Space Symposium, York, N . Y. , June 4-6, 1968. Vibration, S1. Louis, Mo., May 1-2, Venice, Italy, May 27-29, 1968. 1968. C.J. O 'Brien, "Management News­ T.G. Bugenhagen, " Using the Enemy's E.A. Bunt, " Plasma Arc Heating for letters," Workshop, International Council Cost and Effectiveness to Weight Hypersonic Flight Simulation, " of Industrial Editors, Dallas, Texas, Threats," Spring Meeting, Operations Mechanical Engineering Dept., University June to, 1968. of Natal, Durban, South Africa, May Research Society of America, San Fran­ C.J. O'Brien, "Employee Communica­ 27-June 3, 1968. cisco, Calif., May 1-3, 1968. tions in the United States," Address W .H. Avery, "Integrated Urban-Sub­ R.E. Fischell, "Spacecraft Control to a Delegation of the Japanese Federa­ urban Transportation System," Systems," International Colloquium on tion of Employer Associations, Dallas, National Capito! and Baltimore Sections, Attitude E volution and Satellite Stabiliz a­ Texas, June 12, 1968. tion, Centre Nationale d ' Etudes Spatiales, American Institute of Aeronautics and K. Moorjani and C . Feldman, " Optical Paris, France, May 28-31, 1968. Astronautics, Applied Physics Laboratory, Constants of Amorphous Boron, " Howard County, Md., May 16, 1968. W.J. Moore, "Proposed Specification for Third International Symposium on Boron, G.L. Dugger, " Supersonic Combustion Error Signals Resulting from Com­ Warsaw, Poland, June 25-29, 1968. Ramjets," AIAA Section, Colorado State mon Mode Voltage in Passive Signal R . M. Fristrom, " Molecular Beams and University, Denver, Colo., -May 21, Handling Equipment," 74th National Chemical Problems," Physkalische 1968. Aerospace Instrumentation Symposium, Institut, Bonn, Germany, July to, 1968.

18 APL Technical Digest