12/17/2023 0 Comments Parallax errorThese dual antenna radars require a range-dependent beam overlap, or parallax, correction to accurately determine the radar cross section of a target. 1991) and current designs for frequency-modulated continuous wave (FMCW) MMW systems ( Klugmann and Judaschke 1995) use separate transmit and receive antennas as a cost effective means to isolate the receiver from the transmitter. Alignment problems have led to the abandonment of dual-antenna pulsed W-band systems in the cloud remote sensing community, and the current generation of millimeter-wave frequency-modulated continuous wave systems must properly take these problems into consideration.Įarly pulsed millimeter-wave (MMW) radar systems ( Hobbs et al. Observations from a field experiment that include both single- and dual-antenna radar measurements are used to demonstrate these points. ![]() Moreover, parallax errors are essentially independent of range at cirrus altitudes, and it is not possible to separate parallax effects from offsets in calibration at these far ranges. For example, the minimum detectable reflectivity of a W-band radar system may be degraded by more than an order of magnitude for alignment errors on the order of the antenna half-power beamwidth. ![]() Calculations show that dual-antenna parallax errors are extremely sensitive to the alignment of the two antennas, especially for the current generation of W-band radars, which tend to use 0.91- and 1.21-m Cassegrain antennas with half-power beamwidths of typically ≤0.25°. Given precisely aligned antennas, a parallax correction to account for antenna beam overlap, which is range-dependent, must be used with the correct alignment information to produce accurate reflectivities. In order to measure radar reflectivity accurately and to avoid a general decrease in system sensitivity, these systems require precise alignment of their high-gain/narrow-beamwidth antennas, which is difficult. Note that a 0.2° error in the Penn State antenna alignment would produce the error observed on 25 April 1994ĭual-antenna radar designs avoid using a transmit/receive switch. Other points are obtained from cirrus cloud data, as described in section 3b. The first point is obtained from liquid clouds, as described in section 3a. Note that the left-hand edge of the Penn State histogram indicates a reduction in sensitivity of the Penn State radar vs the UMass radarĬomparison of UMass single-antenna and Penn State dual-antenna radar system calibrations throughout the course of the ARM SGP CART site cloud experiment in April 1994. 11, but with corrections applied for attenuation and for the combined effects of calibration offset and dual-antenna parallax. The 140-min dataset spans altitudes from 3600 to 8940 m, and the bin size is 0.25 dB Normalized histograms of uncorrected cirrus cloud reflectivity measurements for the three radar systems. 9, but with corrections applied for attenuation, calibration offset, and dual-antenna parallax The 70-min dataset spans altitudes from 200 to 1200 m, and the bin size is 1 dB Normalized histograms of uncorrected liquid water cloud reflectivity measurements for the three radar systems. Note that the height range extent of the data is limited to 1.2 km, but the fit approaches 0 dB at larger rangesĬontour plot of root-mean-square differences between measured parallax data and model for values of θ s and ϕ s centered on a best-fit solution indicated by dashed lines 6, while the dashed curve is the best fit of f( r) in Eq. The solid curve is the ratio of W-band data plotted in Fig. Ratio of dual-antenna to single-antenna W-band reflectivity vs height. Differences in UMass and Penn State W-band measurements are a result of parallax errors in the two-antenna Penn State system and a calibration difference between the two W-band systems Divergence of UMass Ka-band and W-band data results from frequency dependence of gaseous and cloud extinction rates. UMass and Penn State radar reflectivity profiles averaged between 19 UTC. The antenna diameter ( D) is 0.91 m and θ o = 0.2° The offset ϕ s = 0.0 and Δ = 1 mĭependence of the baseline parallax correction on Δ at several altitudes. The values of θ s are negative in (a) and positive in (b). The range is 900 m, θ s = 0.0, and Δ = 1 mĬomputation of dual-antenna parallax as a function of range ( r) and θ s. Beyond 150 m the spread of values for f( r) is negligibleĬomputation of dual-antenna parallax as a function of ϕ s, which is normalized by the antenna half-power beamwidth ( θ o). Influence of transmit pulse length on baseline parallax correction for a dual-antenna radar system as a function of range with θ o = 0.2° and Δ = 1 m. The angle θ s is the angular offset from zenith in the yz plane, while ϕ s is the offset in the xz plane ![]() Both antennas lie on the y axis separated by a distance Δ. Geometry for two-antenna system with nonparallel beams.
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