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4 Capabilities of Phased Array Radar In contrast to the usual scanning âdishâ antenna illuminated from a single feed point, a phased array antenna uses electronic control of the signal phase at individual array elements to produce constructive interference in the desired beam-pointing direction. Consequently, no mechanical motion, with the associated inertial effects, is necessary; this allows arbitrary steering of the radar beam on a pulse-to-pulse basis, which is typically at intervals of order 0.001 s. The beam can be steered at this rate to any direction in a typical angular range of +/- 45 degrees. This flexible beam steering is in stark contrast to mechanically scanned radars, which must scan in a systematic and angularly continuous pattern to minimize stress on pedestals, motors, gears, and other associated mechanical components. Phased array radars have been used by the military for aircraft surveillance and tracking for some three decades. Figure 4.1a shows a prototype phased array radar for weather observations - the National Weather Radar Testbed (NWRT) located in Norman, Oklahoma (Zrnic et al., 2007; Forsyth et al., 2008). This S-band radar uses a passive phased array antenna (one in which the signals sent to individual elements originate from a single transmitter) from the AN/SPY1-A radar of the Navyâs Aegis system. Other parts of Figure 4.1 illustrate several important capabilities of phased array radars which are briefly discussed below. FIGURE 4.1. Illustration of capabilities of phased array technology for radar observations of distributed targets (weather) and point targets (aircraft). (a) Photograph of National Weather Radar Testbed in Norman, Oklahoma, (b) Rapid beam steering (beam multiplexing), (c) Monopulse tracking, (d) Spaced antenna interferometry (SAI) and sidelobe clutter cancellation (SLC). 19
20 EVALUATION OF THE MPAR PLANNING PROCESS CAPABILITY FOR RAPID UPDATE (BEAM MULTIPLEXING) Conventional scanning radars must rotate at a rate slow enough to allow a number of pulses (typically 20-50 for weather observations) to be transmitted to approximately the same pointing angle. However, since the antenna is continuously rotating, each pulse is actually transmitted to a slightly different direction. Nevertheless, significant overlap in the illuminated volumes (occurring due to finite beamwidth effects) allows the use of these numerous pulses to estimate the reflectivity, radial velocity, and Doppler spectrum width signal moments with estimation errors below the desired thresholds. For WSR- 88D radars, these thresholds are typically set to 1 dB1 and 1 m/s for reflectivity and radial velocity, respectively. Due to the overlap of the resolution volumes and the time required for decorrelation of echoes from atmospheric targets to occur, the echoes from successive pulses are not independent. (In fact, signals with some correlation are required to make Doppler measurements.) To achieve the desired estimation error for reflectivity, the number of pulses required for conventional radar processing is therefore larger than what would be needed if the successive echoes were independent. Time essentially wasted while waiting to acquire the needed independent data could be employed to acquire data from other beam directions by taking advantage of the fast beam steering capability of phased array radars. Beam Multiplexing (BMX) was recently developed for this purpose (Yu et al., 2007). As illustrated in Figure 4.1b, the general idea is to transmit a small number of pulses (typically two) needed for Doppler measurements in one direction, and then steer the beam to a set of spatially diverse pointing angles. After the atmosphere effects (turbulence, shear) have led to decorrelation of the signal, the beam is directed back to the original pointing angle. Thus, numerous pairs of pulses are gathered from each pointing direction and the moment estimation is then performed. Given the independence of these pulse pairs, the estimation scheme will have significantly lower errors for the same total number of pulses, especially for cases with high signal-to-noise ratio (SNR) and small spectrum widths. As a result, it is possible to either produce moment estimates with lower errors or have a more rapid update with consistent errors. Figure 4.2 provides examples from the work of Yu et al. (2007). The upper two panels show the reflectivity and radial velocity fields from BMX while the corresponding conventional scanning results are provided in the lower panels. The scan times here are essentially the same for the two different techniques, and the qualitative improvement in the fields is remarkable. A quantitative analysis has shown a 2-4 times possible improvement in scan time (Yu et al., 2007), with a dependence on SNR and other signal characteristics. 1 Increments and uncertainties should be measured in units of dB, not dBZ. The latter represents an absolute value, specifically 1.26mm^6/(m^3).
CAPABILITIES OF PHASED ARRAY RADAR 21 FIGURE 4.2. Reflectivity (left) and radial velocity (right) fields from BMX (top panels) and conventional (lower panels) scanning. These data were taken over the same time period with the same overall scan time. Note the improved quality in the BMX results, which could be exploited to produce faster updates. Source: Yu et al., 2007. Reprinted with permission from the American Meteorological Society (AMS), copyright 2007. Phased array radars are also useful for the surveillance and tracking of aircraft. Here operation in the surveillance mode differs from the weather surveillance process mainly in the smaller number of pulses (which need not be independent) usually required for aircraft detection. That allows faster coverage of a given volume of the atmosphere for aircraft surveillance. For aircraft tracking functions, the rapid beam steering capability allows the radar time to be allocated to provide frequent updates on tracks of designated targets, according to criteria that might include such things as priority, location or speed of movement. For acquiring and tracking multiple targets simultaneously, phased array radars are highly advantageous (Skolnik, 2001). The tracking function could be interleaved with a more general aircraft surveillance function (probably requiring a slower update rate) to locate new targets that come within the purview of the radar system. In addition, monopulse radar techniques can provide higher track accuracy using either power- or phase-comparison methods. By properly segmenting a phased array antenna, a monopulse configuration can be readily implemented. Non-phased array systems require multiple antennas, or possibly a multiple-lobe feed, in order to implement monopulse tracking; multiple-target tracking would be difficult with such a system.
22 EVALUATION OF THE MPAR PLANNING PROCESS ADAPTIVE CLUTTER SUPPRESSION With the temporal sampling advantage of phased array radars also comes a major challenge. For example, the BMX technique achieves the desired accuracy of reflectivity estimation by using very short dwell times (with the two-pulse minimum) with spatially diverse pointing angles to provide the capability of averaging with independent samples. As a result of the short dwell times, however, ground clutter filtering can be problematic since conventional filters depend on differences in temporal correlation between weather and clutter signals. Such differences are difficult to determine with short dwell times. A phased array radar can exploit the spatial correlation of âauxiliary channelâ signals (ones designed for sidelobe cancellation [SLC]; see Fig. 4.1d) to reduce the effect of clutter contamination through adaptive beamforming techniques. These techniques can place antenna pattern nulls in the directions of the undesired clutter signals (Palmer et al., 1998; Cheong et al., 2006). Larger regions of clutter will be more difficult to suppress using SLC; however, for the weather radar application, clutter is often manifested in very narrow angular regions, especially given the fact that the cancellation is done on each azimuth and range gate independently. The effect of this type of clutter mitigation has been studied for the NWRT configuration using detailed numerical simulations of a tornadic environment (Le et al., in press). Figure 4.3 shows examples of simulated NWRT reflectivity and radial velocity fields for weather only (without clutter), severe clutter contamination (with clutter), and with the use of the adaptive SLC techniques. The SLC algorithm does reduce the clutter contamination for this case, and this benefit has been quantified in the work of Le et al. (2008). FIGURE 4.3. Preliminary results from the SLC algorithm of Le et al. (2008) using a NWRT simulation. The top panels provide a three-frame time sequence of imaged backscattered power (left) and radial velocity (right) uncorrupted by ground clutter. The middle panels show the results when severe ground clutter is present in every imaged pixel. Note the complete loss of the true fields. The bottom panel provides results using the proposed SLC technique with the same severe clutter field. Source: Le et al., 2008; copyright 2008 IEEE.
CAPABILITIES OF PHASED ARRAY RADAR 23 CROSSBEAM WIND ESTIMATION Conventional Doppler weather radar measures the radial velocity component along the beam pointing direction. Phased array radar offers the possibility of measuring wind components perpendicular to the pointing direction (Zhang and Doviak, 2007; 2008). The monopulse capability could facilitate estimates of the transverse components of the wind, though even without this feature the MPAR rapid-scan data would lead to better estimates of the 3D wind field using existing algorithms. The related so-called spaced antenna interferometry (SAI) technique (see Figure 4.1d) has been used for years in the wind profiling radar community (Mitra, 1949; Briggs et al., 1950), but has only recently been applied to weather radar. The technique requires multiple, independent, spatially separated receivers, which is not the norm for weather radar. But a phased array radar is ideally suited for such a technique, since the array can easily be segmented into independent receivers. The fundamental theory behind SAI for weather radar has been developed (Zhang and Doviak, 2007), and the capability has been implemented on the NWRT. ELIMINATION OF BEAM SMEARING Scanning weather radars inherently have so-called beam smearing due to the fact that samples comprising numerous pulses must be collected while the antenna is rotating (Doviak and Zrnic, 2006). As a result, the effective beamwidth of the radar is increased since the resolution volumes corresponding to the individual pulses are not coincident. Furthermore, clutter filtering becomes more difficult due to the widening of the Doppler spectrum associated with the antenna rotation. Finally, the inherent accuracy of the radar products (reflectivity, radial velocity, spectrum width, and future dual-polarization products) is limited by such beam smearing. Since beam steering with phased array radar is not accomplished by mechanical rotation, such beam smearing can be eliminated. Analyzing overlapping samples from a scanning radar does permit so-called âsuper-resolutionâ data, in which the data are output at angular intervals smaller than the antenna beamwidth. While such outputs are not truly independent, they do appear to reveal features in the weather echoes that are of finer scales than the beamwidth resolution would provide. This capability is being implemented on the Next Generation Radar (NEXRAD), and the value of the âsuper-resâ data should be considered in assessing the benefits of the phased array technology. ADAPTIVE SENSING Given the varying nature of aircraft operations and the inhomogeneous nature of weather (spatially and temporally), adaptive sensing (also known as Knowledge-Based Resource Management) holds promise for the optimization of limited radar resources (Miranda et al., 2006). For example, the WSR-88D weather radar system provides essentially similar data about all weather within its surveillance domain with a fixed update rate dependent upon the antenna scan program. A phased array radar would
24 EVALUATION OF THE MPAR PLANNING PROCESS provide an adaptive scanning capability that could be directed to devote more time (and hence provide more accurate data) to more threatening weather locations, or vary the update rate according to the degree of perceived threat. Vertical (âRHIâ) scans could readily be included in the mix. Similarly, todayâs aircraft surveillance radars typically provide rapid updates by scanning rapidly in azimuth with a beam broad in the vertical. This provides no height information; any needed height data is provided by transponders or voice communications. A phased array radar could provide the same rapid updates with a narrower beam that would provide height data directly. It could also adjust the update rate for aircraft being tracked according to the observed characteristics of the targets and the quality of the track data obtained. Currently, only limited research has been conducted on the advantages of adaptive waveforms and scanning strategies for overcoming this challenge for weather targets (Zrnic et al., 2007). One example of where adaptive sensing is already being applied to weather observations is research related to the National Science Foundation (NSF) Engineering Research CenterâCollaborative Adaptive Sensing of the Atmosphere (CASA) (e.g., Brotzge et al., 2006). Adaptive sensing for weather observations and hard-target applications is clearly an important area for making effective use of phased array radars and will be integral to any success in this area. OTHER CAPABILITIES A phased array radar would permit a variety of other enhancements over the capabilities of mechanically scanned radar. For example, digital control of the elements in an active electronically scanned array (AESA) would allow formation of different transmit and receive beam patterns, by appropriate manipulation of the amplitude and phase characteristics of each element in the array. This permits such things as use of a broad transmit beam at high elevation angles, where the targets are necessarily nearby and great sensitivity is not needed. Processing the received signals to yield simultaneous narrower receive beams would retain the inherent spatial resolution capability of the system but reduce the time required to scan that sector of the atmosphere. This capability could also be used at all elevation angles for aircraft observations in the terminal area, where again the targets of interest are nearby. At radar sites with irregular horizons due to topography or nearby obstructions, the beam elevation angles could be programmed to follow the true horizon (including even negative angles where appropriate). This could eliminate gaps in the low-angle coverage, reduce the illumination of ground clutter, and minimize scan time wasted because of beam blockage. GRACEFUL DEGRADATION Individual transmit/receiver (T/R) modules which have dimensions of order half the radar wavelength and separate, solid-state amplifiers for each element are used in active phased array radar systems. Such a phased array antenna would have the advantage of what is called graceful degradation. Since each element of the array has its
CAPABILITIES OF PHASED ARRAY RADAR 25 own T/R module and the entire array is made up of thousands of such modules, limited failures in these components would not significantly affect the performance of the radar. A theoretical example of the expected degradation in the antenna pattern for randomly located T/R failures is shown in Figure 4.4. The top panels provide three examples of failure scenarios (no failures, 20% failure, and 40% failure). The bottom panel shows the one-way antenna patterns for the three cases, with uniform weighting assumed. As the number of failed T/R modules increases, the main lobe decreases, the nulls in the pattern are filled, and the sidelobe envelope is retained. T/R module failures could also have a serious effect on polarization capabilities and should be addressed in the MPAR R&D process. Nevertheless, the general shape of the pattern is retained. Non-random locations of the failed modules, or complete failures in larger groupings or subarrays, could have a more significant effect on the pattern; the overall aperture performance of the array is retained under this simplified scenario. FIGURE 4.4. One-way phased array antenna pattern for varying number of T/R module failures. The top panels provide the locations of the operational T/R modules, with failures randomly dispersed across the array. The bottom panel gives the antenna pattern for the three different failure ratios (no failure, 20% failure, 40% failure).