The electrical engineering handbook CH041

The electrical engineering handbook

Belcher, M.L., Nessmith, J.T., Wiltse, J.C. “Radar” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 © 2000 by CRC Press LLC 41 Radar 41.1Pulse Radar Overview of Pulsed Radars•Critical Subsystem Design and Technology•Radar Performance Prediction•Radar Waveforms•Detection and Search•Estimation and Tracking 41.2Continuous Wave Radar CW Doppler Radar•FM/CW Radar•Interrupted Frequency- Modulated CW (IFM/CW)•Applications•Summary Comments 41.1 Pulse Radar Melvin L. Belcher and Josh T. Nessmith Overview of Pulsed Radars Basic Concept of Pulse Radar Operation The basic operation of a pulse radar is depicted in Fig. 41.1. The radar transmits a pulse of RF energy and then receives returns (reflections) from desired and undesired targets. Desired targets may include space, airborne, and sea- and/or surface-based vehicles. They can also include the earth’s surface and the atmosphere, depending on the application. Undesired targets are termed clutter. Clutter sources include the ground, natural and man- made objects, sea, atmospheric phenomena, and birds. Short-range/low-altitude radar operation is often con- strained by clutter since the multitude of undesired returns masks returns from targets of interest such as aircraft. The range, azimuth angle, elevation angle, and range rate can be directly measured from a return to estimate target position and velocity. Signature data can be extracted by measuring the amplitude, phase, and polarization of the return. Pulse radar affords a great deal of design and operational flexibility. Pulse duration and pulse rate can be tailored to specific applications to provide optimal performance. Modern computer-controlled multiple-func- tion radars exploit this capability by choosing the best waveform from a repertoire for a given operational mode and interference environment automatically. Radar Applications The breadth of pulse radar applications is summarized in Table 41.1. Radar applications can be grouped into search, track, and signature measurement applications. Search radars are used for tracking but have relatively large range and angle errors. The search functions favor broad beam-widths and low bandwidths in order to efficiently search over a large spatial volume. As indicated in Table 41.1, search is preferably performed in the lower frequency bands. The antenna pattern is narrow in azimuth and has a cosecant pattern in elevation to provide acceptable coverage from the horizon to the zenith. Tracking radars are typically characterized by a narrow beamwidth and moderate bandwidth in order to provide accurate range and angle measurements on a given target. The antenna pattern is a pencil beam with approximately the same dimensions in azimuth and elevation. Track is usually conducted at the higher frequency bands in order to minimize the beamwidth for a given antenna aperture area. After each return from a target Melvin L. Belcher Georgia Tech Research Institute Josh T. Nessmith Georgia Tech Research Institute James C. Wiltse Georgia Tech Research Institute © 2000 by CRC Press LLC is received, the range and angle are measured and input into a track filter. Track filtering smooths the data to refine the estimate of target position and velocity. It also predicts the target’s flight path to provide range gating and antenna pointing control to the radar system. Signature measurement applications include remote sensing of the environment as well as the measurement of target characteristics. In some applications, synthetic aperture radar (SAR) imaging is conducted from aircraft or satellites to characterize land usage over broad areas. Moving targets that present changing aspect to the radar can be imaged from airborne or ground-based radars via inverse synthetic aperture radar (ISAR) tech- niques. As defined in the subsection “Resolution and Accuracy,” cross-range resolution improves with increasing antenna extent. SAR/ISAR effectively substitutes an extended observation interval over which coherent returns are collected from different target aspect angles for a large antenna structure that would not be physically realizable in many instances. In general, characterization performance improves with increasing frequency because of the associated improvement in range, range rate, and cross-range resolution. However, phenomenological characterization to support environmental remote sensing may require data collected across a broad swath of frequencies. A multiple-function phased array radar generally integrates these functions to some degree. Its design is usually driven by the track function. Its operational frequency is generally a compromise between the lower FIGURE 41.1 Pulse radar. TABLE 41.1 Radar Bands Band Frequency Range Principal Applications HF 3–30 MHz Over-the-horizon radar VHF 30–300 MHz Long-range search UHF 300–1000 MHz Long-range surveillance L 1000–2000 MHz Long-range surveillance S 2000–4000 MHz Surveillance Long-range weather characterization Terminal air traffic control C 4000–8000 MHz Fire control Instrumentation tracking X 8–12 GHz Fire control Air-to-air missile seeker Marine radar Airborne weather characterization Ku 12–18 GHz Short-range fire control Remote sensing Ka 27–40 GHz Remote sensing Weapon guidance V 40–75 GHz Remote sensing Weapon guidance W 75–110 GHz Remote sensing Weapon guidance © 2000 by CRC Press LLC frequency of the search radar and the higher frequency desired for the tracking radar. The degree of signature measurement implemented to support such functions as noncooperative target identification depends on the resolution capability of the radar as well as the operational user requirements. Multiple-function radar design represents a compromise among these different requirements. However, implementation constraints, multiple- target handling requirements, and reaction time requirements often dictate the use of phased array radar systems integrating search, track, and characterization functions. Critical Subsystem Design and Technology The major subsystems making up a pulse radar system are depicted in Fig. 41.2. The associated interaction between function and technology is summarized in this subsection. Antenna The radar antenna function is to first provide spatial directivity to the transmitted EM wave and then to intercept the scattering of that wave from a target. Most radar antennas may be categorized as mechanically scanning or electronically scanning. Mechanically scanned reflector antennas are used in applications where rapid beam scanning is not required. Electronic scanning antennas include phased arrays and frequency scanned antennas. Phased array beams can be steered to any point in their field-of-view, typically within 10 to 100 m s, depending on the latency of the beam steering subsystem and the switching time of the phase shifters. Phased arrays are desirable in multiple function radars since they can interleave search operations with multiple target tracks. There is a Fourier transform relationship between the antenna illumination function and the far-field antenna pattern. Hence, tapering the illumination to concentrate power near the center of the antenna suppresses sidelobes while reducing the effective antenna aperture area. The phase and amplitude control of the antenna illumination determines the achievable sidelobe suppression and angle measurement accuracy. Perturbations in the illumination due to the mechanical and electrical sources distort the illumination function and constrain performance in these areas. Mechanical illumination error sources include antenna shape deformation due to sag and thermal effects as well as manufacturing defects. Electrical illumination error is of particular concern in phased arrays where sources include beam steering computational error and phase shifter quantization. Control of both the mechanical and electrical perturbation errors is the key to both low sidelobes and highly accurate angle measurements. Control denotes that either tolerances are closely held and maintained or that there must be some means for monitoring and correction. Phased arrays are attractive for low sidelobe applications since they can provide element-level phase and amplitude control. FIGURE 41.2 Radar system architecture. © 2000 by CRC Press LLC Transmitter The transmitter function is to amplify waveforms to a power level sufficient for target detection and estimation. There is a general trend away from tube-based transmitters toward solid-state transmitters. In particular, solid- state transmit/receive modules appear attractive for constructing phased array radar systems. In this case, each radiating element is driven by a module that contains a solid-state transmitter, phase shifter, low-noise amplifier, and associated control components. Active arrays built from such modules appear to offer significant reliability advantages over radar systems driven from a single transmitter. However, microwave tube technology continues to offer substantial advantages in power output over solid-state technology. Transmitter technologies are summarized in Table 41.2. Receiver and Exciter This subsystem contains the precision timing and frequency reference source or sources used to derive the master oscillator and local oscillator reference frequencies. These reference frequencies are used to downconvert received signals in a multiple-stage superheterodyne architecture to accommodate signal amplification and interference rejection. The receiver front end is typically protected from overload during transmission through the combination of a circulator and a transmit/receive switch. The exciter generates the waveforms for subsequent transmission. As in signal processing, the trend is toward programmable digital signal synthesis because of the associated flexibility and performance stability. Signal and Data Processing Digital processing is generally divided between two processing subsystems, i.e., signals and data, according to the algorithm structure and throughput demands. Signal processing includes pulse compression, Doppler filtering, and detection threshold estimation and testing. Data processing includes track filtering, user interface support, and such specialized functions as electronic counter-counter measures (ECCM) and built-in test (BIT), as well as the resource management process required to control the radar system. The signal processor is often optimized to perform the repetitive complex multiply-and-add operations associated with the fast Fourier transform (FFT). FFT processing is used for implementing pulse compression via fast convolution and for Doppler filtering. Fast convolution consists of taking the FFT of the digitized receiver output, multiplying it by the stored FFT of the desired filter function, and then taking the inverse FFT TABLE 41.2 Pulse Radar Transmitter Technology Mode of Maximum Demonstrated Peak/ Typical Typical Technology Operation Frequency (GHz) Average Power (kW) Gain Bandwidth Thermionic Magnetron Oscillator 95 1 MW/500 W @ X-band n/a Fixed–10% Helix traveling Amplifier 95 4 kW/400 W @ X-band 40–60 dB Octave/multioctave wave tube (TWT) Ring-loop TWT Amplifier 18 8 kW/200 W @ X-band 40–60 dB 5–15% Coupled-cavity TWT Amplifier 95 100 kW/25 kW @ X-band 40–60 dB 5–15% Extended interaction Oscillator 220 1 kW/10 W @ 95 GHz n/a 0.2% (elec.) oscillator (EIO) 4% (mech.) Extended interaction Klystron (EIK) Amplifier 140 1 kW/10 W @ 95 GHz 40–50 dB 0.5–1% Klystron Amplifier 35 50 kW/5 kW @ X-band 30–60 dB 0.1–2% (inst.) 1–10% (mech.) Crossed-field Amplifier 18 500 kW/1 kW @ X-band 10–20 dB 5–15% amplifier (CFA) Solid state Silicon BJT Amplifier 5 300 W/30 W @1 GHz 5–10 dB 10–25% GaAs FET Amplifier 30 15 W/5 W @ X-band 5–10 dB 5–20% Impatt diode Oscillator 140 30 W/10 W @ X-band n/a Fixed–5% Source: Tracy V. Wallace, Georgia Tech Research Institute, Atlanta, Georgia. © 2000 by CRC Press LLC of the resulting product. Fast convolution results in significant computational saving over performing the time- domain convolution of returns with the filter function corresponding to the matched filter. The signal processor output can be characterized in terms of range gates and Doppler filters corresponding approximately to the range and Doppler resolution, respectively. In contrast, the radar data processor typically consists of a general-purpose computer with a real-time operating system. Fielded radar data processors range from microcomputers to mainframe computers, depend- ing on the requirements of the radar system. Data processor software and hardware requirements are signifi- cantly mitigated by off loading timing and control functions to specialized hardware. This timing and control subsystem typically functions as the two-way interface between the data processor and the other radar sub- systems. The increasing inclusion of BIT (built-in-test) and built-in calibration capability in timing and control subsystem designs promises to result in significant improvement in fielded system performance. Radar Performance Prediction Radar Line-of-Sight With the exception of over-the-horizon (OTH) radar systems, which exploit either sky-wave bounce or ground- wave propagation modes and sporadic ducting effects at higher frequencies, surface and airborne platform radar operation is limited to the refraction-constrained line of sight. Atmospheric refraction effects can be closely approximated by setting the earth’s radius to 4/3 its nominal value in estimating horizon-limited range. The resulting line-of-sight range is depicted in Fig. 41.3 for a surface-based radar, an airborne surveillance radar, and a space-based radar. FIGURE 41.3 Maximum line-of-sight range for surface-based radar, an airborne surveillance radar, and a space-based radar. © 2000 by CRC Press LLC As evident in the plot, airborne and space-based surveillance radar systems offer significant advantages in the detection of low-altitude targets that would otherwise be masked by earth curvature and terrain features from surface-based radars. However, efficient clutter rejection techniques must be used in order to detect targets since surface clutter returns will be present at almost all ranges of interest. Radar Range Equation The radar range equation is commonly used to estimate radar system performance, given that line-of-sight conditions are satisfied. This formulation essentially computes the signal-to-noise ratio ( S / N ) at the output of the radar signal processor. In turn, S / N is used to provide estimates of radar detection and position measurement performance as described in the subsections “Detection and Search” and “Estimation and Tracking.” S / N can be calculated in terms of the number of pulses coherently integrated over a single coherent processing interval (CPI) using the radar range equation such that (41.1) where P is peak transmitter power output, D is directivity of the transmit antenna, A is effective aperture area of the receive antenna in meters squared, T p is pulse duration, s is radar cross section in square meters, N p is the number of coherently integrated pulses within the coherent processing interval, R is range to target in meters, L t is system ohmic and nonohmic transmit losses, L rn is system nonohmic receive losses, L sp is signal processing losses, k is Boltzmann’s constant (1.38 ´ 10 –23 K), and T s is system noise temperature, including receive ohmic losses (kelvin). At X-band and above it may also be necessary to include propagation loss due to atmospheric absorption [Blake, 1986]. This form of the radar range equation is applicable to radar systems using pulse compression or pulse Doppler waveforms as well as the unmodulated single-pulse case. In many applications, average power is a better measure of system performance than peak power since it indicates the S / N improvement achievable with pulse integration over a given interval of time. Hence, the radar range equation can be modified such that (41.2) where P a is average transmitter power and T c is coherent processing interval (CPI). The portion of time over which the transmitter is in operation is referred to as the radar duty cycle. The average transmitter power is the product of duty cycle and peak transmitter power. Duty cycle ranges from less than 1% for typical noncoherent pulse radars to somewhat less than 50% for high pulse repetition frequency (PRF) pulse Doppler radar systems. High PRF systems are sometimes referred to as interrupted continuous wave (ICW) systems because they operate essentially as a CW radar system with transmitter and receiver alternately turned on and off. The CPI is the period over which returns are collected for coherent processing functions such as integration and Doppler filtering. The CPI can be estimated as the product of the number of coherently integrated pulses and the interval between pulses. Noncoherent integration is less efficient and alters the statistical character of the signal and interference. Antenna Directivity and Aperture Area The directivity of the antenna is (41.3) SN PDATN RLLLkT pp trnsps / = s p()4 24 SN PDAT RLLLkT ac trnsps / = s p()4 24 D A = 4 2 ph l © 2000 by CRC Press LLC where h is aperture efficiency and l is radar carrier wavelength. Aperture inefficiency is due to the antenna illumination factor. The common form of the radar range equation uses power gain rather than directivity. Antenna gain is equal to the directivity divided by the antenna losses. In the design and analysis of modern radars, directivity is a more convenient measure of performance because it permits designs with distributed active elements, such as solid-state phased arrays, to be assessed to permit direct comparison with passive antenna systems. Beamwidth and directivity are inversely related; a highly directive antenna will have a narrow beamwidth. For typical design parameters, (41.4) where q az and q el are the radar azimuth and elevation beamwidths, respectively, in milliradians. Radar Cross Section In practice, the radar cross section (RCS) of a realistic target must be considered a random variable with an associated correlation interval. Targets are composed of multiple interacting scatters so that the composite return varies in magnitude with the constructive and destructive interference of the contributing returns. The target RCS is typically estimated as the mean or median of the target RCS distribution. The associated correlation interval indicates the rate at which the target RCS varies over time. RCS fluctuation degrades target detection performance at moderate to high probability of detection. The median RCS of typical targets is given in Table 41.3. The composite RCS measured by a radar system may be composed of multiple individual targets in the case of closely spaced targets such as a bird flock. Loss and System Temperature Estimation Sources of S / N loss include ohmic and nonohmic (mismatch) loss in the antenna and other radio frequency components, propagation effects, signal processing deviations from matched filter operation, detection thresh- olding, and search losses. Scan loss in phased array radars is due to the combined effects of the decrease in projected antenna area and element mismatch with increasing scan angle. TABLE 41.3 Median Target RCS (m 2 ) Carrier Frequency, GHz 1–2 3 5 10 17 Aircraft (nose/tail avg.) Small propeller 2 3 2.5 Small jet (Lear) 1 1.5 1 1.2 T38-twin jet, F5 2 2–3 2 1–2/6 T39-Sabreliner 2.5 10/8 9 F4, large fighter 5–8/5 4–20/10 4 4 737, DC9, MD80 10 10 10 10 10 727, 707, DC8-type 22–40/15 40 30 30 DC-10-type, 747 70 70 70 70 Ryan drone 2/1 Standing man (180 lb) 0.3 0.5 0.6 0.7 0.7 Automobiles 100 100 100 100 100 Ships-incoming ( ´ 10 4 m 2 ) 4K tons 1.6 2.3 3.0 4.0 5.4 16K tons 13 18 24 32 43 Birds Sea birds 0.002 0.001–0.004 0.004 Sparrow, starling, etc. 0.001 0.001 0.001 0.001 0.001 Slash marks indicate different set. D azel = 10 7 qq © 2000 by CRC Press LLC Search operations impose additional losses due to target position uncertainty. Because the target position is unknown before detection, the beam, range gate, and Doppler filter will not be centered on the target return. Hence, straddling loss will occur as the target effectively straddles adjacent resolution cells in range and Doppler. Beamshape loss is a consequence of the radar beam not being pointed directly at the target so that there is a loss in both transmit and receive antenna gain. In addition, detection threshold loss associated with radar system adaptation to interference must be included [Nathanson, 1991]). System noise temperature estimation corresponds to assessing the system thermal noise floor referenced to the antenna output. Assuming the receiver hardware is at ambient temperature, the system noise temperature can be estimated as T s = T a + 290 ( L ro F – 1) (41.5) where T a is the antenna noise temperature, L ro is receive ohmic losses, and F is the receiver noise figure. In phased array radars, the thermodynamic temperature of the antenna receive beam-former may be signif- icantly higher than ambient, so a more complete analysis is required. The antenna noise temperature is determined by the external noise received by the antenna from solar, atmospheric, earth surface, and other sources. Table 41.4 provides typical loss and noise temperature budgets for several major radar classes. In general, loss increases with the complexity of the radar hardware between the transmitter/receiver and the antenna radiator. Reflector antennas and active phased arrays impose relatively low loss, while passive array antennas impose relatively high loss. Resolution and Accuracy The fundamental resolution capabilities of a radar system are summarized in Table 41.5. In general, there is a trade-off between mainlobe resolution corresponding to the nominal range, Doppler, and angle resolution, and effective dynamic range corresponding to suppression of sidelobe components. This is evident in the use of weighting to suppress Doppler sidebands and angle sidelobes at the expense of broadening the mainlobe and S / N loss. Cross range denotes either of the two dimensions orthogonal to the radar line of sight. Cross-range resolution in real-aperture antenna systems is closely approximated by the product of target range and radar beamwidth in radians. Attainment of the nominal ISAR/SAR cross-range resolution generally requires complex signal processing to generate a focused image, including correction for scatterer change in range over the CPI. The best accuracy performance occurs for the case of thermal noise-limited error. The resulting accuracy is the resolution of the radar divided by the square root of the S / N and an appropriate monopulse or interpolation factor. In this formulation, the single-pulse S / N has been multiplied by the number of pulses integrated within the CPI as indicated in Eqs. (41.1) and (41.2). TABLE 41.4 Typical Microwave Loss and System Temperature Budgets Mechanically Scanned Electronically Scanned Reflector Slotted Solid-State Antenna Array Phased Array Nominal losses Transmit loss, L t (dB) 1 1.5 0.5 Nonohmic receiver loss, L r (dB) 0.5 0.5 0.1 Signal processing loss, L sp (dB) 1.4 1.4 1.4 Scan loss (dB) N/A N/A 30 log [cos (scan angle)] Search losses, L DS Beam shape (dB) 3 3 3 Range gate straddle (dB) 0.5 0.5 0.5 Doppler filter straddle (dB) 0.5 0.5 0.5 Detection thresholding (dB) 1 1 1 System noise temperature (kelvin) 500 600 400 © 2000 by CRC Press LLC In practice, accuracy is also constrained by environmental effects, target characteristics, and instrumentation error as well as the available S / N . Environmental effects include multipath and refraction. Target glint is characterized by an apparent wandering of the target position because of coherent interference effects associated with the composite return from the individual scattering centers on the target. Instrumentation error is minimized with alignment and calibration but may significantly constrain track filter performance as a result of the relatively long correlation interval of some error sources. Radar Range Equation for Search and Track The radar range equation can be modified to directly address performance in the two primary radar missions: search and track. Search performance is basically determined by the capability of the radar system to detect a target of specific RCS at a given maximum detection range while scanning a given solid angle extent within a specified period of time. S / N can be set equal to the minimum value required for a given detection performance, S / N Έ r , while R can be set to the maximum required target detection range, R max . Manipulation of the radar range equation results in the following expression: (41.6) where W is the solid angle over which search must be performed (steradians), T fs is the time allowed to search W by operational requirements, and L os is the composite incremental loss associated with search. The left-hand side of the equation contains radar design parameters, while the right-hand side is determined by target characteristics and operational requirements. The right-hand side of the equation is evaluated to determine radar requirements. The left-hand side of the equation is evaluated to determine if the radar design meets the requirements. The track radar range equation is conditioned on noise-limited angle accuracy as this measure stresses radar capabilities significantly more than range accuracy in almost all cases of interest. The operational requirement is to maintain a given data rate track providing a specified single-measurement angle accuracy for a given number of targets with specified RCS and range. Antenna beamwidth, which is proportional to the radar carrier wave- length divided by antenna extent, impacts track performance since the degree of S/N required for a given measurement accuracy decreases as the beamwidth decreases. Track performance requirements can be bounded as TABLE 41.5 Resolution and Accuracy Dimension Nominal Resolution Noise-Limited Accuracy Angle Range Doppler SAR/ISAR a, taper broadening factor, typically ranging from 0.89 (unweighted) to 1.3 (Hamming); d, antenna extent in azi- muth/elevation; B, waveform bandwidth; K m , monopulse slope factor, typically on the order of 1.5; K i , interpolation factor, typ- ically on the order of 1.8; Dq, line-of-sight rotation of target relative to radar over CPI. al d ------- al dK m 2SN¤ ------------------------------ aC 2B -------- aC 2BK i 2SN¤ --------------------------------- a CPI ---------- a CPIK i 2SN¤ ------------------------------------ al 2Dq ---------- al 2DqK i 2SN¤ ------------------------------------ PA LLL L T S N R T a trsposs r fs ³ æ è ç ö ø ÷ × max k 4 16 W s . range over the CPI. The best accuracy performance occurs for the case of thermal noise-limited error. The resulting accuracy is the resolution of the radar. ranged. The received difference pattern would produce a null return, indicating the target was at the center of the beam. If the target were off the null, the