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Radio Frequency Some basic rules The higher the radiated frequency... the smaller/lighter the required antenna/system the less peak power that can reasonably be radiated by the radar system the more the radiated energy takes on the propagation properties of light The lower the radiated frequency... the larger/heavier the required antenna/system the more peak power that can reasonably be radiated by the radar system the less the radiated energy takes on the propagation properties of light
Frequency Band Designations (MHz) A 0-250 B 250-500 VHF 50-300 UHF 300-1,000 C 500-1,000 L 1,000-2,000 D 1,000-2,000 S 2,000-4,000 Electronic Warfare E 2,000-3,000 F 3,000-4,000 G 4,000-6,000 H 6,000-8,000 I 8,000-10,000 Radar Designers C 4,000-8,000 X 8,000-12,000 Ku 12,000-18,000 K 18,000-27,000 Ka 27,000-40,000 J 10,000-20,000 MMW 40,000-100,000 K 20,000-40,000 L 40,000-60,000 M 60,000-100,000
Reflection Occurs when a wave meets a plane object. The wave is reflected back without distortion. Medium 1 Medium 2 Refraction Occurs when a wave encounters a medium with a different wave speed. The direction and speed of the wave is altered. Medium 1 Medium 2 Diffraction Occurs when the wave encounters an edge. The wave has the ability to turn the corner of the edge. Medium 1 Scattering Catch-all description of wave interactions that are too complex to be described as reflection, refraction or diffraction. Medium 2 Source: www.cs.ucl.ac.uk/staff/s.bhatti/teaching/d51/notes.html
Basic Pulsed Radar System Modulator Transmitter Timer Duplexer Antenna Receiver Indicator/Processor
Pulse-Doppler Radar Exciter Transmitter LO and Reference Signals Signal Processor Duplexer Antenna Display Signal Processor Receiver High PRF results in unambiguous velocity measurements and ambiguous range measurements Doppler measurements require coherency
The Pulse Train Pulse Repetition Interval (PRI) Pulse Repetition Frequency (PRF) Pulse Duration (PD)
Pulse Train PD PRI PRF = 1 PRI 1.0 second PD = normally in usec PRF = normally in pulses per second (pps) PRI = normally in usec PRI = 1 PRF PD is the length of time the illuminating power is on for each transmission PRF is the number of pulses transmitted per second PRI is the time between the start of consecutive pulses
PRF Determines radar data rate Determines Maximum Unambiguous Range (MUR) - The range at which a radar can receive an echo before the next pulse is generated Source: U.S. Navy / NAWC-WD EW Handbook Runamb(nm) = 80 PRF(kHz)
PD Determines range resolution Determines minimum range Remember: ü PD (in feet) = 1000 feet/usec ü PD (in radar feet) = 500 feet/usec
Interpulse Modulation Variation of interval between pulses within the radar s pulse train Used to eliminate MTI blind speeds, main-bang eclipsing and range ambiguities Improves anti-jamming (EP) capabilities
Intrapulse Modulation Involves the process of modulating the RF carrier of a pulsed radar during transmission (within the pulse) Pulses can vary in frequency, phase or amplitude Increases range and range resolution Example: Pulse Compression
Antenna Performance Parameters Gain: Increase/decrease in signal strength as the incoming/outgoing signal is processed by the antenna. Frequency Coverage: The range of frequencies over which the antenna can operate effectively. Bandwidth: Frequency range of the antenna in units of frequency. Polarization: Orientation of E and H waves. Beam Width: Angular coverage of the antenna in horizontal and vertical dimensions. Efficiency: Percentage of signal power transmitted/ received compared to a perfect antenna. Power Rating: The maximum power which can be fed to the antenna without damaging the antenna and/or reducing antenna performance from the desired specifications.
Antenna Gain Antenna gain is the ratio of the power per unit of solid angle radiated in a specific direction, to the power per unit of solid angle had that power been radiated using an isotropic antenna G = A 4π λ e 2 G λ = antenna gain at center of = wavelength mainlobe A e = effective area of aperture Source: Introduction to Airborne Radar (2 nd Edition) Used by permission of SciTech Publishing
Polarization Note: For further information on polarization, see Practical Communications Theory by Dave Adamy Source: U.S. Navy / NAWC-WD EW Handbook
Source: USMC / MAWTS-1 Track-While Scan Radar Beam Pattern
Sidelobe Blanking Concept RF Input (Main Beam - Primary Antenna) RF Input (Secondary Omni - antenna) Duplexer Guard Receiver Receiver B Signal Processor Comparator Signal Processor A Gate A
Coherent Sidelobe Cancellers (CSLC) Uses auxiliary receivers with antennas that have low gain and wide angle coverage Most CSLC radars use 3-6 auxiliary elements In a perfect world, one element (antenna) provides one degree of freedom and can provide one adaptive null The aux receivers operate on the same frequency as the primary radar receiver/ antenna The Howells-Applebaum method is a common CSLC implementation technique Target Return + Sidelobes - CSLC Processor Output
Space Time Adaptive Processing (STAP) STAP exploits the narrow ridge that actually forms the clutter spectrum STAP clutter filters have narrow clutter notches Slower targets fall into the receiver pass band Used for Doppler spread compensation caused by airborne platform motion/tactical maneuvering Uses a priori data to enhance the chosen STAP algorithm(s) Modern processing capabilities are allowing for the increased use (and development) of STAP The Principle of Space-Time Clutter Filtering (Derived from G. Richard Curry)
Knowledge-Based (KB) Radar Systems KB radar systems can dynamically change processing when provided with data from various sources Processing power was the inhibiter in the past (no longer the case) KB-STAP now possible Artificial intelligence (AI) methods can be used to dynamically choose the best STAP algorithm based upon programmable factors, vice a set (single) algorithm based upon a priori data AI has been used to develop an expert system to dynamically modify CFAR Use of KB techniques to perform filtering, detection, tracking and target identification is ongoing NATO has held conferences on KB radar
LPI Systems LPI systems can (roughly) be broken into the following technological/ operational approaches: Reduced ERP Power management based upon current situation requirements Reduced Sidelobes Low and Ultralow sidelobes Broadband Fast becoming common place for COTS marine and battlefield surveillance radar systems Low peak power capabilities» Some < 1 Watt Natural fall-out of waveform diversity Image sources: Lowrance /Kelvin Hughes /Thales Group
Receiver Characteristics Sensitivity Ability to receive weak signals and amplify them to usable level. It is the minimum signal strength that a receiver can receive and still operate effectively. Three components of sensitivity are thermal noise, receiver system noise figure, and signal-to-noise (S/N) ratio. Selectivity Ability of a receiver to tune to a particular station without other signals/ emissions interfering with the reception of the desired signal. Dynamic Range Range of signal levels over which the receiver can successfully operate. The low end of the dynamic range is governed by receiver sensitivity. The high end it is governed by the receiver s ability to handle overload and/or strong signals. Frequency Stability Ability to stay tuned to an incoming signal for a long period of time. 22
Antenna Bandpass Filter Crystal Detector Video Amplifier Low Sensitivity Crystal Video Receiver Antenna RF Pre- amplifier Crystal Detector Video Amplifier High Sensitivity Crystal Video Receiver
The One-way Link Equation P R = P T + G T L + G R 24
Source: U.S. Navy / NAWC-WD EW Handbook 25
Source: U.S. Navy / NAWC-WD EW Handbook 26
Source: U.S. Navy / NAWC-WD EW Handbook 27
Search Dimensions and Impact on POI Source: EW 101 (Dave Adamy)
Simplest Method of Locating Emitters: Triangulation
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