Radar Fundamentals PDF

Radar Fundamentals Christo Cloete [email protected]  Outline Radar Principles Radar Applications Radar Classes Radar Equations Radar Scan Patterns Radar Basics Page 2  Basic Radar Operational Principle Radar - RAdio Det...

Radar Fundamentals Christo Cloete [email protected]  Outline Radar Principles Radar Applications Radar Classes Radar Equations Radar Scan Patterns Radar Basics Page 2  Basic Radar Operational Principle Radar - RAdio Detection And Ranging An electromagnetic wave is transmitted by the radar Some of the energy is scattered when it hits a distant target A small portion of the scattered energy, the radar echo, is collected by the radar antenna C = 3x108 m/s Slant Range vs Ground Range Page 3  Pulse vs. CW Radars with a single antenna can not send and receive signals at the same time This limitation can be solved by Transmitting short pulses and listening for echoes between pulses CW radars – Separate transmit and receive antennas Continuously changing the TX frequency so that the echoes received are at a different frequency to the signal being transmitted when they return to the radar 𝑐𝑇∆𝑓 𝑅= 2𝐵 Both methods have strong and weak points Currently pulsed radars are the most common Transmitting pulses can result in measurement ambiguities Page 4  Pulsed Radar Terminology and Concepts … PRF = 1/ PRI Pulse Repetition Interval (PRI) “Real” pulse Time between pulses For Pulse Doppler radar – determine the maximum unambiguous Doppler velocity of the target Pulse Repetition Frequency Inverse of the PRI Pulse Width (PW or Ʈ) Duration of pulse (pulse length) Determines spatial resolution of radar Page 5  CW & Pulsed Radars examples Reutech Radar Systems (RRS) Page 6  (Some) Radar Applications Civilian Aircraft, Air Traffic Control (ATC) & ground-controlled approach (bad weather landing) Radar Altimeter (Air and space craft) Collision avoidance (aircraft, ships and cars) Border control (people, land, sea and air vehicles) Weather radar Natural resources (Oil and gas exploration) Geology (Ice cover, deforestation, ground composition) Satellite & space debris tracking Space craft docking Speed trap / ball speeds in sports Military Surveillance Early Warning Target tracking Missile Defense (ranging from mortars to ICBMs) Radar guided missiles Terrain / ground mapping / surveillance Terrain following aircraft (fighters and cruise missiles) Locating land mines Page 7 Proximity fuses (AAA, mortar, missiles)  Radar Classes and Functions Radar classes Active radar (primary) - Monostatic - Bi- or multi-static Secondary radar - Erroneously referred to as Identify-Friend-or-Foe (IFF) Not ADS-B (Automatic Dependent Surveillance – Broadcast) Passive radar (Passive Coherent Location - PCL) Typical Functions Detect and track target Measure: - Target range - Direction to the target - Radial velocity between the radar and target - Size and shape of the target (Radar Cross Section - RCS) Imaging - SAR & ISAR (Synthetic Aperture Radar & Inverse SAR) Identification - Cooperative Transponder – Secondary Surveillance Radar (SSR) - Non-Cooperative Target Recognition (NCTR) Weapons Control Page 8  Primary Surveillance Radar (PSR) Page 9  Secondary Surveillance Radar (SSR) SSR Mode C provides altitude information with a resolution of 100 feet and an accuracy of ±50 feet Page 10  Automatic Dependent Surveillance – Broadcast (ADS-B) Page 11  Automatic Dependent Surveillance – Contract (ADS-C) Use SATCOM ADS-B used primarily in continental airspace ADS-C mainly in oceanic airspace ARINC = Aeronautical Radio, Incorporated Page 12 SITA = Societe Internationale de Telecommunications Aeronautiques  Transponder Multilateration (Mlat) Transponder-equipped aircraft SSR & ADS-B Determine aircraft position based on the Time Difference Of Arrival (TDOA) An active MLAT station will also interrogate the aircraft in the area. A Wide Area Multilateration (WAM) system will have mostly passive stations with some active stations to interrogate aircraft Higher update rate enables more efficient separation Page 13  Passive Radar Passive Coherent Location (PCL) Power Densities [dBW/m2] Page 14  Bistatic Radar –Principle Monostatic range resolution: 𝑐 Δrm = 2𝐵 Bistatic range resolution: 𝑐 Δrb = 2𝐵 cos(β/2) Page 15  PCL Systems Lockheed Martin’s Silent Sentry Fraunhofer CORA11 Hensholdt’s Twinvis PCL-PET Thalés; Ground Alerter 100 Leonardo’s AULOS Page 16  Radar Categories Surveillance Radar Imaging radar – Synthetic Aperture Radar (SAR) Can have a Ground Moving Target Indicator (GMTI) mode Search Radar Detects the presence of a target and determines its position (range and bearing) Usually observes the target over a period of time Can designate the target to a - Target Acquisition Radar (TAR) - Target Tracking Radar (TTR) Tracking Radar Provides the track of a target Usually receives target designation from another source (e.g. search radar) - Single Target Track (STT) – Tracks a single target to ID and/or generate a firing solution - Automatic Detection and Tracking (ADT) – Performed by a surveillance radar or a tracking radar using Track-While-Scan (TWS) and can track many targets simultaneously Multi-Function Radar Performed by an AESA radar to search, detect and track multiple targets simultaneously Can do weapon control, SAR, Comms, ES and EA are also possible Deployment Monostatic: the transmitter and receiver are co-located Bistatic / Multistatic: the transmit and receive antennas are at different locations Page 17  Active Radar Block Diagram Pseudo-Coherent (Monostatic) Radar Synchronizer - produce TRIGGER PULSES that start the transmitter, Page 18 indicator sweep circuits, and ranging circuits  Active Radar Block Diagram … Coherent (Monostatic) Radar Page 19  Active Radar Block Diagram … Active Electronic Steered Array (AESA) Waveform Generator/Signal Processor Data Processor Display Page 20  Active Radar Attributes Page 21  Radar Frequency Trade-offs Long range detection/acquisition/surveillance is better at lower frequencies Precision tracking is better at higher frequencies Antennas are smaller at higher frequencies Rain has bigger effect at higher frequencies Clutter is worse at lower frequencies Jamming potentially more effective at lower frequencies (max bandwidth) Page 22  Radar Equation and the Detection Process PtG2λ2σ Pr = (4π)3R4 Swerling Models Model Decorrelation Geometry Several independent 1 Scan-to-Scan reflectors of similar 2 Pulse-to-Pulse intensity Several independent 3 Scan-to-Scan reflectors and one 4 Pulse-to-Pulse predominates Page 23  Radar Maximum Range The basic radar equation can also be expressed in Signal-to-Noise ratio (SNR) 𝑃 𝐺 2 𝜆2 𝜎 𝑆𝑁𝑅 = 4𝜋 3 𝑘𝑇𝐵𝐹𝑅4 where - k is the Boltzmann’s constant, [1.38×10-23 W/(Hz K)] - P is the transmit power [w] - T is the receiver temperature in K - G is the antenna gain - B is the radar bandwidth in Hz - λ is the wavelength [m] - F is the radar receiver noise figure - σ is the Radar Cross Section (RCS) [m2] Then the basic radar range equation is given as 𝑃 𝐺 2 𝜆2 𝜎 𝑅4 = 4𝜋 3 𝑘𝑇𝐵𝐹 𝑆𝑁𝑅 What’s NOT in this (simplified) Radar Equation Losses Time on Target (Power is a function of time) Signal (Pulse) Integration time or Other Processing Gains Peak Power or Average Power (Duty Cycle considerations) Scintillation or Glint Page 24  Radar Cross Section (RCS) Ratio of the power re-radiated (reflected) in a given direction (𝑃𝑟 ) to the power density impinging on the target (𝑃𝑖 ) 𝑃𝑟 𝑅𝐶𝑆 = σ = [𝑚2 ] 𝑃𝑖 Relevant Parameters that influence RCS Target shape Target aspect ratio - Incident & reflected directions Target size Target material Radar RF frequency RCS = 3Dimensional Page 25  Typical RCS Values Page 26  Other effects Page 27  Clutter Clutter is anything that is not a target Rain Sea surface Mountains Trees Note - For SAR these are typically the targets Buildings Jamming Clutter “hides” targets since clutter returns are typically much larger than targets Page 28  Sensitivity Time Control (STC) STC is used to attenuate the very strong signals returned from nearby ground clutter targets in the first few range gates of a Radar receiver Without this signal attenuation, the receiver would routinely saturate due to the strong signals Receiver Gain Time Page 29  Direction Determination The angular determination of the target is determined by the directivity of the antenna By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles from the radar to the object or target can be determined The accuracy of angular measurement is determined by the directivity, which is a function of the size (frequency) of the antenna The shape of the beam influences the echo signal strength Page 30  Dwell Time and Hits per Scan Dwell Time (TD) The time that an antenna beam spends on a target is called dwell time TD The dwell time of a 2D–search radar depends predominantly on the antenna’s horizontally beam width ΘAZ and the turn speed n of the antenna (rotations per minute) Hits per Scan`(m) The hit number stands (e.g. for a search radar with a rotating antenna) for the number of the received echo pulses of a single target per antenna turn. The dwell time TD and the Pulse Repetition Time PRT determine the value of hits per scan Page 31  Radar Antenna Beams (Cosec2) Monopulse Page 32  Search Scan Patterns Circular Scan Sector Scan Helical Scan Spiral Scan Page 33  Multi-Function Scan/Track/Control Page 34  Target Tracking A target that is tracked is said to be “locked on” Key data to maintain on locked targets is Range Azimuth and elevation angle Radial velocity (Doppler radars) Angle tracking techniques are Sequential lobing Conical scan Monopulse Beam steering Range Tracking Early/Late Gate Complete Range Line Page 35  Target Tracking … Angle tracking Sequential lobing Page 36  Target Tracking … Angle tracking Conical scan Page 37  Target Tracking … Angle tracking Monopulse Page 38  Airborne Radar Modes Air-to-Air Search Air-to-Air Tracking (Single Target Tracking – STT) Air-to-Air Track-While-Scan Raster Scan Page 39  Airborne Radar Modes Ground Mapping Continuous Wave Illumination Multimode Page 40  Search Radar Coverage Two-dimensional radar often use an antenna with a cosecant square pattern Most radars are not designed to detect aircraft directly above the radar antenna. This gap is known as the cone of silence. This gap or cone of silence is the inverted cone mapped out by the rotating antenna as a result of the antenna back angle being less than 90 degrees Aircraft flying in a radar's cone of silence may, however, be detected by another, or several other radar sites a hundred or so miles away due to their overlapping coverage Page 41  Radar Operator Displays PPI = Plan Position Indicator Page 42  Radar Operator Displays … Weather Radar Displays Reflectivity image Velocity image Page 43  Radar Resolution Cell The radar’s pulse width, horizontal beam and vertical beam width form a 3D Resolution Cell (RC) The horizontal and vertical dimensions of the RC vary with range 𝑐 𝑃𝑊 The RC is the smallest volume of airspace Range resolution is given as Δ𝑅 = in which a radar cannot distinguish the 2 presence of more than one target Function of Pulse Compression Angular resolution is function of Antenna Beamwidth Page 44  Range Resolution The range resolution of a radar is its ability to distinguish between targets that are very close in range Pulses must be shorter than the time it takes for the signal to travel between targets - Otherwise, the returned pulses overlap in the receiver and the radar cannot distinguish between them Weapons-control radars should be able to distinguish between targets that are only meters apart Search radars are usually less precise and only distinguishes between targets that are hundreds of meters or even kilometers apart Page 45  Radar Unambiguous Range Maximum range at which a target can be unambiguously detected Leading edge of the backscattered pulse must be received before the transmission of the next pulse begins Maximum unambiguous range is related to the PRI of the radar as Cannot send out 𝑐 (𝑃𝑅𝐼 − 𝑃𝑊) another pulse until 𝑅𝑢𝑎𝑚𝑏 = total time window 2 (range) has passed, in which it is expected to see a return echo Staggered PRI may be used to solve the unambiguity PRI changes in a controlled way The ambiguous target does not have stable range over multiple staggered PRIs Page 46  Pulse Compression What is Pulse Compression Method to increase range resolution and signal-to-noise ratio (SNR) of radar measurements simultaneously Basis of operation: - Transmit (long) coded pulse – more energy on the target - Compress return through matched filtering Why Maximum detection range - Radar detection range is dependant on the amount of energy on target High range resolution - Accurate target position - Target separation for multiple targets - Clutter reduction Robustness against various forms of jamming Radar Equation, SNR SNR depends on energy = PTx τ Psignal PTxGTx GRx 2 - Increase peak power PTx = Expensive, makes radar more detectable Pnoise (4 )3 R 4 kTs - Increase pulse duration τ? Two broad classes of pulse compression codes Phase coded Frequency Modulated Page 47  Doppler Relative movement with a radial component between a transmitter source and a receiver will cause an apparent shift in frequency This apparent change in frequency is called the Doppler Frequency Where ft = Transmit frequency vr = radial velocity When not directly radial Pulse Doppler (PD Radar combines the advantages of both pulse and Doppler radar systems ( Range, Azimuth, Elevation and radial Velocity ) Page 48  Doppler Ambiguity The frequency spectrum of the transmitted pulsed signal is a comb-shaped line spectrum The line spacing of the spectrum is equal to the PRF or fPRF The received frequency spectrum (subject to the Doppler Effect) can only be used for unambiguous velocity measurements when the displacement of the received spectrum is smaller than the line spacing in the spectrum i.e. the Doppler frequency must be lower than the PRF Unambiguous velocity or the PRF must be higher than transmitted frequency Page 49  Range/Doppler Maps Page 50  PRF and Range & Doppler Ambiguity PRF Categories depends on the operational situation Low PRF is one for which the maximum required operational range falls within the first range zone – unambiguous in range High PRF is one for which all significant targets are unambiguous in velocity Medium PRF is one for which both range and Doppler frequency are ambiguous Example Typical PRFs Low PRF < 3 kHz Medium PRF = 3 kHz to 30 kHz High PRF > 30 kHz Page 51  Duty Cycle Ratio of the PW and the PRI 𝜏 𝑃𝑊 𝐷𝑢𝑡𝑦𝐶𝑦𝑐𝑙𝑒 = = = 𝑃𝑊 𝑃𝑅𝐹 𝑃𝑅𝐼 𝑃𝑅𝐼 The following equations are also widely used 𝑃𝑎𝑣𝑔 𝐷𝑢𝑡𝑦𝐶𝑦𝑐𝑙𝑒 = 𝑃𝑝𝑒𝑎𝑘 𝑃𝑊 𝑃𝑎𝑣𝑔 = 𝑃𝑝𝑒𝑎𝑘 𝐷𝑢𝑡𝑦𝐶𝑦𝑐𝑙𝑒 = 𝑃𝑝𝑒𝑎𝑘 𝑃𝑅𝐼 Duty Cycle of a CW radar = 100% Page 52  Detection False Alarm Erroneous radar target detection decision caused by noise or other interfering signals exceeding the detection threshold False Alarm Rate (FAR) 𝐹𝑎𝑙𝑠𝑒 𝑡𝑎𝑟𝑔𝑒𝑡𝑠 𝑝𝑒𝑟 𝑃𝑅𝑇 𝐹𝐴𝑅 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑟𝑎𝑛𝑔𝑒 𝑐𝑒𝑙𝑙𝑠 Target detection - Compares the signal to a threshold Threshold - function of probability of detection and probability of false alarm Constant false-alarm rate (CFAR) schemes vary the detection threshold as a function of the sensed environment Detection is needed for a given cell (Cell Under test - CUT) Noise power is estimated from neighbouring cells Detection threshold, T, is given by T=αPn - where Pn is the noise power estimate - α is a scaling factor called the threshold factor With the appropriate threshold factor, α, the resulting probability of false alarm can be kept at a constant, hence the name CFAR Page 53  Detection CFAR can be Range Acting Doppler Acting Range & Doppler acting Different type of CFAR Cell-Averaging CFAR (CA-CFAR) Greatest-Of CFAR (GO-CFAR) Order Statistics (OS-CFAR) Cell-Averaging Greatest-Of (CAGO-CFAR) Least-Of CFAR (LO-CFAR) Cell-Averaging Smallest Of (CASO-CFAR) Cell Averaging CFAR Most widely used CFAR detector Noise samples are extracted from both leading and lagging cells (called training cells) around the CUT. The noise estimate can be computed as N is the number of training cells and xm is the sample in each training cell Page 54  Radar Classification by Waveform (Coherent) Pulse Doppler Page 55  Any Easy Questions? Page 56  Assignment Page 57