DSTO. DSTO High Frequency Over-the-Horizon Radar. IEEE Lecture Atlanta, GA, May 2012

Transcription

1 High Frequency Over-the-Horizon Radar IEEE Lecture Atlanta, GA, May 2012 Dr. Giuseppe A. Fabrizio Senior Research Scientist, High Frequency Radar Branch, Intelligence, Surveillance and Reconnaissance Division, DSTO Australia. D E P A R T M E N T O F D E F E N C E DEFENCE SCIENCE & TECHNOLOGY ORGANISATION DSTO DEFENCE: PROTECTING AUSTRALIA 1 Commonwealth of Australia 2010

2 Presentation Outline 1. Fundamental Principles 2. Sky-Wave OTH Radar 3. HF Radar Sub-Systems 4. HF Signal Environment 5. Conventional Processing 2

3 1. Fundamental Principles Section Outline: Surveillance Radar & Frequency Bands Interest in the High Frequency Region Essential OTH Radar Concepts 3

4 Surveillance Radar & Frequency Bands Radar Frequencies The choice of frequency band has a pronounced influence on the characteristics and performance of a radar system. Ionospheric Effects Meteorological Effects Over-the-Horizon Radar Microwave Radar ( GHz) Band HF VHF UHF L S C X Ku K Ka Frequency 3-30 MHz MHz MHz 1-2 GHz 2-4 GHz 4-8 GHz 9-12 GHz GHz GHz GHz Wavelength m 1-10 m m ~20 cm ~10 cm ~5 cm ~3 cm ~2 cm ~1.4 cm ~0.8 cm Physically Larger Range Coverage Physically Smaller Resolution/Accuracy 4

5 Target Types Surveillance Radar & Frequency Bands Focus on radars used primarily for surveillance of man-made targets. Conventional & OTH surveillance radars have many common target types Example: Surveillance radar targets & mission priority: Large Aircraft Fighter-Sized & Helicopters Missiles Aircraft (Primary Mission) Large Ships Destroyers & Patrol Boats Go-Fast Boats Ships (Secondary Mission) Remote sensing radars (e.g. sea-state mapping) target natural scatterers. Remote sensing applications of OTH radar not explicitly considered here 5

6 Surveillance Radar & Frequency Bands Surveillance Functions Conventional & OTH surveillance radars share the main functions. Target detection, localization & tracking Example: Main surveillance radar functions: 1) Target detection-estimation: Clutter Noise Jamming Discriminate target echoes against disturbance signals and estimate target parameters of interest to infer target geographical position and velocity. Range Direction Radial Velocity Coordinate Registration 2) Target track-while-scan: Establish, maintain and display detected target tracks while continuing to search the coverage area for new targets. 6

7 Surveillance Radar & Frequency Bands Early HF Radar British Chain Home radar, the first used for air-defence in wartime [2]. HF technology was only available means to generate sufficient power (1935) Radar designed for line-of-sight ranges, not for over-the-horizon detection. Echoes from very long distances constituted interference for radar operators Later during world war II, microwave radars were successfully employed. Regarded as the most competitive frequency band for line-of-sight applications Example Chain Home radar station (East UK coast). Frequency MHz Robert Watson-Watt ( ). 7

8 Surveillance Radar & Frequency Bands Conventional Radar Great majority of line-of-sight radars implemented at microwave frequency. main technical reasons (at a glance) LINE-OF-SIGHT PROPAGATION-PATH TARGET RCS (OFTEN IN OPTICAL REGION) (TARGET LOCALIZATION ACCURACY) PHYSICALLY SMALL HIGH GAIN ANTENNAS (EASIER TO SATISFY SITE CONSTRAINTS) LOW AMBIENT NOISE LEVEL (INTERNAL NOISE LIMITED) CONVENTIONAL RADAR GREATER USEABLE BANDWIDTHS (FINE RANGE RESOLUTION) POTENTIAL FOR CLUTTER REDUCTION (e.g. UP-LOOKING GEOMETRY) 8

9 Surveillance Radar & Frequency Bands Line-of-Sight Coverage Microwave radar coverage is mostly restricted to line-of-sight (LOS). Propagation is shadowed by mountains & limited by the Earth s curvature Earth s Surface Low-flying targets escape early detection Range increased by raising radar platform (or by anomalous propagation). Doubling range requires quadrupling the platform height (e.g. airborne radar) Atmospheric ducting is not predictable (and may also degrade performance) 9

10 Beyond Line-of-Sight Interest in the High Frequency Region High frequency signals (3-30 MHz) propagate beyond the line-of-sight. 1. A sky-wave mode involving reflection(s) from the ionosphere 2. A surface-wave mode guided by a conductive sea-surface Different physical mechanisms that are essentially unique to the HF band. Exploited by OTH radar & short-wave communicators since G. Marconi (1901) MICROWAVE IONOSPHERE Guglielmo Marconi TX HF Surface-Wave HF Sky-Wave EARTH S S SURFACE 10

11 Interest in the High Frequency Region About the Ionosphere Ionized gas (plasma) formed by the Sun s extreme UV radiation [3]. Electron density distribution with height exhibits local maxima (regions) No direct radiation at night but plasma in ionosphere never decays fully Courtesy of James Clerk Maxwell describes theory of electromagnetic radiation and predicts existence of radiowaves 1887 Heinrich Hertz proves existence of radiowaves 1895 Guglielmo Marconi demonstrates wireless (radio) communication in Bologna, Italy 1899 Marconi transmits radio signal across English Channel Dec. 12, Marconi transmits radio signal across Atlantic Ocean from Cornwall, England to St. John's, Newfoundland Oliver Heaviside; Arthur Kennelly propose existence of conducting layer in upper atmosphere 1909 Marconi awarded Nobel Prize 1924 Edward Appleton and others develop ionosonde & begin ground-based soundings; prove existence of ionosphere 1925 Appleton discovers second layer (the F region) Robert Watson-Watt (later developer of radar) coins word "ionosphere" Sydney Chapman describes theory for formation of ionosphere 1947 Appleton awarded Nobel Prize 1958 Incoherent Scatter Radar developed 11

12 Useful Coverage Interest in the High Frequency Region Ray-tracing through a model ionosphere using simulation software. Escape rays at high elevations produce a skip zone Earth not illuminated Reflected rays at lower elevation useful range extent beyond the skip-zone 600 km Altitude (km) 300 km Single 200 Frequency 100 f c φ 500 CONCEPTUAL REPRESENTATION ESCAPE RAYS f > c Range (km) f p sin φ f c f p sin φ REFLECTED RAYS Escape Rays 1 st Hop 2 st Hop Backscattered Power (Two-way path) Skip Zone Radar Footprint Useful Coverage Leading Edge Focusing Surface Clutter Target Range (km) 12

13 Essential OTH Radar Concepts Sky-wave OTH Radar Sky-wave OTH radar exploits oblique reflection over a two-way path. Cost-effective early-warning (long-distance) & wide area surveillance Monitor strategic areas where it is not possible to install conventional radar Concept of Operation Receive Ionosphere Transmit Transmit Skip-Zone Limit Potential Radar Coverage Dwell Time (CPI) Radar Footprint Resolution Cells Radar Footprint Higher Frequency TX & RX Beam Steering 13

14 Radar Equation Essential OTH Radar Concepts Noise-limited radar equation for OTH and conventional radar systems. Same form but range increases by order of magnitude (for all target altitudes) Target echo received by OTH radar experiences additional 40dB spreading loss Transmit Power (Average) Transmit Antenna Gain Target Cross Section Receive Antenna Gain Effective Integration Time Operating Wavelength Output Signal-to-Noise Ratio S N = P N ave o G L TA σ F P G G T λ σ F 2 t e p ave t r = ( 4π ) R N o L ( 4π ) R 4 p Propagation Factor External Noise Power per unit bandwidth Losses (Path and System) Slant Range (Radar-to-Target) Radar Type Range Coverage (km) Surface Coverage (sq km) Sky-wave OTH Radar Millions Ground-Based Microwave Tens of thousands 14

15 Essential OTH Radar Concepts Resolution & Accuracy OTH radar range resolution limited by useable bandwidths in HF spectrum. 1. User-congestion in HF band limits availability of clear frequency channels 2. Frequency dispersion in ionosphere places a limit on coherence bandwidth Antenna gain & beamwidth are dependent on aperture size in wavelengths. HF radar wavelengths are three orders of magnitude greater than microwave Antenna apertures 3 km long needed for beamwidths in the order of 1 degree Spatial resolution comparison order of magnitude. Radar Type Useable Bandwidths Range Resolution Aperture Size Antenna Beamwidth OTH Radar 5-50 khz 3-30 km 3000 m deg. Microwave khz m 6 m deg. Target location accuracy determined by propagation-path knowledge Propagation through ionosphere is much more uncertain than line-of-sight Target location accuracy for OTH radar may at times be up to km 15

16 Target Scattering Essential OTH Radar Concepts Target RCS in Rayleigh-resonance scattering regimes for OTH radar. 1. Influenced mainly by gross target dimension (conductive segments) 2. Depends on operating frequency, aspect angle & TX/RX polarization 3. Stealth by energy absorbing materials and shaping ineffective at HF Example: Missile (Length = 10 m) σ Optical 3 MHz (100 m wavelength) Rayleigh RCS falls very rapidly with frequency 30 MHz (10 m wavelength) Resonance Cross Section Rayleigh Resonance variable but often higher target RCS Operating Frequency f OTH radar point targets contained within single resolution cell. Target physical size << spatial dimensions of radar resolution cell Radial velocity produces steady phase progression (Doppler shift) 16

17 Clutter & Interference-plus-Noise Essential OTH Radar Concepts 1. External interference-plus-noise often dominates internal receiver noise. Atmospheric noise (e.g. lightning) propagated long-distances by the ionosphere Anthropogenic (man-made) interference from other users of the HF spectrum 2. OTH radar prone to high clutter levels (40-70 db > than target echoes). Look down geometry illuminates Earth s surface coincidently with targets Large resolution cell sizes increases effective clutter RCS relative to targets Noise Spectral Density versus Time of Day Backscattered clutter power versus range and frequency Operating frequency km range coverage Skip Zone Internal noise db lower 17

18 Doppler Processing Essential OTH Radar Concepts 1. Doppler processing is essential for target detection in OTH radar. Resolves Doppler shifted target echoes from clutter in same resolution cell Provides coherent gain (time-on-target) to improve signal-to-noise ratio 2. OTH radar CPI are much longer than microwave radar (ms). A few seconds for aircraft and tens of seconds for ship detection To compensate for spreading loss & smaller Doppler shifts at HF 3. Limits on OTH radar CPI length arise from factors including: Temporal instability of ionosphere or manoeuvring targets in CPI Region revisit rate for tracking (also trade-off with coverage area) Target Sea Clutter Range Cells (one beam only) Doppler Frequency (0 Hz at centre of display) 18

19 Power & Waveforms Essential OTH Radar Concepts OTH radar transmit power X higher than microwave radar. Average transmit power from 10kW - 1MW sensitivity against noise Frequency modulated continuous waveforms reduces peak powers Transmitter & receiver separation ~100 km (continuous waveform). Referred to as a quasi-monostatic configuration (described later) Relaxes receiver dynamic requirements by attenuating direct wave Quasi-monostatic OTH radar configuration TX RX ~ 100 km separation 19

20 Propagation-Path Characteristics Essential OTH Radar Concepts Propagation via the ionosphere is very complex & challenging to model Unpredictable variation of path characteristics over a very wide range of scales Real-time radar management techniques indispensable for successful operation Characteristic Temporal Spatial Frequency Polarization Multipath Attenuation Ionospheric Propagation-Path Variability Dynamic: Intra-CPI, intra-mission, diurnal, seasonal and solar cycle (11 year) Heterogeneous: Intra-region, over coverage area, latitudinal variability Dispersive: in time-delay (range), Doppler frequency & ray angle-of-arrival Anisotropic: Magneto-ionic components, Faraday rotation (polarization fading) Ever present: E and F regions over two-way path, variable number of modes High: D-layer absorption in day time may cause significant signal attenuation Use of auxiliary sounders to select & update optimum radar frequency Appropriate illumination of the coverage area + minimize interference-plus-noise Updates to reflect changes in ionosphere over time & different coverage areas 20

21 2. Sky-wave OTH Radar Section Outline: Example Systems Skywave OTH Radar Characteristics The Ionosphere & Propagation Effects 21

22 Early Research & Current Systems Example Sky-wave Systems OTH radars exhibit significant diversity in architecture (no standard system). US Navy OTH Radar ROTHR (NRL) Two-site linear FMCW Maximum power 200 kw Receive aperture 2.6 km 372 elements & receivers Counter-drug application French OTH Radar Nostradamus (ONERA) Mono-static (coded pulse) Maximum Power 50 kw Y-Array, 384 m arm length 288 elements, 48 sub-arrays First reported detection 1994 Russian OTH Radar Steel Yard (NIDAR) Two-site (coded pulse) Average Power ~1 MW Vertical array, height 140 m Horizontally polarized dipoles Operational in the mid 1970 s 22

23 Early Research & Current Systems Australian OTH Radar - Jindalee DSTO Australia Jindalee Team (1975) Team Leader: John Strath (circled below) History of OTH radar development in Australia Research to determine ionospheric stability Jindalee Stage A (Detections in one direction) Jindalee Stage B ( Track while scan, 90 deg) Jindalee Stage C ( Operational capability ) 1986 Announcement of JORN 1987 Defence White Paper on broad area surveillance 1990 JFAS Transferred from DSTO to RAAF 1991 JORN Contract Signature 1998 Contract to RLM 2002 JORN Commissioned Dr Malcolm Golley Dr. Fred Earl Jindalee Bare Bones OTH Radar Receiver Array (Central Australia) 23

24 Australian OTH Radar - JORN Early Research & Current Systems Australian Jindalee Operational Radar Network two additional radars Longreach (Queensland), Laverton (Western Australia), Control centre (Adelaide) JORN Laverton OTH Radar TX & RX Site Transmitter Array Receiver Array Coverage Frequencies Typical Characteristics Separation ~100km Dual Band, Linear VLPA (~150 m) 480 monopole pairs (~3 km aperture) +/- 90 degrees 5-30 MHz RX Site TX Site Waveform Bandwidth Average Power Linear FMCW 5-50 khz 250 kw TX Site PRF CIT 4-80 Hz seconds RX Site 24

25 Configuration & Site Selection Skywave OTH Radar Characteristics Radar configuration refers to relative transmitter and receiver locations. Monostatic economical (single radar site & no need for inter-site links) Bistatic Allows use of continuous waveforms (two propagation paths) Quasi-monostatic High sensitivity but essentially one propagation path Multi-static: De-couples ionosphere from target localization & tracking Site selection for OTH sky-wave radar takes several factors into account. 1. Land Needs flat wide open spaces with relatively homogeneous surface 2. Electrically Quiet Avoid strong HF noise near industrial/residential areas 3. Self-Interference Isolation to protect RX from TX continuous waveform 4. Skip-Zone Minimum detection range of ~1000 km (surveillance region) 25

26 Skywave OTH Radar Characteristics Pulsed & Continuous Waveforms Use of pulsed/continuous waveforms depends on OTH radar requirements. Waveform Type TX-RX Configuration Spectral Behaviour Average-to-Peak Power Pulsed Single site Poor out-of-band Low (sensitivity in noise) Continuous Two sites Better out-of-band Higher radar sensitivity Frequency f c + B 2 Linear Frequency Modulated Continuous Waveform (FMCW). Coherent Integration Time (CIT) T CIT f c Time f c B 2 T p Pulse Repetition Frequency (PRF) f = 1 / T p p Range Resolution c ΔR = 2 B R amb Ambiguity ct p = = 2 2 c f p Velocity Δ v Resolution c Δ f d = = 2 f 2 c c f T c CIT Ambiguity cf v amb = 26 ± 4 f p c

27 Skywave OTH Radar Characteristics Typical Missions OTH radar surveillance missions broadly classed as air & surface tasks. Task coverage divided into number of Dwell Interrogation Regions (DIRs) Task A: Barrier Task wide area surveillance mainly used for aircraft C Task B: Stare Task surveillance of airports air/ship lanes, missile sites A Time on each DIR = CIT, all DIR s revisited in turn. Task C: Force Protection air route surveillance navy fleet protection Task D: Remote Sensing D DIR s Radar steps through DIRs in a scheduled sequence. sea-state mapping cyclone tracking B 27

28 Skywave OTH Radar Characteristics Dwell Interrogation Region (DIR) Each DIR consists of many radar resolution cells in range & azimuth. Example dimensions: Rx array aperture (D) = 3 km, TX array aperture 150 m, Range (R) = 2250 km Carrier Frequency (F) = 15 MHz, Waveform Bandwidth (B) = 10 khz Transmitter Footprint ΔR = c 2B = 15 km 300 km (20 Beams) DIR contains 1200 resolution cells ΔL = RΔθ Range-Azimuth Resolution Cell Rλ = 15 D km 900 km (60 range cells) Transmitter D=150m (8 deg at 15 MHz) Receiver D=3000m (0.4 deg at 15 MHz) Receiver Finger Beams 28

29 Skywave OTH Radar Characteristics Aircraft & Ship Detection Aircraft & ships typically detected against noise & clutter respectively. f c Air Maximize Signal-to-Noise Ratio (SNR) for high velocity targets Ship Minimize clutter Doppler spectrum contamination for slower targets B f p T CIT Air Low to find clear frequency channels (with adequate range resolution) Ship High to reduce range cell size and increase sub-clutter visibility (SCV) Air High to avoid velocity ambiguities for fast moving aircraft targets Ship Low to avoid range-folded spread-doppler clutter (unambiguous targets) Air Short for rapid region revisit rates (allows tracking over many DIR s) Ship Long for fine Doppler resolution to resolve targets from strong clutter Example waveform parameters (Assume carrier frequency=15 MHz, detection range=1500 km). Mode B Δ R D Δ L Air Surface Units khz km m km Hz km km/h s m/s f p R amb v amb T CIT Δ v 29

30 Signal & Data Processing Skywave OTH Radar Characteristics Rudimentary OTH radar signal and data processing steps. Signal Processing Pulse Compression Note: Beam Forming Doppler Processing CFAR Early-warning allows more time to decide about target presence compared with certain conventional radars. Higher false detection rates can be tolerated & filtered by the tracker in time before targets declared present. Peak Detection Tracking Coordinate Registration Display Data Processing More details on signal and data processing to follow. 30

31 The Ionosphere & Propagation Effects Ionospheric Regions The ionosphere may be broadly divided into three altitude regions. where electron-density versus height profile tends reach local maxima Ionosphere Height (km) Main Region Characteristics Relevance to OTH Radar D Region E Region F Region Formed during the day-light hours Ionization too low for HF reflection May contain anomalous Sporadic-E Generally stable propagation layer Highest layer with maximum ionization Splits into F1 and F2 layers in the day F1 peak ( km) is sun following F2 peak ( km) present at night Attenuation of radar signals Electron-neutral collisions One-hop paths to ~2000km Allows signals to penetrate Fundamental to OTH Radar 1-Hop F1 can reach 3000km 1-Hop F2 can reach 4000km F2 less stable in space & time Ionosphere exhibits significant variability in structure in space & time. Temporal variations occur diurnally, seasonally and over the 11 year solar cycle Significant spatial variations occur across mid-latitude, equatorial & polar region 31

32 Multipath Propagation The Ionosphere & Propagation Effects Simple illustration of two-way one-hop reflections from E and F layers. Target multiple echoes often resolved in cone angle, range & Doppler shift Clutter contamination of Doppler frequency spectrum (mode superposition) Interference a single source can spread over a significant number of beams F-Layer Simple One-Hop Modes 1F 1F 1F 1E 1E 1E E-Layer Mixed or Hybrid Modes 1E 1F TX-RX Earth Target More complex modes involving multi-hop propagation, top-side layer reflections and trans-equatorial (chordal) modes also exist. 32

33 The Ionosphere & Propagation Effects Propagation-Path Information 1. Synoptic information about ionosphere useful for radar design. Statistical forecasts of diurnal, seasonal, solar-cycle & global variations 2. Real-time information is useful for optimizing radar operation. Mission-to-mission propagation-path data for DIR s in all radar tasks Updates from Ionospheric Sounders Real-Time Propagation-Path Information Radar Parameter Optimization Backscatter sounding Spectrum surveillance Clutter Doppler profile VI & OI Sounders Clutter power levels Noise spectral density Spectral purity Mode structure Carrier frequency Waveform parameters Track association Coordinate registration 33

34 4. HF Radar Sub-systems Section Outline: Transmitter Receiver Radar Management 34

35 Transmitter Vertical Log-Periodic Array Vertically polarized log-periodic monopole arrays with ground-screen. 1. Simultaneously covers all useful elevation angles at reasonable cost. Elevations of 5-40 degrees for one-hop illumination to ranges km Exploits illumination of very large range depths when the ionosphere permits 2. Broadband operation over required frequency range. Use of two (or more) VLPA matched to different sub-bands in HF spectrum JORN VLPA ~ 40 m tall and mechanically stabilized to reduce Aeolian noise 35

36 Ground Screen Transmitter HF antenna radiation patterns depend heavily on ground properties. Ground mesh-screens provide two main benefits: A. Increase antenna gain at low elevations for long range coverage B. Stabilize ground impedance to reduce antenna pattern distortion JORN site approximately 300,000 sqm of galvanised earth-mat. JORN Laverton Transmit Site Ground-Screen High-Band Low-Band 36

37 Transmit Aperture Transmitter Uniform linear arrays containing 8-16 transmitting elements per band. Transmitter aperture length trades off sensitivity with coverage rate. Larger apertures provide higher antenna gain (to increase radar sensitivity) Short apertures provide a broader beam (increases coverage & revisit rate) JORN Longreach Transmitter Site 37

38 Transmitter Elevation Control Two-D transmit apertures permit the beam to be steered in elevation. Enhanced transmit directivity in elevation has positives & negatives. + Improves sensitivity against clutter, facilitates mode selection & CR - Can reduce range depth & azimuth resolution for a fixed # channels Elevation control with ground distributed or vertically raised antennas. Good resolution at low elevations Expensive and difficult to stabilize Less expensive & easier to stabilize Poorer resolution at low elevations 38

39 Element Design Receiver 1. Matched antennas less important for externally noise-limited receivers. Antenna efficiency experienced by targets & noise no SNR improvement Match elements at high end (lower noise) with graceful frequency response 2. Reduce cost by using small end-fire antenna element doublets. Antenna heights of 4-6 meters (less susceptible to Aeolian noise effects) Twin elements combined with time-delay cable for front-to-back ratio Jindalee Rx Antennas (980 installed by Jim McMillan & Wife in 32 days) JORN Receive Antennas 39

40 Array Aperture Receiver Uniform linear arrays (ULA) Best spatial resolution for cost but elevation-azimuth ambiguity (cone angle) Two dimensional arrays Elevation control & mode filtering (with 2D TX) plus wider azimuth coverage Wide receive apertures improve: Gain (sensitivity) & spatial resolution Target detection, location & tracking Jindalee Uniform linear array (~ 2.8 km, 90 deg. ) Upper limit on RX aperture size: Greater expense & additional land Need greater # of coherent beams JORN L-shaped array ( ~ 3 km apertures, 180 deg. ) 40

41 Receiver Reception Channel Traditional heterodyne receiver and sub-array beamforming architecture. Fine resolution finger beams formed by digital combination of all RX outputs Jindalee groups 462 elements into 32 over-lapped sub-arrays (of 28 doublets) Antenna doublet Front-to-back ratio Wide-band RX front-end Rapid frequency changes High Dynamic range 16 bit I&Q sampling Network of switched delay-lines Steers sub-arrays over footprint Tuneable local oscillator Fixed IF filter bandwidth Conversion to base-band Calibrated freq. response Limiting factors: Linearity, A/D conversion, reciprocal mixing & image rejection. 41

42 Frequency Management Radar Management Sub-systems providing real-time frequency advice for main OTH radar. Backscatter Sounder Returned clutter power in group-range & frequency for different beams Vertical/Oblique Incidence Sounders Mode content & virtual heights versus frequency for point-to-point links HF spectrum surveillance Power spectral density of natural & man-made noise across HF band Mini-radar Clutter Doppler profile in group-range & azimuth at selected frequencies Channel Scattering Function Mode distribution in time-delay and Doppler for a narrowband HF circuit 42

43 Backscatter Sounder Radar Management 1. Backscattered clutter power versus frequency, group-range & beam. Original system resolutions: 200 khz, 50 km and 8 beams over 90 deg. Update intervals in order of 5-10 min, and sounder is co-located with radar 2. Concurrent ionograms recorded in early evening ~45 degrees apart. Note significant azimuth dependence, and possibility of range-folded clutter Range extent of km illuminated most by frequencies MHz 18 MHz 17 MHz Range Ambiguity Range-folded clutter 2 nd Hop 1 st Hop Range Coverage Skip-Zone 43

44 Spectrum Surveillance Radar Management 1. Identify unoccupied frequency channels in real-time at receiver site. Avoid interference to other HF services (e.g. broadcasting/communications) Omni-directional antenna measures noise power in 2 khz wide channels 2. SCV combine spectral surveillance & clutter power measurements. Both databases acquired at time of radar operation and in the radar location Sub-clutter visibility (clutter-to-noise ratio) good indicator of radar sensitivity Entire HF Spectrum Protected emergency channels forbidden 1 MHz Wide Zoom Other HF Users Background noise level in clear channels & in azimuth Clear channels (> 100 khz) Background noise level 44

45 Radar Management Vertical & Oblique Incidence Sounders 1. Maintain a real-time ionospheric model (RTIM) of mode structure. Enables propagation modes to be identified and reflection heights estimated Propagation-path information for track association & coordinate registration 2. Network of sounders with rapid (5 min) updates near dawn & dusk. OI Ionogram Darwin-Alice Springs (1260 km path) VI Ionogram Similar time near path mid-point Frequency Dispersion F-layer (high rays) X-ray O-ray F-layer (low rays) E-layer Virtual height F RFI Virtual height E 45

46 5. HF Signal Environment Section Outline: Composite Signal Environment Land & Sea Surface Clutter Ionospheric Clutter & Meteors Noise & Radio Frequency Interference 46

47 Composite Signal Environment Signal Environment Composite received signal for OTH radar is a superposition of: Radar waveform echoes and interference-plus-noise. OTH Radar Signal Environment Radar Echoes Interference-plus-noise Clutter Returns Target Echoes Anthropogenic Naturally Occurring (e.g. Land, Sea) skin echoes (Man-Made) Atmospherics Galactic Unintentional Intentional (e.g. Lightning) (e.g. Stars) (e.g. Electrical machinery) (e.g. Radio stations) 47

48 Land & Sea Surface Clutter Clutter Power Received power of clutter backscattered from Earth s surface. Function of resolution cell area & normalized backscatter coefficient Effective Clutter RCS Single resolution cell σ = σ 0 A Resolution Cell Area Aperture, range & bandwidth General Characteristics: Normalized Backscatter Coefficient Surface properties & grazing angle Sea clutter often more powerful than land clutter (order of magnitude) Higher conductivity of sea-surface and resonant (Bragg) scattering mechanism High seas towards/away from radar significantly increase clutter power More resonant backscatter, while very flat seas produce near specular reflection Spatial RCS variations gradual over sea, but can be very sharp over land Presence of cities and other topographical discontinuities can enhance RCS Received clutter power may be db stronger than target echoes Receiver dynamic range must be sufficiently high to capture both signals 48

49 Land & Sea Surface Clutter First Order Clutter Resonant clutter two orders of magnitude stronger than higher order. Advancing & receding Bragg wave-trains Doppler spectrum Bragg lines EM wavefronts ψ λ 2 Bragg Wave-Trains Advancing Wave Receding Wave Bragg wavelength L cos ψ = Radar wavelength λ 2 L Grazing angle In deep water, the Bragg wave trains move with radial velocity (i.e. gravity waves): v = ± gl 2 π = ± 1 2 π g λ cos ψ 1 / 2 Without surface currents, this imposes a Doppler shift on two clutter Bragg Lines f b = 2 v cos λ ψ = ± g cos πλ ψ 1 / 2 49

50 Higher Order Continuum Land & Sea Surface Clutter Mainly due to double scattered echoes from pairs of wave-trains. Second-order clutter continuum is distributed in Doppler frequency May impede target detection, especially slow ships in high sea-states Target visibility depends on echo strength & Doppler shift Bragg Lines (first-order clutter) Blind speeds (solid lines) SCR limits target detection High RCS Medium RCS Lower RCS Higher Order Clutter Continuum 50

51 Land & Sea Surface Clutter Ship Detections 10 Beams Doppler Shift Nested Range Cells Land clutter Sea clutter Hz + Note Doppler spreading (transit via ionosphere) 51

52 Spectral Density Noise & Radio Frequency Interference Background noise includes atmospheric (i.e. lightning) & galactic noise. Received noise depends not only on sources, but also propagation conditions Atmospheric dominates at low frequencies and galactic at higher frequencies Frequency and (diurnal) time dependencies determined partly by ionosphere Day Time (~Noon) D-layer absorption in the day attenuates long-range noise at lower frequencies (lossy propagation paths) Anthropogenic RFI 50 db above background Night Time (~Midnight) Powerful RFI & congested lower HF spectrum No sky-wave paths for higher frequencies at night Background Noise Level Higher Background Noise Mainly Atmospheric Mainly Galactic 52

53 Background Noise Variations Noise & Radio Frequency Interference Background noise spectral density variations (monthly median figures). Higher noise levels in geographical areas of strong thunderstorm activity Many databases recorded by omni-directional antennas; CCIR report Background noise is directional level depends on radar look direction Median Noise Spectral Density Local Noon Local Midnight High frequencies penetrate ionosphere at night Internal receiver noise spectral density 53

54 6. Conventional Processing Section Outline: Range, Beam & Doppler Processing CFAR Detection & Peak Estimation Tracking and Radar Displays 54

55 Processing Stages Signal & Data Processing Stages Flow chart of OTH radar signal/data processing steps and displays. Conventional processing traditionally based on FFT Radar data and target track displays for operators Time for processing and display in the order ~ CPI Signal Processing Stages A/D Conversion Pulse Compression Beam Forming Doppler Processing Data Processing Stages CFAR Peak Detection Tracking Coordinate Registration Radar Data and Track Displays 55

56 Pulse Compression Range/Beam & Doppler Processing Separates received echoes on basis of time-delay. Target group-range estimated for localization Resolves targets from each other & multipath Well-known principle of pulse-compression Reference Chirp Δf Returned Echo Difference Frequency B Taper Function FFT Energy Δ R = c 2 B τ T p Δf τb = T p R c = τ = 2 ct p Δ f 2 B Range Bins Example for sky-wave OTH radar. Task Pulse Period Bandwidth Ambiguity Resolution No. Bins Range Depth Air 0.02 seconds 10 khz 3,000 km 15 km km Surface 0.2 seconds 30 khz 30,000 km 5 km km 56

57 Doppler Processing Separates received echoes on basis of Doppler shift. Range/Beam & Doppler Processing Isolates targets/clutter in separate frequency bins Coherent gain to improve target detection in noise Estimates target radial velocity to improve tracking Chirp 1 Processed Range Bin Chirp 2 Chirp K Taper Function Range Range Range Doppler Processing FFT Range-Doppler Doppler Bins Range Bins Example for sky-wave OTH radar. Task Pulse Period CIT Ambiguity Resolution No. Pulses Frequency Air 0.02 seconds 2 seconds +/- 900 km/h 5 m/s MHz Surface 0.2 seconds 40 seconds +/- 90 km/h 0.25 m/s MHz 57

58 Beamforming Range/Beam & Doppler Processing Separates received signals on the basis of angle-of-arrival. Provides coherent gain in surveillance beam direction Estimates the (cone) angle-of-arrival of target echoes Attenuates sidelobe disturbance due to clutter & noise Receivers Processed Cell Range Beam Cell Range ULA Narrowband Sensors Rx 1 Rx N Range & Doppler Processing Range & Doppler Processing Doppler Doppler Range Beam FFT or DFT Doppler Doppler Range θ 1 θ N Beams Example for sky-wave OTH radar. Taper Function Task Aperture Frequency Ambiguity Resolution No. Beams Coverage Air 1500 m 15 MHz d / λ <0.5 ~ 1 deg deg. Surface 3000 m 15 MHz d / λ <0.5 ~ 0.5 deg deg. 58

59 Processing Example Range/Beam & Doppler Processing Pulse Compression Doppler Processing Beamforming Receivers, ranges, & pulses Receiver Range-Doppler maps Beam Range-Doppler maps Strong clutter masks target Clutter confined close to 0 Hz Target becomes clearly visible Range Rx 1 Rx 2 Receivers, Nested Range Cells Receivers, Nested Range Cells Beams, Nested Range Cells Clutter Target Time (Chirp Number) Doppler Frequency 59 Doppler Frequency

60 Multipath Echoes BEAM SPECTRUM Array Boresight Coning Effect Range/Beam & Doppler Processing Zoom (beam containing target) 1F 1F 1F 1E 1E 1F 1E 1E } Group Range Hypothesized mode structure - 0 Hz Doppler Frequency + F-Layer Key observations: E-Layer TX-RX Earth Single target multiple echoes Distinct range, Doppler & beam F-F mode c.f. E-E mode Longer group-range Smaller Doppler shift Target 60 Higher coning effect

61 Window Functions Range/Beam & Doppler Processing Importance of window functions to control spectral leakage. Without Doppler window With Doppler window Target Target masked by Clutter sidelobes 61

62 CFAR Processing CFAR Detection & Peak Estimation Constant false alarm rate (CFAR) processing applied to ARD data. To reduce the number of false detections made on clutter & noise Variety of CFAR techniques: Definition of cell under test (CUT) Window in range-doppler & beam Cell-averaging or ordered statistics Estimation of a background level Normalization of CUT by this level Repeat for all radar resolution cells Doppler Window Range Window CUT Guard Cells CFAR window dimensions may be changed to suit local disturbance features. Cell Averaging (CA) or Greatest of Ordered Statistics (GOOS) methods. 62

63 CFAR Detection CFAR Detection & Peak Estimation Peak detections on the CFAR output are passed onto tracker if: Cell under test is a local peak in range, Doppler & beam dimensions Magnitude of this peak exceeds a pre-set target detection threshold CFAR Output High Threshold (low detection probability) Low Threshold (high false alarm rate) Possible Clutter False Alarm Target } Suitable detection threshold range Target 63

64 CFAR Detection & Peak Estimation Peak Estimation Peak parameters estimated using ARD data (before CFAR): Quadratic interpolation using peak and immediate neighbours Non-integer estimates range, Doppler, beam & SNR to tracker Step must be repeated for all detected peaks in CPI data cube Target Peak Quadratic Interpolation Target Beam Estimate 64

65 Tracking Tracking and Radar Displays Target presence may be declared on basis of established target tracks. Early-warning time for tracker to filter out many false (noise/clutter) peaks Permits use of low peak detection thresholds to capture weaker target echoes Single target may produce several distinct echoes due to multipath. Tracking usually performed in radar coordinates on all propagation modes Separate processing to associate multipath tracks & covert them to ground Probability data association (PDA) filter successful for OTH radar. Track updated by combined influence of multiple peaks in neighbourhood Simultaneous tracking of multiple targets with multiple hypothesis models 65

66 Coordinate Registration Tracking and Radar Displays Challenging problem of converting from radar to ground coordinates. Uncertain propagation via the ionosphere Several possible CR techniques: Ray tracing with real-time ionospheric model (RTIMs) Transponders at known locations in surveillance area Detection of sea-land clutter boundaries in coverage Association of detections with available GPS reports Detections on commercial aircraft & shipping lanes Registering airports where tracks begin or terminate Correlation of clutter RCS enhancements with cities Effective fusion of different CR techniques 66

67 Tracking and Radar Displays Radar Data Displays Geographical Track Display Detections filtered in time (CPI) Displays multipath target tracks Tracks for single target 3 tracked ionospheric modes With possible TID presence Whitened ARD Display Target position + Doppler For a single DIR and CPI Stare Scroll Display Localized area (one beam) Clearly shows manoeuvres Target Detections Range x Range x+1 Manoeuvring Target Time Doppler 67 Doppler