dsto high frequency over-the-horizon radar · high frequency over-the-horizon radar dr. giuseppe a....
TRANSCRIPT
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DEFENCE: PROTECTING AUSTRALIADSTOD E P A R T M E N T O F D E F E N C EDEFENCE SCIENCE & TECHNOLOGY ORGANISATION
High Frequency Over-the-Horizon Radar
Dr. Giuseppe A. Fabrizio
Senior Research Scientist, High Frequency Radar Branch,
Intelligence, Surveillance and Reconnaissance Division,
DSTO Australia.
IEEE LectureAtlanta, GA, May 2012
© Commonwealth of Australia 2010
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Presentation Outline
1. Fundamental Principles
2. Sky-Wave OTH Radar
3. HF Radar Sub-Systems
4. HF Signal Environment
5. Conventional Processing
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1. Fundamental Principles
Section Outline:
• Surveillance Radar & Frequency Bands
• Interest in the High Frequency Region
• Essential OTH Radar Concepts
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Radar Frequencies
Band HF VHF UHF L S C X Ku K Ka
Frequency
Wavelength
3-30 MHz
30-300 MHz
300-1000 MHz
1-2 GHz
2-4 GHz
4-8 GHz
9-12 GHz
12-18 GHz
18-27 GHz
27-40 GHz
10-100 m
1-10 m
0.3-1 m
~20cm
~10 cm
~5 cm
~3 cm
~2 cm
~1.4 cm
~0.8 cm
Surveillance Radar & Frequency Bands
The choice of frequency band has a pronounced influence on the characteristics and performance of a radar system.
Resolution/AccuracyRange Coverage
Microwave Radar (0.4 – 40 GHz)Over-the-Horizon Radar
Meteorological EffectsIonospheric Effects
Physically Larger Physically Smaller
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Target Types
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:
Surveillance Radar & Frequency Bands
Remote sensing radars (e.g. sea-state mapping) target natural scatterers.
Remote sensing applications of OTH radar not explicitly considered here
Large Ships
Fighter-Sized & Helicopters
Aircraft(Primary Mission)
Large Aircraft Missiles
Go-Fast BoatsDestroyers & Patrol Boats
Ships(Secondary Mission)
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Surveillance Functions
Discriminate target echoes against disturbance signals and estimate target parameters of interest to infer target geographical position and velocity.
Range
Establish, maintain and display detected target tracks while continuing to search the coverage area for new targets.
JammingClutter Noise
Direction Radial Velocity Coordinate Registration
Conventional & OTH surveillance radars share the main functions.
Target detection, localization & tracking
1) Target detection-estimation:
2) Target track-while-scan:
Surveillance Radar & Frequency Bands
Example: Main surveillance radar functions:
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Early HF Radar
Later during world war II, microwave radars were successfully employed.Regarded as the most competitive frequency band for line-of-sight applications
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
Surveillance Radar & Frequency Bands
Example Chain Home radar station (East UK coast).
Frequency 20-30 MHz Robert Watson-Watt (1892-1973).
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Conventional Radar
Great majority of line-of-sight radars implemented at microwave frequency.
main technical reasons (at a glance)
CONVENTIONALRADAR
PHYSICALLY SMALL HIGH GAIN ANTENNAS
(EASIER TO SATISFY SITE CONSTRAINTS)
LOW AMBIENT NOISE LEVEL
(INTERNAL NOISE LIMITED)POTENTIAL FOR CLUTTER REDUCTION
(e.g. “UP-LOOKING” GEOMETRY)
LINE-OF-SIGHT PROPAGATION-PATH
(TARGET LOCALIZATION ACCURACY)
GREATER USEABLE BANDWIDTHS
(FINE RANGE RESOLUTION)
TARGET RCS(OFTEN IN OPTICAL REGION)
Surveillance Radar & Frequency Bands
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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
Surveillance Radar & Frequency Bands
Earth’s Surface
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)
Low-flying targets escape early detection
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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
IONOSPHEREIONOSPHERE
TXTX
EARTHEARTH’’S SURFACES SURFACE
HF Sky-Wave
MICROWAVE
HF Surface-Wave
Different physical mechanisms that are essentially unique to the HF band.
Exploited by OTH radar & short-wave communicators since G. Marconi (1901)
Guglielmo Marconi
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About the IonosphereIonized 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
Interest in the High Frequency Region
Courtesy of http://www.windows.ucar.edu
1864 - 73 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, 1901
Marconi transmits radio signal across Atlantic Ocean from Cornwall, England to St. John's, Newfoundland
1902 Oliver Heaviside; Arthur Kennelly propose existence of conducting layer in upper atmosphere
1909 Marconi awarded Nobel Prize
1924 Edward Appleton and others develop ionosonde & beginground-based soundings; prove existence of ionosphere
1925 Appleton discovers second layer (the F region)
1926 Robert Watson-Watt (later developer of radar) coins word "ionosphere"
1927 Sydney Chapman describes theory for formation of ionosphere
1947 Appleton awarded Nobel Prize
1958 Incoherent Scatter Radar developed
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Useful Coverage 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
φsinp
c
ff >
φsinp
c
ff ≤
100
200
300
400
Altitude (km)500
1000 15002500
3000
2000 Range (km)
cf
φ
100
200
300
400
ESCAPE RAYS
CONCEPTUAL REPRESENTATION
REFLECTED RAYS
600 km
300 km
Interest in the High Frequency Region
Escape Rays
1st Hop
2st Hop
Backscattered Power(Two-way path)
Skip Zone
Useful Coverage
Surface Clutter
Radar Footprint
Leading Edge Focusing
Target
0 500 1000 1500 2000 2500 3000Range (km)
SingleFrequency
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Sky-wave OTH Radar
Transmit
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
RadarFootprint
Radar FootprintHigher Frequency
TX & RX Beam Steering
Ionosphere
Concept of OperationConcept of Operation
PotentialRadar Coverage
Transmit
Receive
Essential OTH Radar Concepts
Skip-ZoneLimit
ResolutionCells
Dwell Time (CPI)
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Radar Equation
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
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2
42 )4()4( RLNFTGGP
RLNFTAGP
NS
o
prtave
o
petave
πσλ
πσ
==Output
Signal-to-Noise Ratio
Transmit Antenna Gain
Receive Antenna Gain
Effective Integration Time
Operating Wavelength
Target Cross Section
Propagation Factor
Losses (Path and System)
Slant Range (Radar-to-Target)
Transmit Power (Average)
Radar Type Range Coverage (km) Surface Coverage (sq km)
Sky-wave OTH Radar 1000-3000 Millions
Ground-Based Microwave 1-300 Tens of thousands
Essential OTH Radar Concepts
External Noise Power per unit bandwidth
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Resolution & Accuracy
OTH radar range resolution limited by useable bandwidths in HF spectrum.
Radar Type
Useable Bandwidths
Range Resolution
Aperture Size
3-30 km 3000 m
6 m30-300 m
Antenna Beamwidth
OTH Radar 5-50 kHz 0.2-2.0 deg.
0.1-1.0 deg.Microwave 500-5000 kHz
Antenna gain & beamwidth are dependent on aperture size in wavelengths.
Spatial resolution comparison – “order of magnitude”.
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 10-40 km
Essential OTH Radar Concepts
1. User-congestion in HF band limits availability of clear frequency channels 2. Frequency dispersion in ionosphere places a limit on coherence bandwidth
• 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
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Example: Missile (Length = 10 m)
• 3 MHz (100 m wavelength) “Rayleigh”
RCS falls very rapidly with frequency
• 30 MHz (10 m wavelength) “Resonance”
variable but often higher target RCS
Target Scattering Target RCS in Rayleigh-resonance scattering regimes for OTH radar.
• Target physical size << spatial dimensions of radar resolution cell
• Radial velocity produces steady phase progression (Doppler shift)
Essential OTH Radar Concepts
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
OTH radar “point” targets contained within single resolution cell.
Cros
s Se
ctio
n
Operating Frequency
Optical
RayleighResonance
f
σ
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Clutter & Interference-plus-Noise
1. External interference-plus-noise often dominates internal receiver noise.
2. OTH radar prone to high clutter levels (40-70 dB > than target echoes).
Internal noise 20-30 dB lower
Skip Zone
Essential OTH Radar Concepts
Atmospheric noise (e.g. lightning) propagated long-distances by the ionosphere
Anthropogenic (man-made) “interference” from other users of the HF spectrum
“Look down” geometry illuminates Earth’s surface coincidently with targets
Large resolution cell sizes increases effective clutter RCS relative to targets
2000-3000 km range coverage
Backscattered clutter powerversus range and frequency
Noise Spectral Density versus Time of Day
Operating frequency
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Doppler Processing
2. OTH radar CPI are much longer than microwave radar (ms).
1. Doppler processing is essential for target detection in OTH radar.
• Temporal instability of ionosphere or manoeuvring targets in CPI
• Region revisit rate for tracking (also trade-off with coverage area)
Doppler Frequency (0 Hz at centre of display)
Range Cells(one beam only)
Sea ClutterTarget
Essential OTH Radar Concepts
• Resolves Doppler shifted target echoes from clutter in same resolution cell
• Provides coherent gain (time-on-target) to improve signal-to-noise ratio
• 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:
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Power & Waveforms
Transmitter & receiver separation ~100 km (continuous waveform).
• Referred to as a “quasi-monostatic” configuration (described later)
• Relaxes receiver dynamic requirements by attenuating direct wave
OTH radar transmit power 10-100 X higher than microwave radar.
• Average transmit power from 10kW - 1MW sensitivity against noise
• Frequency modulated continuous waveforms reduces peak powers
RX
TX
Essential OTH Radar Concepts
~ 100 kmseparation
Quasi-monostatic OTH radar configuration
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Propagation-Path Characteristics
Characteristic Ionospheric Propagation-Path Variability
Temporal Dynamic: Intra-CPI, intra-mission, diurnal, seasonal and solar cycle (11 year)
Spatial Heterogeneous: Intra-region, over coverage area, latitudinal variability
Frequency Dispersive: in time-delay (range), Doppler frequency & ray angle-of-arrival
Polarization Anisotropic: Magneto-ionic components, Faraday rotation (polarization fading)
Multipath Ever present: E and F regions over two-way path, variable number of modes
Attenuation High: D-layer absorption in day time may cause significant signal attenuation
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
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
Essential OTH Radar Concepts
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2. Sky-wave OTH Radar
Section Outline:
• Example Systems
• Skywave OTH Radar Characteristics
• The Ionosphere & Propagation Effects
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Example Sky-wave Systems
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
OTH radars exhibit significant diversity in architecture (no standard system).
Early Research & Current Systems
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Australian OTH Radar - Jindalee
DSTO Australia “Jindalee Team” (1975) History of OTH radar development in Australia1952 – 1972 Research to determine ionospheric stability 1974 – 1978 Jindalee Stage A (Detections in one direction)1979 – 1985 Jindalee Stage B (“Track while scan”, 90 deg)1986 – 1989 Jindalee Stage C (“Operational capability”)1986 Announcement of JORN1987 Defence White Paper on broad area surveillance1990 JFAS Transferred from DSTO to RAAF 1991 JORN Contract Signature1998 Contract to RLM 2002 JORN Commissioned
Jindalee “Bare Bones” OTH Radar Receiver Array(Central Australia) Dr Malcolm Golley Dr. Fred Earl
Team Leader: John Strath (circled below)
Early Research & Current Systems
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Australian OTH Radar - JORN
JORN Laverton OTH Radar
Typical Characteristics
TX & RX Site Separation ~100km
Transmitter Array
Dual Band, LinearVLPA (~150 m)
Receiver Array
480 monopole pairs(~3 km aperture)
Coverage +/- 90 degrees
Frequencies 5-30 MHz
Waveform Linear FMCW
Average Power 250 kW
PRF 4-80 Hz
CIT 1.5-30 seconds
Bandwidth 5-50 kHz
Australian Jindalee Operational Radar Network two additional radars
• Longreach (Queensland), Laverton (Western Australia), Control centre (Adelaide)
TX Site
RX Site
RX Site
TX Site
Early Research & Current Systems
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Configuration & Site Selection
Skywave OTH Radar Characteristics
Radar configuration refers to relative transmitter and receiver locations.
Site selection for OTH sky-wave radar takes several factors into account.
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
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)
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Pulsed & Continuous Waveforms
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
Use of pulsed/continuous waveforms depends on OTH radar requirements.
Frequency
Timecf
2Bf c +
pT
CITT
Pulse Repetition Frequency (PRF) pp Tf /1=
Coherent Integration Time (CIT)
2Bf c −
Resolution
BcR
2=Δ
p
pamb f
ccTR
22==
Ambiguity
Range CITcc
d
Tfc
ffcv
22=
Δ=ΔVelocity
c
pamb f
cfv
4±=
Resolution Ambiguity
Linear Frequency Modulated Continuous Waveform (FMCW).
Skywave OTH Radar Characteristics
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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
• air route surveillance • navy fleet protection
Task B: “Stare” Task
• wide area surveillance• mainly used for aircraft
Task C: Force Protection
• surveillance of airports • air/ship lanes, missile sites
Task D: Remote Sensing
A
B
C
D• Radar steps through DIRs
in a scheduled sequence.
• Time on each DIR = CIT, all DIR’s revisited in turn.
• sea-state mapping • cyclone tracking
DIR’s
Skywave OTH Radar Characteristics
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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
km 15=≈Δ=ΔD
RRL λθ
km 152
==ΔBcR
Range-AzimuthResolution Cell
300 km
(20 Beams)
900 km (60 range cells)
DIR contains1200 resolution cells
Transmitter D=150m(8 deg at 15 MHz)
Receiver D=3000m(0.4 deg at 15 MHz)
1
20
20 Receiver“Finger Beams”
Skywave OTH Radar Characteristics
Transmitter Footprint
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Aircraft & Ship Detection
Aircraft & ships typically detected against noise & clutter respectively.
CITTpf
cf
B
• 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
Mode
Air 10 15 1000 30 50 3000 900 1 10
Surface 30 5 3000 10 5 30000 90 20 0.5
Units kHz km m km Hz km km/h s m/s
CITTRΔB pfD LΔ ambR ambv vΔ
Example waveform parameters (Assume carrier frequency=15 MHz, detection range=1500 km).
• 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 Maximize Signal-to-Noise Ratio (SNR) for high velocity targets• Ship Minimize clutter Doppler spectrum contamination for slower targets
Skywave OTH Radar Characteristics
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Pulse Compression
Signal & Data Processing
Rudimentary OTH radar signal and data processing steps.
CoordinateRegistration
Doppler Processing CFAR Peak
Detection
Tracking
BeamForming
Signal Processing
Data Processing
Higher false detection rates can be tolerated & filtered by the tracker in time before targets declared present.
Early-warning allows more time to decide about target presence compared with certain conventional radars.
Display
Note:
Skywave OTH Radar Characteristics
More details on signal and data processing to follow.
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Ionospheric Regions
Ionosphere Height (km) Main Region Characteristics Relevance to OTH Radar
D Region 50-90Formed during the day-light hours Ionization too low for HF reflection
Attenuation of radar signalsElectron-neutral collisions
E Region 90-140May contain anomalous Sporadic-E Generally stable propagation layer
One-hop paths to ~2000kmAllows signals to penetrate
F Region 140-400
Highest layer with maximum ionization Splits into F1 and F2 layers in the dayF1 peak (140-210 km) is sun following F2 peak (210-400 km) present at night
Fundamental to OTH Radar 1-Hop F1 can reach 3000km 1-Hop F2 can reach 4000km F2 less stable in space & time
The ionosphere may be broadly divided into three altitude regions.where electron-density versus height profile tends reach local maxima
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
The Ionosphere & Propagation Effects
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Multipath Propagation
Earth
E-Layer
F-Layer
EE 11 −FF 11 − EF 11 −
FE 11 −
Simple One-Hop Modes Mixed or “Hybrid” Modes
Simple illustration of two-way one-hop reflections from E and F layers.
More complex modes involving multi-hop propagation, top-side layer reflections and trans-equatorial (chordal) modes also exist.
TX-RX Target
The Ionosphere & Propagation Effects
• 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
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Propagation-Path Information
1. Synoptic information about ionosphere useful for radar design.Statistical forecasts of diurnal, seasonal, solar-cycle & global variations
The Ionosphere & Propagation Effects
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
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4. HF Radar Sub-systems
Section Outline:
• Transmitter
• Receiver
• Radar Management
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Vertical Log-Periodic ArrayVertically polarized log-periodic monopole arrays with ground-screen.
• Elevations of 5-40 degrees for one-hop illumination to ranges 1000-3000 km • Exploits illumination of very large range depths when the ionosphere permits
• 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
Transmitter
1. Simultaneously covers all useful elevation angles at reasonable cost.
2. Broadband operation over required frequency range.
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Ground ScreenHF antenna radiation patterns depend heavily on ground properties.
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
High-Band
Low-Band
Ground-Screen
Transmitter
Ground mesh-screens provide two main benefits:
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Transmit Aperture
• Larger apertures provide higher antenna gain (to increase radar sensitivity)
• Short apertures provide a broader beam (increases coverage & revisit rate)
Uniform linear arrays containing 8-16 transmitting elements per band.
JORN Longreach Transmitter Site
Transmitter
Transmitter aperture length trades off sensitivity with coverage rate.
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Elevation Control
• Good resolution at low elevations • Expensive and difficult to stabilize
Transmitter
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
• Less expensive & easier to stabilize • Poorer resolution at low elevations
Elevation control with ground distributed or vertically raised antennas.
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Element Design
Receiver
2. Reduce cost by using small end-fire antenna element doublets.
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
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
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Array ApertureUniform 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
Receiver
Jindalee Uniform linear array (~ 2.8 km, 90 deg. )
• Gain (sensitivity) & spatial resolution
• Target detection, location & tracking
• Greater expense & additional land
• Need greater # of coherent beams
Wide receive apertures improve: Upper limit on RX aperture size:
JORN L-shaped array ( ~ 3 km apertures, 180 deg. )
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Reception ChannelTraditional 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)
• High Dynamic range • 16 bit I&Q sampling
• Wide-band RX front-end• Rapid frequency changes
• Conversion to base-band• Calibrated freq. response
• Tuneable local oscillator • Fixed IF filter bandwidth
• Antenna doublet • Front-to-back ratio
• Network of switched delay-lines • Steers sub-arrays over footprint
Limiting factors: Linearity, A/D conversion, reciprocal mixing & image rejection.
Receiver
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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
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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 2000-3000 km illuminated most by frequencies 17-18 MHz
Range Coverage
18 MHz
1st Hop
17 MHz
2nd HopRange-folded
clutterRange Ambiguity
Skip-Zone
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Spectrum Surveillance1. 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
Other HF Users
Background noise level
Clear channels(> 100 kHz)
1 MHz Wide ZoomEntire HF Spectrum
Protected emergencychannels forbidden
Background noise levelin clear channels & in azimuth
Radar Management
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Vertical & Oblique Incidence Sounders
Radar Management
OI Ionogram Darwin-Alice Springs (1260 km path) VI Ionogram Similar time near path mid-point
E-layer
F-layer (low rays)
F-layer(high rays) X-rayO-ray
RFI
Virtual height F
Virtual height E
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.
Frequency Dispersion
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5. HF Signal Environment
Section Outline:
• Composite Signal Environment
• Land & Sea Surface Clutter
• Ionospheric Clutter & Meteors
• Noise & Radio Frequency Interference
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Signal Environment
Composite Signal Environment
Composite received signal for OTH radar is a superposition of:
Radar waveform echoes and interference-plus-noise.
Anthropogenic (Man-Made)
OTH Radar Signal Environment
Radar Echoes Interference-plus-noise
Clutter Returns
(e.g. Land, Sea)
Target Echoes
skin echoes
Unintentional
(e.g. Electrical machinery)Intentional
(e.g. Radio stations)
Naturally Occurring
Atmospherics
(e.g. Lightning)
Galactic(e.g. Stars)
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Clutter Power
General Characteristics:
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 40-80 dB stronger than target echoes
Receiver dynamic range must be sufficiently high to capture both signals
Received power of clutter “backscattered” from Earth’s surface. Function of resolution cell area & normalized backscatter coefficient
A×= 0σσEffective Clutter RCS
• Single resolution cell
Resolution Cell Area• Aperture, range & bandwidth
Normalized Backscatter Coefficient
• Surface properties & grazing angle
Land & Sea Surface Clutter
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DSTO
First Order Clutter
In deep water, the Bragg wave trains move with radial velocity (i.e. gravity waves): 2/1
cos21
2 ⎥⎦
⎤⎢⎣
⎡±=±=
ψπλ
πggLv
2/1coscos2⎥⎦⎤
⎢⎣⎡±==
πλψ
λψ gvf b
Bragg wavelength
Resonant clutter two orders of magnitude stronger than higher order.Advancing & receding Bragg wave-trains Doppler spectrum Bragg lines
2cos λψ =L
Radarwavelength
Grazing angle
Without surface currents, this imposes a Doppler shift on two clutter “Bragg Lines”
ψ2λ
L
Bragg Wave-Trains
RecedingWave
Advancing Wave
EM wavefronts
Land & Sea Surface Clutter
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DSTO
Higher Order Continuum
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)
Higher OrderClutter Continuum
Blind speeds(solid lines)
High RCS
Lower RCS
Medium RCS
SCR limits target detection
Land & Sea Surface Clutter
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DSTO
Ship Detections
Sea clutter
Land clutter
Doppler Shift10 Beams
Note Doppler spreading(transit via ionosphere)
1
2
3
10
4
5
6
7
8
9
Nested Range Cells
Land & Sea Surface Clutter
0 Hz +-
4
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DSTO
Spectral Density
Noise & Radio Frequency Interference
• 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) Night Time (~Midnight)
Anthropogenic RFI 50 dB above background
Background Noise Level
Higher Background NoiseMainly
Atmospheric
D-layer absorption in the day attenuates long-range noise at lower frequencies (lossy propagation paths)
No sky-wave paths for higher frequencies at night
MainlyGalactic
Powerful RFI & congested lower HF spectrum
Background noise includes atmospheric (i.e. lightning) & galactic noise.
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DSTO
Background Noise VariationsBackground noise spectral density variations (monthly median figures).
Median Noise Spectral Density
Internal receiver noise spectral density
Noise & Radio Frequency Interference
• 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
Local Noon Local Midnight
High frequencies penetrate ionosphere at night
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DSTO
6. Conventional Processing
Section Outline:
• Range, Beam & Doppler Processing
• CFAR Detection & Peak Estimation
• Tracking and Radar Displays
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DSTO
Processing Stages
Signal & Data Processing Stages
A/D Conversion
CoordinateRegistration
Doppler Processing
CFARPeak
Detection Tracking
BeamForming
Signal Processing Stages
Data Processing Stages
Pulse Compression
Radar Data and Track Displays
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
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DSTO
Pulse Compression
B
Range/Beam & Doppler Processing
Reference ChirpReturned Echo
pTτ
FFT
TaperFunction
fΔ
Range Bins
Energy
pTBf τ
=Δ
BfcTcR p
22Δ
==τ
BcR
2=Δ
Difference Frequency
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”
Task Pulse Period Bandwidth Ambiguity Resolution No. Bins Range Depth
Air 0.02 seconds 10 kHz 3,000 km 15 km 40 600 km
Surface 0.2 seconds 30 kHz 30,000 km 5 km 80 400 km
Example for sky-wave OTH radar.
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DSTO
Doppler Processing Separates received echoes on basis of Doppler shift.
Isolates targets/clutter in separate frequency bins
Coherent gain to improve target detection in noise
Estimates target radial velocity to improve tracking
Chirp 1 ProcessedRange Bin
Chirp 2 Chirp K
Doppler Processing FFTTaperFunction
Range Range Range Range-Doppler
Task Pulse Period CIT Ambiguity Resolution No. Pulses Frequency
Air 0.02 seconds 2 seconds +/- 900 km/h 5 m/s 100 15 MHz
Surface 0.2 seconds 40 seconds +/- 90 km/h 0.25 m/s 200 15 MHz
Range Bins
Doppler Bins
Range/Beam & Doppler Processing
Example for sky-wave OTH radar.
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DSTO
Beamforming
Task Aperture Frequency Ambiguity Resolution No. Beams Coverage
Air 1500 m 15 MHz ~ 1 deg. 10 10 deg.
Surface 3000 m 15 MHz ~ 0.5 deg. 20 10 deg.
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
Beam
FFT
or
DFT
Taper Function
Range & DopplerProcessingRx 1
Receivers
ULANarrowband
Sensors
Range
Doppler
Processed Cell
Range & DopplerProcessingRx N
Range
Doppler
Range
Doppler
Beam Cell
Range
Doppler
1θ
Nθ
Beams
5.0/ <λd5.0/ <λd
Range/Beam & Doppler Processing
Example for sky-wave OTH radar.
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DSTO
Processing Example
Rec
eive
rs, N
este
d Ran
ge C
ells
Beam
s, N
este
d Ran
ge C
ells
Time (Chirp Number) Doppler Frequency
Rec
eive
rs, N
este
d Ran
ge C
ells
Doppler Frequency
Receivers, ranges, & pulses
Strong clutter masks target
Receiver Range-Doppler maps
Clutter confined close to 0 HzBeam Range-Doppler maps
Target becomes clearly visible
Pulse Compression Doppler Processing Beamforming
TargetClutter
Rx 1
Rx 2
Range
Range/Beam & Doppler Processing
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DSTO
Multipath Echoes
EE 11 −
FF 11 −EF 11 −FE 11 −
Hypothesized mode structure
“Zoom” (beam containing target)
Doppler Frequency
GroupRange
Earth
E-Layer
F-Layer
TX-RXTarget
+- 0 Hz
}
Array Boresight
• Single target multiple echoes
• Distinct range, Doppler & beam
F-F mode c.f. E-E mode• Longer group-range• Smaller Doppler shift • Higher coning effect
Key observations:
Range/Beam & Doppler Processing
“Coning Effect”
BEAM SPECTRUM
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DSTO
Window Functions
Importance of window functions to control spectral leakage.
Without Doppler window With Doppler window
Target
Target masked by Clutter sidelobes
Range/Beam & Doppler Processing
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DSTO
CUT
Guard Cells
Range Window
Doppler Window
CFAR Processing
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
Constant false alarm rate (CFAR) processing applied to ARD data.
To reduce the number of false detections made on clutter & noise
CFAR window dimensions may be changed to suit local disturbance features.
Cell Averaging (CA) or Greatest of Ordered Statistics (GOOS) methods.
CFAR Detection & Peak Estimation
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DSTO
CFAR Detection 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 Low Threshold(high false alarm rate)
High Threshold(low detection probability)
Suitable detectionthreshold range}
Possible Clutter False Alarm
Target
Target
CFAR Detection & Peak Estimation
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DSTO
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 Beam Estimate
Quadratic InterpolationTarget
Peak
CFAR Detection & Peak Estimation
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DSTO
Tracking
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
Tracking and Radar Displays
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DSTO
Coordinate Registration
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
Tracking and Radar Displays
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DSTO
Radar Data Displays
“Whitened” ARD Display• Target position + Doppler
• For a single DIR and CPI
“Stare” Scroll Display• Localized area (one beam)
• Clearly shows manoeuvres
Target Detections
Doppler Doppler
Range x Range x+1
ManoeuvringTarget
Time
Geographical Track Display• Detections filtered in time (CPI)
• Displays multipath target tracks
Tracks for single target• 3 tracked ionospheric modes
• With possible TID presence
Tracking and Radar Displays