dsto high frequency over-the-horizon radar · high frequency over-the-horizon radar dr. giuseppe a....

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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.

[email protected]

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:


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:


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


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)


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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


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


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


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


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


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)


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


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


• 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


~ 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


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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)


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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


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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).


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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


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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”


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


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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:


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


• 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


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


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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


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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


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



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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



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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


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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:


“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


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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


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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


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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


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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


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