DSTO High Frequency Over-the-Horizon Radar

DSTO

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. [email protected]

DEPARTMENT OF DEFENCE DEFENCE SCIENCE & TECHNOLOGY ORGANISATION

DSTO

DEFENCE: PROTECTING AUSTRALIA 1

© Commonwealth of Australia 2010

DSTO

Presentation Outline 1.

Fundamental Principles

2.

Sky-Wave OTH Radar

3.

HF Radar Sub-Systems

4.

HF Signal Environment

5.

Conventional Processing

2

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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 The choice of frequency band has a pronounced influence on the characteristics and performance of a radar system. Meteorological Effects

Ionospheric Effects Over-the-Horizon Radar

Microwave Radar (0.4 – 40 GHz)

Band

HF

VHF

UHF

L

S

C

X

Ku

K

Ka

Frequency

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

Wavelength

10-100 m

1-10 m

0.3-1 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

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Target Types Focus on radars used primarily for surveillance of man-made targets. ¾

Conventional & OTH surveillance radars have many common target types

Example: Surveillance radar targets & mission priority: Large Aircraft

Fighter-Sized & Helicopters

Missiles

Large Ships

Destroyers & Patrol Boats

Go-Fast Boats

Aircraft (Primary Mission)

Ships (Secondary Mission) Remote sensing radars (e.g. sea-state mapping) target natural scatterers. ¾ Remote sensing applications of OTH radar not explicitly considered here

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Surveillance Functions Conventional & OTH surveillance radars share the main functions. ¾ Target detection, localization & tracking

Example: Main surveillance radar functions: Clutter

Noise

Jamming

1) Target detection-estimation: 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

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Early HF Radar British “Chain Home” radar, the first used for air-defence in wartime [2]. Æ HF technology was only available means to generate sufficient power (1935) Radar designed for line-of-sight ranges, not for over-the-horizon detection. Æ Echoes from very long distances constituted “interference” for radar operators Later during world war II, microwave radars were successfully employed. Æ Regarded as the most competitive frequency band for line-of-sight applications 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. Æ 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)

GREATER USEABLE BANDWIDTHS (FINE RANGE RESOLUTION)

LOW AMBIENT NOISE LEVEL (INTERNAL NOISE LIMITED) CONVENTIONAL RADAR

POTENTIAL FOR CLUTTER REDUCTION (e.g. “UP-LOOKING” GEOMETRY)

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Line-of-Sight Coverage Microwave radar coverage is mostly restricted to line-of-sight (LOS). ¾ Propagation is shadowed by mountains & limited by the Earth’s curvature

Earth’s S u r f ac e

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

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Beyond Line-of-Sight 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. ¾ Exploited by OTH radar & short-wave communicators since G. Marconi (1901) MICROWAVE

IONOSPHERE

HF Surface-Wave

Guglielmo Marconi

HF Sky-Wave

TX EARTH’S SURFACE

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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 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 & begin ground-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” Æ Earth not illuminated • Reflected rays at lower elevation Æ useful range extent beyond the skip-zone Escape Rays

CONCEPTUAL REPRESENTATION m 600 k

500

Altitude (km)

fc >

400

φ

300

1st Hop

2000

ESCAPE RAYS

m 300 k

Single 200 Frequency

1500

1000

Range (km) 2st3000 Hop

2500

fp

fc ≤

sin φ

fp

400

sin φ

300 REFLECTED RAYS

fc

100

200 100

Radar Footprint Useful Coverage

Backscattered Power (Two-way path)

Leading Edge Focusing Surface Clutter Target

Skip Zone Range (km) 0

500

1000

1500

2000

2500

3000

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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 Dwell Time (CPI)

Potential Radar Coverage Radar Footprint

Resolution Cells

Radar Footprint Higher Frequency TX & RX Beam Steering

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Radar Equation Noise-limited radar equation for OTH and conventional radar systems. ¾ Same form but range increases by order of magnitude (for all target altitudes) ¾ Target echo received by OTH radar experiences additional 40dB spreading loss Transmit Power (Average)

Output Signal-to-Noise Ratio

Transmit Antenna Gain

Receive Antenna Gain

Target Cross Section

Pave G t TA e σ F p

S = N N o L ( 4π ) 2 R External Noise Power per unit bandwidth

4

=

Effective Integration Time

Pave G t G r T λ 2 σ F p N o L ( 4π )3 R Losses (Path and System)

Operating Wavelength

Propagation Factor

4

Slant Range (Radar-to-Target)

Radar Type

Range Coverage (km)

Surface Coverage (sq km)

Sky-wave OTH Radar

1000-3000

Millions

Ground-Based Microwave

1-300

Tens of thousands 14

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Resolution & Accuracy OTH radar range resolution limited by useable bandwidths in HF spectrum. 1. 2.

User-congestion in HF band limits availability of clear frequency channels 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

0.2-2.0 deg.

Microwave

500-5000 kHz

30-300 m

6m

0.1-1.0 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 10-40 km 15

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Target Scattering 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

σ Optical

• 3 MHz (100 m wavelength) “Rayleigh” Æ RCS falls very rapidly with frequency • 30 MHz (10 m wavelength) “Resonance”

Cross Section

Example: Missile (Length = 10 m)

Rayleigh

Resonance

Æ 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)

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Clutter & Interference-plus-Noise 1. External interference-plus-noise often dominates internal receiver noise. ¾ Atmospheric noise (e.g. lightning) propagated long-distances by the ionosphere ¾ 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). ¾ “Look down” geometry illuminates Earth’s surface coincidently with targets ¾ 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 2000-3000 km range coverage

Skip Zone

17 Internal noise 20-30 dB lower

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Doppler Processing 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)

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Power & Waveforms OTH radar transmit power 10-100 X higher than microwave radar. • Average transmit power from 10kW - 1MW Æ sensitivity against noise

• Frequency modulated continuous waveforms Æ 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

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Propagation-Path Characteristics 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

Ionospheric Propagation-Path Variability

Temporal

Dynamic: Intra-CPI, intra-mission, diurnal, seasonal and solar cycle (11 year)

Spatial Frequency Polarization

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)

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

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 Æ over time & different coverage areas 20

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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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Early Research & Current Systems

Example Sky-wave Systems OTH radars exhibit significant diversity in architecture (no standard system). US Navy OTH Radar • “ROTHR” (NRL)

French OTH Radar • “Nostradamus” (ONERA)

Russian OTH Radar • “Steel Yard” (NIDAR)

• • • • •

• • • • •

• • • • •

Two-site Æ linear FMCW Maximum power 200 kW Receive aperture 2.6 km 372 elements & receivers Counter-drug application

Mono-static (coded pulse) Maximum Power 50 kW Y-Array, 384 m arm length 288 elements, 48 sub-arrays First reported detection 1994

Two-site (coded pulse) Average Power ~1 MW Vertical array, height 140 m Horizontally polarized dipoles Operational in the mid 1970’s

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Australian OTH Radar - Jindalee DSTO Australia “Jindalee Team” (1975) Team Leader: John Strath (circled below)

Dr Malcolm Golley

Dr. Fred Earl

History of OTH radar development in Australia 1952 1974 1979 1986 1986 1987 1990 1991 1998 2002

– – – –

1972 Research to determine ionospheric stability 1978 Jindalee Stage A (Detections in one direction) 1985 Jindalee Stage B (“Track while scan”, 90 deg) 1989 Jindalee Stage C (“Operational capability”) Announcement of JORN Defence White Paper on broad area surveillance JFAS Transferred from DSTO to RAAF JORN Contract Signature Contract to RLM JORN Commissioned

Jindalee “Bare Bones” OTH Radar Receiver Array (Central Australia)

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Australian OTH Radar - JORN Australian Jindalee Operational Radar Network Æ two additional radars • Longreach (Queensland), Laverton (Western Australia), Control centre (Adelaide) JORN Laverton OTH Radar

Typical Characteristics

TX & RX Site

Separation ~100km

Transmitter Array

Dual Band, Linear VLPA (~150 m)

Receiver Array

480 monopole pairs (~3 km aperture)

Coverage

+/- 90 degrees

Frequencies

5-30 MHz

Waveform

Linear FMCW

Bandwidth

5-50 kHz

Average Power

250 kW

PRF

4-80 Hz

CIT

1.5-30 seconds

RX Site

TX Site

TX Site

RX Site

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Configuration & Site Selection Radar configuration refers to relative transmitter and receiver locations. ¾

Monostatic Æ economical (single radar site & no need for inter-site links)

¾

Bistatic Æ Allows use of continuous waveforms (two propagation paths)

¾

Quasi-monostatic Æ High sensitivity but essentially one propagation path

¾

Multi-static: De-couples ionosphere from target localization & tracking

Site selection for OTH sky-wave radar takes several factors into account. 1.

Land Æ Needs flat wide open spaces with relatively homogeneous surface

2.

Electrically Quiet Æ Avoid strong HF noise near industrial/residential areas

3.

Self-Interference Æ Isolation to protect RX from TX continuous waveform

4.

Skip-Zone Æ Minimum detection range of ~1000 km (surveillance region) 25

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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

Linear Frequency Modulated Continuous Waveform (FMCW). Frequency

B fc + 2

T CIT

Coherent Integration Time (CIT)

Time

fc fc −

B 2

Ambiguity Resolution cT p c c R = = amb Range Δ R = 2 2 fp 2B

T

p

Pulse Repetition Frequency (PRF)

Resolution

Velocity

Δv =

cΔ fd c = 2 fc 2 f c T CIT

f p = 1/ Tp Ambiguity cf p v amb =26± 4 fc

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Typical Missions OTH radar surveillance missions broadly classed as air & surface tasks. Æ 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

Task C: Force Protection

A

• Radar steps through DIRs in a scheduled sequence.

D

• air route surveillance • navy fleet protection

DIR’s

Task D: Remote Sensing • sea-state mapping • cyclone tracking

• Time on each DIR = CIT, all DIR’s revisited in turn.

B 27

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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 Transmitter Footprint

ΔR =

Range-Azimuth Resolution Cell

300 km (20 Beams)

c = 15 km 2B

DIR contains 1200 resolution cells

ΔL = RΔθ ≈

Rλ = 15 km D

900 km (60 range cells)

Transmitter D=150m (8 deg at 15 MHz) Receiver D=3000m (0.4 deg at 15 MHz)

1 20 Receiver “Finger Beams” 20

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Aircraft & Ship Detection Aircraft & ships typically detected against noise & clutter respectively.

fc

• Air Æ Maximize Signal-to-Noise Ratio (SNR) for high velocity targets • Ship Æ Minimize clutter Doppler spectrum contamination for slower targets

B

• Air Æ Low to find clear frequency channels (with adequate range resolution) • Ship Æ High to reduce range cell size and increase sub-clutter visibility (SCV)

fp

• Air Æ High to avoid velocity ambiguities for fast moving aircraft targets • Ship Æ Low to avoid range-folded spread-Doppler clutter (unambiguous targets)

TCIT

• Air Æ Short for rapid region revisit rates (allows tracking over many DIR’s) • Ship Æ 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

ΔR

D

ΔL

f

p

R amb

v amb

Air

10

15

1000

30

50

3000

Surface

30

5

3000

10

5

Units

kHz

km

m

km

Hz

TCIT

Δv

900

1

10

30000

90

20

0.5

km

km/h

s

m/s 29

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Signal & Data Processing Rudimentary OTH radar signal and data processing steps. Signal Processing Beam Forming

Doppler Processing

CFAR

Note:

Peak Detection Tracking

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.

Display

Data Processing

Pulse Compression

More details on signal and data processing to follow.

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Ionospheric Regions The ionosphere may be broadly divided into three altitude regions. ¾ where electron-density versus height profile tends reach local maxima Ionosphere

Height (km)

Main Region Characteristics

D Region

50-90

Formed during the day-light hours Ionization too low for HF reflection

Attenuation of radar signals Electron-neutral collisions

E Region

90-140

May contain anomalous Sporadic-E Generally stable propagation layer

One-hop paths to ~2000km Allows signals to penetrate

Highest layer with maximum ionization Splits into F1 and F2 layers in the day F1 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

F Region

140-400

Relevance to OTH Radar

Ionosphere exhibits significant variability in structure in space & time. ¾ Temporal variations occur diurnally, seasonally and over the 11 year solar cycle ¾ Significant spatial variations occur across mid-latitude, equatorial & polar region 31

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Multipath Propagation Simple illustration of two-way one-hop reflections from E and F layers. • Target Æ multiple echoes often resolved in cone angle, range & Doppler shift • Clutter Æ contamination of Doppler frequency spectrum (mode superposition) • Interference Æ a single source can spread over a significant number of beams F-Layer

Simple One-Hop Modes

Mixed or “Hybrid” Modes

1F − 1F

1F − 1E 1E − 1F

1E − 1E E-Layer

TX-RX

Earth

Target

More complex modes involving multi-hop propagation, top-side layer reflections and trans-equatorial (chordal) modes also exist.

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Propagation-Path Information 1. Synoptic information about ionosphere useful for radar design. Æ Statistical forecasts of diurnal, seasonal, solar-cycle & global variations

2. Real-time information is useful for optimizing radar operation. Æ Mission-to-mission propagation-path data for DIR’s in all radar tasks

Updates from Ionospheric Sounders

Real-Time Propagation-Path Information

Backscatter sounding

Clutter power levels

Carrier frequency

Spectrum surveillance

Noise spectral density

Waveform parameters

Clutter Doppler profile

Spectral purity

Track association

VI & OI Sounders

Mode structure

Coordinate registration33

Radar Parameter Optimization

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

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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 1000-3000 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

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Transmitter

Ground Screen 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 Æ approximately 300,000 sqm of galvanised earth-mat. JORN Laverton Transmit Site Ground-Screen High-Band Low-Band

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Transmitter

Transmit Aperture 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

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Transmitter

Elevation Control Two-D transmit apertures permit the beam to be steered in elevation. Enhanced transmit directivity in elevation has positives & negatives. + Æ Improves sensitivity against clutter, facilitates mode selection & CR

- Æ 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

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Receiver

Element Design 1. Matched antennas less important for externally noise-limited receivers. ¾ Antenna efficiency experienced by targets & noise Æ no SNR improvement ¾ Match elements at high end (lower noise) with graceful frequency response

2. Reduce cost by using small end-fire antenna element doublets. ¾ Antenna heights of 4-6 meters (less susceptible to Aeolian noise effects) ¾ Twin elements Æ 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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Receiver

Array Aperture Uniform linear arrays (ULA) ¾ Best spatial resolution for cost but elevation-azimuth ambiguity (cone angle)

Two dimensional arrays ¾ Elevation control & mode filtering (with 2D TX) plus wider azimuth coverage

Wide receive apertures improve:

Upper limit on RX aperture size:

• Gain (sensitivity) & spatial resolution

• Greater expense & additional land

• Target detection, location & tracking

• Need greater # of coherent beams

Jindalee Æ Uniform linear array (~ 2.8 km, 90 deg. )

JORN Æ L-shaped array ( ~ 3 km apertures, 180 deg. )

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Receiver

Reception Channel Traditional heterodyne receiver and sub-array beamforming architecture. ¾ Fine resolution “finger beams” formed by digital combination of all RX outputs ¾ 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

• Network of switched delay-lines • Steers sub-arrays over footprint

• Tuneable local oscillator • Fixed IF filter bandwidth

• High Dynamic range • 16 bit I&Q sampling

• Conversion to base-band • Calibrated freq. response

Limiting factors: Linearity, A/D conversion, reciprocal mixing & image rejection.

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Radar Management

Frequency 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

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Radar Management

Backscatter Sounder 1. Backscattered clutter power versus frequency, group-range & beam. ¾ Original system resolutions: 200 kHz, 50 km and 8 beams over 90 deg. ¾ 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. ¾ Note significant azimuth dependence, and possibility of range-folded clutter ¾ Range extent of 2000-3000 km illuminated most by frequencies 17-18 MHz 17 MHz

18 MHz Range Ambiguity

Range-folded clutter

2nd Hop

1st Hop

Range Coverage

Skip-Zone 43

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Radar Management

Spectrum Surveillance 1. Identify unoccupied frequency channels in real-time at receiver site. ¾ Avoid interference to other HF services (e.g. broadcasting/communications) ¾ Omni-directional antenna Æ measures noise power in 2 kHz wide channels

2. SCV Æ combine spectral surveillance & clutter power measurements. ¾ Both databases acquired at time of radar operation and in the radar location ¾ Sub-clutter visibility (clutter-to-noise ratio) good indicator of radar sensitivity Entire HF Spectrum

1 MHz Wide Zoom Other HF Users

Protected emergency channels forbidden

Background noise level in clear channels & in azimuth

Clear channels (> 100 kHz)

Background noise level 44

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Radar Management

Vertical & Oblique Incidence Sounders 1. Maintain a real-time ionospheric model (RTIM) of mode structure. ¾ Enables propagation modes to be identified and reflection heights estimated ¾ Propagation-path information for track association & coordinate registration

2. Network of sounders with rapid (5 min) updates near dawn & dusk.

OI Ionogram Æ Darwin-Alice Springs (1260 km path) Frequency Dispersion

VI Ionogram Æ Similar time near path mid-point

F-layer (high rays) X-ray O-ray F-layer (low rays)

Virtual height F

E-layer RFI

Virtual height E

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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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Composite Signal Environment

Signal Environment Composite received signal for OTH radar is a superposition of: ¾ Radar waveform echoes and interference-plus-noise.

OTH Radar Signal Environment

Radar Echoes

Interference-plus-noise

Clutter Returns

Target Echoes

(e.g. Land, Sea)

Æ skin echoes

Anthropogenic (Man-Made)

Naturally Occurring

Unintentional

Intentional

(e.g. Electrical machinery)

(e.g. Radio stations)

Atmospherics

Galactic

(e.g. Lightning)

(e.g. Stars) 47

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Land & Sea Surface Clutter

Clutter Power Received power of clutter “backscattered” from Earth’s surface. ¾ Function of resolution cell area & normalized backscatter coefficient

σ = σ0 × A

Effective Clutter RCS • Single resolution cell

Resolution Cell Area • Aperture, range & bandwidth

Normalized Backscatter Coefficient • Surface properties & grazing angle

General Characteristics: Sea clutter often more powerful than land clutter (order of magnitude) Æ Higher conductivity of sea-surface and resonant (Bragg) scattering mechanism

High seas towards/away from radar significantly increase clutter power Æ More resonant backscatter, while very flat seas produce near “specular reflection”

Spatial RCS variations gradual over sea, but can be very sharp over land Æ Presence of cities and other topographical discontinuities can enhance RCS Received clutter power may be 40-80 dB stronger than target echoes Æ Receiver dynamic range must be sufficiently high to capture both signals

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First Order Clutter Resonant clutter two orders of magnitude stronger than higher order. ¾ Advancing & receding Bragg wave-trains Æ Doppler spectrum Bragg lines EM wavefronts

λ 2

ψ

Bragg Wave-Trains Advancing Wave

Receding Wave

Bragg wavelength

Radar wavelength

L cos ψ =

In deep water, the Bragg wave trains move with radial velocity (i.e. gravity waves):

v = ±

⎤ 1 ⎡ gλ = ± 2 ⎢⎣ π cos ψ ⎥⎦

1 / 2

Without surface currents, this imposes a Doppler shift on two clutter “Bragg Lines”

fb =

2 v cos ψ

λ

⎡ g cos ψ ⎤ = ±⎢ ⎥⎦ πλ ⎣

2

Grazing angle

L gL 2π

λ

1/2

49

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) High RCS

Blind speeds (solid lines)

SCR limits target detection Medium RCS Lower RCS

Higher Order Clutter Continuum 50

DSTO


Ship Detections Doppler Shift

10 Beams 10 9 8 7 6 Nested Range Cells

Land clutter

5 4

Sea clutter

3

Note Doppler spreading (transit via ionosphere)

2 1 -

0 Hz

+

51

DSTO

Noise & Radio Frequency Interference

Spectral Density 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

Background Noise Level

Night Time (~Midnight) Powerful RFI & congested lower HF spectrum

No sky-wave paths for higher frequencies at night

Higher Background Noise Mainly Mainly Atmospheric Galactic

52

DSTO

Noise & Radio Frequency Interference

Background Noise Variations 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 Æ 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

DSTO

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

54

DSTO

Signal & Data Processing Stages

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

Tracking


Data Processing Stages CFAR

Peak Detection

Radar Data and Track Displays

55

DSTO

Range/Beam & Doppler Processing

Pulse Compression 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

Taper Function

Returned Echo

Energy

Difference Frequency Δf

FFT

B Δf =

τ

T

c 2B

ΔR =

τB Tp

R =

p

cT p Δ f cτ = 2 2B

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

40

600 km

Surface

0.2 seconds

30 kHz

30,000 km

5 km

80

400 km 56

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

Chirp K

Chirp 2 Processed Range Bin

Range-Doppler Range

Range

Taper Function

Range

Range Bins

Doppler Processing FFT Doppler 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

100

15 MHz

Surface

0.2 seconds

40 seconds

+/- 90 km/h

0.25 m/s

200

15 MHz 57

DSTO


Beamforming 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

Rx 1

Beam Cell

Range

Beam FFT or DFT

Processed Cell

Receivers Range & Doppler Processing

Doppler

ULA Narrowband Sensors

Range Rx N

Range & Doppler Processing Doppler

Example for sky-wave OTH radar.

Range

θ1 Doppler Beams Range

θN Doppler

Taper Function

Task

Aperture

Frequency

Ambiguity

Resolution

No. Beams

Coverage

Air

1500 m

15 MHz

d / λ < 0.5

~ 1 deg.

10

10 deg.

Surface

3000 m

15 MHz

d / λ < 0.5

~ 0.5 deg.

20

10 deg. 58

DSTO


Processing Example Pulse Compression

Doppler Processing

Beamforming

Æ Receivers, ranges, & pulses

Æ Receiver Range-Doppler maps

Æ Beam Range-Doppler maps

Æ Strong clutter masks target

Æ Clutter confined close to 0 Hz

Æ Target becomes clearly visible

Range

Rx 1

Beams, Nested Range Cells

Receivers, Nested Range Cells

Receivers, Nested Range Cells

Rx 2

Clutter

Target

59 Time (Chirp Number)

Doppler Frequency

Doppler Frequency

DSTO


Multipath Echoes BEAM SPECTRUM

“Zoom” (beam containing target) Group Range

Array Boresight

1F − 1F “Coning Effect”

1F − 1E 1E − 1F 1E − 1E

}

-

0 Hz Doppler Frequency

Hypothesized mode structure F-Layer

Key observations: • Single target Æ multiple echoes • Distinct range, Doppler & beam

E-Layer

TX-RX Earth

Target

F-F mode c.f. E-E mode • Longer group-range • Smaller Doppler shift 60 • Higher coning effect

+

DSTO


Window Functions Importance of window functions to control spectral leakage. Without Doppler window

With Doppler window

Target

Target masked by Clutter sidelobes

61

DSTO

CFAR Detection & Peak Estimation

CFAR Processing Constant false alarm rate (CFAR) processing applied to ARD data. ¾ 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

CUT

Guard Cells Range Window

CFAR window dimensions may be changed to suit local disturbance features. ¾ Cell Averaging (CA) or Greatest of Ordered Statistics (GOOS) methods. 62

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

Possible Clutter False Alarm

Low Threshold

High Threshold

(high false alarm rate)

(low detection probability)

Target

}

Suitable detection threshold range

Target

63

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 Peak

Quadratic Interpolation

Target Beam Estimate

64

DSTO

Tracking and Radar Displays

Tracking Target presence may be declared on basis of established target tracks. • Early-warning Æ 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

DSTO


Coordinate Registration Challenging problem of converting from radar to ground coordinates. Æ Uncertain propagation via the ionosphere Several possible CR techniques: ¾

Ray tracing with real-time ionospheric model (RTIMs)

¾

Transponders at known locations in surveillance area

¾

Detection of sea-land clutter boundaries in coverage

¾

Association of detections with available GPS reports

¾

Detections on commercial aircraft & shipping lanes

¾

Registering airports where tracks begin or terminate

¾

Correlation of clutter RCS enhancements with cities

Æ Effective fusion of different CR techniques 66

DSTO


Radar Data Displays Geographical Track Display

“Whitened” ARD Display

• Detections filtered in time (CPI)

• Target position + Doppler

• Displays multipath target tracks

• For a single DIR and CPI

Tracks for single target

“Stare” Scroll Display

• 3 tracked ionospheric modes

• Localized area (one beam)

• With possible TID presence

• Clearly shows manoeuvres Range x

Target Detections

Range x+1

Manoeuvring Target

Time

67 Doppler

Doppler

DSTO High Frequency Over-the-Horizon Radar

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