Ch 8

Radar Signal Processing

@Prof Y Kwag 2

Lecture 8 : Radar Signal Processing

Objective - - MTI, MTD - SAR

- Introduction - Signal Integration - Correlation / Convolution - Moving Target Indicator (MTI) - Moving Target Detection (MTD) Doppler Processing - PRF Ambiguity - Improvement Factor - High Resolution Radar - SAR - Reference

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Introduction

RSP objective - Improve S/N and Pd of target - High clutter rejection - High Interference/jamming rejection - Exact information extraction : characteristics

Environments - Clutter - surface, volume clutters - Interference jamming, ECM, spiky noise - Target RCS scintillation SW 1~4 - Noise & noise jamming = randomness(amp/phase) - Desired target = small, orderliness phase

Differences between signal and noise - orderliness vs randomness in phase & amp - rate of changes of the phase of orderly signal

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Introduction

Processes - signal integration : vector sum, orderly from hit-to-hit - correlation(pulse compression) : matching the desired signal to the reference

- filtering & spectrum analysis windowing used in correlation & spectral analysis to reduce leakage error

convolution : windowing in time convolution in freq. Domain

Block diagram - Digital pulse compression

< Typical signal Processor, Digital Pulse Compression >

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Introduction

- Analog pulse compression

< Typical signal Processor, Analog Pulse Compression >

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Sampling Range &Doppler

Range Bin Rate - PRF : rate at which an individual target can change

target sampling freq.

phase shift from hit to hit caused by the Doppler shift.

sample at a rate or equal to at least twice the highest Doppler

frequency, otherwise Doppler ambiguity

- Range/Doppler trade-off

Range bin rate = A/D sampling

= Range resolution

Doppler sampling rate = PRF

16 FFT (ex)

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PRI Dwell Time, CPI,

Burst, Scan

SCAN

DWELL TIME

CPI

RANGE

CELL

P1 P2 Pa P1 P2 Pa P1 P2 Pa

Scani-1 Scani Scani+1

DT1 DT2 DTk DTm-1 DTm

R1 R2 Ri Rk Rj Rl

CPI1 CPI2 CPI3

Effective Range Guard Time

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Radar Range-Gated Data Structure

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TN

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3 D Structure

Rangel/Azimuth/Doppler

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(1)

(2)

1st PRF 2nd PRF

PRF

f

f

Radar Echo Signal

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(4) AMTI

PRF

f

(3) MTI

PRF

f

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PRF f

PRF/N

f

(5) Doppler

Filter Bank

(FFT)

(6) CFAR

/

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Signal Integration

Non-Coherent Integration - Signal plus Noise

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Signal Integration

Non-Coherent Integration - Signal plus Clutter

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Signal Integration

Coherent Integration - Stationary Target

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Signal Integration

Coherent Integration - Bin-1 Moving Target

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Signal Integration

Integration Loss - Type of integration (coherent or non-coherent) - Number of pulse integrated - Required detection & false alarm probability - Target fluctuation statistics - Processing window used

Coherent integration loss is determined by - processing window used

Window loss for most window is less than 3 dB

- target fluctuation statistics

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Correlation

Correlation - process of matching two waveforms in time domain - determine the time at the maximum correlation coefficient

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Convolution

Continuous Convolution

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< Convolution >

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Gated CW Convolution

Gated CW Convolution

< Spectrum of Gated CW Wave from Convolution >

Clutter Rejection

MTI and Pulse Doppler Processing

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Air Defense Scenario

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Terminology

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Doppler Frequency

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Example Clutter Spectra

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MTI and Pulse Doppler Waveforms

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MTI Processing

Separate MTI Process

< Separate MTI Process for Each Range Bin >

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Two Pulse MTI Canceller

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MTI Processing

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MTI and Doppler Processing

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MTI and Doppler Processing

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Moving Target Indicator (MTI) Processing

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Clutter Spectrum Characteristics

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MTI Improvement Factor

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MTI Improvement Factor Examples

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Pulse Doppler Processing

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Moving Target Detector (MTD)

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MTI and Doppler Processing

PRF Stagger - Blind Doppler occurs when the freq. shift is an integer multiple of sample rate (PRF)

pulse-to-pulse PRF stagger

look-to-look & scan-to-scan stagger

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Staggered PRFs to Increase Blind Speed

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Blind Rejection Filter

Staggering Filter Response

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< Three Delay Non-Recursive Filter Response >

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Range Ambiguities

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Doppler Ambiguities

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Unambiguous Range and Doppler Velocity

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Limitation on Improvement Factor

Limitations on the Improvement Factor With good canceller or filter bank design, cancellation can be

essentially perfect if

- The antenna is stationary (not scanning)

- The clutter is totally stationary, with a zero width spectrum

- Enough rang sweeps are gathered to totally charge the canceller,

or in the case of a filter bank, the number of points processed is large

- The system is totally linear

- Pulse-to-pulse stagger is not necessary to avoid blind Doppler shifts

Many MTI systems are specified and tested with the antenna stationary.

Scanning is, of all the factors listed, the most important in limiting the

improvement of MTI and MTD. Without scanning or with step-scanning,

the same antenna gain is pointed at the clutter throughout the dwell

and the echo from non-moving clutter is constant.