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Radar Systems Design > Week 5 > Flashcards

Week 5 Flashcards

1
Q

Targets are detected when

A

The signal out of envelope detector crosses threshold

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2
Q

Detection criterion

A

Established using N received pulses from target

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3
Q

Analog radar displays

A

A-scope and PPI use persistence on tube to integrate signals

PPI tube persistence&raquo_space; pulse repetition interval

Returns from target contribute to brightness of spot on screen

Analog integration - no automatic detection

A-scope, PPI display requires a human observer (read range and angle from PPI)

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4
Q

Analog displays - observers

A

Can interpret signals

Get fatigued when staring at screen

Can fail to notice targets

Can misread range or angle

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5
Q

Automatic target detection

A

Removes observer as a detector

Threshold circuit deletes all targets below threshold

Has no intelligence, does not get fatigued, always reports range and angle correctly

Used by modern radar systems (present processed info to observer for interpretation, i.e., removes noise from targets and screen and labels target with additional info)

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6
Q

Air traffic control radar labels target on screen with

A

ID, altitude

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7
Q

Range gating

A

Dividing up received signal in time

A sample of the receiver output at an instant of time

Integration cannot be carried out without range gates

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8
Q

Natural time step for range gating

A

τ, the pulse width (corresponds to a step in range equal to ΔR the range resolution)

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9
Q

How does range gating work

A

Output of receiver is sampled at discrete instants of time after each pulse

Integration proceeds by adding up the samples returned by the pulses for each delay time

Threshold then applied to decide whether each range gate contains a target

Range dimension of the radar display will be divided into segments (cells)

For each cell a decision will be made as to whether a target is present - the cell might ‘light up’

There will be no ability to resolve multiple targets within a cell

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10
Q

M out of N detector

A

Signal pulses appear in same range gate, noise pulses appear randomly in time

Require M detections in N tries to declare that there is a target

In each range gate try N times (usually number of pulses per target)

1 for detection, 0 for no detection

Threshold at M detections

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11
Q

M out of N detector - Pfa

A

Pfa: Probability that a noise spike occurs in the same range gate on successive radar transmission falls rapidly with increase in N

Pd: probability that a target pulse appears in the same range gate on successive radar transmission falls slowly with increase in N

Probability of N noise pulse crossing threshold in the same range gate

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12
Q

m-out-of-n tries

A

Cost of improvement is an extended dwell time in order to conduct test N times

Improvement to Pd and Pfa

With M out of N detection we can meet spec with a smaller radar (can meet spec with a lower single-pulse S/N with M out of N processing than with single-pulse detection

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13
Q

Processing gain

A

Reduction in the S/N from M out of N detection required to meet spec

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14
Q

Target RCS Fluctuations

A

Targets typically have dimensions much larger than wavelength (ships, aircraft, people, except for raindrops where D

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15
Q

Two scatterer case

A

Equal RCS

Electric fields of reflected waves from target 1 and 2 can:

  • add in phase (E1+E2=2E)
  • add in anti phase (E1-E2=0)

Received power is proportional to E^2

Received power varies between 0 and 4P

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16
Q

Meaning of RCS for complex targets

A

RCS not single-valued

Because its RCS is the average value of σ over its PDF, the variability of σ needs to be handled statistically

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17
Q

Statistical models for RCS fluctuation

A

Case 0 - no fluctuating σ

2 PDFs x 2 fluctuation rates = 4 models (SW1, SW2, SW3, SW4)

Choice depends on the nature of the target

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18
Q

2 types of PDFs

A

Rayleigh and 4th degree chi-square

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19
Q

Two types of fluctuations

A

Fast, with σ varying from pulse to pulse

Slow, with σ varying from scan to scan (dwell to dwell)

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20
Q

SW1

A

Rayleigh, slow

Most commonly applied

Higher detection probabilities require larger increases in (S/N) - curve diverges

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21
Q

SW3

A

Chi-square, degree 4; slow

22
Q

SW2

A

Rayleigh, fast

23
Q

SW4

A

Chi-square, degree 4; fast

24
Q

Targets made up of many small scatterers

A

SW1, SW2

25
Q

Target made up of a combination of one large scatterer and many small scatterers

A

SW3, SW4

26
Q

Finding (S/N) with Swerling Targets

A

Find the (S/N) needed to meet spec with a non fluctuating model

Decide which swerling model best fits target

Read off additional S/N needed for the Swerling target

Add additional S/N to that needed for SW0 target

27
Q

Typical values of additional S/N needed to meet the Pd spec with a fluctuating target

A

SW1 or 2: 7-8dB

SW3 or 4: 3-4dB

28
Q

Impact of target fluctuations on pulse integration

A

Coherent integration requires ‘coherence’ in the pulse-to-pulse returns (the returns need to be highly correlated in phase)

Fast fluctuations of SW2 and SW4 targets defeat coherent integration of pulse returns

Does not affect non-coherent integration

29
Q

Efficiency of envelope detection

A

The output SNR from an envelope detector is less than the input S/N

Efficiency of detector gets worse as (S/N)in approaches 1 or 0 dB

30
Q

When noise voltage > signal voltage

A

Noise ‘captures’ the detector

Does not allow the single pulse S/N for non-coherent integration to fall below +3dB at threshold (does not apply to coherent detection)

31
Q

range ambiguity

A

PRF corresponds to a max unambiguous range

R = Run + x with x < Run, R’ = x = R-Run; real target can have any value {R} = mRun + x; correct target is R=R’=x

Echo from the first pulse is associated with the second pulse (R>Run)

All ranges > Run are aliased to the measurement interval {0, Run}

{R} < Rmax if targets do not occur beyond Rmax

Only if Rmax < Run have we eliminated the range ambiguity

32
Q

Resolving range ambiguity

A

Using two PRFs can resolve the range ambiguity that arise with individual PRFs

mRun1 = nRun2

If new Run is too far away, there is no possibility of a target being present of or of it having a large enough RCS to be detected

33
Q

Up to what range can we avoid ambiguity by combining observations with two PRFs

A

First occurrence of mRun1 = nRun2 = Run_eff

All target ranges beyond this range will aliase to nearer ranges

Avoid choosing PRFs with common multiples

34
Q

The atmosphere as a propagation medium includes the effects of

A 
Absorption 
Refraction
Volumetric scattering
Turbulence 
Called atmospherics
35
Q

In order of increasing altitude, the temperature

A

False through the troposphere (0-10km)
Rises through the stratosphere (10-50km)
Falls through the mesosphere (50-90km)
Riss through the thermosphere (above 90km)

36
Q

Structure of the atmosphere

A

Each layer has an upper boundary where the temp reaches a minimum (tropopause, mesopause) or maximum (stratopause)

At a pause the vertical gradient in temp is zero (dT/dz = 0)

37
Q

Refraction in the atmosphere

A

Refraction in the troposphere generally bends radar signal downwards

Causes errors in knowing target’s position but extends the range of radar systems

Obeys Snell’s Law applied to the index of refraction

Fairly predictable but can produce bizarre anomalies and what amount to mirages

38
Q

Snell’s Law

A

nsin(i)

n - index of refraction, i - angle of incidence

39
Q

Effect of earth’s surface

A

Effectively blocks propagation of E/M waves

Plane of the horizon defines two regions

40
Q

For a target in the line-of-sight region

A

Transmissions have direct access

41
Q

For a target in the shadow region

A

Transmissions only have access via a diffraction or refraction mechanism

42
Q

Distance to horizon

A

Microwave radars are line of sight devices

Some bending of rays occur due to refraction and targets can be detected beyond the physical horizon

43
Q

The ionosphere and refraction

A

Ionosphere begins about 80km altitude and extends through a peak at about 300 km to about 1000 km

Has electrical properties because it has ions and electrons

Refraction in the ionosphere is strong on HF frequencies and can cause signals to reflect and propagate downwards

Basis for over-the-horizon (OTH) radar

44
Q

What reveals the ionosphere

A

Airglow

45
Q

How can HF signal skip to great distances

A

By ‘bouncing’ off the ionosphere and Earth’s surface

46
Q

Atmospheric attenuation and absorption

A

An E/M wave loses energy from the intended direction of propagation by absorption and scattering (amounts to loss)

47
Q

Absorption is largely due to

A

Oxygen and water vapor

48
Q

Scattering is due to

A

Particles that deflect wave energy away from the intended direction of propagation

49
Q

Major factors in attenuation

A

Water content and temperature (highly variable)

50
Q

Clear air and water vapor attenuation

A

Attenuation in clear air varies strongly with frequency above 10 GHz
-significant peaks in attenuation occur within absorption bands that are due to molecular oxygen and water vapor

Water vapor produces a minor absorption band between 20 and 24 GHz (k-band)
-oxygen produces a major observation band that peaks near 60 GHz at almost 20 dB/km

Radar Systems Design (6 decks)

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