Principles of Modern Radar: Basic Principles

1. INTRODUCTION AND RADAR OVERVIEW

 

1.1 INTRODUCTION

radar applications: the traditional military and civilian tracking of aircraft and vehicles, two- and three-dimensional mapping, collision avoidance, Earth resources monitoring

 

1.2 THERADARCONCEPT

Interference comes in four different forms:

(1) internal and external electronic noise

(2) reflected EM waves from objects not of interest, often called clutter

(3) unintentional external EM waves created by other human-made sources, that is, electromagnetic interference (EMI)

(4) intentional jamming from an electronic countermeasures (ECM) system, in the form of noise or false targets

 

1.3 The physics of EM waves

Superposition - (constructive or destructive interference)

 

Polarization

In general, the polarization is linear if the x and y components differ in phase by any integer multiple of π radians;

If Ex = Ey and the phases differ by an odd multiple of π/2, the tip of E traces out a circle as the wave propagates and the EM wave is said to be circularly polarized;

Ex ≠ Ey polarization is elliptical.

 

1.4 INTERACTION OF EMWAVE SWITH MATTER

diffraction (antenna); attenuation, refraction and depolarization (atmosphere); and reflection (target).

 

Diffraction

 

 

Narrow antenna beamwidths are desired in applications such as tracking, mapping, and others where good angular resolution is desired.

Applications in which large antenna beamwidths are advantageous are (1) in the search mode and (2) in strip-map synthetic aperture radars (SARs).

 

Atmospheric Attenuation

 

 

Attenuation peaks: 22 GHz (due to water vapor absorption), 60 GHz (due to oxygen absorption)

high frequency radar: 35GHz(Ka-band), 94GHz(W-band), etc.

low frequency radar: L-band, S-band

 

 

At radar frequencies, rain and cloud attenuation is small, giving radar systems their famous “all-weather capability” not seen in electro-optical and infrared (IR) systems.

 

Atmospheric Refraction

altitude increase → index of refraction reduce → bend back toward the earth

this allow the EM wave to propagate over the horizon

Over land, long-range propagation can be achieved by using the refractive effect at the earth’s ionosphere.

→ works at high-frequency (HF) region (3–30 MHz) and is generally not encountered above 150MHz. Radars that use this phenomenon are called over-the-horizon (OTH) radars.

 

Reflection

reradiation types: scattering, reflection

scattering phenomenology is quantified by the target parameter RCS(radar cross section)

three mechanisms determine the RCS: interception, reflection, directivity

 

1.5 BASIC RADAR CONFIGURATIONS AND WAVE FORMS

 

Monostatic versus Bistatic

Monostatic configuration: one antenna serves both the transmitter and receiver

Bistatic configuration: separate antennas for the transmit and receive radar functions

Use of two antennas alone does not determine whether a system is monostatic or bistatic. If the two antennas are very close together, say, on the same structure, then the system is considered to be monostatic. The system is considered to be bistatic only if there is sufficient separation between the two antennas.

Most modern radars are monostatic — a more practical design since only one antenna is required. The isolation is provided by a T/R device, such as a circulator or switch. For a radar using a pulsed waveform, the transmitter and receiver do not operate at exactly the same time.

 

Continuous Wave versus Pulsed

CW radar: RX/TX 동시 동작 → bistatic configuration → low transmitting power → short-range

Pulsed radar: monostatic configuration → RX/TX 번갈아가며 동작

Pulsed radar cycle time: interpulse period (IPP) or pulse repetition interval (PRI)

Pulse Repetition Frequency (PRF): 1/PRI

Duty cycle: fraction of transmit time during radar cycle time ($d_t = \frac{\tau}{PRI}$)

Average power: $P_{avg} = P_t \cdot d_t$ where $P_t$ is peak transmitted power

Range sampling → time between samples must be no more than a pulse width (todo: why?)

 

 

(todo: 여기 부분 노트북으로 채울 것!)

 

 

1.6 NOISE, SIGNAL-TO-NOISE RATIO, AND DETECTION

Thermal noise: Environmentally generated (from >0K material), internally generated (from antenna)

For microwave radar: internal noise > environmental noise

random variable nature: noise alone can spike up (false alarm), target plus noise drop below the threshold (detection fail)

$P_D$: probability of detection

$P_{FA}$: probability of false alarm

radar range equation (RRE): equation to predict the SNR (Chapter 2)

 

1.7 Basic Radar Measurements

 

(todo: 여기 resolution 부분을 간단하게라도 채우는 게 어떨까)

- Azimuthal angle, $\theta$
- Elevation angle, $\phi$
- Range, $R$
- Range rate, $\dot{R}$: (by measuring Doppler frequency, $f_d$) → suppress clutter, determine multiple targets at the same range, improve the cross-range resolution in SAR radar
- Polarization: can discriminate clutter (e.g., returns from rain) vs. target

polarization scattering matrix (PSM)

$$\mathbf{S} = 
\begin{bmatrix}
\sqrt{\sigma_{11}} e^{j\phi_{11}} & \sqrt{\sigma_{12}} e^{j\phi_{12}} \\
\sqrt{\sigma_{21}} e^{j\phi_{21}} & \sqrt{\sigma_{22}} e^{j\phi_{22}}
\end{bmatrix}$$

Polarizations 1 and 2 are orthogonal (horizontal, vertical or right-hand-circular, left-hand-circular)

Measuring PSM:

1) need: polarization-agile transmitter + dual-polarized receiver

2) transmit vertical polarization → horizontal, vertical reciever → results: two amplitude, relative phase

3) transmit horizontal polarization → horizontal, vertical reciever → results: two amplitude, relative phase

* horizontal, vertical 편광을 동시에 보내지 않으면 uncertainty 문제가 있으나, 실제 구현에서는 두 편광을 보내는 시간 차이가 무시할 만큼 작기 때문에 문제 없다.

 

1.8 Basic Radar Functions

Three basic functions: (1) search/detect (2) track (3) image

(1) search/detect

- Mechanical scanning (continuous rotation)

- ESA (Electronic Scanning Array) with discrete beam positions

(2) track

Tracking radars measure target states as a function of time.

Track filtering: multiple measurements + target dynamics model → smooth target position

Antenna beamwidth: wide - search mode, narrow - track mode, medium - compromise solution (search & track simultaneously)

(3) image

SAR resolution affected by: resolution of range and cross-range (angular) profile, Doppler shift resolution

Target identification methods

- Measure a one-dimensional high-range-resolution (HRR) image

- Two-dimensional range/cross-range imaging

- High-resolution Doppler spectrum analysis

- Target polarization characteristics measurement

- ATR (Automatic Target Recognition) techniques for analysis and identification

 

1.9 RADAR APPLICATIONS

ground-based or surface ship: search & track performed by two independent radar systems

airborne application: limited power → two functions performed by one system

 

(todo: 어플리케이션 궁금하면 읽어볼 것)

 

 

 

 

16.6 ROBUST CFARs

Greatest-of CA-CFAR: 경계부분에서 안전한 성능, 간섭 target이 있을 때는 성능이 저하

Smallest-of CA-CFAR: 앞이나 뒤 window의 간섭 target을 제거할 수 있음, 두 window에 모두 간섭 target이 있을 경우 성능 급격히 저하

censored CFAR: 가장 큰 $N_C$​ 개의 샘플에는 간섭 표적의 반사가 포함될 수 있으므로 이를 검열한다.

Trimmed CFAR: 가장 큰 $N_{TL}$ ​ 개의 샘플과 가장 작은 $N_{TS}$ ​ 개의 샘플을 제거, CS-CFAR의 일반적인 형태, 일반적으로 $N_{TL}$ ​ 은 간섭 표적을 제거하기 위해 선택되고, $N_{TS}$​ 는 클러터 경계 오경보를 억제하기 위해 선택

Order Statistics CFAR: N개의 샘플 정렬, k-번째 샘플을 CFAR 통계로 선택 → N−k개의 간섭 표적을 배제

 

16.7 ALGORITHMCOMPARISON

heterogeneous clutter estimating (HCE) CFAR

generalized censored mean level detector (GCMLD)