1.Basic Def in Radar:
(1).What is Radar?
DEF: is a detection system that uses radio waves to determine the distance (ranging), angle, or
velocity of objects
(2)What is Phased Array?
DEF:y usually means an electronically scanned array, acomputer-controlled array of antennas which
creates a beam of radio waves that can be electronically steered to point in different directions
without moving the antennas(調整相位去改變波束方向)
(3)active electronically scanned array (AESA)主動相位陣列或稱為有源相位陣列:
DEF:is a type of phased array antenna, which is a computer-controlled array antenna
2.Block Diagram of A Radar System:
3.Atmospheric Attenuation of EM Waves:
上圖為EM Waves在大氣中的衰減，可看出某些頻段會衰減特別多，所以要挑選carrier frequencies
如何挑選適合的carrier frequencies，依據以下三個原則:
(1)長距離的用Lower frequencies，因為其low atmospheric attenuation and high available powers特性
(2)短距離的用Higher frequencies，因為其有finer resolution, higher attenuation, and lower available powers
(3)A radar in high frequency bands usually operates at an atmospheric window
frequency where attenuation is relatively low
4.Intensity and Far-Field Plane Wave:
Intensity(equivalent to power density):
DEF:the power per unit area of the propagating wave
The locus of all points having the peak amplitude at a given moment in a spherical wavefront
球面波前在給定時刻具有峰值振幅的所有點的軌跡
若R>(2D^2)/λ則為far field plane wave
(近場和遠場是物體（例如發射天線）周圍電磁場的區域，或者是物體輻射散射的結果。非輻射近場行為在靠近天線或散射物體處
占主導地位，而電磁輻射遠場行為在較遠的距離處占主導地位)
transmitted power density:total radiated transmitted power PT
divided by the surface area of the sphere
5.Radar Cross Section (RCS)
DEF:denoted by σ in the unit of square meter
it is a measure of
(1)how much of the incident EM wave is reflected from the target
(2)how much of the wave is intercepted by the target, and
(3)how much is directed back toward the radar receiver
若一個target被設計成invisible to a radar(例如F35)，則其RCS要盡可能小
(1)最小化target的RCS
(2)在其表面使用radar-absorbing material
(3)shaping the target
6.Non-Coherent Versus Coherent
1.Non-Coherent:只偵測大小
2.Coherent:A radar system 偵測received signals的大小和相位
(Coherent可以去測量phase是否改變，所以可已做移動目標特徵提取和beamforming)
Most of the modern phased array radar systems are coherent because the phase
information is very critical to know target characteristics
