Principles of Modern Radar - Volume 2 1891121537

6.1 Introduction 213• Image quality is predicted using deterministic quantities, such as resolution, as well asstatistical quantities, like the contrast ratio, that depend on the average scene content.• The quality of a particular image can be judged using standardized rating scales.• Image quality can be degraded by phase errors occurring over the course of the collection.These phase errors can affect the resolution and contrast of an image. They canalso introduce false copies of the scene content.• Measuring the platform motion and and compensating for it in the image reconstructionprocessing is the best way to reduce phase errors. Autofocus algorithms are then appliedto the imagery to remove residual defocus.6.1.3 NotationThe notation used in this chapter is summarized in this section. In certain cases a symbol isused several times with a subscript added to impart specific meaning evident from context.For example, τ c is used to indicate the length of the transmitted radar signal, while τ g isused to represent the time gating of the receiver.Symbols used in this chapterδφ Angular sampling period (rad)δk x ,δk y Wavenumber sampling period (rad/m)δr Range resolution (m)δx Cross-range resolution (m)θ Angle (rad)λ Wavelength (m)σ Backscatter coefficient (dimensionless)τ Time duration (s)φ Angle (rad)ω Radian frequency (rad/s)B Bandwidth (Hz)c Speed of light (m/s)D SAR image diameter (m)f Frequency (Hz)f D Doppler frequency (Hz)G TX , G RX Transmit and receive gain (dimensionless)H Radar altitude (m)K c Chirp rate (Hz/s)k, k Wavenumber, wavenumber vector (rad/m)L Distance (m)R, r Range (m)T, t Time (s)v Aircraft speed (m/s)x, x 1-D position, 3-D position (m)6.1.4 AcronymsAcronyms used are collected in this section. Aside from standard acronyms such as SNRand FFT, this chapter introduces many terms commonly used in the SAR literature.