TABLE OF CONTENTS 'The Earth, the air, the sea, the snow; the rain, the fog, the winds that blow.'
Anon
The previous chapter looked at a radar and target in unbounded free space. In reality, the space traversed is bounded by the sea surface, which acts as an imperfect mirror. Oblique incident energy is forward-reflected specularly (Chapter 2, Section 2.1.1, Figure 2.1) with an efficiency which depends on surface smoothness. Phase is also shifted at the point of reflection, which is called the grazing point. Arriving via this longer route some nanoseconds later than the direct ray, the indirect rays are delayed in phase as well as being reduced in amplitude. Constructive or destructive interference results from the system geometry and modifies the signal strength at the target.
The sea surface follows the curvature of the Earth, introducing a horizon, whose range depends on the heights of the scanner and target, and on the refractive index variation with height of the atmosphere. At long range, multipath interference is replaced by a diffraction mechanism which carries some energy to targets beyond the horizon. Heavy seas may physically obscure or screen the target from time to time, reducing the maximum available probability of detection. This effect is discussed in Chapter 12.
The atmosphere is lossy, particularly during precipitation. Near the end of this chapter, we develop the atmospheric loss term L A to account for the attenuation of air, water vapour and precipitation in the radar-target path.
The processes recur...
