Imaging radar

The two main advantages of radar imaging over visual imaging sensors are 24 hour capability (radar can "see" equally well in daylight and darkness) and all weather capability (radar can "see" through clouds). Another advantage radar has over other sensors is that radar can penetrate slightly beneath the surface of the earth (mine detection capability).

a. Basics

An SBR satellite moving through space in orbit sends microwave radiation pulses through its antenna at the speed of light. The pulses are directed in the range, look or across-track direction. Figure 1 illustrates the following definitions:

• Slant Range – the line-of-site distance measured from the antenna to the target

• Ground Range – the horizontal distance measured along the surface from the ground track to the target

• Near Range – the area closest to the ground track at which a radar pulse intercepts the terrain

• Far Range – the area of pulse termination farthest from the ground track

• Depression Angle (β) – the angle measured from a horizontal plane downward to a specific part of the radar beam

• Look Angle (θ) – the angle measured from a vertical plane upward

When measured to the same part of the beam, the depression angle and the look angle are complementary angles (β + θ = 90°).

• Incidence Angle (φ) – the angle measured between the axis of the radar beam and a line perpendicular to the local ground surface that the beam strikes

• Grazing Angle (γ) – the complement of the incidence angle

Consequently, the incidence angle and the grazing angle are a function of both the illumination angle (β or θ) and the slope of the terrain. When the terrain is horizontal, the depression and grazing angles are equal (β= γ) and the look and incidence angles are equal (θ=φ).

• Resolution – the minimum separation between two objects of equal reflectivity that will enable them to appear individually in a processed radar image

• Pulse Rectangle – the surface area covered by the energy radiated from the sensor

When two or more objects fall within the same pulse rectangle they cannot be resolved as separate entities. Rather, they are presented as one echo to the radar system. If objects are separated by a distance exceeding the corresponding dimension of the pulse rectangle, they will be imaged separately.

• Range Resolution – determines resolution cell size perpendicular to the ground track

• Azimuth Resolution – establishes the cell size parallel to the ground track

Figure 3. Definitions of Terms for Imaging Radar [From 12]

b. Detection

Radar detection is defined as any object that reflects enough energy to be distinguished from the background noise by the receiver (a blip on the scope). Objects are categorized based on their ability to reflect microwave radiation. Highly reflective objects create large radar signatures. Flat metal surfaces produce large signatures; a significant portion of the microwave radiation is reflected back to sensor. Objects with multiple surface angles

produce small signatures; most of the microwave radiation is reflected away from the sensor.

c. Range Resolution

Range resolution is determined by the length of the emitted microwave pulse (pulse length). Pulse length is determined by multiplying the pulse duration (τ) by the speed of light.

Two objects will appear as one unless all parts of their reflected signals reach the radar sensor at different times. Consequently, objects must be separated by a slant-range distance greater than one half of a pulse length to be seen as separate entities.

Ground range resolution is half the pulse length divided by the cosine of the depression angle. Therefore, ground range resolution can be improved by increasing the distance from the ground track and by shortening the pulse length.

d. Signal Shape

Target resolution is determined based on the pulse length (“t” in the figure below). Related to pulse length, pulse repetition interval (PRI, “T” in the figure below) is the interval between pulses. As illustrated, the PRI duration is generally much longer than the pulse length. The pulse repetition frequency (PRF) is the inverse of the PRI.

Figure 4. Radar Pulse [From 13]

Pulse length is also related to spectrum. Range resolution is proportional to the length of the pulse. Essentially, a short pulse length contains a wide spectrum and a long pulse length is restricted to a narrow spectrum.

One solution to the pulse vs. spectrum conundrum is using frequency differential. By modulating the frequency of the pulses and monitoring the frequencies of the returns, the two objects can be discerned even if they overlap in time.

e. Azimuth Resolution

Beam width is determined by antenna size and wavelength.

Azimuth, or along-track, resolution is a function of the beam width. The beam width increases with range, therefore the greater the range the poorer the azimuth resolution. Two objects at the same range within the beam will appear as one because their returns will be received at the same time. Therefore, to distinguish between two objects, their ground separation distance in azimuth must be greater than the width of the radar beam.

f. Resolution

Azimuth resolution is the slant range multiplied by the wavelength divided by the length of the antenna. Therefore, azimuth resolution improves as range decreases, antenna length increases and wavelength decreases. To improve azimuth resolution, use a long antenna, a short operating wavelength, a close-in interval, or a combination of these factors. The problems are antenna size is limited and the all weather capability of radar is reduced when wavelength is less than 3 cm. The solution is Synthetic Aperture Radar (SAR).