However, the actual length of the antenna is limited by what can be carried on an airborne or spaceborne platform. Finer azimuth resolution can be achieved by increasing the antenna length. The radar beamwidth is inversely proportional to the antenna length (also referred to as the aperture) which means that a longer antenna (or aperture) will produce a narrower beam and finer resolution.įiner range resolution can be achieved by using a shorter pulse length, which can be done within certain engineering design restrictions. In this illustration, targets 1 and 2 in the near range would be separable, but targets 3 and 4 at further range would not. As the radar illumination propagates to increasing distance from the sensor, the azimuth resolution increases (becomes coarser). This beamwidth (A) is a measure of the width of the illumination pattern. The azimuth or along-track resolution is determined by the angular width of the radiated microwave beam and the slant range distance. Thus, for fixed slant range resolution, the ground range resolution will decrease with increasing range. However, when projected into ground range coordinates, the resolution in ground range will be dependent of the incidence angle. Slant range resolution remains constant, independent of range. For example, targets 1 and 2 will not be separable while targets 3 and 4 will. Two distinct targets on the surface will be resolved in the range dimension if their separation is greater than half the pulse length. The range or across-track resolution is dependent on the length of the pulse (P). In this case, the resolution is dependent on the effective length of the pulse in the slant range direction and on the width of the illumination in the azimuth direction. If a Real Aperture Radar (RAR) is used for image formation (as in Side-Looking Airborne Radar) a single transmit pulse and the backscattered signal are used to form the image.

$efine altitude geometry$

Unlike optical systems, a radar's spatial resolution is a function of the specific properties of the microwave radiation and geometrical effects. The ground range distance (D) is the true horizontal distance along the ground corresponding to each point measured in slant range. At all ranges the radar antenna measures the radial line of sight distance between the radar and each target on the surface. In the near range, the viewing geometry may be referred to as being steep, relative to the far range, where the viewing geometry is shallow. The look angle (B) is the angle at which the radar 'looks' at the surface. The incidence angle is the angle between the radar beam and ground surface (A) which increases, moving across the swath from near to far range. The portion of the image swath closest to the nadir track of the radar platform is called the near range (A) while the portion of the swath farthest from the nadir is called the far range (B). This side-looking viewing geometry is typical of imaging radar systems (airborne or spaceborne). Range (D) refers to the across-track dimension perpendicular to the flight direction, while azimuth (E) refers to the along-track dimension parallel to the flight direction. The microwave beam is transmitted obliquely at right angles to the direction of flight illuminating a swath (C) which is offset from nadir. Similar to optical systems, the platform travels forward in the flight direction (A) with the nadir (B) directly beneath the platform. The imaging geometry of a radar system is different from the framing and scanning systems commonly employed for optical remote sensing described in Chapter 2.

Efine altitude geometry