3.3 Imagery standards
103
The Lidar sensor emits laser pulses and records the time until a reflection is received. It is mostly operated from an aircraft in order to determine the shape of the
earth's surface. Airbome Lidar is often called Laser Scanning. Some systems include
seabed surveys in shallow water.
Synthetic Aperture Radar (SAR) and Interferometric SAR (InSAR) type sensors
are operated from satellite and aircraft platforms. The sensor emits radiation in the
microwave band and records the time and the phase of the reflection. The system
works under any light conditions (i.e., day or night). As microwave frequencies can
penetrate clouds, the system's operation is not restricted to good weather conditions.
Hn = flyi ng height
ß = depression angle
0n = near edge incidence angle
0 f = far edge incidence angle
Rs = slant range
Rg = ground range
a = ground range resolution (x direction)
b = azimuth resolution (y direction)
e = slant range resolution
Fig. 3.24. Geometry of a side-Iooking Synthetic Aperture Radar (IS057 2003)
Fig. 3.25. 1nterferometric point positioning by range and phase difference
Bis the base between two independent antennas. The circles correspond to "equi-range" !ines
to sensor sI and the hyperbolas to "equi-difference of range" !ines to both sensors. The flight
direction is vertical to the drawing plane. (1S057 2003)
The hydrographie sonar is used to measure the depth of the seabed and emits
sound as weil as measuring the time until a reflection is received.