Figure 1: Band designation of the radio spectrum and designation of the microwave range
Radar systems (Radio Detection and Ranging) were developed in the 1950s mainly by the armed forces. Radar is an active remote sensing system which means that it provides its own source of energy to produce an image. Thus, it does not require sunlight (as do optical systems) and data can be acquired either by day or by night. Furthermore, due to the specific wavelength of radar (for the microwave spectrum see Figure 1) cloud cover can be penetrated without any effect on the imagery.
Figure 1: Band designation of the radio spectrum and
designation of the microwave range
Radar is based on the transmission and reception of pulses in a narrow beam in the cm bands of the electromagnetic spectrum; the returning echoes are then recorded, taking into consideration their strength, time interval and phase. The power received by the antenna on board from each radar pulse transmitted is directly connected with the physical characteristics of the target through the backscattering coefficient. The value of this backscatter coefficient (corresponding to grey values in optical images) is basically dependent on three factors:
A surface is considered as being rough in the radar sense if its structure or shape has dimensions which are an appreciable fraction of the incident radar wavelength. For example, gravel surfaces exhibit stronger scatter than smooth clays. Another example of the effect of surface roughness can be observed when looking at the difference between water surfaces with and without waves. Over water surfaces which are not moved by wind effects, almost no energy is scattered back to the antenna, which means that this area appears dark. Land surfaces are usually rougher (higher backscatter) than water surfaces as they contain structures with vertical faces and corners.
Moisture content influences the electrical properties of a target (soil, vegetation, etc.) and the backscatter increases with humidity. A wet ground surface is characterised by a stronger backscatter than a dry one having the same roughness.
The importance of both factors is also dependent on the chosen wavelength. A given surface may appear smooth at a wavelength of 25 cm (S band) and rough at 5 cm (C band). Furthermore, the longer the wavelength the higher the penetration capabilities. Thus, a 100 cm wave (P band) penetrates vegetation better than a 3 cm (X band wave) one.
Furthermore, a distinction can be made between active systems (radar) and passive systems (radiometers), as well as non-imaging systems (scatterometers, altimeters) and imaging systems (synthetic or real aperture radar, scanning scatterometers). For example, the radar instrument mounted on ERS is a SAR (Synthetic Aperture Radar) which means that the antenna is made to appear longer than it really is by exploiting the relative movements between the platform and the Earth with complex processing. Through this, a higher resolution can be achieved. Moreover, both the polarization and the incidence angle play a very important role in the detection of a target with the radar instrument.
More detailed information on radar principles can be found via the bibliography given in the Annex.
1) This chapter has been based on the ESA/FAO publication "Principles of Radar Imagery" , Roma 1989
SP-1199