Answers to the self-test

Answers to the self-test Microwave Remote Sensing:

Tropical forests are very humid, causing cloud cover nearly all the time. Optical systems then fail and radar is the only possibility for monitoring these areas since radar can 'look' through clouds.

The main difference lies in the fact that radar is an active system, operating day and night, and being insensitive to cloud cover. An optical system mostly is a passive system, which is not able to penetrate clouds.

The amount of energy reflected or emitted by the Earth is very small in the microwave region. In order to obtain a signal larger than the noise level, the signal must be integrated over larger areas. Active systems can produce higher energy amounts.

Much the same as with optical sensors that have different bands or channels of data, multi-wavelength and multi-frequency radar images can provide complementary information. Radar data collected at different wavelengths is analogous to the different bands of data in optical remote sensing. Similarly, the various polarizations may also be considered as different bands of information. Depending on the wavelength and polarization of the radar energy, it will interact differently with features on the surface. As with multi-band optical data, we can combine these different "channels" of data together to produce colour images which may highlight subtle variations in features as a function of wavelength or polarization.

With resolution we mean here the spatial resolving power: how far should objects be apart in order to be observable separately. Range resolution refers to the separability perpendicularly to the flight direction, while azimuth resolution refers to the separability in the flight direction.

Generally, image brightness increases with increased moisture content. However, in the case of flooding, the surface is completely saturated and results in standing water. Areas where the water has risen above the height of the crops will likely appear dark in tone, as the water acts as a specular reflector (mirror) bouncing the energy away from the radar sensor. Flooded areas would generally be distinguishable by a darker tone from the surrounding agricultural crops which are not flooded and would scatter more diffusely. However, if the wheat and corn stalks are not completely submersed, then these areas may actually appear brighter on the image. In this situation, specular reflections off the water which then bounce and hit the wheat and corn stalks may act like corner reflectors and return most of the incoming energy back to the radar. This would result in these areas appearing quite bright on the image. Thus, the degree of flooding and how much the crops are submersed will impact the appearance of the image.

For an appropriate visual interpretation it is important to reduce or even eliminate the speckle in a radar image. The human eye is very distracted by the effect of speckle.

The backscatter response, and thus the appearance of an object or feature on a radar image, is dependent on several things.

Different radar wavelengths or frequencies will result in variations due to their differing sensitivities to surface roughness, which controls the amount of energy backscattered.
Using different polarizations will also affect how the energy interacts with a target and the subsequent energy that is reflected back to the radar.
Variations in viewing geometry, including look/incidence angle, the look direction and orientation of features to the radar, and the local incidence angle at which the radar energy strikes the surface, play a major role in the amount of energy reflected. Generally, these differences can be quite significant between airborne and spaceborne platforms.
Changes in the moisture content of an object or feature will also change the amount of backscatter.

Speckle is caused by the coherent summing of radar backscatter from scatterers (e.g. plants or leaves) within one resolution cell, which causes interference. In principle radar is a coherent source of radiation, with only one wavelength and all waves being in phase (no phase differences while emitting).

A SAR system (synthetic aperture radar) simulates a very long antenna by special data recording and processing techniques. As a result a high spatial resolution (azimuth) is obtained.