Abstract Because of the all-weathers and whole-day-long monitoring capabilities, radar remote sensing have many advantages on the application of agriculture. It is necessary to objectively and reasonably have an analysis of the advantage and shortcoming of Radar RS for agricultural application. There are quite a few unknown fields on the effects of microwave upon crops or plants,it is a difficult task to mine more agricultural information from a Radar RS image.
The image processing of radar RS, to develop more suitable software for the radar RS , and the foundational research are needed to pay more attention to. The radar RS has a very bright prospects future.
1. Introduction
Scientists and officials of the government in agricultural section pay more and more attention to the radar RS, which is because of the reason for radar RS not only have the all-weathers and whole-day-long monitoring capabilities, but also have many other advantages on the application of agriculture. It is necessary to objectively and reasonably have an analysis of the advantage and shortcoming of Radar RS for agricultural application in order to push the development of radar RS forward. In fact, agriculture is a very large and complicated field. It is not enough to only use the conventional RS, because of its shortcomings, some of which are crucial, for instance, in the cloudy weather conditions, it can not get the image for the surveying work by using the conventional satellite RS. The cloudy weather, in most eastern part of China, the monsoon weather areas, often happens from May to August, the time when the crops growing and needed to be monitored by RS. It is already the uniform understanding that radar RS
has the unreplaceable technological advantage for agriculture.
But, after all, Radar RS for agriculture is a new field. There are quite a few unknown fields on the effects of microwave upon crops or plants, even on the statistical effects. Therefore, it is a difficult task to mine more agricultural information from a Radar RS image.
Chinese scientists and technologists on RS application for agriculture have done a lot of works on radar RS for agriculture, including land use monitoring, status monitoring for crop growth, flood and drought monitoring, and the estimation of crop yield.
2. Comparing Radar Rs with Conventional RS
The Table1 shows the differences between the Radar RS and conventional RS. The detailed explanation of its content is given as below:
- In regard to resource of electric-and- magnetic wave: Radar RS uses artificial wave (compared with sun light), so it is called the active RS. Conventional RS uses natural wave, so it is called the passive RS. This means RS technologists are able to select the suitable wave- band and polarization model based on the monitoring objects by using Radar RS. But the noise of radar wave is definitely larger than natural wave.
- With regard to wavelength: Radar wave is a microwave, the wavelength range is from 0.3cm to 100cm. Within the range, the majority of surfaces of common objects are at the critical condition, i.e., some are rough surfaces, and other ones are mirror surfaces, according to Rayligh Criteria:
Where
h is the roughness,
l is the wavelength,
b is depression angle.
For the radar wave (microwave), the surface of water-body, road surface, leaves of plant and so on, are considered as mirror surfaces. And the surfaces of agriculture field, tree's canopy and so on are considered as rough surfaces. We can identify quite a few kinds of objects on radar RS image by using the effect of roughness.
On grayness of pixel depends on, for radar RS, not only roughness, but also back-scattering coefficient of target in stead of only depending on Albedo of target for conventional RS. It makes the
image of radar RS more complicated, and interpretation of the image becomes more difficult.
- On color composition model of image: for conventional RS, the color composition model is false-color. It means by the model the color of pixel is closed to the condition or status of responding plant growing, which is benefit to monitoring crops and land-use. But for radar RS, the color composition model is pseudo-color. It means the color is totally not related to the condition of plant growing.
The image data give expression to body message for radar RS, but for conventional RS to surface message of target. It is because microwave has the penetrating power, but the visible or infrared light has not. This causes the increase of interpretation difficulties of radar RS imageries.
Generally speaking, radar RS data have got more information, but more difficult to interpret the image. It needs more advanced technology to process image. And it is also hopefuly to get more information on agricultural field.
Table. To compare radar RS with conventional RS
| |
Radar RS |
Conventional RS |
| Resource of electrical magnetic wave |
Artificial wave |
Solar light or radiating of heat on the earth |
| Wavelength |
0.3cm-100cm (micro-wave) |
0.3mm-12mmvisual light and infrared light |
| Polarization model |
H-H,H-V,V-H,V-V |
No-polarization |
| Imaging model |
Side looking |
Orthogonal looking |
| Projection |
-- |
Central projection |
| Collect message of |
Scattering wave or mirror reflecting wave |
Scattering |
| Noise level |
High |
Low |
| Geometrical distortion of image |
Large |
Small |
| Grayness of pixel depends on |
Roughness and back scattering coefficient of target |
Albedo of target objective |
| Colour composition model of image |
Pseudo-colour composition |
False-colour or real-colour composition |
| Data give expression to |
Body message |
Surface message |
