Symposium on Remote Sensing for Resources Development and Environmental Management/Enschede/August 1986 
© 1987Balkema, Rotterdam. ISBN 90 6191 674 7 
The microwave remote sensing program for agriculture and forestry in 
the Netherlands 
L.Krul 
Delft University of Technology, Netherlands 
ABSTRACT: The microwave remote sensing work in The Netherlands is based on a multidisciplinary approach. The 
work is done under the aegis of a national program and consists of three components i.e. research on the inter 
action between microwaves and objects, the development of instrumentation to support this theoretical work by 
measurements and projects which are focused on potential applications. A brief overview of the first two activ 
ities is presented here, some of the projects are described by others. 
RESUME: Au Pays-Bas le développement de la télé-détection est fondé sur une coopération multidisciplinaire. On 
travaille sous le drapeau d'un programme national comprenant trois aspects: l'interaction des micro-ondes avec 
des objets, la construction des instruments pour mesurer les propriétés électromagnétiques des objets et des 
projects dirigés vers des applications futures. Cette présentation s'adresse à l'interaction et à la construc 
tion des instruments. Des autres causeront des applications possibles. 
1 INTRODUCTION 
At several places in the world work is going on to in 
vestigate the potentialities of microwave remote sen 
sing. In general the applications to be studied are 
selected according to national priorities. In connec 
tion with their geographical location it therefore 
seems logical that The Netherlands are interested in 
applications related to the sea. At the same time how 
ever a great deal of effort is put in agriculture and 
forestry since these are also areas of great economic 
importance. 
The majority of those entering the field of micro- 
wave remote sensing have acquired their experience 
working in the visible or infrared windows. Therefore 
they are likely to see the microwave region as just 
another window to be used in addition to the ones al 
ready existing in the other wavelength regions. Obvi 
ously this viewpoint is based on the rather superfi 
cial idea that each channel will add some information 
and in this way will help to meet the general remote 
sensing objectives. This idea is supported by the fact 
that sometimes microwave images have been used in this 
way with some success (Megier e.a. 1985). 
Strictly speaking however this approach can only be 
justified by proving that additional observations in 
the microwave region are physically or statistically 
independent of the observations at other wavelengths. 
For this reason the remote sensing work in The Nether 
lands starts from the idea that microwave data can 
only be used in an optimum way if the underlying phys 
ical processes are well understood. In practice this 
will mean that research on the interaction between 
microwaves and objects is an important aspect of the 
work. The studies are performed in such a way that 
the theoretical (modelling) work is always supported 
by practical measurements, which implies the develop 
ment of suitable instrumentation. 
Preceded by a SLAR flight program the experimental 
work on radar backscatter started in The Netherlands 
in 1968. At first a pulse type X-band radar was used 
to measure backscatter of vegetation and crops uti 
lizing TV-towers as measuring platforms (de Loor 1974) 
these measurements showed that the radar return para 
meter y °r (0 ) may serve as a useful classifier. 
Apart from the fact that only low grazing angles are 
covered, TV-tower measurements suffer from a number 
of other limitations. Fortunately, however, these 
measurements have stimulated the interest in this type 
of work and in 1973 it was decided to build a trans 
portable ground-based, so-called, scatterometer work 
ing at 3 cm wavelength. In 1980 a second ground-based 
scatterometer working in the 8 mm band became avail 
able . 
Ground-based scatterometry however cannot answer all 
the questions arising when applications of radar re 
mote sensing are taken into consideration. In order 
to eliminate such limitations as the relatively small 
illuminated area, the limited number of testfields and 
the local character of ground-based measurements the 
development of a multiband airborne scatterometer was 
started. This instrument will be operational in 1986. 
After a short introduction to the basic ideas under 
lying radar remote sensing (section 2) the instrumen 
tation developed to support the theoretical work by 
measurements will be described in section 3. In the 
final section some important results will be discuss 
ed. 
Ultimately basic research has to result in practical 
applications. On the other hand however it can be 
expected that, by considering possible applications, 
questions arise which require new research. In order 
to stimulate this interaction a number of projects 
were initiated. During this symposium some of them 
will be presented. 
2 SOME FUNDAMENTAL ASPECTS OF MICROWAVE REMOTE SENSING 
A common aspect of all remote sensing systems is that 
they depend on the use of electromagnetic waves to 
cover the distance between the sensor and the object. 
Consequently the physical and/or biological quantities 
have to be extracted from the electromagnetic wave 
parameters. A straightforward solution of this problem 
in radar remote sensing would require a detailed de 
scription of the interaction between microwaves and 
objects. 
The theoretical quantification of the influence of 
the different object parameters on the radar signal 
however turns out to be extremely difficult. One of 
the major complications is that some of the variables 
are described by measures that are only meaningful 
in a statistical sense. 
A rigorous theoretical description being impossible 
a semi-empirical approach is followed instead. This 
type of approach is based on a simplified description 
of the interaction phenomena by the introduction of 
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