111 
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
The determination of optimum parameters for identification 
of agricultural crops with airborne SLAR data 
P.Binnenkade 
National Aerospace Laboratory NLR, Amsterdam, Netherlands 
ABSTRACT: The SLAR campaigns of the Dutch ROVE-Team "Crop Identification" in the years 1983-1985 are 
discussed. From data—acquisition through classification the determining parameters are elaborated. The 1985 
campaign which used steeper incidence angles in order to simulate satellite data, and is thus different from 
previous ROVE-campaigns, is discussed. 
1 INTRODUCTION 
Microwave remote sensing has played an important 
role in The Netherlands Remote Sensing community 
since the late sixties. Especially the work on the 
radar backscatter of vegetation canopies has always 
been given great emphasis. The ROVE-team (ROVE 
stands for Radar Observation of VEgetation), which 
has been involved in all land-applications of micro- 
wave remote sensing in The Netherlands, then and 
today consists of representative scientists and 
technicians from the following institutes: 
- Agricultural University of Wageningen and its 
associated institutes 
- Delft University of Technology, Microwave Group 
- Physics and Electronics Laboratory TNO 
- National Aerospace Laboratory NLR. 
Starting with a pulse-type X-band measuring radar 
operating from a TV-tower in 1968, the ROVE-group 
nowadays makes use of a calibrated digital SLAR 
(Fig. 1) mounted in the NLR Metro II Laboratory air 
craft and a multiband airborne scatterometer 
(DUTSCAT) installed in the NLR Queen Air laboratory 
aircraft in combination with supporting ground truth 
instrumentation. 
One of the principal aims of the ROVE-group is the 
identification and classification of agricultural 
crops from microwave data. 
Such data (obtained from airborne or spaceborne 
sensors) contain only one type of information: radar 
backscatter as a function of position. This radar 
backscatter is the condensed result of a great 
number of contributing factors. Even when confined 
to only one frequency the radar return is influenced 
by: 
- polarization effects 
- illumination function 
- incidence (grazing) angle 
- crop height 
- crop coverage 
- vegetation-structure parameters 
- soil roughness 
- moisture content. 
Some of these factors may be considered fixed and 
measurable whereas others can be influenced and 
changed by setting the radar and/or antenna and by 
flight-geometry modifications. 
Nevertheless, due to the complexity of the above 
parameters, work on theoretical models became indis- 
pendable (Ulaby, Attema, Hoekman). At an early stage 
it was recognized that this modelling work had to be 
supported by ground-based measurements on crops and 
soils. Special emphasis was given to the need for 
simplicity in the models to limit the number of un 
known parameters (Ref. 1). 
2 CROP IDENTIFICATION 
One of the specialized sub-groups of ROVE is the 
workinggroup "Crop Identification" in which the 
following institutes participate: 
- Centre for Agrobiological Research CABO 
- Delft University of Technology, Information Theory 
group 
- Physics and Electronics Laboratory TNO 
- National Aerospace Laboratory NLR. 
The workinggroup has chosen to undertake two 
parallel paths of (applied) research: 
1. Research into (hierarchical) classification 
methods for radar data. 
2. Initiation and execution of semi-operational 
application programmes. 
As for the latter, upon request from The Nether 
lands agricultural authorities an application pro 
gramme has been carried out in 1983 and 1984 to 
monitor crop rotation, especially potatoes, in a 
number of (ecologically-different) regions within 
The Netherlands. Crop disease control authorities 
allow potatoes to be grown only once every three 
years on any one field (if no other protective 
measures are taken); thus it has become desirable 
to monitor adherence to this practice. 
The group was asked to devise a method to uniquely 
discriminate potatoes from all other crops, hence 
time-consuming ground investigations be reduced. 
Three test-sites with an area each of 20*4 km 
were chosen in the Flevo-polders, Groningen and 
Brabant province respectively. 
3 THE DUTCH DIGITAL SLAR 
Sideways-looking airborne radar (SLAR) is an in 
strument which allows for data to be acquired on a 
line- to-line basis. The calibrated digital SLAR 
(X-band) uses a digital recording system where 
each line is formed on a point-to-point basis 
(Table 1). The received radar backscatter varies 
widely in a stochastic way when natural surfaces 
and -objects are observed; good estimates of the 
backscatter coefficient can only be made by com 
bining a large number of independent measurements. 
The Netherlands SLAR has been designed to do so 
(Table 1).