MSc Remote Sensing

Vegetation Science: Practical 2b

Theoretical Vegetation Scattering Model

Dr Paul Saich
Remote Sensing Unit
UCL Geography

1    Purpose

The aim of this part of the practical is to investigate the properties of a theoretical model for microwave scattering.

1.1    Model and Input Data

The scattering model that will be used is called "rt2". The simplest way to gain access to it is to use an alias, e.g.

venice% alias rt2 /data/leiden/scat_software/rt2

If you type this command, then it will assign the executable to the command "rt2". If you now type "rt2" you should get the following:-

venice% rt2
BAE SYSTEMS
RT2 version 3.3
Enter name of input file >

At this point, the model requires you to enter the name of an input file. Some examples of these are contained in the directory /home/psaich/data2/MScII/Model/ The names of the input files all have the form "name.rt". Copy these example files into your local directory.

WARNING: if you get the error message:
ld.so.1: /data/leiden/scat_software/rt2: fatal: libF77.so.3: open failed: No such file or directory

Then the required FORTRAN library files are not present on the machine you are on. Let the system manager know about this, but log on to a different machine for now (e.g. london).
 

2    Familiarising Yourself

2.1    Surface Scattering

Look at the contents of the input file ground.rt. The file contains some header information including wavenumber, k (=2 pi / wavelength) and incident angle (along with some additional pieces of information (algo, etc) which you should ignore). The next section of the file refers to the ground properties, and begins with the name of the surface scattering approximation used. Whenever you use the model for L- and P-band, the hashed line should read "#spm_g" but for C-band you should change this to "#ksp_g". The two models referred to here are the Small Perturbation Model (SPM) and the Kirchhoff Stationary Phase approximation (KSP), and their validity depends on the relative wavelength and roughness. If the model is not valid, you'll get a message to the screen during execution to this effect.

The remaining information in the "ground" section of the file includes roughness (ls and sigmah), the way in which moisture content is related to soil permittivity (a model called "Hallik" based on a paper by Hallikainen et al 1985) and then the volumetric moisture content, sand content (%) and clay content (%). The latter two of these relate to the soil texture (sand + clay + silt = 100%).The only parameter you need be concerned with here is the volumetric moisture content, mv, (which varies between 0 and 1).

Try executing rt2 with this input file. The results are written to the screen and also to a file called "ground.out".
 

2.2 Predictions as a function of an independent variable

Now make the following modifications to the input file:- The header to the file should now look like this:-

rt

 runs 10
 range0 5. 75.
 k  26.0
 theta (variable0)
 alg0 std
  loops 5.0e3
  tol 1.0e-5
 alg1 fresnel
 

Now try executing rt2 again. You should find this time that the output is a set of predictions as a function of incident angle. Note that there are ten incident angles ("runs") in the range 5 to 75 degrees ("range0"). Also notice that in the input file, the value of the incident angle has been replaced with "(variable0)" to tell the model that it is this which varies over range0.

You can look at variations in most of the other parameters (or even several parameters at once). An example of this is given below.
 

2.2    Vegetation Scattering

Now look at the contents of the file "needle.rt". This file contains similar header information to the previous exercise, and a similar set of data for the ground. However, it now also contains a new set of data relating to "layer0". (Additional layers can be built on top of this one, see the following section.) The first piece of information in the layer section must be the thickness of the layer (called "height"). Thereafter, any number of scatterer types can be included within the layer. In "needle.rt" there is only one type of scatterer - the file will simulate the scattering from a single layer of thin needles.

"len" is actually the half-length of the scatterer types - i.e. half-length of the needles, or half-thickness in the case of discs. For needles and discs you can use the Generalised Rayleigh-Gans approximation ("#needle_rg" or "#disc_rg"). For both these cases you must include rad1 and rad2 (the semi-major and semi-minor radii) even if these are the same. You will see later that you can also model cylindrical scatterers using the finite cylinder approximation ("#cylinder"). This is generally better over a wide range of wavelengths (especially for short wavelengths) and in this case, you need only give a single radius value (see next section).

Try running rt2 with the input file "needle.rt" and then look also at "needle_var1.rt" and "needle_var2.rt". For these files, rt2 will give predictions as a function of (i) soil moisture content, and (ii) needle moisture content. Make sure you understand the formats of the input and output files, and the ways in which the two files "needle_var1.rt" and "needle_var2.rt" are different from the original "needle.rt" file.
 

2.3    Contributions to the Total Backscattering Coefficient

When you run rt2, the output data are written into the file "name.out". A binary file called "name.mul" is also created which contains the contributions to the total scattering arising from ground scattering, vegetation scattering, and interactions between vegetation and ground. In order to view the contents of this file, you need to run a program called "rw". Again, the best way is to first set up an alias:-

venice% alias rw /data/leiden/scat_software/rw

When you execute rw, the contributions are all written into the file "name.out" (and if this file already exists, then it is overwritten). As an exampe, run rt2 using "needle_var1.rt". This will give the backscattering coefficients as a function of soil moisture (and the total backscattering coefficients are in the file "needle_var1.out"). Now execute rw in the following way:-

venice% rw needle_var1.mul

The output from this is in the file needle_var1.out". Look at the contents of this file and check that you understand them.

2.4    The Other Input Files

Look at the contents of the other files (those with "forest" in the name). These are (approximately) representative of 20 yr old Pine Trees at the Landes forest in France (and have been taken from a European Space Agency contract report by Le Toan et al 1992). Check the location of the incident angle and wavenumber in the files. Make sure that you understand the location of the definitions for each scatterer type, and for the ground. Also make sure you understand how these files control whether the program will make a single set of polarimetric predictions, or whether it will give predictions as a function of one (or more!) of the independent parameters. (Notice that in "forest_var2.rt" the predictions will be made as a function of four variables. These are actually the gravimetric moisture contents of the four scatterer types - trunks in the lower layer, trunks in the upper layer, branches in the upper layer and leaves in the upper layer. The moisture contents have been set so that they vary over the range 0.4 to 0.9, but always remain equal.)

3.    Simulations

Copy the shell /home/plewis/public_html/rt2/processMe and the awk program /home/plewis/public_html/rt2/SUGBEET.awk.

Running the shell processMe allows you to loop over:

within rt2, and produce comprehensible files of backscatter (dB).

The leaf angle distribution is assume spherical in the shell for a single layer canopy. Note that soil texture (sand,clay) and roughness properties (length scale ls and height RMS sigmah) are set in the program SUGARBEET.awk. The disc thickness is set to 3.000e-4 m.

Modify this shell to create a set of experiments, e.g., examining wavelength dependence of scattering for a given set of variables. Examine also, e.g. leaf moisture effects, say for a high and a low number density.
 

Try to relate the results to the theoretical understanding developed through the lecture material and any reading around the subject. The sort of questions you may wish to investigate could include:-


Finally, the most important aspects we wish to address with the theoretical model:-

In order to answer these questions, it may help to think about how you would calculate the "total biomass density" of a stand (in tons/ha or kg/m2) from input data such as above.

4.    Notes

Any write-up should focus mostly on Parts 2 and 3 but a better paper would also contain some of the material from Part 1 (properties of the data).