A coherent bistatic vegetation scattering model, based on a Monte Carlo simulation, is being developed to simulate polarimetric bi-static reflectometry at VHF/UHF-bands (240-270 MHz). The model is aimed to assess the value of geostationary satellite signals of opportunity to enable estimation of the Earth's biomass and root-zone soil moisture. An expression for bistatic scattering from a vegetation canopy is derived for the practical case of a ground-based/low altitude platforms with passive receivers overlooking vegetation. Using analytical wave theory in conjunction with distorted Born approximation (DBA), the transmit and receive antennas effects (i.e., polarization, orientation, height, etc.) are explicitly accounted for. Both the coherency nature of the model (joint phase and amplitude information) and the explicit account of system parameters (antenna, altitude, polarization, etc) enable one to perform various beamforming techniques to evaluate realistic deployment configurations. In this paper, several test scenarios will be presented and the results will be evaluated for feasibility for future biomass and root-zone soil moisture application using geostationary communication satellite signals of opportunity at low frequencies

Deshpande, Manohar

Eroglu, Orhan

Joseph, Alicia T.

Kurum, Mehmet

Lang, Roger H.

O'Neill, Peggy E.

NASA Technical Reports Server

DEVELOPMENT OF A COHERENT BISTATIC VEGETATION MODEL FOR SIGNAL OF 
OPPORTUNITY APPLICATIONS AT VHF/UHF-BANDS 
 
Mehmet Kurum (1), Manohar Deshpande (2), Alicia T. Joseph (3),  
Peggy E. O’Neill (3), Roger H. Lang (4), Orhan Eroglu (1) 
 
(1) Department of Electrical and Computer Engineering, Mississippi State University,  
Mississippi State, MS, 39762 
(2) Microwave Instrument Technology Branch (Code 555), NASA Goddard Space Flight Center, 
Greenbelt, MD 20771 
(3) Hydrological Sciences Laboratory (Code 617), NASA Goddard Space Flight Center, 
Greenbelt, MD 20771 
(4) Department of Electrical and Computer Engineering, George Washington University,  
Washington, DC, 20052 
 
 
ABSTRACT 
 
A coherent bistatic vegetation scattering model, based on a 
Monte Carlo simulation, is being developed to simulate 
polarimetric bi-static reflectometry at VHF/UHF-bands 
(240-270 MHz).  The model is aimed to assess the value of 
geostationary satellite signals of opportunity to enable 
estimation of the Earth’s biomass and root-zone soil 
moisture.  An expression for bistatic scattering from a 
vegetation canopy is derived for the practical case of a 
ground-based/low altitude platforms with passive receivers 
overlooking vegetation.  Using analytical wave theory in 
conjunction with distorted Born approximation (DBA), the 
transmit and receive antennas effects (i.e., polarization, 
orientation, height, etc.) are explicitly accounted for.  Both 
the coherency nature of the model (joint phase and 
amplitude information) and the explicit account of system 
parameters (antenna, altitude, polarization, etc) enable one 
to perform various beamforming techniques to evaluate 
realistic deployment configurations.  In this paper, several 
test scenarios will be presented and the results will be 
evaluated for feasibility for future biomass and root-zone 
soil moisture application using geostationary 
communication satellite signals of opportunity at low 
frequencies.  
 
Index Terms— Bistatic, UHF, root-zone soil moisture, 
biomass, signal of opportunity, discrete scatterer.  
 
1. INTRODUCTION 
 
In the past decade, there has been increasing interest in 
using signals of opportunity (SoOp) for remote sensing of 
Earth [1].  Most of these past studies have focused on 
collection or modeling of Global Navigation Satellite 
Systems (GNSS) signal reflections over ocean surface for 
estimating wind direction and speed or altimetry.  On the 
other hand, application of GNSS-Reflectometry technique 
to land is rather scarce and relies mostly on experimental 
observations [2-4].  Sensitivity to soil and vegetation by 
GNSS’s bistatic signals at L-band has been, to some extent, 
experimentally demonstrated and have been also supported 
by few theoretical studies [5-9].  However, L-band signals 
are limited to penetrate through dense vegetation and deep 
into the soil column.  Thereby, GNSS-based approaches can 
provide estimate of surface soil moisture up to a depth of 
~0.05 meter through moderate amounts of vegetation.  In 
recent past, to address this issue, a new SoOp methodology 
is suggested, where the signals transmitted by the Military 
Satellite Communication (MilSatCom) System’s UHF (240–
270MHz) Follow-On program) are utilized to measure 
properties of reflecting targets by recording reflected signals 
using a simple passive microwave receiver [10, 11]. 
The low frequency SoOP approach has the potential of 
measuring root-zone soil moisture (top 1-m) through heavy 
vegetation. These measured quantities would provide 
information critical for understanding global water 
resources and the vertical moisture gradient responsible for 
evapotranspiration and runoff.  In this study, a low 
frequency (VHF/UHF) bi-static electromagnetic scattering 
model is being developed to supplement recent 
experimental efforts and to exploit further their potential.  
Since the low-frequency SoOP experiments are so far 
carried out from either ground based or airborne platforms, 
we will restrict the modeling study to such geometries, but it 
can be fairly easily extended to the receivers in low Earth 
orbit.  The model will be used to conduct sensitivity 
analysis and to determine optimum system configuration for 
https://ntrs.nasa.gov/search.jsp?R=20170007424 2019-08-29T22:19:15+00:00Z
estimating root-zone soil moisture and biomass parameters 
for the Earth surface covered with heavy forest/vegetation.  
The antenna characteristics and orientation play a key 
role in received signal. For ground-based systems, the 
antenna radiation pattern projected on the surface is not 
uniformly distributed in phase, amplitude, or polarization. 
The bistatic radar coefficient calculated by the model thus 
needs to have the antenna characteristics embedded in it as 
well as the statistical and physical properties of the terrain to 
mimic the real measurement setting.  Previous studies have 
usually assumed plane wave illumination/scattering [5-7], 
and ignored such antenna effects.  In the following section, 
a brief description of the model will be provided for 
calculating the response of the antenna due to the coherent 
bistatic scattered field from a vegetated surface.   
 
2. FORMULATION 
 
In this paper, a general bistatic scattering case is considered, 
where two antennas do not have their main beam axis 
pointing at each other and both of which are overlooking the 
vegetation.  One antenna system is associated with the 
signal of opportunity transmitter while the other refers to the 
passive receiver.  The vegetation is represented as ensemble 
of canonical scatterers located above a flat ground.  The 
transmitter is located far away.  The local incident angles 
are assumed to be constant (parallel) and spreading loss 
effects due to the incoming wave are ignored, but the 
spreading loss and sphericity of the scattered wave are 
considered due to proximity of the platforms that operate 
close to the ground.  This configuration will represent 
“plane wave incidence” and “spherical wave scattering”. 
It is important to realize that antennas cannot be 
constructed to produce pure polarization states.  There will 
always be some nonzero cross polarization level that could 
degrade the measurement.  Additionally, the polarization 
state of the antenna is usually defined along the main beam 
direction.  The polarization, however, varies for increased 
off-axis angles over the radiation pattern.  Polarization 
states of the antennas are taken into account through a 
polarization (field) rotation matrices.  As a result, the 
formulation considers variations of both the strength and 
polarization states of the received wave along the beam 
direction. 
Applying ‘discrete scatter’ modeling approach [12], the 
forest layer is replaced by a slab of equivalent homogenous 
medium by the Foldy theory [13].  An expression for the 
scattered field due to a single particle immersed in the slab 
of mean medium over a flat ground surface is derived.  The 
received signal is then expressed as a weighted coherent 
sum involving the antenna spatial spectrum and bistatic 
scattering.  The results of the single particle scattering are 
summed over all types and particles by taking into account 
of the polarization mixing (rotation matrices), the antenna 
impurities (antenna pattern matrices with co- and cross-pol 
components), and the spreading loss. 
The received field has three main terms: (1) direct term, 
(2) specular term, (3) diffuse term.  The first one represents 
the line of sight or the shortest path between the antennas 
while the second term denotes the forward scattering along 
the specular direction, where the incoming and the 
scattering waves make the same angle with respect to the 
surface normal.  Lastly, the diffusely scattered waves arrive 
at the receiver antenna from a wide range of angles in both 
azimuth and elevation due to scattering from the illuminated 
volume.  An average diffuse term is obtained by a sufficient 
number of realizations of vegetation through Monte Carlo 
simulations.   
Each term is a “complex” field quantity and thus 
includes both amplitude and phase information. This 
enables one to decompose the field into a orthogonal 
polarization basis in order to simulate polarimetric bi-static 
reflectometry for various polarization combinations. 
 
3. SIMULATION SETTINGS 
 
In the simulation, several scenarios will be tested for the 
recently developed 4-channel VHF receiver hardware [11]. 
The instrument is shown in anechoic chamber for 
measurement of radiation characteristics in Fig. 1.  Each 
channel receives VHF signal picked up by the linear dipole 
tuned to frequency range 240-270 MHz.  Due to broad 
beam dipole patterns, each channel will receive the direct as 
well as reflected signal (reflected from the vegetation) 
transmitted by the MilSatCom satellites.  The instrument is 
capable of recording complex field quantities that allows to 
apply beamforming.  Using digital beam forming in the post 
processing, the direct signal and the reflected signals are 
separated for soil moisture and root zone soil moisture 
extraction.    
 
 
Figure 1: 4-channel VHF receiver. 
Zenith, direct signal, horizon, and reflected signal 
directions will be considered for antenna orientations. For 
each configuration, the 4 channel array beam will be steered 
into direct and reflected direction to isolate the signals.  The 
model will be simulated for considering various 
combination of system and target parameters.  Among the 
antenna parameters, effects of antenna height, antenna 
cross-polarization, and beamwidth width will be analyzed to 
understand platform and antenna requirements for such 
applications.  For each system configuration, the target 
parameters such as soil moisture, vegetation type, height, 
and biomass level, will be varied to conduct sensitivity 
analysis.  
  
4. CONCLUSION 
 
Knowledge of the Earth biomass, root zone soil moisture 
(including surface soil moisture) are important earth science 
parameters because of their strong influence on the Earth’s 
climate system.  Currently, these Earth science parameters 
are measured using airborne and space borne microwave 
radiometers/scatterometers operating at L-band (1.4 GHz) 
frequencies.  However, the L-band frequency range is 
inadequate for measuring these parameters for the Earth 
surface covered with heavy forest/vegetation.  In the recent 
past, NASA has shown great interest in using lower 
frequency band such as VHF/UHF for these parameter 
measurements. In this study, newly developed coherent 
bistatic vegetation scattering model, based on a Monte Carlo 
simulation, will be utilized to simulate polarimetric bi-static 
reflectometry at VHF/UHF-band. Preliminary performance 
of the model and sensitivity analysis by the model will be 
presented. 
 
5. REFERENCES 
 
[1] U. Zavorotny, S. Gleason, E. Cardellach, and A. Camps, 
“Tutorial onremote sensing using GNSS bistatic radar of 
opportunity,” IEEE Geosci. Remote Sens. Mag., vol. 2, no. 4, pp. 
8–45, Dec. 2014. 
 
[2] A. Alonso-Arroyo, A., Camps, A. Aguasca, G.F. Forte, A. 
Monerris, C. Rudiger, J.P. Walker, H. Park, D. Pascual, R. 
Onrubia, “Dual-polarization GNSS-R interference pattern 
technique for soil moisture mapping,” IEEE J. Sel. Topics Appl. 
Earth Obs. Remote Sens., vol. 7, no. 5, pp. 1533–1544, May 2014. 
 
[3] A. Egido, S. Paloscia, E. Motte, L. Guerriero, N. Pierdicca, M. 
Caparrini, E. Santi, G. Fontanelli, N. Floury, “Airborne GNSS-R 
polarimetric measurements for soil moisture and above-ground 
biomass estimation,” IEEE J. Sel. Topics Appl. Earth Obs. Remote 
Sens., vol. 7, no. 5, pp. 1522–1532, May 2014. 
 
[4] A. Camps, H.  Park, M. Pablos, G. Foti, C.  P.  Gommenginger, 
P. W. Liu, and J. Judge, “Sensitivity of GNSS-R spaceborne 
observations to soil moisture and vegetation,” IEEE J. Sel. Topics 
Appl. Earth Observ. Remote Sens., vol. 9, no. 10, pp. 4730–4742, 
Oct. 2016. 
 
[5] P. Liang, L.E. Pierce, and M. Moghaddam, “Radiative transfer 
model for microwave bistatic scattering from forest canopies,” 
IEEE Trans. Geosci. Remote Sens., vol. 43: pp. 2470-2483, 2005. 
 
[6] P. Ferrazzoli, L. Guerriero, N. Pierdicca, and R. Rahmoune, 
“Forest biomass monitoring with GNSS-R: Theoretical 
simulations,” Adv. Space Res., vol. 47, no. 10, pp. 1823–1832, 
2010. 
 
[7] L. Guerriero, N. Pierdicca, L. Pulvirenti, and P. Ferrazzoli, 
“Use of satellite radar bistatic measurements for crop monitoring: 
A simulation study on corn field,” Remote Sens., vol. 5, pp. 864–
890, 2013 
 
[8] N. Pierdicca, L. Guerriero, R. Giusto, M. Brogioni, and A. 
Egido, “SAVERS: A simulator of GNSS reflections from bare and 
vegetated soils,” IEEE Trans. Geosci. Remote Sens., vol. 52, no. 
10, pp. 6542–6554, Oct. 2014. 
 
[9] X. R. Wu and S. G. Jin, “GNSS-Reflectometry: Forest canopies 
polarization scattering properties and modeling,” Adv. Space Res., 
54(5), 863-870, 2014. 
 
[10] J. Knuble, J. Piepmeier, M. Deshpande, C. D. Toit, J. 
Garrison, Y-C. Lin, G. Stienne, S. Katzberg, G. Alikakos, 
“Airborne P-band Signal of Opportunity (SoOP) demonstrator 
instrument; status update,” in Proc. IEEE Int. Geoscience Remote 
Sensing Symp., Beijing, China, 2016. 
 
[11] A. T. Joseph, M. Deshpande, P. E. O’Neill, and L. Miles, 
“Development of VHF (240–270 MHz) Antennas for SoOp (Signal 
of Opportunity) Receiver for 6U Cubesat Platforms,” in Proc. 
Progress In Electromagnetic Research Symposium (PIERS), 
Shanghai, China, 2016. 
 
[12] M. Kurum, (2009). L-band Estimation of Forest Canopy 
Attenuation by a Time-Domain Analysis of Radar Backscatter 
Response. Chapter 3, Ph.D. dissertation. The George Washington 
University. Washington DC, US. 
 
[13] L. Tsang, J. Kong, and R. Shin, Theory of Microwave Remote 
Sensing. ch. 3, New York, Wiley, 1985. 


Development of a Coherent Bistatic Vegetation Model for Signal of Opportunity Applications at VHF UHF-Bands

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server