The author has identified the following significant results. Terrain microwave backscatter and emission response to soil moisture variations were investigated using Skylab's 13.9 GHz RADSCAT (radiometer/scatterometer) system. Data acquired on June 5, 1973, over a test site in west-central Texas indicated a fair degree of correlation with composite rainfall. The scan made was cross-track contiguous (CTC) with a pitch of 29.4 deg and no roll effect. Vertical polarization was employed with both radiometer and scatterometer. The composite rainfall was computed according to the flood prediction technique using rainfall data supplied by weather reporting stations

Barr, J. C.

Moore, R. K.

Sobti, A.

Ulaby, F. T.

English

NASA Technical Reports Server

NASA CR-
Title of Investigation: Design Data Collection with Skylab/^REP Microwave
Instrument S-193
Title of ReporS: Soil Moistrue Detection by Skylab's Microwave Sensors
RSL Advance Report of Significant Results
October, 1974
NASA Contract NAS 9-13331
Prepared for:
Principal Investigations Management Office
Technical Monitor: Mr. Larry B. York
NASA Lyndon B. Johnson Space Center
Houston, Texas 77058
Prepared by:
Fawwaz T. Ulaby, Co-Investigator
University of Kansas Center for Research, Inc.
Remote Sensing Laboratory
Lawrence, Kansas 66045
Type of Report: Advance Report of Significant Results
THE UNIVERSITY OF KANSAS CENTER FOR RESEARCH, INC.
2385 Irving Hill Drive—Campus West Lawrence, Kansas 66045
https://ntrs.nasa.gov/search.jsp?R=19750008878 2020-03-22T22:55:39+00:00Z
1. Report No. 2. Government Accession No.
243-9 . ' .
4. Title one! SubtilTe
Soil Moisture Detection by Skylob's Microwave
Sensors
5. Report Dote
October. 1974
3. Recipient's Catalog No.
6.. Performing Organization Code
7. Author(s)
F. T. Ulaby/ J. Barr, A. Sobti, and R. K. Moore
8. Performing Organization Report No.
TR 243-9
9. Performing Organization Name and Address 10. Work Unit No.
The University of Kansas Center for Research, Inc.
Remote Sensing Laboratory
2291 Irving Hill-Road—Campus West
11. Contract or Grant No.
IN AS 9H:333-1
2. Sponsoring Agency Name and Address
NASA Lyndon B. Johnson Space-Center
Houston, Texas 77058
,13. Type-of Report, and, Period. Covered
Advance Report of
Significant Results
14. Sponsoring Agency Code
15. Supplementary Notes
16. Abstract
Terrain microwave backscatter-and;emission response to-soil moisture
variations are investigated using Skylab's 13.9 GHz RADSGAT (Radiometer-
icatterometer) system.. Data.acquired on June 5>; 1973,, over a; test site in.
ifVest-ceritral Texas indicates a fair degree of correlation with composite rain-
all. The scan made was cross-track contiguous (CTC) with a pitch of 29.4°
and no roll offset. Vertical polarization was employed with both radiometer
and scatterometer. The composite rainfall was computed according to the
lood prediction technique using rainfall data supplied by weather reporting
fations.
17. Key Words (Selected by Author(s))
Skylab
Microwave backscatter
18. Distribution Statement
Unclassified
19. Security Oossif. (of this report)
Unclassified
20. Security Classif. (of .this page)
Unclassified
21. No. of Pages 22. Price
'For sale by the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151.
SOIL MOISTURE DETECTION BY SKYLAB'S MICROWAVE SENSORS
Fowwoz T. Uloby, John Ban-, Arun Sobtiond Richord K. Moore
University of Konscs Center for Research, Inc.
Remote i^<..,.'...g Lcborotory • •
Lawrence, Kansas 66045
ABSTRACT
Terroin microwave backscatfer and emission response to soil moisture variations are
investigated using Skylab's 13.9 GHz PAr>SCAT (Radiometer-Scatterometer) system. Dr»a
acquired on June 5-, 1973 over a test site in west-central Texas indicates a fair degree of
correlation with composite rainfall. The scon-mode-was cross-track contiguous (CTC) with
o pitch of 29.4 and no roll offset. Vertical polarization was employed wirh both radiometer
end scatterometer. The composite rainfall was computed according to the flood prediction
technique using rainfall data supplied by weather reporting stations.
. INTRODUCTION
The dielectric properties of soil are strongly dependent on the soil water content. The surface
backseattering coefficient a and emissivity e are functions of the surface reflection coefficient, which
in turn is o function of the dielectric properties. The scattering and emission processes are modified,
however, by other surface features such as roughness, vegetation cover and soil type.
In an earlier reporting by Eagleman and Ulaby fl] data obtained by Skylab's microwave sensors
was compared with soil moisture contents determined from samples collected by ground crews. In the
present paper, a review of some-of the results of fhe'above study will be presented., followed by an
extension from comparing the. microwave response to.soli moisture of the few footprints for which giound
samples were available, to the entire test site area of 173 km x 358 km. Since soil moisture samples are
not available for the entire test site, composite rainfall is used instead.
TEST SITE DESCRIPTION
The Texas site, shown in Figure 1, lies in the west-central portion of the state. It covers an
area approximately 108 miles (173 km) wide and 224 miles (358 km) long extending from near Lubbock
southwest to San Saba, Texas. The topography varies from f lot plains to gently rolling hills. The major
land uses in the area are grassland used for grazing, cropland, the major crop being cotton, scrub
brushland, and scattered areas of woodland.
SKYLAB MICROWAVE DATA
The S-193 RADSCAT data used for this onalysis wos obtoined on June 5, 1973 berween 18:00:13
ond 18:01:16 GMT. The instrument was operoted in the.RAD/SCAT mode, thus gathering radiometer ond
jcofteromcter data alternately. The scon mode was cross-track contiguous (CTC) with a pitch of 29.4
from nadir und no roll of fset. Defined according to ths- effect ive antenna beomwidth, the size c* fhe
rcd'oir.iler elliptically shaped footprint on the g.Ow-r.d is opproximately 14 x 21 km. The effe.,;!.,. antenna
bearrtwidth of the scatterometer is calculated from the squared ontenno pattern, ond hence yields a smaller
footprint, opproximately 12 x 16 km. The radiometer data gathered over this test site was vertically
polarized covering an antenna temperature range of 52.43 K, extending between a high of 28G.73 K
ond o low of 236.29 K. The scatterometer doto which also was vertically polarized (both transmit ond
receive), produced o scattering coefficient o—_ with a dynamic range of 4.08 dB from a low ot -II .4 dB '
too high of-7.38 dB. vv
Incorporated in the data recorded by S-193 are contributions and/or losses caused by atmospheric
constituents, particularly clouds. Approximately 60% of the test site was cloud covered during the pass.
The oreol extent and degree of cloud cover over the test site was estimated from Skylab's S-190A
plujioyrophy token during the Texas pass. Four categories of cloud cover were determined: no clouds,
light clouds, medium clouds, and heavy clouds. The cloud ceiling was obtained from radar weather
information and cloud thickness wos estimated by measuring shadow length on the photography. For the
four cloud categories mentioned above, water contents of 0, 0.3 g/m^, 0.6 g/m1^, ond 1 .0 g/m^, were
assigned. Surface humidity, temperature ond pressure were obtained from weather reporting stations.
This information was then used to calculate the attenuation and upward ond downward emission by each
cloud category [2], For each scatterometer footprint, the "measured" scattering coefficient was corrected
for the Kvo way attenuation by the atmosphere (atmospheric gases and clouds). The radiometer data were
converted to emissivity by assuming a plane surface model:
_ _ W L T d
where T is the recorded radiometer antenna temperature, T represents upward atmospheric emission and
Ti is the downward atmospheric emission which, upon reflection by a ground reflectivity (1 -C), is
propagated up towards the radiometer. T is the ground temperature and L is the atmospheric tronsmitfance.
T was determined from a temperature data provided by weather reporting stations modified by a correction
fccror based on ground temperature samples collected by the ground crew.
Figures 2 ond 3 show the emissivity and scattering coefficient as a function of soil moisture content
in (he fop 2.5 cm (1]. The moisture content of each point shown in Figures 2 and 3 represents the average
moisture of two or more soil samples collected within or very close to the footprint. Considering the poor
representation of the soil moisture content of an approximately 231 km2 radiometer footprint ond 132 km'
scafterometer footprint by a few soil samples, the correlations indicated in Figures 2 and 3 are indeed
encouraging.
i
1.90
FIGURE 2. THE EMISSIVITY AS A FUNCTION
OF SOIL MOISTURE CONTENT IN. THE TOP
2.5 CM. The emissivily was determined from
S-193 dolo operating ot 13,9 GHz, vertical
polarization ond 29.4 pilch angle.
FIGURE 3. THE SCATTERING COEFFICIENT
AS A FUNCTION OF SOIL MOISTURE
CONTENT IN THE TOP 2.5 CM. The
scattering coefficient was determined from
S-193 data operating ol 13.9 GHz,
vertical polarization and 29.4 pitch angle.
J
CORRELATION WITH COMPOSITE RAINFALL
The onolysis in the previous section was limited to a few footprints, namely those for vtr.ich
ground data wos available. To utilize Ihe microwave data available for Ihe entire test site, it was
decided to represent soil moisture by on indirect, yet related, parameter which can be readily calculated
from weather data. This parameter is composite rainfall. The water content in a given soil is a function
of v^varc! parameters including rainfall history, '.r.;! type ond management ond the evopotransrirction
rote. Of all these factors, the only readily available one is rainfall history.
After dividing the test site into 46 rows x 49 columns of grid nodes, the rainfall for earH cf the
four days preceding the pass and the day of ihe pass were calculated for each grid node by extrapolating
doiu of Ihe four nearest weather stations |2], A similar program was employed to estimate the emissiviry
ond scattering coefficient at each grid node from data of the neighboring footprints. Composite rainfall
R was calculated by a method analogous to flood prediction techniques in that it utilizes an antecedent
precipitation index K in an attempt to account for the general soil permeability ond run-off character-
istics. The expression used is:
! K5 R. ]
i=0
where i is an integer representing the number of days prior to the pass (the day of pass is i=0), R. is the
rainfall on the i'h day and K is the antecedent precipitation index having a value smaller than 1. The
value of K used in this analysis is 0.85.
Figures 4 and 5 are scaltergrams of the emissivity and scattering coefficient, respectively, of all
grid nodes within the test site as a function of R. The calculated correlation coefficients are -0.81 for
emissivity ond .62 for scattering coefficient.
a 97
195
19}
a 91
0.89
£•
f 0.87
Efej
0185
a o
a si
a 77,
6-2.1 X lCfV-8.02 X 10'3R»0.945
P--0.81
0.0 0.4 0.8 1.2 1.6 2.0
Composite Rainfall R in Inches
2.4
FIGURE 4 . SCATTERGRAM OF S-193 CALCULATED EMISSIVITY VERSUS
COMPOSITE RAINFALL FOR THE JUNE 5,1973 PASS OVER V/ESTERN KANSAS.
-11.41-
0.0
c ' -O.WR-'J.Wc'B
P-0.6?
-ia?r
<U CU 1.? 1.6 J.O
Conp«it« Rainfall R in lncr*i
2.4
FIGURE 5. SCATTERGRAM OF S-193 SCATTERING COEFFICIENT VERSUS
; * » t r/-M-» TIIP n »». i r c 1CV71 n A C f /^\/rp t 'T fTrpKI TfT V A C
CONCLUSION
The result* presented in this paper demonstrate the potential of microwave sensors as tools for
mapping soil moisture content from satel l i te platforms. Based on data collected from ground [3-6]
ond aircraft platforms |7-8|, the sensitivity to soil moisture variations can be improved considerably by
operating at smaller incidence angles (10 -20 ) end lower microwave freouencies (1-5 GHz range).
REFERENCES
(1) toglemon, J. ondF. T. Ulaby, "Radiometer-Scatterometer Soil Moisture Detection," Proceedings
of the American Astronomical Society ^°^>'<ng, Los Angeles, California, August, 1 °~4.
|2] Borr, J. and F. T. Ulaby, "Analysis of Skylab S-193 Data of West-Central Texas," RSL Technical
Report 243-6, University of Kansas Center for Research, Inc., Lawrence, Kansas, October, 1974.
13] Lee, Siu Lim, "Dual Frequency Microwave Radiometer Measurements of Soil Moisture for Bare
ond Vegetated Rough Surfaces," Technical Report RSC-56, Texas A£M University Remo'e Sensing
Center, College Station, Texas, August, 1974.
(4] Uloby, F. T., "Radar Measurement of Soil Moisture Content," IEEE Transactions on Antennas ond
Propagation, vol. AP-22, no. 2, March, 1974.
[5] Ulaby, F. T., J. Cihlar and R. K. Moore, "Active Microwave Measurement of Soil Water
Confent," Journal of Remote Sensing of Environment, January, 1975.
[6] Ulaby, F. T., "Radar Response to Vegetation," IEEE Transactions on Antennas ond Propagation,
vol. AP-23, no. 1, January, 1975.
[7] Schmugge, T., P. Gloersen, T. Wilheit, and F. Geiger, "Remote Sensing of Soil Moisture with
Microwave Radiometers," Journal of Geophysical Research, vol. 79, no. 2; pp. 317-323, 1974.
18] Dickey, F. M., C. King, J. C. Holrzman and R. K. Moore, "Moisture Dependency of Radar
Backscatter from Irrigated and Non-Irrigated Fields at 400 MHz and 13.3 GHz," IEEE Transactions
on Geoscience Electronics, vol. GE-12, no. 1, pp. 19-22, 1974.
ACKNOWLEDGEMENT
The research reported in the paper was supported by NASA Lyndon B. Johnson Space Center Contract
NAS 9-13331.


Soil moisture detection by Skylab's microwave sensors

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