Microwave remote sensing measurements at L-band (~1.2-1.6 GHz) of geophysical parameters such as soil moisture will need to be at higher spatial resolution than current systems (SMOS/ SMAP/ Aquarius) in order to meet the requirements of land surface, ocean, and numerical weather prediction models in the near future, which will operate at ~9-15 km global grids and 1-3 km regional grids in the next few years. In order to make progress toward these needed spatial resolutions, advancements in technology are necessary which would lead to improved effective (i.e. equivalent) antenna size. An architecture trade study was conducted to quantitatively define the value and limits of different microwave technology paths, and to select the most appropriate path to achieve the high spatial resolution required by science in the future without sacrificing performance, accuracy, and global coverage

Bindlish, R.

Cruz-Ortiz, G.

Hudson, D.

Le Vine, D.

Li, L.

O'Neill, P.

Olney, D.

Piepmeier, J.

NASA Technical Reports Server

 DETERMINATION OF BEST LOW-FREQUENCY MICROWAVE ANTENNA 
APPROACH FOR FUTURE HIGH RESOLUTION MEASUREMENTS FROM SPACE 
 
 
P. O’Neill, R. Bindlish, J. Piepmeier, D. Le Vine, D. Hudson, L. Li, G. Cruz-Ortiz, and D. Olney 
 
NASA Goddard Space Flight Center, Greenbelt, MD  20771  USA 
tel:  1-301-614-5773, fax:  1-301-614-5808,  Peggy.E.ONeill@nasa.gov 
 
 
 
ABSTRACT 
Microwave remote sensing measurements at L-
band (~1.2-1.6 GHz) of geophysical parameters such 
as soil moisture will need to be at higher spatial 
resolution than current systems (SMOS/ SMAP/ 
Aquarius) in order to meet the requirements of land 
surface, ocean, and numerical weather prediction 
models in the near future, which will operate at ~9-
15 km global grids and 1-3 km regional grids in the 
next few years.  In order to make progress toward 
these needed spatial resolutions, advancements in 
technology are necessary which would lead to 
improved effective (i.e. equivalent) antenna size.  An 
architecture trade study was conducted to 
quantitatively define the value and limits of different 
microwave technology paths, and to select the most 
appropriate path to achieve the high spatial 
resolution required by science in the future without 
sacrificing performance, accuracy, and global 
coverage. 
 
Keywords (Index Terms) -- soil moisture, passive 
microwave, antenna technology, aperture synthesis, high 
spatial resolution, active/passive. 
 
1.  INTRODUCTION 
Soil moisture is the key land surface hydrologic 
state variable that helps to control the Earth’s water and 
energy balance, and it provides critical information to 
such applications as weather and climate predictions, 
agricultural forecasts, and hazards monitoring.  
Although NASA’s SMAP and ESA’s SMOS missions 
have pushed the spatial resolution at L-band for accurate 
soil moisture determination in all-weather conditions to 
~40 km [1], the next generation of numerical weather 
prediction and land surface models will operate at 
global scales of 9-15 km in the next few years (regional 
scales of 1-3 km), and will likely push global resolution 
scales to 1-5 km in the next decade (limited only by 
available computing power and the resolution of the 
needed input data streams) [2].  Additionally, desired 
temporal resolution requirements are approaching 1 day 
for many users.  For NASA measurements from space 
to keep pace with the requirements of the global 
modeling community, advances in spatial resolution are 
needed while still maintaining radiometric performance 
and complete global coverage in a timely manner. 
In addition to soil moisture, high-quality L-band 
radiometer measurements from space have been used to 
provide very useful information to other parameter 
estimations such as sea surface salinity, sea ice, and 
ocean winds [3,4].  The improvements in resolution 
targeted by this study for soil moisture investigations 
are mirrored by similar requirements for improved 
resolution in other disciplines (such as the need for 
better remote sensing of salinity in coastal zones and 
along ice edges).  The results of this study are relevant 
across a wide range of Earth science application areas. 
The expected outcome of the proposed study is a 
quantitative assessment of the limits to growth of three 
different microwave technologies while maintaining the 
needed science performance, and how such limits match 
up with the spatial resolution of science measurements 
needed by global models in the next decade.   
 
2.  OBJECTIVE 
The main goal of this study was to determine the 
technology path which shows the most promise through 
a trade study analysis which determines the effective 
limits of three different antenna approaches: (1) what is 
the largest real aperture mesh reflector that is feasible to 
spin and control (mechanically and thermally)?  [SMAP 
example in Fig. 1a]; (2) what is the largest aperture 
synthesis radiometer in either 1-D or 2-D that can meet 
the requirements for radiometric accuracy in a cost-
https://ntrs.nasa.gov/search.jsp?R=20180005175 2019-08-31T14:40:31+00:00Z
 
 
       Fig. 1a.  Large Spinning Mesh Reflector                              Fig. 1b.  Large 2-D Aperture Synthesis Antenna 
                         (SMAP example)                                                                             (SMOS example) 
 
effective manner?  [SMOS example in Fig. 1b];  and (3) 
where is the accuracy boundary between active-passive 
and passive-only approaches while reaching spatial 
resolution targets?  While all three of these approaches 
have produced viable microwave measurements from 
space at relatively coarse resolution for accurate 
retrieval of soil moisture and other geophysical 
parameters, the limits to growth in the size of these 
antennas and hence the achievable spatial resolution in 
the future is currently unknown.   
This study will examine the trade space between 
size and complexity versus performance, accuracy, 
swath coverage, etc. for each antenna approach.  It will 
focus on defining the elements in common for large 
aperture designs (such as mechanical and thermal 
control of very large apertures) as well as those factors 
which are unique to the growth of a specific approach, 
such as location/design of the feed array for large mesh 
antennas, mutual coupling and impacts of thinning the 
array for aperture synthesis antennas, and the effects of 
radar noise on disaggregation algorithms for the 
active/passive approach.  Each antenna technology will 
also be evaluated in terms of its ability to accommodate 
a radar if needed.  Once a realistic limit to the size of a 
particular antenna type is determined, issues of cost, 
complexity, engineering control, and ability to meet the 
resolution and performance demands of the user 
communities will be factored in to achieve a 
recommendation for the most effective technology path 
to follow. 
Although these efforts are focused on achieving the 
best path forward for a future high resolution L-band 
(1.2-1.4 GHz) mission (both ocean and land surface), 
the antenna technology trade results are also applicable 
to other low microwave frequencies and could help 
define the best approach for multi-frequency missions 
(e.g., P- and L-band combination for accuracy and better 
penetration depth in the soil and vegetation, or L- and 
Ka-band combination to obtain coincident land surface 
temperature information needed in the soil moisture 
retrieval algorithm).  However, for a multi-frequency 
mission, which is beyond the proposed scope of this 
study, the aperture design approach would have to be 
assessed to ensure that the specific microwave 
frequencies could be accommodated on the same 
antenna (for example, one early study found that an L-
C band combination would work on the basic 
HYDROSTAR 1-D aperture synthesis antenna, while 
an L-S band combination would not work [5]). 
It is expected that the results of the proposed trade 
study will be relevant to the release of the next Earth 
Science Decadal Survey early in 2018.  It is also 
relevant to an ongoing informal effort among European 
and U.S. scientists to coordinate L-band continuity 
activities. 
 
3.  APPROACH 
Prior to studying the various microwave antenna 
technology approaches for a future high resolution 
mission, the basic science requirements for such a 
mission were laid out.  These requirements included: (1) 
a revisit interval of 2-3 days, (2) complete global 
mapping within the revisit interval with no data gaps, 
(3) a spatial resolution of 10 km or better, including any 
incidence angle effects, and (4) a mission life of 2-3 
years or longer.  A future goal is a 1-day revisit with 1-
5 km spatial resolution. 
 
3.1  Large Mesh Antennas 
The first technology approach is to utilize the 
SMAP spinning mesh reflector architecture (Fig. 1a) as 
a starting point and grow the antenna size, with or 
without a radar.  The primary axis of the trade space is 
the reflector diameter.  SMAP works very well with a 
6-m diameter spinning mesh antenna.  A larger antenna, 
however, would put additional demands on the 
spacecraft attitude control system (ACS) because of 
increased moment-of-inertia.  One mitigation approach 
is to move the radiometer electronics and feed network 
off the spun side to the despun side of the spacecraft – 
the resulting rotating polarization basis can be 
electronically compensated using existing techniques.  
One part of the study will be to determine if an ACS 
system could counter the dynamics of the larger 
spinning platform by requiring larger or more reaction 
wheels.  At the same time, more balance mass might be 
required on the despun spacecraft such that the natural 
spinning motion of the entire system is more favorable 
for control purposes.  Another part of the study will 
examine the trade between increased reflector size, spin 
rate, number of radiometer detectors, and instrument 
noise.  An outcome of the analyses will be a high-level 
relationship between spacecraft resources, reflector 
size, and expected instrument performance.  
 
3.2  1-D or 2-D Aperture Synthesis Antennas 
The second technology approach involves the 
design of aperture synthesis antennas to provide large 
equivalent antenna sizes with less bulk, either in 1-D 
(HYDROSTAR example, Fig. 2a) or in 2-D (similar to 
SMOS, Fig. 2b).   While these approaches permit 
efficient packing of the aperture in launch vehicle 
fairings for later deployment on orbit, there are a 
number of issues which limit their practical growth 
potential for high-quality science measurements.  These 
include the thinning algorithm and number of baselines, 
the large number of small antenna elements/ 
correlations, the complexity of digital signal processing 
and image reconstruction, and noise/NEDT concerns.  
In particular, the major tradeoffs between spatial 
resolution and coverage will be explored since there is 
an inverse relationship between the aperture baselines 
(separation of antenna elements) and decorrelation at 
the Earth’s surface which limits the field of view within 
which accurate science measurements can be obtained. 
 
As part of the analysis of the aperture synthesis 
approach to large antennas, an effort will be made to 
quantify relationships between spatial resolution, 
positional knowledge of and number of antenna 
baselines, and accurate brightness temperature 
mapping.  How minimizing NEDT for this antenna 
approach translates into engineering design 
considerations will also be examined, as well as lessons 
learned from SMOS [6].  The analyses should result in 
general formulas which will ingest a quantitative high-
level description of a microwave interferometer and 
output a rough estimate of its science performance. 
 
3.3  Adding a Radar to Microwave Radiometry 
The third technology approach is based on the 
assumption that a radiometer and radar can share the 
same antenna, and that there is a defined relationship 
between the radiometer and radar response at the same 
frequency for soil moisture or another geophysical 
parameter of interest that enables the process of 
disaggregating the radiometer brightness temperatures 
using simultaneous radar measurements to achieve 
improved spatial resolution (Fig. 2).  Besides the added 
cost and complexity of flying a radar with a radiometer, 
radar observations have an inherent noise which 
impacts the disaggregation process and the accuracy of 
the resulting soil moisture estimates.  As part of this 
study, assuming that a radar can be accommodated with 
a radiometer on the maximum size of each type of 
antenna studied, potential radar performance of each of 
the antenna types will be analyzed to determine if an 
active/passive approach is a simpler, less costly way of 
meeting the needed science performance and resolution 
requirements. 
 
 
    Fig. 2.  Schematic diagram of the SMAP active/passive 
                  disaggregation algorithm methodology [7, 8]. 
 
 
4.  CONCLUSIONS 
 
Low frequency passive microwave missions require 
large apertures on orbit to achieve even moderate spatial 
resolution.  Current concepts for a future high resolution 
soil moisture mission call for an overall effective 
antenna aperture of ≥20 m, regardless of whether a real 
aperture or aperture synthesis approach is used, in order 
to achieve 10 km or better spatial resolution.  Table 1 
Table 1.  Comparison of Microwave Antenna Approaches 
for Future High Resolution Measurements from Space 
 
 
 
presents a comparison of some antenna characteristics 
for real versus synthetic aperture approaches.  Other 
considerations for future missions include the addition 
of another frequency (such as 37 GHz for surface 
temperature estimation, a key parameter needed in soil 
moisture retrieval algorithms), the option of going 
broadband in frequency (L to P band, for example), and 
the costs/benefits of adding an unfocused SAR to the 
primary radiometer instrumentation.  
 
 The expected outcome of this trade study when 
fully completed is an assessment of the limits to growth 
of three different microwave antenna technologies (real 
aperture, aperture synthesis, and active/passive 
approaches) and a recommendation to the science 
community on how best to proceed to enable future 
higher-resolution low frequency missions.  However, 
the immediate path forward is likely to be dictated by 
the recommendations to NASA of the next Earth 
Science Decadal Survey (expected release in January 
2018) as well as international efforts to coordinate L-
band continuity or follow-on missions to SMAP, 
SMOS, and Aquarius. 
 
 
6.  REFERENCES 
[1] Chan, S., R. Bindlish, P. O’Neill, E. Njoku, T. Jackson, 
A. Colliander, F. Chen, M. Burgin, S. Dunbar, J. Piepmeier, 
S. Yueh, D. Entekhabi, M. Cosh, T. Caldwell, J. Walker,  X. 
Wu, A. Berg, T. Rowlandson, A. Pacheco, H. McNairn, M. 
Thibeault, J. Martínez-Fernández,  Á. González-Zamora, M. 
Seyfried, D. Bosch, P. Starks, D. Goodrich, J. Prueger, M. 
Palecki, E. Small, J. Calvet, W. Crow, and Y. Kerr, 
“Assessment of the SMAP Level 2 Passive Soil Moisture 
Product,”  IEEE Trans. on Geoscience and Remote Sensing, 
vol. 54, no. 8, August 2016, pp. 4994-5007, doi: 
10.1109/TGRS.2016.2561938.   
 
[2] Personal communication: S. Belair/ Environment Canada, 
X. Zhan/ NOAA, R.Reichle/ GSFC GMAO, S. Kumar/ GSFC 
LIS, C. Peters-Lidard/ GSFC LIS, National Water Model;  
also ECMWF roadmap: 
https://www.ecmwf.int/sites/default/files/ECMWF_Roadma
p_to_2025.pdf 
 
[3] Le Vine, D. M., E. P. Dinnat, T. Meissner, S. H. Yueh, 
F.J. Wentz, S.E. Torrusio,  (2015). “Status of Aquarius-
SACD and Aquarius Salinity Retrievals”, IEEE Journal of 
Selected Topics in Applied Earth Observations and Remote 
Sensing, 8(12): 5401-5415. 
 
[4] Yueh, S., A. Fore, W. Tang, A. Hayashi, B. Styles, N. 
Reul, Y. Weng, and F. Zhang, “SMAP L-Band Passive 
Microwave Observations of Ocean Surface Wind During 
Severe Storms,” IEEE Trans. on Geo. Rem. Sens., vol. 54, no. 
12, Dec., 2016, pp. 7339-7350. 
 
[5] HYDROSTAR design, Earth Science System Pathfinder 
proposal follow-on study, 1999. 
 
[6] M. Martin-Neira et al., “Lessons Learnt from SMOS after 
7 Years in Orbit,” Proc. of IGARSS’17, IEEE, Fort Worth, 
Texas, July 23-28, 2017. 
 
[7] Entekhabi, D., N. Das, E. Njoku, S. Yueh, J. Johnson, and 
JC Shi, SMAP Algorithm Theoretical Basis Document: Level 
2 & 3 Radar/Radiometer Soil Moisture (Active/Passive) Data 
Products. SMAP Project, D-66481, JPL, Pasadena, CA, 
Revision A, Dec. 9, 2014. 
 
[8] Das, N., D. Entekhabi, and E. Njoku, “An Algorithm for 
Merging SMAP Radiometer and Radar Data for High-
Resolution Soil-Moisture Retrieval,” IEEE Trans. on Geo. 
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Determination of Best Low-Frequency Microwave Antenna Approach for Future High Resolution Measurements from Space

Determination of Best Low-Frequency Microwave Antenna Approach for Future High Resolution Measurements from Space

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