A new airborne synthetic aperture radar (SAR) system was recently developed for the estimation of snow water equivalent (SWE). The radar is part of the SWESARR (Snow Water Equivalent Synthetic Aperture Radar and Radiometer) instrument, an active passive microwave system specifically designed for the accurate estimation of SWE. The dual polarization (VV, VH) radar operates at three frequency bands (9.65 GHz, 13.6 GHz, and 17.25 GHz), with bandwidths of up to 200 MHz. The radar flew its first flight campaign in November 2019, along with SWESARRs - already operational  radiometer. The radar collected comprehensive data sets over various terrains that show a successful system performance. The inst slated to participate in future SnowEx campaigns

Bonds, Quenton

Brucker, Ludovic

Kielbasa, Chase

Osmanoglu, Batuhan

Perrine, Martin

Racette, Paul

Rincon, Rafael

Seufert, Steve

English

NASA Technical Reports Server

TRI-FREQUENCY SYNTHETIC APERTURE RADAR FOR THE MEASUREMENTS OF 
SNOW WATER EQUIVALENT  
Rafael Rincon, Batuhan Osmanoglu, Paul Racette, Quenton Bonds, Martin Perrine,                             
Ludovic Brucker, Steve Seufert, Chase Kielbasa 
NASA Goddard Space Flight Center 
 
 
ABSTRACT 
 
A new airborne synthetic aperture radar (SAR) system 
was recently developed for the estimation of snow water 
equivalent (SWE).  The radar is part of the SWESARR (Snow 
Water Equivalent Synthetic Aperture Radar and Radiometer) 
instrument, an active passive microwave system specifically 
designed for the accurate estimation of SWE. The dual 
polarization (VV, VH) radar operates at three frequency 
bands (9.65 GHz, 13.6 GHz, and 17.25 GHz), with 
bandwidths of up to 200 MHz. The radar flew its first flight 
campaign in November 2019, along with SWESARR’s -
already operational – radiometer. The radar collected 
comprehensive data sets over various terrains that show a 
successful system performance. The instrument is slated to 
participate in future SnowEx campaigns. 
 
Index Terms— Snow, SAR, SWE 
 
1. INTRODUCTION 
Snow is an important source of fresh water in many areas 
of the world. One-sixth of the world’s population (1.2 billion 
people) rely on seasonal snowpack and glaciers as a source of 
fresh water [1][2]. With rising global temperatures, the 
world’s snow resources are predicted to change [3]. Snow 
water equivalent (SWE), the measurement of how much 
water is present as snow, is thus an important parameter for 
water resource management and climate studies. Developing 
the tools to remotely measure SWE is one of NASA’s 
priorities. To that end, SnowEx, a multi-year airborne snow 
campaign, allow us to test the performance of various 
instrument to get a better understanding of how much water 
is stored in the Earth’s terrestrial snow. The ultimate goal of 
these studies is to develop robust remote sensing techniques 
by collecting multi-sensor datasets that will advance the 
science needs. SWESARR (Snow Water Equivalent 
Synthetic Aperture Radar and Radiometer) is an airborne 
instrument designed to measured co-located active and 
passive microwave signatures from snow. The instrument’s 
unique measurement approach can provide critically needed 
data for the accurate estimation of SWE and make a valuable 
contribution to future SnowEx efforts. Furthermore, 
SWESARR will provide a test-bed for measurement 
 
 
 
 
 
 
techniques that can be extended to satellite observation, 
setting a path for future spaceborne snow missions. In 
addition, the Canadian Space Agency is conducting  phase 0 
snow mission concept study for dual band Ku band SAR. 
SWESARR could serve as  an airborne simulator for that 
mission concept 
SWESARR’s radar and radiometer systems (fig. 1) were 
developed at the NASA Goddard Space Flight Center 
(GSFC).  The radar development was completed in Oct 2018, 
while the radiometer has been operational since 2013 and has 
been flown several campaigns. The radar conducted its first 
test flights over the Grand Junction area in Colorado between 
December 1st and 4th, 2018. The area covered during the 
flights included the Grand Junction airport, where corner 
reflectors were deployed for radar calibration, and the Grand 
Mesa, a large flat top mountain which has been selected as 
test site for SnowEx campaigns. SAR images acquired at 9.65 
GHz over the Grand Junction airport are shown in figure 2. 
2. RADAR SCPECIFICATION 
The radar system is made up of three independent radar 
units that operate at 9.65 GHz, 13.6 GHz, and 17.25 GHz, 
respectively, with bandwidths up to 200 MHz, utilizing the 
maximum ITU allocations. The three radars transmit 
vertically (V) polarized waveforms and receive returns with 
vertical and horizontal (VV, VH) polarizations (dual-  
Figure 1.  SWESARR, a tri-band radar and 
radiometer airborne system designed to retrieve 
snow water equivalent (SWE), during testing at 
NASA GSFC’s anechoic chamber. 
https://ntrs.nasa.gov/search.jsp?R=20200001353 2020-03-28T18:57:09+00:00Z
 polarization mode). The radar can operate at all three 
frequency bands simultaneously or interleaving them, 
depending on the application. The radiometer operates at 10.6 
GHz, 18.7 GHz, and 36.5 GHz. Table 1 lists the radar 
nominal characteristics and a block diagram illustrating the 
radar architecture is shown in fig. 3. 
 
SWESARR’s waveform generation and data acquisition 
is performed by a multi-channel Radar Digital Unit (RDU) 
developed for EcoSAR [4]. The RDU, capable of arbitrary 
waveform generation, data acquisition, and onboard 
processing, generates the transmit waveforms and coherently 
acquires all six radar returns.   
The radar and the radiometer share an offset-fed reflector 
antenna (fig. 1), making the acquisition of collocated 
active/passive measurements possible. The reflector, 
designed by Harris Corporation [5], is fed by a “current sheet 
array” feed, designed by Nuvotronics, that operates from 8 to  
 
    Table 1 SWESARR’s radar main characteristics 
 
 
 
 
 
 
40 GHz [6]. The antenna is nominally pointed at 45 degree 
depression angle and perpendicular to the flight track. 
3. PERFORMANCE 
The radar flew on board a Twin Otter International (TOI) 
aircraft (fig. 3) at three altitudes (1.5 km, 2 km, 3 km) above 
ground. The flights were spread over several days and about 
a total of 8 hours of data were acquired at different 
bandwidths and acquisition modes. Four trihedral corner 
 
Figure 2 Architecture of the SWESARR instrument that 
operates at three radar bands, and three radiometer 
bands, using a shared an offset fed reflector antenna. 
Frequency (GHz) 9.65  13.6 17.25 
Aircraft Height (km) 2 2 2 
PRF (kHz) 1  1 1 
Look Angle (degrees) 45 45 45 
Pulse Width 10 10 10 
Max Bandwidth (per ITU)  200 200 100 
Slant Range Resolution (m) 0.75 0.75 1.5 
NESO (dB) -32.6  -30.8 -29 
Swath  (m) 446             316              249 
Figure 2. SWESARR’s dual polarization (VV, VH) images acquired at 9.65 GHz over the Grand Junction 
airport from an altitude of 2 km. 
  
 
reflectors (CRs) were deployed adjacent to the airport runway 
for calibration. The CRs were 0.9 m, 0.5 m, 0.2 m, and 
0.16 m in size. Figure 4 shows an expanded area of figure 2 
where the corner reflectors were deployed.  
 
The Co-pol image in figure 2 was used here for a 
preliminary performance assessment. The impulse response 
of the 0.9 m CR is shown in figure 5 where a Kaiser 
smoothing window (2.5 beta value) has been used in both 
range and azimuth dimensions. The impulse response shows 
the radar resolution is 0.35 m and 2 m in azimuth and range, 
respectively, as expected with the use of a Kaiser smoothing 
window. The ISLR (Integrated Sidelobe Ratio) is -14 dB and 
-16 dB in azimuth and range, respectively. The larger ISRLR 
in azimuth is likely caused by errors in the focusing of the 
image due to aircraft motion. An improvement of the motion 
correction algorithm is under way which should yield a lower 
ISLR. 
The PSLR (Peak-to-Siledlobe Ratio) is -13 db and -16 
dB in azimuth and range, respectively. The higher PSLR in 
azimuth is also due the motion errors and is expected to 
decrease with the more robust motion correction algorithm. 
 
Figure 4 Four corner reflectors were deployed adjacent to 
the airport for radar calibration and performance 
analysis.   
 
Figure 5 Impulse response of the radar at 9.65 GHz using 
the 0.9 m Corner Reflector. 
The large sidelobe in range, which has also been observed in 
loop back calibration data, is likely caused by artifacts in the 
waveform introduced by the digital waveform generator. 
These issues will be addressed prior to the next SWESARR 
flight. The noise equivalent sigma naught is better than – 30 
dB, as expected.    
4. CONCLUSION 
The new airborne synthetic aperture radar (SAR) 
designed for the SWESARR instrument demonstrated its dual 
polarization operation at 9.65 GHz, 13.6 GHz, and 17.25 
GHz. Analysis of the radar data acquired during its first flight 
campaign indicated the radar operated successfully meeting 
the design performance metrics. The radar will fly again in 
2019 as part of the SnowEx campaigns when the radar will 
be expected to operate at optimum performance. 
 
4. REFERENCES 
[1] National Aeronautics and Space Administration, “Got Snow”, 
https://neptune.gsfc.nasa.gov/uploads/images_db/Got_SnowSM.pdf 
[2] J. S Mankin, D. Viviroli, D. Singh, A. Y. Hoekstra, N. S. Diffenbaugh, 
“The potential for snow to supply human water demand in the present 
and future”, November 2015. doi.org/10.1088/1748-
9326/10/11/114016.  
[3] Räisänen, J. Clim Dyn (2008) 30: 307. https://doi.org/10.1007/s00382-
007-0289-y. 
[4] R. Rincon, T. Fatoyinbo, K. Ranson, B. Osmanoglu, G. Sun, M. 
Deshpande, M. Perrine, C. Du Toit, Q. Bonds, J. Beck, and D. Lu, “The 
ecosystems sar (EcoSAR) an airborne P-band polarimetric InSAR for 
the measurement of vegetation structure, biomass and permafrost,” 
Proc. 2014 IEEE Radar Conference, pp. 1443–1445, May 2014, 
Cincinnati OH, USA. 
[5] T. E. Durham, K. J. Vanhille, C. R. Trent, K. M. Lambert and F. A. 
Miranda, "Design of an 8–40 GHz antenna for the wideband instrument 
for snow measurements (WISM)," 2015 IEEE International 
Symposium on Antennas and Propagation & USNC/URSI National 
Radio Science Meeting, Vancouver, BC, 2015, pp. 1999-2000. 
doi: 10.1109/APS.2015.7305389. 
[6] J. W. Jordan et al., "PolyStrata® X/Ku/K/Ka-band, Dual-Polarized, 
Tightly Coupled Dipole Scannable Focal Plane Array," 2018 IEEE 
International Symposium on Antennas and Propagation & 
USNC/URSI National Radio Science Meeting, Boston, MA, 2018, pp. 
817-818. doi: 10.1109/APUSNCURSINRSM.2018.8608507.  
Figure 3.  The SWESARR instrument during 
integration to the Twin Otter aircraft prior to the 
radar’s first test flights. 


Tri-Frequency Synthetic Aperture Radar for the Measurements of Snow Water Equivalent

Tri-Frequency Synthetic Aperture Radar for the Measurements of Snow Water Equivalent

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