Data Set ID:

SMAPVEX12 PALS Backscatter Data, Version 1

This data set contains backscatter data obtained by the Passive Active L-band System (PALS) microwave aircraft instrument as part of the Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12).

This is the most recent version of these data.

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Data: Data integrity and usability verified

Documentation: Key metadata and user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools

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Data Format(s):
  • ASCII Text
Spatial Coverage:
N: 49.96, 
S: 49.44, 
E: -97.85, 
W: -98.51
Spatial Resolution:
  • 500 m to 1500 m x 500 m to 1500 m
Temporal Coverage:
  • 7 June 2012 to 19 July 2012
Temporal Resolution1 day to 5 daysMetadata XML:View Metadata Record
Data Contributor(s):Andreas Colliander

Geographic Coverage

Other Access Options

Other Access Options


As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Colliander, A. 2014. SMAPVEX12 PALS Backscatter Data, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: [Date Accessed].
11 March 2020
Last modified: 
12 March 2020

Data Description

This data set contains backscatter data obtained by the Passive Active L- and S-band (PALS) microwave aircraft instrument. The data were collected as part of the Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12).


The parameter for this data set is normalized radar cross-section (dB). Valid parameter values range between -40 and 0 dB.

File Information

Format and File Contents

Data are provided in 32 ASCII text files (two files for each day in which data were collected). Table 1 provides descriptions for each column in the data files:

Table 1. Contents of Data Fields
Column Number Description
1 UTC time in seconds
2 Latitude of the boresight (footprint center) [°]
3 Longitude of the boresight [°]
4 UTM x-coordinate of the boresight [m]
5 UTM y-coordinate of the boresight [m]
6 VV normalized radar cross-section [dB]
7 HH normalized radar cross-section [dB]
8 HV normalized radar cross-section [dB]
9 VH normalized radar cross-section [dB]
10 Heading uncertainty flag [0/1]

File Naming Convention

Files are named according to the following convention, and as described in Table 2:



Table 2. File Naming Convention



SV12PLBK Data Set Short Name
PALS Passive Active L- and S-band (PALS) data
S0 Sigma Nought (dB)
2012 2012 (representing SMAPVEX12 campaign)
MM 2-Digit Month
DD 2-Digit Day
[Hi/Lo]Alt Indicates whether this is a high- or low-altitude file
vXXX Data version (v101 = version 1.01)
.txt Indicates this is an ASCII text file

Example: SV12PLBK_PALS_S0_20120629_LoAlt_v101.txt

Spatial Information


Southernmost Latitude: 49.44°N
Northernmost Latitude: 49.96°N
Westernmost Longitude: 98.51°W
Easternmost Longitude: 97.85°W


The low-altitude radiometer footprint size is approximately 500 m, and the high-altitude radiometer footprint size is approximately 1500 m.


Data are provided in Universal Transverse Mercator (UTM), Zone 14 N, World Geodetic System 1984 (WGS84) coordinates.

Temporal Information

Coverage and Resolution

Data were collected every 1 to 5 days from 07 June 2012 through 19 July 2012.

Software and Tools

No special tools are required to view these data. Any word-processing program or Web browser will display the data.

Data Acquisition and Processing

Current microwave models and retrieval algorithms have significant limitations in their treatment of different vegetation types and heterogeneous scenes (mixtures of grass, crops, trees, streams, lakes) and quantitative treatment of algorithm scaling and error analysis for such heterogeneous scenes. Measurements over wide varieties of terrain are needed, with joint active and passive sensors, to develop algorithms and parameterizations that can work across all terrain types, and extract optimum information from the combined data. This will have direct impact on the design of dedicated soil moisture missions and development of methods to assimilate such data into land surface models.

Microwave radiometry and radar are well-established techniques for surface remote sensing. Combining passive and active sensors provides complementary information contained in the surface emissivity and backscatter signatures, which can improve the accuracy of retrieval of geophysical parameters. Over land, it has been demonstrated that the radiometer and the radar both provide information for estimating soil moisture and vegetation water content (Bolten et al. 2003, Njoku et al. 2002, Narayan et al. 2004).

Sensor or Instrument Description

The campaign deployed by the Jet Propulsion Laboratory (JPL), with NASA support, designed, built and tested a precision Passive/Active L/S-band (PALS) aircraft instrument for measurements of soil moisture and ocean salinity (Wilson et al. 2001). PALS provides radiometer products, vertically and horizontally polarized brightness temperatures, and radar products, including normalized radar backscatter cross-section for V- transmit/V-receive, V-transmit/H-receive, H-transmit/H-receive, and H-transmit/V-receive. In addition, it can also provide the polarimetric third Stokes parameter measurement for the radiometer and the complex correlation between any two of the polarized radar echoes (VV, HH, HV and VH). Table 3 provides the key characteristics of PALS.

Table 3. Description of the PALS instrument
Passive Frequency 1.413 GHz
Polarization V, H, +45, -45
Calibration stability 1 K (bias); 0.2 K (stability)
Active Frequency 1.26 GHz
Polarization VV, HH, VH, HV
Calibration accuracy <2 dB (bias); 0.2 dB (stability)
Antenna Half Power Beamwidth 20° (passive); 23°(active)
Beam Efficiency 94%
Directivity 18.5 dB
Polarization isolation > 35 dB

The PALS instrument was flown in four major soil moisture experiments (SGP99, SMEX02, CLASIC and SMAPVEX08 [Colliander et al. 2012]) before deployment in SMAPVEX12. Beginning with CLASIC, a new flat-panel antenna array was substituted for the large horns. The planar antenna consists of 16 stacked-patch microstrip elements arranged in a four by- four array configurations. Each stacked-patch element uses a honeycomb structure with extremely low dielectric loss at L-band to support the ground plane and radiating patches. The measured antenna pattern shows better than 33 dB polarization isolation, far exceeding the need for the polarimetric measurement capability. This compact, lightweight antenna has enabled PALS to transition to operating on small aircraft, such as the Twin Otter (Yueh et al. 2008).

PALS was mounted at a 40° incidence angle looking to the rear of the aircraft. The 3dB spatial resolutions of the instrument at the minimum and maximum altitudes are 500 m (1000 m altitude, minimum for the radar operation) and 1500 m (3000 m altitude, maximum for Twin Otter operation without oxygen supply). It is important to note that PALS provides a single beam of data along a flight track and that any mapping must rely upon multiple flight lines at a spacing of the footprint width.

SMAP PALS Instrument Image
Figure 1. Images of Three Different Aircraft Installations of the PALS Combined Active and Passive L-band Instrument

Error Sources

When the aircraft heading uncertainty flag is set to 1 the uncertainty of the boresight geolocation exceeds the nominal. The uncertainty in the heading of the aircraft was caused by drifting navigation unit. Except for the very first days of the campaign this drift was compensated for.

Quality Assessment

The quality of the normalized radar cross-section relies on internal calibration utilizing a calibration loop. The external calibration utilizes predetermined coefficients of the antenna and front-end and comparisons to concurrent UAVSAR measurements. These references assure generally good quality of the data.

References and Related Publications

Bolten, J., V. Lakshmi, and E. Njoku. 2003. Soil Moisture Retrieval using the Passive/Active L- and S-Band Radar/Radiometer. IEEE Trans. Geosci. Rem. Sens., 41:2792-2801.

Colliander, A., S. Chan, S. Kim, N. Das, S. Yueh, M. Cosh, R. Bindlish, T. Jackson, and E. Njoku. 2012. Long Term Analysis of PALS Soil Moisture Campaign Measurements for Global Soil Moisture Algorithm Development. Rem. Sens. of Environ. 121:309-322.

Narayan, U., V. Lakshmi, and E. Njoku. 2004. Retrieval of Soil Moisture from Passive and Active L/S Band Sensor (PALS) Observations during the Soil Moisture Experiment in 2002 (SMEX02). Rem. Sens. Environ., 92:483-496.

Njoku, E., W. Wilson, S. Yueh, S. Dinardo, F. Li, T. Jackson, V. Lakshmi, and J. Bolten. 2002. Observations of Soil Moisture using a Passive and Active Low Frequency Microwave Airborne Sensor during SGP99. IEEE Trans. Geosci. Rem. Sens., 40:2659-2673.

McNairn,H., T. Jackson, G. Wiseman, S. Belair, A. Berg, P. Bullock, A. Colliander, M. Cosh, S. Kim, R. Magagi, M. Moghaddam, J. Adams, S. Homayouni, E. Ojo, T. Rowlandson, J. Shang, K. Goita, M. Hosseini. 2013. In Review. The Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12): Pre-Launch Calibration and Validation of the SMAP Satellite. IEEE Trans. Geosci. Rem. Sens.

Wilson, W.J., S.H. Yueh, S.J. Dinardo, S. Chazanoff, F.K. Li, and Y. Rahmat-Samii. 2001. Passive Active L- and S-band (PALS) Microwave Sensor for Ocean Salinity and Soil Moisture Measurements, IEEE Trans. Geosci. Rem. Sens. 39, 1039-1048.

Yueh, S., S. Dinardo, S. Chan, E. Njoku, T. Jackson, and R. Bindlish. 2008. Passive and Active L-Band System and Observations during the 2007 CLASIC Campaign. Proc. IEEE IGARSS08, (2) II-241 - II-244, July 7-11, 2008.


Andreas Colliander
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Dr, Pasadena, CA 91109 USA

Document Information


October 2013


March 2020

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