Data Description

Parameters

The Level-1A product includes scaled radiometer counts for the first four statistical moments of the vertically and horizontally polarized signals, as well as the complex cross-correlation between the polarizations. In subsequent processing, the first and second raw moments are used to compute a second central moment, which is the output of a conventional radiometer. The kurtosis is computed using all four raw moments, which is used in RFI detection and mitigation. The complex cross-correlation is used to compute the third and fourth Stokes parameters. All data have an effective 12 bits of resolution and are scaled to single-precision floating point numbers with consistent units.

Refer to the Data Fields document for details on all parameters.

File Information

Format

Data are in HDF5 format. For software and more information, including an HDF5 tutorial, visit the HDF Group's HDF5 Web site.

File Contents

As shown in Figure 1, each HDF5 file is organized into five main groups, which contain additional groups and/or data sets. Files contain both fullband and subband data, referred to as moments data and high resolution moments data, respectively. Fullband moments data are acquired over the entire 24 MHz radiometer bandwidth, and subband high resolution moments data are acquired for each of the 16 subbands within the 24 MHz bandwidth. Each subband has a 1.5 MHz bandwidth.

The five main groups are summarized below. For a complete list and description of all data fields within these groups, refer to the Data Fields document. Note that data array dimensions and sizes vary for this product.

High Resolution Moments Data

Includes the first four sample raw moments of the 16 subband signals parsed into five radiometric states. The moments are provided for both vertical (V) and horizontal (H) polarizations, and separately expressed in terms of the in-phase (real) and quadrature (imaginary) components of the signals. The complex cross-correlations of the two polarizations are also included for each of the 16 subbands.

Housekeeping Data

Includes housekeeping telemetry or engineering data in both digital numbers and engineering units for each scan.

Moments Data

Includes the first four sample raw moments of the fullband signal parsed into five radiometric states. The moments are provided for both vertical and horizontal polarizations and separately expressed in terms of the in-phase (real) and quadrature (imaginary) components of the signals. Also included are the complex cross-correlations of the two polarizations.

Spacecraft Data

Includes data for an entire antenna scan in the instrument swath, such as geometric and geographic information, spacecraft attitude, spacecraft nadir longitude and latitude, as well as representative time stamps.

Metadata Fields

Includes all metadata that describe the full content of each file. For a description of all metadata fields for this product, refer to the Metadata Fields document.

File Naming Convention

Files are named according to the following convention, which is described in Table 1:

SMAP_L1A_RADIOMETER_[Orbit#]_[A/D]_yyyymmddThhmmss_RLVvvv_NNN.[ext]

For example:

SMAP_L1A_RADIOMETER_00934_D_201510245T195920_R12242_001.h5

Where:

File Size

Each half-orbit file is approximately 1.4 GB.

The daily data volume is approximately 36.4 GB.

Spatial Information

Coverage

Coverage spans from 180°W to 180°E, and from approximately 86.4°N to 86.4°S. The gap in coverage at both the North and South Pole, called a pole hole, has a radius of approximately 400 km. The swath width is 1000 km, enabling nearly global coverage every three days. Fullband moments and cross-correlation data are collected globally.High resolution subband moments and cross-correlation data are collected over all land areas and two regions used for calibration—one in the South Pacific Ocean and one in Antarctica. If a portion of the radiometer scan covers land, the entire scan will contain high resolution data, resulting in some ocean coverage near coastal areas.

Coverage Map

Figure 2 shows the spatial coverage of the SMAP L-Band Radiometer for one descending half orbit, which comprises one file of this data set.

Resolution

The native spatial resolution of the radiometer footprint is approximately 40 km.

Temporal Information

Coverage

Coverage spans from 31 March 2015 to present.

Satellite and Processing Events

Due to instrument maneuvers, data downlink anomalies, data quality screening, and other factors, small gaps in the SMAP time series will occur. Details of these events are maintained on two master lists:

• SMAP On-Orbit Events List for Instrument Data Users
• Master List of Bad and Missing Data

A significant gap in coverage occurred between 19 June and 23 July 2019 after the SMAP satellite went into Safe Mode. A brief description of the event and its impact on data quality is available in the SMAP Post-Recovery Notice.

Latencies

FAQ: What are the latencies for SMAP radiometer data sets?

Resolution

Each half orbit spans approximately 49 minutes. The data sampling interval is approximately 350 μs for fullband moments data and 1.2 msec for subband high resolution moments data. The subband sampling interval represents a science packet, which covers four Pulse Repetition Intervals (PRIs).

Data Acquisition and Processing

Acquisition

SMAP Level-1A time-ordered instrument counts are processed from Level-0, Version 1 science packet data.

Processing

Overview

The SMAP Science Data System (SDS) at the Jet Propulsion Laboratory in Pasadena, California generates the Level-1A radiometer product from downlinked radiometer telemetry. Each Level-1A product contains a time-ordered series of instrument counts. These counts are extracted and scaled from instrument packets that conform to Consultative Committee on Space Data Systems (CCSDS) standards. The Level-1A sorts the packets based on the radiometric states, which are described below. Each data set associated with a specific packet of radiometer counts is labeled with a time stamp that records the instant of instrument acquisition. The Level-1A product also contains a set of housekeeping telemetry converted to engineering units for each scan.

The science telemetry includes the first four raw moments of the fullband (24-MHz wide) and 16 subband (each 1.5 MHz wide) signals, for both vertical and horizontal polarization. These data are separately expressed in terms of the in-phase and quadrature components of the signals. The instrument also outputs complex cross-correlation of the two polarizations for the fullband moments data as well as for the moments data that represent the 16 subband/high resolution moments data. Every science data packet therefore contains 360 elements of time-frequency data in high-rate mode and 72 elements of low-rate-mode time data. The subband data provide time and frequency diversity. These data improve detection and mitigation of RFI. Since RFI is expected mostly over land, the SMAP spacecraft downlinks high-rate-mode data over land and low-rate-mode data over oceans.

Radiometer data include science data packets that are generated once every four PRIs. For every PRI of the radar, the radiometer integrates approximately 300 μs within the receive window. The exact integration time varies based on the radar PRI length and blanking time length chosen by the instrument designers. Radiometer packets are made up of four PRIs. Each science data packet includes both fullband moments data and subband high resolution moments data for each of the four PRIs. The subband data are further integrated over each set of four PRIs yielding an integration time of approximately 1.2 ms. The radiometer timing diagram is shown in Figure 3.

Radiometric States

The radiometer switching scheme indicates the radiometric state for each particular science data packet. A switch in state can occur once every packet or every four PRIs. The radiometer digital electronics control each instance when the instrument state changes during an antenna scan. The Level-1A processor employs the switching scheme to parse the raw science data. The switches that incorporate the use of the reference load and noise sources are necessary for calibration of science data. The calibration network can produce different combinations of switch and noise diode states. The default radiometer switching sequence produces five states, including:

• Antenna: Data acquired when the radiometer is switched to the antenna to observe the scene.
• Reference: Data acquired when the radiometer is switched to the reference load.
• Reference with Internal Noise Diode: Data acquired when the radiometer is switched to the reference load and the internal noise diode is turned on. The internal noise diode is coupled into both of the V and H channels.
• Antenna with External Noise Diode: Data acquired when an external noise diode is used to inject noise into the radio frequency path.
• Antenna with Internal Noise Diode: Data acquired when the radiometer is switched to the antenna to observe the scene and the internal noise diode is turned on.

Quality, Errors, and Limitations

Error Sources

L-band anthropogenic RFI, principally from ground-based surveillance radars, can contaminate radiometer measurements. Early measurements and results from the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission indicate that, in some regions, RFI is present and detectable. The SMAP radiometer electronics and algorithms have been designed to include features to mitigate the effects of RFI. The SMAP radiometer implements a combination of time and frequency diversity, kurtosis detection, and the use of 3rd and 4th Stokes parameter thresholds to detect and where possible mitigate RFI. Data elements associated with subbands are included in the SMAP L1B Radiometer Time-Ordered Brightness Temperatures, Version 3 to track and enable RFI detection and mitigation.

Level-1A radiometer data can also contain bit errors caused by noise in communication links and memory storage devices. The CCSDS packets include error-detecting Cyclic Redundancy Checks (CRCs), which the Level-1A processor uses to flag errors.

Quality Assessment

SMAP data sets provide multiple means to assess quality. Each data set contains bit flags, uncertainty measures, and file-level metadata that provide quality information. The Data Fields and Metadata Fields documents describe the specific bit flags, uncertainty measures, and file-level metadata contained in this data set.

Each SMAP HDF5 data file contains metadata with Quality Assessment (QA) metadata flags. These QA metadata flags are calculated and set by the SDS at JPL prior to delivery to the National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC). A separate, ISO 19115-compliant metadata file with an .xml file extension is also delivered to NSIDC DAAC with the HDF5 data file; it contains the same information as the file-level metadata.

A separate QA file with a .qa file extension is also associated with each data file. QA files are ASCII text files that contain statistical information in order to help users better assess the quality of the associated data file.

In addition, various levels of QA are conducted with the Level-1A data. If a file passes QA, the SDS applies that file for higher-level processing, browse generation, active science QA, and data archive and distribution. If a product fails QA, it is never delivered to NSIDC DAAC.

Instrumentation

Description

For a detailed description of the SMAP instrument, visit the SMAP Instrument page at Jet Propulsion Laboratory (JPL) SMAP Web site.

Software and Tools

For tools that work with SMAP data, refer to the Tools Web page.

Version History

Document Creation Date

October 2015

Document Revision Date

April 2016

Related Data Sets

SMAP Data at NSIDC | Overview

SMAP Radar Data at the ASF DAAC

Related Websites

SMAP at NASA JPL

Contacts and Acknowledgments

Investigators

Jeffrey R. Piepmeier, Ed Kim, Priscilla N. Mohammed, Jinzheng Peng, Chris Ruf
NASA Goddard Space Flight Center
Microwave Instrument & Technology Branch
Greenbelt, MD 20771 USA

References

Entekhabi, D. et al. 2014. SMAP Handbook–Soil Moisture Active Passive: Mapping Soil Moisture and Freeze/Thaw from Space. Pasadena, CA USA: SMAP Project, JPL CL#14-2285, Jet Propulsion Laboratory. (https://smap.jpl.nasa.gov/files/smap2/SMAP_Handbook_FINAL_1_JULY_2014_Web.pdf, 4.09 MB)

Mohammed-Tano, P. 2015. Soil Moisture Active Passive (SMAP) Project Level 1A Radiometer Product Specification Document. Pasadena, CA USA: SMAP Project, JPL D-92340, Jet Propulsion Laboratory. (D-92340-A_SMAP_Radiometer_Level1A_Product_Specification_Document_150720_with sigs.pdf, 979 KB)

Piepmeier, J. R. et al. 2016. SMAP Algorithm Theoretical Basis Document: L1B Radiometer Product: Includes L1A and L1B. SMAP Project, NASA GSFC SMAP-006, NASA Goddard Space Flight Center, Greenbelt, MD. (SMAP_ATBD_RadiometerL1B_TB_REV-B_20150401.pdf, 1.94 MB)