«Algorithm Theoretical Basis Document Level 3 Radar Freeze/Thaw Data Product (L3_FT_A) Revision A December 9, 2014 Scott Dunbar, Xiaolan Xu, Andreas ...»
Soil Moisture Active Passive (SMAP)
Algorithm Theoretical Basis Document
Level 3 Radar Freeze/Thaw Data Product
December 9, 2014
Scott Dunbar, Xiaolan Xu, Andreas Colliander, Kyle McDonald, Erika Podest, Eni Njoku
Jet Propulsion Laboratory
California Institute of Technology
John Kimball and Youngwook Kim
Flathead Lake Biological Station
University of Montana
Climate Research Division, Environment Canada
Toronto, Canada Jet Propulsion Laboratory California Institute of Technology © 2014. All rights reserved.
The SMAP Algorithm Theoretical Basis Documents (ATBDs) provide the physical and mathematical descriptions of algorithms used in the generation of SMAP science data products.
The ATBDs include descriptions of variance and uncertainty estimates and considerations of calibration and validation, exception control and diagnostics. Internal and external data flows are also described.
The SMAP ATBDs were reviewed by a NASA Headquarters review panel in January 2012 with initial public release later in 2012. The current version is Revision A. The ATBDs may undergo additional version updates after SMAP launch.
2 Table of Contents ACRONYMS AND ABBREVIATIONS
1.1! THE SOIL MOISTURE ACTIVE PASSIVE (SMAP) MISSION
1.1.1! BACKGROUND AND SCIENCE OBJECTIVES
1.1.2! MEASUREMENT APPROACH
1.2! SMAP REQUIREMENTS RELATED TO FREEZE/THAW STATE
2! BACKGROUND AND HISTORICAL PERSPECTIVE
2.1! PRODUCT/ALGORITHM OBJECTIVES
2.2! L3_FT_A PRODUCTION
2.3! DATA PRODUCT CHARACTERISTICS
3! PHYSICS OF THE PROBLEM
3.1! SYSTEM MODEL
3.2! RADIATIVE TRANSFER AND BACKSCATTER
4! RETRIEVAL ALGORITHM
4.1! THEORETICAL DESCRIPTION
4.1.1! BASELINE ALGORITHM: SEASONAL THRESHOLD APPROACH
4.1.2! VARIANCE AND UNCERTAINTY ESTIMATES
4.2! PRACTICAL CONSIDERATIONS
4.2.1! PROCESSING AND DATA FLOW CONSIDERATIONS
4.2.2! ANCILLARY DATA AVAILABILITY/CONTINUITY
4.2.3! UPDATING AND OPTIMIZATION OF REFERENCES AND THRESHOLDS
4.2.4! CALIBRATION AND VALIDATION
4.2.5! ALGORITHM BASELINE SELECTION
5! CONSTRAINTS, LIMITATIONS, AND ASSUMPTIONS
APPENDIX 1: GLOSSARY
3 ACRONYMS AND ABBREVIATIONS
AMSR Advanced Microwave Scanning Radiometer ASF Alaska Satellite Facility ATBD Algorithm Theoretical Basis Document CONUS Continental United States CMIS Conical-scanning Microwave Imager Sounder DAAC Distributed Active Archive Center DCA Dual Channel Algorithm DEM Digital Elevation Model EASE Equal Area Scalable Earth [grid] ECMWF European Center for Medium-Range Weather Forecasting EOS Earth Observing System ESA European Space Agency GEOS Goddard Earth Observing System (model) GMAO Goddard Modeling and Assimilation Office GSFC Goddard Space Flight Center JAXA Japan Aerospace Exploration Agency JPL Jet Propulsion Laboratory LTAN Local Time of Ascending Node LTDN Local Time of Descending Node MODIS MODerate-resolution Imaging Spectroradiometer NCEP National Centers for Environmental Prediction NDVI Normalized Difference Vegetation Index NEE Net ecosystem exchange NPOESS National Polar-Orbiting Environmental Satellite System NPP NPOESS Preparatory Project NSIDC National Snow and Ice Data Center NWP Numerical Weather Prediction OSSE Observing System Simulation Experiment PDF Probability Density Function PGE Product Generation Executable RFI Radio Frequency Interference RVI Radar Vegetation Index SAR Synthetic Aperture Radar SDT (SMAP) Science Definition Team SDS (SMAP) Science Data System SMAP Soil Moisture Active Passive SMOS Soil Moisture Ocean Salinity (mission) SNR Signal to Noise Ratio SRTM Shuttle Radar Topography Mission USGS United States Geological Survey VWC Vegetation Water Content 4
1 INTRODUCTIONThe L3_FT_A product provides a daily classification of freeze/thaw state for land areas north of 45°N derived from the SMAP high-resolution radar output to 3 km polar and global EASE grids. This document provides a complete description of the algorithm used to generate the SMAP Level 3 land surface freeze/thaw product, including the physical basis, theoretical description and practical considerations for implementing the algorithm. Details of the algorithm implementation and validation approach for determining algorithm performance against the mission requirement are also included.
1.1 THE SOIL MOISTURE ACTIVE PASSIVE (SMAP) MISSION
1.1.1 BACKGROUND AND SCIENCE OBJECTIVES The National Research Council’s (NRC) Decadal Survey, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, was released in 2007 after a two year study commissioned by NASA, NOAA, and USGS to provide them with prioritization recommendations for space-based Earth observation programs [National Research Council, 2007]. Factors including scientific value, societal benefit and technical maturity of mission concepts were considered as criteria. SMAP data products have high science value and provide data towards improving many natural hazards applications. Furthermore SMAP draws on the significant design and risk-reduction heritage of the Hydrosphere State (Hydros) mission [Entekhabi et al., 2004]. For these reasons, the NRC report placed SMAP in the first tier of missions in its survey. In 2008 NASA announced the formation of the SMAP project as a joint effort of NASA’s Jet Propulsion Laboratory (JPL) and Goddard Space Flight Center (GSFC), with project management responsibilities at JPL. The target launch date is January 2015 [Entekhabi et al., 2010].
The SMAP science and applications objectives are to:
• Understand processes that link the terrestrial water, energy and carbon cycles;
• Estimate global water and energy fluxes at the land surface;
• Quantify net carbon flux in boreal landscapes;
• Enhance weather and climate forecast skill;
• Develop improved flood prediction and drought monitoring capabilities.
1.1.2 MEASUREMENT APPROACH Table 1 is a summary of the SMAP instrument functional requirements derived from its science measurement needs. The goal is to combine the attributes of the radar and radiometer observations (in terms of their spatial resolution and sensitivity to soil moisture, surface roughness, and vegetation) to estimate soil moisture at a resolution of 10 km, and freeze/thaw state at a resolution of 3 km.
The SMAP spacecraft is designed for a 685-km circular, sun-synchronous orbit, with equator crossings at 6 AM and 6 PM local time. The instrument combines radar and radiometer subsystems that share a single feedhorn and parabolic mesh reflector (Fig. 1). The radar operates with VV, HH, and HV transmit-receive polarizations, and uses separate transmit frequencies for the H (1.26 GHz) and V (1.29 GHz) polarizations. The radiometer operates with polarizations V, H, and the third and fourth Stokes parameters, T3, and T4, at 1.41 GHz. The T3-channel measurement is used to assist in the correction of Faraday rotation effects. The reflector is offset from nadir and rotates about the nadir axis at 14.6 rpm, providing a conically scanning antenna beam at a surface incidence angle of approximately 40°. The provision of constant incidence angle across the swath simplifies the data processing and enables accurate repeat-pass estimation of soil moisture and freeze/thaw change. The reflector diameter is 6 m, providing a radiometer footprint of approximately 40 km (root-ellipsoidal area) defined by the one-way 3-dB beamwidth. The two-way 3-dB beamwidth defines the real-aperture radar footprint of approximately 30 km. The real-aperture (‘lo-res’) swath width of 1000 km provides global coverage within 3 days or less equatorward of 35°N/S and 2 days poleward of 55°N/S. The realaperture radar and radiometer data will be collected globally during both ascending and descending passes.
6 Figure 1. The SMAP observatory is a dedicated spacecraft with a rotating 6-m light-weight deployable mesh reflector. The radar and radiometer share a common feed.
To obtain the desired high spatial resolution the radar employs range and Doppler discrimination. The radar data can be processed to yield resolution enhancement to 1-3 km spatial resolution over the 70% outer parts of the 1000 km swath. Data volume prohibits the downlink of the entire radar data acquisition. Radar measurements that allow high-resolution processing will be collected during the morning overpass over all land regions and extending one swath width over the surrounding oceans. During the evening overpass data poleward of 45° N will be collected and processed as well to support robust detection of landscape freeze/thaw transitions.
The baseline orbit parameters are:
Orbit Altitude: 685 km (2-3 days average revisit and 8-days exact repeat) !
Inclination: 98 degrees, sun-synchronous !
Local Time of Ascending Node: 6 pm !
At L-band anthropogenic Radio Frequency Interference (RFI), principally from ground-based surveillance radars, can contaminate both radar and radiometer measurements. Early measurements and results from the SMOS mission indicate that in some regions RFI is present and detectable. The SMAP radar and radiometer electronics and algorithms have been designed to include features to mitigate the effects of RFI. To combat this, the SMAP radar utilizes selective filters and an adjustable carrier frequency in order to tune to pre-determined RFI-free portions of the spectrum while on orbit. The SMAP radiometer will implement a combination of time and frequency diversity, kurtosis detection, and use of T4 thresholds to detect and where possible mitigate RFI.
The SMAP L1-L4 data products are listed in Table 2. Level 1B and 1C data products are calibrated and geolocated instrument measurements of surface radar backscatter cross-section and brightness temperatures derived from antenna temperatures. Level 2 products are geophysical retrievals of soil moisture on a fixed Earth grid based on Level 1 products and 7 ancillary information; the Level 2 products are output on half-orbit basis. Level 3 products are daily composites of Level 2 surface soil moisture and freeze/thaw state data. Level 4 products are model-derived value-added data products that support key SMAP applications and more directly address the driving science questions.
Table 2. SMAP Data Products
1.2 SMAP REQUIREMENTS RELATED TO FREEZE/THAW STATEThe primary science objectives for SMAP directly relevant to the freeze/thaw measurement include linking terrestrial water, energy and carbon cycle processes, quantifying the net carbon flux in boreal landscapes and reducing uncertainties regarding the so-called missing carbon sink
on land. This leads to the following requirements on the freeze/thaw measurement:
1) surface freeze/thaw measurements shall be provided over land areas where these factors are primary environmental controls on land-atmosphere exchanges of water, energy and carbon;
2) the freeze/thaw status of the aggregate vegetation-soil layer shall be determined sufficiently to characterize the low-temperature constraint on vegetation net primary productivity and surface-atmosphere CO2 exchange;
3) SMAP shall measure landscape freeze/thaw with a spatial resolution of 3 km;
Current SMAP baseline mission requirements specific to terrestrial freeze/thaw science
activities state that:
[Level 1 mission requirement] The baseline science mission shall provide estimates of surface binary freeze/thaw state for the region north of 45° N latitude, which includes the boreal forest zone, with a spatial classification accuracy of 80% at 3 km spatial resolution and 2-day average intervals.
The above requirements form the basis of the design of the L3_FT_A product. This document includes a description of the baseline freeze/thaw state classification algorithm, and discussion of theoretical assumptions and procedures for refining and testing the algorithm to achieve the mission requirement.
2 BACKGROUND AND HISTORICAL PERSPECTIVEThe terrestrial cryosphere comprises cold areas of Earth's land surface where water is either permanently or seasonally frozen. This includes most regions north of 45°N latitude and most areas with elevation greater than 1000 meters. Within the terrestrial cryosphere, spatial patterns and timing of landscape freeze/thaw state transitions are highly variable with measurable impacts to climate, hydrological, ecological and biogeochemical processes.
Landscape freeze/thaw state influences the seasonal amplitude and partitioning of surface energy exchange strongly, with major consequences for atmospheric profile development and regional weather patterns (Betts et al., 2000). In seasonally frozen environments, ecosystem responses to seasonal thaw are rapid, with soil respiration and plant photosynthetic activity accelerating with warmer temperatures and the abundance of liquid water (e.g., Goulden et al., 1998; Black et al., 2000; Jarvis and Linder, 2000). The timing of seasonal freeze/thaw transitions can generally be related to the duration of seasonal snow cover, frozen soils, and the timing of lake and river ice breakup and flooding in the spring (Kimball et al., 2001, 2004a). The seasonal non-frozen period also bounds the vegetation growing season, while annual variability in freeze/thaw timing has a direct impact on net primary production and net ecosystem CO2 exchange (NEE) with the atmosphere (Vaganov et al., 1999; Goulden et al., 1998).
Satellite-borne microwave remote sensing has unique capabilities that allow near real-time monitoring of freeze/thaw state, without many of the limitations of optical-infrared sensors such as solar illumination or atmospheric conditions. The SMAP L3_FT_A product is designed to provide the most accurate remote sensing-based characterization of landscape freeze/thaw state for land areas north of 45°N latitude. The unique design of the SMAP L-band radar allows a 9 combined spatial and temporal characterization of terrestrial freeze/thaw transitions previously unavailable.