NASA’s SMAP Program and Cal/Val
The forthcoming January 29, 2015 launch of NASA’s SMAP satellite will produce global estimates of soil moisture to 5 cm depth and freeze/thaw state every 2-3 days at 3, 9, and 36 km spatial scales to an accuracy of ±0.04 m3 m-3 [Entekhabi and others, 2010]. The data will be expressed as a digital array at these varying gridding schemes projected onto the Equal-Area Scalable Earth Grid (EASE-2). The EASE-2 grid is static: each corner is defined in space at 3, 9 and 36 km and the final data will coincide within these locations [Brodzik and others, 2012]. Soil moisture will be retrieved globally at a spatial resolution of 3 km using the active L-band microwave radar and 36 km using a passive L-band radiometer; the final product will combine the two into an 9 km grid (with geodetic distortion) and freeze-thaw state at a spatial resolution of 3 km [Shi and others, 2010]. Data assimilation of SMAP 0-5 cm soil moisture into a land surface model will drive a root-zone soil moisture model at hourly time steps.
The initial purpose behind TxSON is to improve soil moisture estimates in Texas using the SMAP satellite, and this requires calibration and validation of the SMAP signal to conditions on the ground. Calibration is the relationship between quantities obtained by a measuring instrument or system and the corresponding values realized by standards taken under specified conditions. Validation is the process of assessing—by independent means—the quality of the data products derived from the system outputs. NASA is establishing a calibration and validation (Cal/Val) phase during the science mission that will follow the 90-day post-launch, in-orbit commissioning (IOC) phase [Jackson and others, 2012]. The duration of the Cal/Val phase is 6 months for Level 1 products and 12 months for Level 2 through Level 4 products. Cal/Val has become synonymous in the context of remote sensing with the suite of processing algorithms that convert raw data into accurate and useful geophysical or biophysical quantities that are verified to be self-consistent. In situ soil moisture, surface and air temperature, surface flux, and additional land surface characteristics observations will be important in validating science products from the SMAP mission. Based upon the SMAP mission requirements, an ideal in situ soil moisture resource would include verified (±0.04 m3 m-3) retrieval of surface soil moisture (0 to 5 cm soil depth) and root zone to 100 cm.
The SMAP program differentiates Cal/Val sites as being either sparse, containing only a few measurements, or dense, providing multiple measurements within a satellite footprint. Sparse networks include the USDA’s Soil Climate Assessment Network (SCAN), NOAA’s Climate Reference Network (CRN) or the Oklahoma Mesonet. Few dense networks exist with the exception of nine USDA-ARS Watersheds; however, many of these sites span multiple EASE-2 cells making them less than ‘ideal’ for the SMAP footprint [Jackson and others, 2012]. NASA is using existing stations; they could not invest in additional in situ validation sites. For those entities that choose to invest and participate in an approved Cal/Val site (known as early-adopters), NASA offers beta data products 9 months before full release.
Brodzik, M. J., Billingsley, B., Haran, T., Raup, B., and M. H. Savoie, M. H., 2012, EASE-Grid 2.0: Incremental but Significant Improvements for Earth-Gridded Data Sets, ISPRS International Journal of Geo-Information, 1, 32-45.
Entekhabi, D., and others, 2010, The Soil Moisture Active Passive (SMAP) Mission, Proc. IEEE, 98, 704-716, doi.org/10.1109/jproc.2010.2043918.
Jackson, T., Colliander, A., Kimball, J., Reichle, R., Crow, W. T., Entekhabi, D., O' Neill, P., and Njoku, E., 2012, Soil Moisture Active Passive (SMAP) Mission: Science Data Calibration and Validation Plan, Jet Propulsion Laboratory, California Institute of Technology, Rep. JPL D-52544, Pasadena, CA, July 6, 2012.
Shi, J. C., and others, 2010, Deriving Soil Moisture with the Combined L-Band Radar and Radiometer Measurements, International Geoscience and Remote Sensing Symposium, 812-815, doi.org/10.1109/IGARSS.2010.5652424.