Soil Moisture Active Passive (SMAP) Satellite Royce Bohnert

SMAP Overview Objectives Timeline Image from SMAP Handbook Objectives Estimate soil moisture in the top 5 cm of soil at 10-km spatial resolution with 3-day average intervals In regions north of 45N latitude, measure freeze/thaw state transitions These measurements are important for weather forecasting, predicting agricultural productivity, and flood area mapping [1]. Timeline In a report released in 2007, the National Research Council recognized the importance of global soil moisture and freeze/thaw state measurements to various earth science applications. Formulation of the project was started by NASA in 2008. Was launched into orbit in January 2015. SMAP’s radar instrument failed in July 2015 [2]. Radiometer only data was collected for the remainder of the mission. Designed for a 3 year mission. Decommissioning was scheduled for early 2018.

Orbit Low earth orbit with altitude of 685 km above equator A polar, sun-synchronous orbit allows global coverage every 3 days Image from SMAP Handbook

Geometry of Data Collection The 3 dB antenna footprint is 40 km on the ground. Antenna look angle is 35.5° and incidence angle is 40.0° (due to spherical earth). A 1000 km wide swath of data is collected as the satellite passes over land. Higher resolution radar data is collected over about 70% of the swath using SAR processing. Image from SMAP Handbook

SMAP Instruments SMAP was equipped with an L-band radiometer and an L-band SAR radar The instruments share a common feed horn and a 6-meter conically scanning reflector antenna [1]. Image from SMAP Handbook

SMAP Instrument - Radiometer 24Mhz bandwidth at center frequency of 1.4135 GHz Integration is performed for roughly 350 μs during the RX window for the radar The fullband data is divided into 16 subband signals RFI mitigition was a major consideration in the radiometer design. Algorithms are used to detect RFI on the individual subbands so that corrupted data can be selectively removed. Image from SMAP Handbook

Radiometer Timing Image from SMAP Handbook

SMAP Instrument - Radar PRF is approximately 2850Hz Transmit pulses are 15μs long, 1 MHz LFM chirps Peak Transmit Power: 500 W H- and V-polarized pulses are transmitted consecutively with 9 μs in between. Center frequencies are 3 MHz apart. During the RX window, co- and cross- polarization is measured on H and V channels Image from SMAP Handbook

Radar Time and Frequency Domain Representations Images from SMAP Handbook

Image source: NASA/JPL-Caltech/GFSC

Surface Soil Moisture 2015-3-31

You can explore the data yourself at: https://worldview. earthdata

References 1) Dara Entekhabi et al, SMAP Handbook, NASA JPL, 2014. 2) https://www.nasa.gov/press-release/nasa-soil-moisture-radar-ends-operations-mission-science-continues

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