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SMAP - Soil Moisture Active Passive - Mapping soil moisture and freeze/thaw state from Space
Measurement Approach

Frequency & Polarizations
The SMAP instrument incorporates an L-band radar (VV, HH, and HV polarizations) and an L-band radiometer (V, H, and 3rd and 4th Stokes parameter polarizations).

Artist rendition of swath path
The L-band frequency enables observations of soil moisture through moderate vegetation cover, independent of cloud cover and night or day. Multiple polarizations enable accurate soil moisture estimates to be made with corrections for vegetation, surface roughness, Faraday rotation, and other perturbing factors.

Instrument Configuration & Spatial Resolution
The radar and radiometer share a single feedhorn and reflector. The deployable mesh reflector is offset from nadir and rotates about the nadir axis at 14.6 rpm, providing a conically scanning antenna beam with a surface incidence angle of 40°.

The reflector diameter of 6 m yields a radiometer footprint spatial resolution at the surface of 39 km x 47 km, and a real-aperture radar footprint resolution of 29 km x 35 km ('low-resolution' radar).
To obtain high spatial resolution the radar employs range and Doppler discrimination. Unfocused SAR processing yields spatial resolution that varies from 1 km to 3 km over the outer 70% of the swath ('high-resolution' radar).

To mitigate radio-frequency interference (RFI) from ground transmitters the radiometer employs a digital backend and sub-banding approach. The radiometer 'high-rate' mode acquires sub-band data; the radiometer 'low-rate' mode acquires data averaged over the full band only.

Detailed Instrument Description

Soil Moisture Simulation
Science Accuracy
The science goal is to combine the attributes of the radar observations (high spatial resolution but lower soil moisture accuracy) and radiometer observations (higher soil moisture accuracy but coarse spatial resolution).
By joint processing of the radar and radiometer data soil moisture will be retrieved at a spatial resolution of 10 km, and freeze-thaw state at a spatial resolution of 3 km, to meet the mission science requirements.

The provision of constant incidence angle across the 1000-km swath simplifies the data processing and enables accurate repeat-pass estimation of soil moisture and freeze/thaw.

Orbit Characteristics
The SMAP orbit is a 685-km altitude, near-polar, sun-synchronous 6am/6pm, 8-day exact repeat, frozen orbit.

  • Near-polar provides global land coverage up to high latitudes including all freeze/thaw regions of interest
  • Sun-synchronous provides observations of the surface at close to the same local solar time each orbit throughout the mission, enhancing change detection algorithms and science accuracy
  • Consistent 6am observation time is optimal for science, minimizes effect of Faraday rotation, and minimizes impact on S/C design.
  • Frozen orbit provides minimal altitude variation during an orbit, benefitting radar design and accuracy
  • 685-km altitude is an exact 8-day repeat orbit, advantageous for radar change detection algorithms
  • Orbit provides optimum coverage of global land area at three-day average intervals, and coverage of land region above 45N at two-day average intervals
Orbit Overpass Calculator
Orbit Overpass Calculator GUIDE (PDF, 46 KB)

Data Acquisition & Processing
Low-rate radiometer data and low-resolution radar data will be acquired continuously over fore and aft portions of the scan (full 360 degrees) and ascending and descending portions of the orbit.

High-resolution radar data will be acquired to include at a minimum:

  • 360 degrees of the antenna scan (fore and aft looks) for the AM (6 am Equator crossing) half-orbit over the global land region (excluding the Antarctic)
  • 180 degrees of the antenna scan (fore look) for the PM (6 pm Equator crossing) half-orbit over the Boreal land region (north of 45-degrees N latitude)
  • 180 degrees of the antenna scan (fore look) for the AM half-orbit over the coastal ocean region (within 1000 km of continental boundaries)
The Science Operations Phase (SOP) begins after completion of the 90-day post-launch in-orbit commissioning and lasts for 3 years. The first part of the SOP is the Calibration and Validation (Cal/Val) phase which lasts for 12 months. Following the Cal/Val phase is the Routine Observations Phase (ROP) which lasts for 24 months.

Science Data Products & Archiving

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