Soil Moisture Passive Active (SMAP) is a remote-sensing observatory with two instruments — a synthetic aperture radar (SAR) and a radiometer — that map soil moisture and determine the freeze or thaw state of the area being mapped. Both instruments help map soil-moisture content, and unique properties of SAR enable the freeze-thaw mapping. Externally, the instruments share a common, 20-foot mesh antenna and a feed assembly. Inside the spacecraft, their electronics differ. When combined, the SMAP radar and radiometer deliver high-accuracy, high-resolution global maps of the Earth’s soil moisture and freeze-thaw state.
The radar actively sends pulses of radio waves down to a spot on Earth and measures the echo that returns microseconds later. The strength and “shape” of the echoes can be interpreted to indicate the moisture level of the soil, even through moderate levels of vegetation. The radiometer passively detects radio waves emitted by the ground from the same small area. The strength of the emission indicates temperatures.
Artist’s concept of SMAP observatory and its antenna-beam footprint. Credit: NASA.
Dimensions, bus only: 1.5 x .9 x .9 m
Weight: 944 kg
Power: 1,450 watts
Altitude: 685 km
Orbit path: Near-polar, sun-synchronous, equator crossings 6 a.m. and 6 pm. local time
Orbital inclination: 98.1 degrees
Frequency: 1.2 to 1.3 GHz
Polarizations: VV, HH, HV (not fully polarimetric)
Relative accuracy (3 km grid): 1 dB (HH and VV), 1.5 dB (HV)
- High-resolution (SAR) data acquired over land
- Low-resolution data acquired globally
SMAP’s antenna-beam footprint measures 1,000 km. Image credit: NASA/JPL.
Frequency: 1.41 GHz
Polarizations: H, V, 3rd and 4th Stokes
Relative accuracy (30 km grid): 1.3 K
- High-rate (sub-band) data acquired over land
- Low-rate data acquired globally
Conically scanning deployable mesh reflector shared by radar and radiometer
Diameter: 6 m
Rotation rate: 14.6 RPM
Beam efficiency: ~90%
Beam surface incidence angle: 40°
- SAR: 1-3 km (over outer 70% of swath; ‘high-resolution’ radar)
- Radiometer (IFOV): 39 km x 47 km
- Real-aperture radar footprint resolution: 29 km x 35 km (‘low-resolution’ radar)
Swath width: 1,000 km
Radar Resolution and Gridding
The SMAP radar employs unfocused synthetic aperture radar (SAR) processing. The range and azimuth resolutions are determined by the unique antenna scan geometry.
The SAR single-look samples (time-ordered) are averaged (multi-looked) onto a swath-oriented 1-km grid to form the L1C_S0_HiRes product. The grid posting of the L1C_S0_HiRes product is fixed at 1 km, but the spatial resolution, number of looks, and signal-to-noise ratio (SNR) vary across the swath.
This panel illustrates the radar-measurement geometry showing the range and Doppler contours. The real aperture radar footprint ellipse is shown at two representative azimuth-scan angles. The radar 1-MHz bandwidth yields a ground-range resolution of ~250 m. The doppler diversity is maximum at a scan angle perpendicular to the satellite velocity (swath edge), leading to an azimuth single-look resolution of ~450 m. The single-look resolution degrades as the scan angles approach the satellite velocity vector. Image Credit: NASA/JPL.
This schematic, which is not drawn to scale, illustrates how the single-look data samples from successive fore-look scans are oriented and overlap relative to the 1-km gred. Image Credit: NASA/JPL.
This panel illustrates the variation as a function of swath position (distance from center track). Image Credit: NASA/JPL.
Frequency and Polarizations
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.
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).
To obtain high spatial resolution, the radar employs range and Doppler discrimination.
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.
The radar high-resolution measurement samples are created within the radar real-aperture footprint by synthetic aperture processing in range and azimuth. The synthesized single-look samples have variable spatial resolution in the azimuth direction. The single-look samples are averaged (multi-looked) onto 1-km grid pixels to form the L1C_S0_HiRes data product.
The L1C_S0_HiRes HH and VV data have uncertainty from all sources (excluding rain) of 1.0 dB or less (1-sigma) defined at 3-km spatial resolution and for surfaces of radar cross-section greater than -25 dB. The HV data have uncertainty from all sources (excluding rain) of 1.5 dB or less (1-sigma) defined at 3-km spatial resolution and for surfaces of HV radar cross-section greater than -30 dB.
The radiometer instantaneous field of view (IFOV) or 3-dB footprint is 39 km x 47 km. The radiometer L1B_TB data product includes compensation for effects of antenna sidelobes (outside the main beam), cross-polarization, Faraday rotation, atmospheric effects (excluding rain), and solar, galactic, and cosmic radiation.
The L1B_TB have mean uncertainty from all sources (excluding rain) of 1.3 K or less (1-sigma) in the H and V channels, defined on the basis of binning the fore- and aft-look samples onto hypothetical, swath-oriented, 30-km x 30-km grid cells (a different grid is used for the actual L1C_TB data product).