The SMAP spacecraft carries two instruments, a radar (active) and a radiometer (passive), that together will make global measurements of land surface soil moisture and freeze/thaw state.
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 6 m (20 foot) antenna weighs 65.5 kg (144 lbs.) and is made of gold-coated mesh. The feedhorn sends out radar pings and receives radar returns. It also collects microwave emission from the same points Earth’s surface for the radiometer.
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).
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.
Radar Resolution and Gridding
The SMAP radar employs unfocused 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 Range-Range plot illustrates the radar measurement geometry showing the range and Doppler contours. The real aperture radar footprint ellipse is show 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 approaches the satellite velocity vector. The measurement loop geometry of a radar swath showing the field of view orientations at two azimuths is shown. Vg is the observatory’s velocity vector.
Azimuth resolution varies with the field of view distance from the center of the track (satellite nadir path on Earth’s surface). The single-look data samples from successive fore-look scans are oriented and overlap relative to the 1-km grid. Figure 3 is a schematic illustration and is not drawn accurately to scale. This is a schematic representation, not to scale, of the orientation and overlap of single-look data samples from leading segments of successive loops. The grid is 1 km squares.
Why combined active and passive?
The science goal is to combine the attributes of the radar observations (high spatial resolution but lower soil moisture sensitivity) 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. Lower resolution (40 km) but more accurate soil moisture data from the radiometer are combined with higher resolution (3 km) but less accurate moisture data from the radar to provide accurate maps with resolution of 10 km.