The SMAP instrument architecture incorporates an L-band radar and an L-band radiometer that share a single feedhorn and parabolic mesh reflector. The 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 approximately 40°. The reflector has a diameter of 6 m, providing a radiometer footprint of 40 km. The real-aperture radar footprint is 30 km, defined by the two-way antenna beamwidth.
To obtain the desired high spatial resolution the radar employs range and Doppler discrimination. The radar data can be processed to yield resolution enhancement to 1-3 km spatial resolution.
The science goal is to combine the attributes of the radar and radiometer observations in terms of their spatial resolution and sensitivity to soil moisture, surface roughness, and vegetation. Soil moisture will be estimated optimally at a resolution of 10 km and freeze-thaw state at a resolution of 1-3 km.
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 change.
Radar
- Frequency: 1.26 GHz
- Polarizations: VV, HH, HV
- Data collection:
- High-resolution/high-rate data collected for ground SAR processing
- Low-resolution real-aperture data collected continuously
Radiometer
- Frequency: 1.41 GHz
- Polarizations: H, V, U
- Relative accuracy: 1.3 K
- Data collection: Continuous over full scan
Antenna
- Configuration: Conically-scanning reflector
- Forms 1000 km wide swath
- Shared by both radar and radiometer
- Diameter: 6 meters
- Resolution:
- 40 km radiometer
- 1-3 km SAR
- Rotation rate: 14.6 RPM
- Beam efficiency: 90%
