Alaska Satellite Facility
Delivering Remote Sensing Data Since 1991

News: NASA Soil Moisture Radar Ends Operations; Mission Science Continues
News: NASA Focused on Sentinel as Replacement for SMAP Radar 

Instrument Events Timetable

Synthetic aperture radar (SAR) SMAP artist rendering

     Artist's concept of SMAP observatory
     and its antenna-beam footprint.
     Credit: NASA. Click to enlarge.


1. Overview
2. Specifications
3. Radar Resolution & Gridding
4. Frequency & Polarizations
5. Spatial Resolution
6. Radar Data
7. Radiometer Data


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.


Synthetic aperture radar (SAR) SMAP beam footprint
SMAP's antenna-beam footprint measures
1,000 km.
Image credit: NASA/JPL.

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 

Radar: L-band
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)
Data acquisition:

  • High-resolution (SAR) data acquired over land
  • Low-resolution data acquired globally

Radiometer: L-band
Frequency: 1.41 GHz
Polarizations: H, V, 3rd & 4th Stokes
Relative accuracy (30 km grid): 1.3 K
Data collection:

  • 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°
Spatial Resolution:

    • 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 & 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.

Synthetic aperture radar SMAP variation of azimuth resolution
Variation of azimuth resolution
as a function of swath
Click for more. 


synthetic aperture radar SMAP radar measurement geometry
Radar-measurement geometry
showing the range and
Doppler contours. 
for more. 

Synthetic aperture radar SMAP single-look data samples
Single-look data samples from
successive fore-look scans. 
Click for more. © NASA/JPL. 

Frequency & 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).

Spatial Resolution

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.

Radar Data

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.

Radiometer Data

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).

Get SAR Data

Get SAR Data

Select and download SAR data online using Vertex.

Use the ASF API for downloading SAR data via a command line.