SMAP Instruments

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.

The SMAP orbit is a 685-km altitude, near-polar, sun-synchronous, 6 a.m. / 6 p.m., eight-day, exact-repeat, frozen orbit.

• Near-polar orbit provides global land coverage up to high latitudes including all freeze-thaw regions of interest.
• Sun-synchrony provides observations of the surface close to the same local solar time each orbit throughout the mission, enhancing change-detection algorithms and scientific accuracy.
• Consistent 6 a.m. observation time is optimal because it minimizes the effect of Faraday rotation and 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 3-day average intervals, and coverage of land region above 45N at 2-day average intervals.

Low-rate radiometer data and low-resolution radar data will be acquired continuously over fore and aft portions of the scan (full 360 degrees), as well as 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 morning (6 a.m. equator crossing) half-orbit over the global land region (excluding the Antarctic)
• 180 degrees of the antenna scan (fore look) for the evening (6 p.m. 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 morning half-orbit over the coastal ocean region (within 1,000 km of continental boundaries)
• The Science Operations Phase (SOP) begins after completion of the 90-day post-launch, in-orbit commissioning and lasts for three 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.