SMAP Overview
NASA's Soil Moisture Active Passive (SMAP) mission's purpose is to measure the amount of water in the top 5cm of soil everywhere on Earth's surface every 2 to 3 days. SMAP orbits Earth at 685 km (426 mi) above the surface and has a period of 98.5 minutes. Its 6-meter (20 ft) antenna spins at 14.6 revolutions per minute, providing a 1000 km (621 mi) wide loop.
SMAP uses an L-band radar and an L-band radiometer for concurrent soil moisture measurements. This takes advantage of both active and passive microwave remote sensing in order to measure soil moisture more precisely. The SMAP L-band Synthetic Aperture Radar (SAR) sends out very short radio frequency (RF) pulses and measures how much they return. The purpose of using the L-band is to ensure that atmospheric variables are not measured. This resolution (1-3km) is much greater than the coarser radiometer instrument. The L-band radiometer differs in that it measures the natural RF energy given off by Earth's surface. To ensure that there is as little interference as possible, the radiometer records a 1.41 GHz frequency. SMAP combines these two measurements, producing a highly accurate soil moisture product.
The off-nadir-angle that creates the cone is set to 48.5 degrees, which represents an Earth incidence angle of 52.8 degrees. The Earth viewing scan sector is about 140 degrees and is centered along the velocity vector. The remaining 260 degrees is used for housekeeping and instrument calibration.
This information was taken from https://smap.jpl.nasa.gov/mission/description