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.

This information was taken from https://smap.jpl.nasa.gov/mission/description