SMAP: A World-Changing Satellite

Ethan Wong

July 14, 2023

The Soil Moisture Active and Passive mission, also known as SMAP, is a satellite that orbits Earth and surveys the entire area of land on the planet, observing and measuring moisture levels in the planet’s soil. After a couple of days, the innovation is capable of fully recording the moisture levels within every inch of soil on Earth (I assume not the precision of every inch but you get the idea). Soil dampness measurements will help scientists understand flows of water and waves of energy that interact between the Earth’s surface and the atmosphere, affecting our weather and climate. The satellite uses a radiometer to see through clouds, allowing it to precisely measure the amount of water levels that are in between minerals, and organic particles/rocks that are typically found in soils. With this innovation, the creation can predict several ecological instances that are useful to both farmers and forecasts; for instance, soil that lacks water will often result in droughts–a condition that can affect the production of food for farmers. On the contrary, soil that has an extreme amount of moisture/water might suggest future mudslides, landslides, or floods if heavy rainfall “over-absorbs” the soil. 

As soon as the spacecraft is deployed from the fairing of the Delta II rocket, the solar panels are unfolded and face the Sun, allowing the satellite to generate power. The satellite has two different approaches to detecting moisture in soil on its polar orbit flight around the planet. The more active approach uses a radar that funnels radio frequencies directly to the feedhorn on the satellite. The radar receives echoes (microwave radio frequencies) that are emitted by the Earth’s surface–stronger echoes are a sign of high soil moisture and vice versa. While much of Earth’s soil is exposed, the radar is not hindered by vegetation or weather that might “get in the way”. A more passive approach is with a radiometer. Despite being over 400 miles above Earth’s surface, it can measure the energy of radio frequencies given off by the soil detected, which allows the satellite to piece together the temperature indirectly. Measuring this warm or cold temperature can help the radiometer collect accurate data on the soil moisture on Earth, as wetter soil will appear colder than dry soil, which would be detected at a higher temperature.  

While these two instruments provide scientists with beneficial data on the state of the soil, the main feature of the satellite is the antenna. This device allows the entire Earth to be covered in 3 days and is mounted on top of the spacecraft, spinning around a gold-mesh reflector that spans 6 meters in length when fully expanded. It spins around 14.6 times a minute to collect microwave emissions from the soil, where it is then channeled down to its feedhorn. 

The actual spacecraft has incredible and reliable movement to guarantee constant feedback on Earth’s soil moisture. Once in orbit, there are a multitude of sensors that keep the satellite’s array of solar panels facing the Sun and the instruments toward the planet. Thrusters are added onto the probe that uses propellant stored on the spacecraft to maneuver itself–every once in a while–-to stay on course and fix its orientation when orbiting the planet.