Have you ever wondered how the SMAP satellite, with an altitude of 685 km (425 miles) above the Earth, can take measurements as precise as the amount of water in the Earth's soil?
The quick answer is that it requires precisely calibrated instruments, along with a technological marvel in satellite design.
SMAP was designed to use a combination of two precise instruments, one with high resolution, one with high accuracy.
Active (High Resolution): SMAP’s radar was designed accurately measure the echoes of very short radio frequency (RF) pulses that bounce (“backscatter”) off the Earth’s surface. The amount of backscatter that is returned to the radar changes with the amount of moisture in the soil – wetter soil causes more backscatter to reach the radar. The radar measurement is also quite sensitive to whether the soil is frozen or thawed – which in turn is used by SMAP to determine when different regions of the Earth under spring thaw or fall freeze.
Passive (High Accuracy): SMAP’s radiometer is a very sensitive receiver that was designed accurately measure the naturally occurring radio frequency (RF) energy given off by the Earth’s surface. The radiometer operates like an infrared camera (or night vision goggles), where warmer objects appear proportionally “brighter” than colder objects and allow their temperature to be accurately measured without being in direct contact. The radiometer receives energy in a narrow microwave band. This frequency has been set aside by international agreement for applications which involve only a receiver (and no transmitting is allowed in that band). This allows SMAP’s radiometer (and, in other frequencies, ground-based radio telescopes) to operate without interference. This frequency band also allows the radiometer to be not much affected by weather or by a moderate amount of vegetation that may cover the soil.
Within this frequency band (L-band), water appears relatively ‘cold’ (about 100K) and dry soil appears relatively ‘warm’ (about 300K) to the radiometer. With this great difference between wet and dry soils, the radiometer allows SMAP to produce very high soil moisture accuracy (4%) by simply measuring the microwave ‘temperature’ of the land surface.
Unfortunately, approximately 3 months after launch, the SMAP radar encountered some problems, and has ceased working. Currently, SMAP is still returning high accuracy passive radiometer data, allowing the world an accurate look at its global soil moisture. Read more about the radar anomaly HERE.
THE TECHNOLOGICAL MARVEL:
The Antenna: The antenna (see image below) is the ‘eye’ of the instrument. Its large size – 6m (20 ft) – and gold-plated wire mesh (or screen) surface focuses the radio frequency (RF) energy collected by SMAP’s radar and radiometer. The area measured is a ‘spot’ on the earth’s surface only 40 km (25 miles) diameter.
The antenna works like a flashlight where the feedhorn, located at the base of the antenna boom, illuminates the large reflector that in turn produces a narrow beam that illuminates the Earth. The beam is tilted at a 40° angle so that the ‘spot’ on the Earth is shifted about 500 km (310 miles) from directly under the observatory. When the antenna spins, the spot moves in a circle 1000 km (620 miles) in diameter around the observatory – this forms the very wide measurement swath of the instrument that is key to enabling SMAP to measure the entire Earth every 2-3 days (depending on latitude).
You can see an animation of the SMAP satellite, in orbit, HERE!