Other Active Microwave Systems
Introduction
While imaging radar systems are most commonly associated with active microwave remote sensing, there are also two other types of non-imaging active sensors: Scatterometers and Radar Altimeters. All of these systems produce pulses of microwave energy that are directed at a surface.
Scatterometers
Similar to radar systems, scatterometers measure the amount of reflected microwave energy, or backscatter, from the Earth's surface. Scatterometers are primarily used in remote sensing to measure wind speed and direction over oceans. The returned radar pulses from waves on the surface of the ocean allow for wind speed and direction to be derived. Scatterometers generally do not have the same level of detail as a SAR sensor and scatterometers tend to have the same level of detail as passive microwave imagery. Over ocean surfaces, measurements of backscattered radiation in the microwave spectral region can be used to derive maps of surface wind speed and direction.
The SeaWinds sensor aboard NASA's Quick Scatterometer (QuikSCAT) satellite provided daily, global views of ocean winds and sea ice. More recently the RapidScat scatterometer has launched in 2014 and installed on the International Space Station (ISS). RapidScat is a replacement for the QuikSCAT satellite.
Radar Altimeters
Radar Altimeters send pulses of radar energy toward the Earth and measures the time it takes to return to the sensor. The pulse's round-trip time determines how far the satellite is from the reflecting surface. With a known reference, this information is used to measure the altitude of various features at the Earth's surface. With enough precision, a radar altimeter can determine the height of the sea ice surface above sea level, which scientists use to calculate the total thickness of the sea ice. Radar altimeters are also used to measure the height of the ocean surface.
Jason-1, Jason-2 and Jason-3 have all been equipped with radar altimeters to measure the height of the ocean surface. Jason-3 was recently launched in early 2016. Jason-3 will maintain observations of global sea surface height that began in 1992. The data helps provide wave forecasts, tides and current forecasts, El Niño forecasts, and improve hurricane forecasts. The ocean climatology data produced is also integral for studying climate change factors like sea level rise and long term variability of the ocean.
Sea level rise from 1994 to 2014 with data acquired from TOPEX/Poseidon and Jason-1 and Jason-2