In active imagery systems, instruments are composed of a transmitter that sends out a specific electromagnetic signal and of a sensor receiving the interaction of the signal with the Earth’s surface. Such observations are not dependent on solar illumination. ### **1 Synthetic Aperture Radar** The most common active sensor used for Earth Observation is the Synthetic Aperture Radar (SAR). This instrument transmits electromagnetic pulses towards the Earth’s surface where they are reflected or scattered by the surface features. The instrument’s antenna can detect and record the return pulses. The intensity of the return pulse and the time it takes to arrive back at the antenna are used to generate SAR imagery. The main advantage of radar imaging is that it is insensitive to the day/night cycle and most of the time to the meteorological conditions (shorter wavelength signals such as X-band can be degraded by heavy intense rain cells). The selected radio band impacts what is observed from the scene by influencing the level at which the incident radiation will backscatter.  Applications include (for instance) ship detection, oil spill detection, sea ice monitoring (see Figure 6), forest monitoring, soil moisture, critical infrastructure, etc.  **Figure 6:** Example of SAR imagery for monitoring formation of Icebergs (Source: ESA ENVISAT) By using a technique known as SAR  interferometry,  highly accurate measurements  of  geophysical  parameters  such  as  surface  topography,  ground  deformation  and  subsidence and glacier movements can be made. In SAR interferometry, the  phase  of  two  or  more  complex  radar  images are compared that have been acquired from slightly different positions or at different times. Since the phase of each SAR image pixel contains range information that is accurate to a small fraction of the radar wavelength, it is possible to detect and measure path length differences with centimetric or even millimetric precision.  With  across-track  interferometry the  radar  images  are  acquired  from  mutually  displaced  flight  tracks , enabling (for instance)  a  precise  measurement  of  the  surface  topography. By  using  an  external  DEM,  the  topographic  information can be subtracted from the interferogram, leading to a differential SAR interferometric  measurement  where subtle (mm) changes  of  the  range  distance  between  the  two acquisitions (e.g. due to subsidence) can be detected. Further potential is possible by comparison of the coherence between several data acquisitions, which can be used for land classification and change detection. With  along-track  interferometry, the  radar  images  are  acquired  from  one  and  the  same  flight  track  but  at  different  times, enabling (for instance) the observation of ocean surface currents. #### **2 Lidar** Lidar (Light Detection And Ranging) EO uses the same principle as SAR but works in the IR, visible or UV wavelengths. Lidars are used for precise measurement of topographic features, monitoring growth or decline of glaciers, profiling clouds, measuring winds, studying aerosols and quantifying various atmospheric components. The Atmospheric Lidar ATLID on ESA’s EarthCare mission will provide vertical profiles of aerosols and thin clouds. It operates at a wavelength of 355nm and has a high-spectral resolution receiver and depolarisation channel. More information on ATLID can be found at: [http://download.esa.int/docs/EarthObservation/EarthCARE_instrument_factsheet.pdf](http://download.esa.int/docs/EarthObservation/EarthCARE_instrument_factsheet.pdf). The Atmospheric Laser Doppler Lidar Instrument ALADIN on ESA’s Aeolus-ADM mission will measure Line-of-Sight wind profiles at different levels in the atmosphere from the troposphere to the lower stratosphere with vertical resolution of 250 m - 2 km. It operates at a wavelength of 355nm, with spectrometers for molecular Rayleigh and aerosol/cloud Mie backscatter.  ALADIN will be the first wind lidar in space to obtain aerosol/cloud optical properties (backscatter and extinction coefficients). More information on ALADIN can be found at: [http://esamultimedia.esa.int/docs/EarthObservation/AEOLUS_factsheet_March2016.pdf](http://esamultimedia.esa.int/docs/EarthObservation/AEOLUS_factsheet_March2016.pdf) ### **3 Radar Altimetry** | | |---| || || Radar altimeters are active sensors that use the ranging capability of radar to measure the surface topography profile along the satellite track. They provide precise measurements of a satellite's height above the ocean by measuring the time interval between the transmission and reception of very short electromagnetic pulses. A variety of parameters may be inferred using the information from radar altimeter measurements, such as time-varying sea-surface height (ocean topography), the lateral extent of sea ice and altitude of large icebergs above sea level, as well as the topography of land and ice sheets, and even that of the sea floor. Satellite altimetry also provides information for mapping sea-surface wind speeds and significant wave heights. Jason-3 and Jason-CS (Sentinel 6) are contributing radar altimetry missions of the Copernicus programme, which will provide the continuity of critical high precision observations of ocean surface topography until 2030+, in full synergy with the marine mission of the Copernicus Sentinel 3. **Figure 7:** Altimetry-derived mean dynamic topography (Source: [http://www.esa.int/spaceinimages/Images/2005/06/Altimetry-derived_mean_dynamic_topography](http://www.esa.int/spaceinimages/Images/2005/06/Altimetry-derived_mean_dynamic_topography); Copyright CLS) ### **4 GNSS-R** | | |---| || GNSS reflectometry (GNSS-R) is a relatively new category of satellite navigation applications which entails a method of remote sensing to receive and process microwave signals reflected from various surfaces to extract information about those surfaces. In this process, the GNSS satellite acts as the transmitter and an airplane or Low Earth Orbit (LEO) satellite as the receiving platform. For altimetry applications, a GNSS-R receiver can also be placed on the land. An advantage of GNSS-R remote sensing is the ubiquity of signal sources, including GPS, Galileo, GLONASS, and Beidou/Compass. A wide range of applications is possible such as wide-swath altimetry, sea-wind retrieval, and measurement of seawater salinity and ice-layer density, as well as humidity measurements over land. **Figure 8:** Starlab GNSS-R Sensor Oceanpal for monitoring lake level in ESA Business Applications project [INTOGENER](https://business.esa.int/projects/intogener-feasibility-study). ### **5 Radar Scatterometry** | | |---| || A radar scatterometer is a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of the Earth from an aircraft or a satellite. It provides a measure of wind speed and direction near the sea surface. The radar scatterometer measures the backscatter from small (cm) waves at the sea surface, at skew incidence angles. From these sea roughness measurements the wind vector at 10 m height is calculated. Radar scatterometer data are important sources of information for Numerical Weather Prediction (NWP), oceanography and climate studies. Radar scatterometers can also provide information such as sea ice cover.  Sea ice typically reflects more of the radar energy emitted by the sensor than the surrounding ocean, so it appears brighter in a radar scatterometer image. **Figure 9:** Composite radar scatterometer image of Antarctica, 19 July 2003, from the QuikSCAT satellite (Source: David Long, Brigham Young University Center for Remote Sensing, [https://nsidc.org/cryosphere/seaice/study/active_remote_sensing.html](https://nsidc.org/cryosphere/seaice/study/active_remote_sensing.html))