Space-Based Radars (SBRs) are radar systems that are deployed on satellites or spacecraft to observe Earth atmosphere, oceans, or other space objects from the orbit. Unlike ground station radars, SBRs operate from space, enabling extensive global coverage, high revisit rates, and all-weather monitoring capabilities. Space-based radar systems are designed to operate in the microwave and RF spectrum, which penetrates clouds, rain, fog, and even vegetation to a great extent. These radar systems typically consist of high-performance RF front-ends, phased array antennas, power amplifiers, and advanced onboard signal processing units. SBRs are designed to withstand radiation, thermal extremes, and vacuum conditions, while maintaining calibration accuracy over long mission durations.  

When radar systems are mounted on satellites or spacecraft, they operate from Low Earth Orbit (LEO), Medium Earth Orbit (MEO), or Geostationary Orbit (GEO) rather than fixed terrestrial sites. This elevated vantage point allows them to illuminate and observe vast swathes of the Earth’s surface or space environment in a single pass, something ground-based radars cannot achieve due to curvature of the Earth and line-of-sight limitations.

Unlike terrestrial radars that are constrained by horizon limits and infrastructure placement, SBRs leverage orbital motion to provide near-global coverage. As satellites continuously orbit the Earth, they revisit the same geographic locations at regular intervals which is referred to as the revisit rate. Constellations of multiple satellites can further reduce revisit times from days to hours or even minutes, enabling continuous monitoring.

Working principle of space based radar system:

Space-based radar systems operate similarly as traditional radar systems. They transmit, reflect, receive, and process the signals but with additional complexity due to orbital motion and advanced signal processing. These radars transmit electromagnetic waves toward a target such as Earth surface, ocean, or object in space and analyzes the reflected signals to extract information such as distance, velocity, shape, motion, and material properties.

They are integrated with an active remote-sensing payload carried on a satellite. The payload does not wait for sunlight or external illumination. Instead, it generates its own RF energy, and transmits that towards a target area, receives the echoes, and processes those echoes into usable intelligence such as images, motion data, elevation, surface roughness, or target tracks.

Key subsystems used in space radars - 

• Power System - The system provides the electrical power needed for radar transmission..
• Radar Transmitter: They generate high-frequency microwave/RF pulses or waveforms.
• Power Amplifier: They boost the signals to a level strong enough to illuminate the Earth’s surface or another target from orbit.
• Antenna: They radiate the signal toward the target and collects the returning echo. In many systems this is a phased-array or deployable antenna.
• Receiver Chain: They capture the extremely weak reflected signals and amplifies them with very low added noise.
• Timing and frequency reference: They maintain precise synchronization, which is critical because radar measurements depend on very accurate time and phase information.
• Signal processor / onboard computer: They handle waveform control, data sampling, compression, storage, and sometimes partial image formation before downlink.
• Data handling and communication subsystem: They store radar data and transmits it to ground stations.

The radars emit electromagnetic energy in RF/microwave frequency bands such as L, S, C, X, Ku, or Ka band, depending on the mission. L-band penetrates vegetation better and is useful for biomass, soil moisture, and deformation studies, C-band is used for Earth observation and balanced performance, and X-band provides finer spatial resolution and is common in high-resolution imaging. 

A space-based imaging radar usually creates an image using two coordinate directions such as range direction, and azimuth direction. Range direction is measured from pulse time delay. This is perpendicular to the flight path in radar geometry. Azimuth direction is measured using Doppler and synthetic aperture processing. This is along the direction of travel.

Space-based radar systems do not always operate in a single fixed mode. They can switch among several observation modes depending on mission needs. 

• Stripmap mode: The radar antenna points at a fixed angle relative to the satellite’s flight path. As the satellite moves forward, the radar continuously transmits pulses and receives echoes. This results in imaging a long, continuous strip (swath) of terrain parallel to the satellite ground track.
• Spotlight mode: The radar locks onto a specific ground patch (target area). The antenna beam is electronically steered backward and forward to keep illuminating that same area. This increases the effective synthetic aperture length, leading to much finer resolution than stripmap mode.
• ScanSAR mode: The radar does not continuously illuminate a single strip (like stripmap) or a single point (like spotlight). Instead, it cycles through several sub-swaths, illuminating each one briefly in a burst sequence. This allows the satellite to cover a much larger ground area, at the cost of reduced resolution.
• Interferometric mode: Interferometric mode (InSAR – Interferometric Synthetic Aperture Radar) is used to measure very small changes in distance (phase differences) between the radar and the Earth’s surface. It enables 3D terrain mapping and millimeter-scale surface deformation detection.
• Polarimetric mode:  PolSAR (Polarimetric Synthetic Aperture Radar) is a operating mode in which the radar transmits and receives signals in multiple polarizations to extract detailed information about the physical and structural properties of targets.