A Synthetic Aperture Radar (SAR) is a coherent, active microwave imaging method that improves natural radar resolution by focusing the image through a process known as synthetic aperture processing. Typically, this requires a complex integrated array of onboard navigational and control systems with location accuracy provided by both Doppler and inertial navigation equipment. Environmental monitoring, Earth-resource mapping and military systems require broad area imaging at high resolutions. Many times the imagery must be acquired in inclement weather or during night as well as day. SAR provides such a capability. SAR systems take advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high resolution imagery. Synthetic Aperture Radar complements photographic and other optical imaging capabilities because of the minimum constraints on time of-day and atmospheric conditions and because of the unique responses of terrain and cultural targets radar frequencies.

SAR uses microwave frequency radiation. Microwave radiation penetrates cloud and haze. So SAR views the Earth's surface (land and sea) in all weathers. This is the major advantage of SAR. Being an active microwave instrument, it produces high resolution imagery of the Earth's surface.

Advantages of using Synthetic Aperture Radar::
1. Works equally day and night.
2. Polarised-can be used to gain additional information (specially, when different polarizations are available on the same platform-as on the most recent shuttle missions).
3. Needs a lot more power than passive sensors, and can therefore only operate intermittently.
4. Synthetic Aperture Radar generates its own illumination of the scene to be viewed, in the manner of a camera with flash. The satellite's illumination is coherent, i.e., all the light in any flash is exactly in phase, in the manner of a laser, so it does not simply disperse over the distance between the satellite and the Earth's surface. A SAR instrument can measure both intensity and phase of the reflected light, resulting not only in a high sensitivity to texture, but also in some three-dimensional capabilities.

Spying Process Using Remote Sensing: Remote sensing is the small or large-scale acquisition of information of an object or phenomenon by the use of real-time sensing devices that are not in physical contact with the object (such as by way of aircraft, satellites). Earth observation, or weather satellite collection platforms, ocean and atmospheric observing weather buoy platforms and space probes are all examples of remote sensing.

There are two kinds of remote sensing:

Passive sensors detect natural radiation that is emitted or reflected by the object or surrounding area being observed. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography; infra-red and radiometers.

Active collection, on the other hand, emits energy in order to scan objects and areas whereupon a passive sensor then detects and measures the radiation that is reflected or back scattered from the target. RADAR is an example of active remote sensing, where the time delay between emission and return is measured, establishing the location, height, speed and direction of an object. Remote sensing makes it possible to collect data and photographs on dangerous or inaccessible areas, like border areas. Radar is widely used in remote sensing. Conventional radar is mostly associated with aerial traffic control, early warning and certain large scale meteorological data. Doppler radar is used by local law enforcement's monitoring of speed limits and in enhanced meteorological collection such as wind speed and direction within weather systems. Other type of active collections includes plasma in the ionosphere. Interferometric Synthetic Aperture Radar is used to produce precise digital elevation models. In Indian spy satellite, RISAT, India has been using Synthetic Aperture Radar.