RADAR

Introduction

Radar (Radio Detection and Ranging) is the most common type of active microwave remote sensing. Radar systems sends out pulses of microwave energy towards a target and detects the energy that is reflected back . Radar systems may or may not produces images. In this section we will be discussing imaging radar. Non-imaging radar can include Doppler radar to measure speed and plan position indicator (PPI) to observe weather and air traffic. Imaging radar produces images similar to a black and white photography, but is produced by measuring the level of microwave energy reflected back from the surface.

How does Radar Work?

In a radar system the sensor sends out microwave pulses towards a target and detects the energy that is reflected back. In radar the energy that is reflected back is known as backscatter and is similar to the idea of reflectance or laser pulses that are reflected back in a lidar system. Radar systems measure the strength and time it takes for the microwave signals to return to the sensor. In most imaging radar systems approximately 1500 high powered microwave pulses per second are transmitted towards the surface. At the surface, the microwave energy in the radar pulse is scattered in all directions, with some reflected back toward the sensor antenna.

Most radar systems look off to the side, known as Side Looking Radar or Side Looking Airborne Radar. For military applications this allows planes to fly over friendly territory and look into enemy territory. More importantly for remote sensing, it provides gives us more information about surface characteristics. In a SLAR system the radar energy is transmitted from the side of the aircraft during data acquisition. Since the radar pulses are transmitted at an angle, the pulses hit the terrain at an oblique angle enhancing geologic features. Some surface features, such as subtle faults and folds, may be more clearly seen on radar imagery than on conventional aerial photographs or satellite images.

The pulse duration and the length of the radar antenna determines the spatial resolution or the radar image. The longer the antenna the finer the resolution. In traditional real aperture radar the resolution is limited by the size of antenna a plane or satellite can carry.

Synthetic Aperture Radar

More commonly referred to just as Synthetic Aperture Radars (SARs), synthetic aperture side-looking airborne radar systems combine radar
and signal processing to form high resolution backscatter images. A SAR can simulate the effect of a large antenna by taking advantage of the Doppler effect. A synthetic aperture is constructed by moving a real aperture or antenna through a series of positions along the flight track. As the radar moves, a pulse is transmitted at each position; the return echoes pass through the receiver and are recorded. Because the radar is moving relative to the ground, the returned echoes are Doppler-shifted (negatively as the radar approaches a target; positively as it moves away). Comparing the Doppler-shifted frequencies to a reference frequency allows many returned signals to be "focused" on a single point, effectively increasing the length of the antenna that is imaging that particular point. This focusing operation, commonly known as SAR processing, is now done digitally on fast computer systems. The SAR technique was developed in the 1950s and is now widely used.

Space-borne Radar Systems

While many imaging radar system are aircraft based, there are many imaging radar systems on space-borne platforms including satellites and spacecraft. There are a variety of SAR satellites that have been launched by NASA and other international space agencies. Seasat launched in 1978 was one of the first space-borne SAR systems. RADARSAT-1 (1995) and more recently RADARSAT-2 were launched by the Canadian Space Agency. RADARSAT has provided data for resource management, ice, ocean and environmental monitoring.

The European Space Agency Sentinel-1 program is a two satellite constellation with the prime objectives of Land and Ocean monitoring. The Sentinel-1 satellites have synthetic aperture radar imaging systems that operate day and night, enabling them to acquire imagery regardless of the weather.

Radar Derived Terrain Models

Topographic mapping traditionally has been created by using stereo pairs of photographs taken from aircraft and satellites. Radar systems now are frequently used to generate DEMs. Radar is especially useful because it can penetrate through clouds, allowing for areas to be mapped that couldn't be mapped previously due to high cloud cover (e.g. tropical areas). Three dimensional topographic data can be generated from radar systems with multiple passes or by systems equipped with two antennas. The Shuttle Radar Topography Mission (SRTM) was a joint project between the National Geospatial-Intelligence Agency (NGA) and the National Aeronautics and Space Administration (NASA). The goal of this project was to produce digital topographic data for the majority of the Earth's surface. The SRTM consisted of a special radar system that flew on-board the Space Shuttle Endeavour during an 11-day mission in February of 2000. This mission used single-pass radar interferometry, which acquired two radar signals at the same time by using two different radar antennas. Differences between the two signals allowed for the calculation of surface elevation. The SRTM worldwide data has been processed and released and is available for download through EarthExplorer.

 

 

 

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