23057page 1 Physics of SAR Summer 2003

23057page 2 Synthetic-Aperture Radar SAR Radar - Transmits its own illumination a "Microwave flashlight" RAdio Detection And Ranging SAR Radar

23057page 3 Azimuth Form a terrain image using a radar in a moving airborne/orbital vehicle Problem Simplest Approach - Real-Beam Imaging Radar Example: P P lan P P osition I I ndicator (PPI) Individual image points (pixels) must be discriminated in two dimensions, range and azimuth Range Azimuth 0° 90° 180° 270° PPI Display Radar Imaging

23057page 4         d d   d d  d d The transmitted pulse travels at the speed of light 10 9 feet/second  1 nanosecond/foot Round trip "radar time"  2 nanoseconds/foot (  d = 2 feet   = 4 nanoseconds) But target returns overlap if targets are separated by less than  /2 The transmitted pulse travels at the speed of light 10 9 feet/second  1 nanosecond/foot Round trip "radar time"  2 nanoseconds/foot (  d = 2 feet   = 4 nanoseconds) But target returns overlap if targets are separated by less than  /2 Range Discrimination

23057page 5 So for better range resolution, just make the transmitted pulse SHORTER However, the shorter pulses must somehow transmit the SAME ENERGY to the target As the pulse gets SHORTER, the peak power gets HIGHER Peak power gets MUCH too high before pulse length even approaches high resolution Problem = = = = = = Shorter Pulses

23057page 6 Linear F.M. (Frequency Modulation) Transmit a long coded pulse that can be decoded (compressed) after reception into a much shorter pulse Solution Linear Swept Frequency "Chirp" Note:A typical 200 microsecond pulse extends over more than 16 nautical miles in radar space f f 1 1 f f 2 2   Coded Pulses

23057page 7 Transmitted/Received Pulse f 1 f 2  Pulse compression ratio = pulse "time-bandwidth product" Variable Delay Line "Compression" Filter Decoded / "Compressed“ Output     Time Frequency f 1 f 2  f 1 f 2 Delay Time Frequency Time Frequency  f  f  f 0  f 1 Pulse Compression

23057page 8 Pulse Compression Advantages Range resolution independent of transmit pulse length – Transmit long pulses – Keep peak power comfortably low Set range resolution with transmitted bandwidth – Resolution inversely proportional to bandwidth  300 MHz ñ 2-foot resolution  600 MHz ñ 1-foot resolution – Resolution independent of slant range Range resolution independent of transmit pulse length – Transmit long pulses – Keep peak power comfortably low Set range resolution with transmitted bandwidth – Resolution inversely proportional to bandwidth  300 MHz ñ 2-foot resolution  600 MHz ñ 1-foot resolution – Resolution independent of slant range

23057page 9 Synthetic-Aperture Radar SAR Antenna beamwidth is inversely proportional to the number of wavelengths in its length (aperture)  = L radians = = c c f f L L Azimuth Considerations

23057page 10 Real-beam imaging radar As the collection vehicle moves along the flight path, targets are detected as they move in and out of the antenna pattern But target returns overlap if the targets are separated in azimuth by less than the antenna beamwidth So Achievable azimuth resolution decreases with range As the collection vehicle moves along the flight path, targets are detected as they move in and out of the antenna pattern But target returns overlap if the targets are separated in azimuth by less than the antenna beamwidth So Achievable azimuth resolution decreases with range L L L L R R R L R L   d d Flight Path Azimuth Discrimination

23057page 11 Problem Antennas get MUCH too long and frequencies MUCH too high before the beamwidth even approaches high resolution Narrower Beamwidth So for better azimuth resolution, just make the antenna beam NARROWER! – Generate more wavelengths in the antenna aperture by lengthening the antenna or by shorting the wavelength (increasing the frequency) However, very LONG antennas are difficult to carry and position, and very HIGH frequencies limit performance in weather and at long ranges So for better azimuth resolution, just make the antenna beam NARROWER! – Generate more wavelengths in the antenna aperture by lengthening the antenna or by shorting the wavelength (increasing the frequency) However, very LONG antennas are difficult to carry and position, and very HIGH frequencies limit performance in weather and at long ranges

23057page 12 Store the data collected sequentially and coherently across a long aperture and then process the data to synthesize a full aperture collection Synthesize a long antenna aperture using a physically short antenna Solution Synthetic-Aperture Synthetic-Aperture Radar SAR

23057page 13 LPLP LPLP /L S Real Beam Synthetically Processed Beam Synthetically Processed Aperture (L S ) Physical Antenna (L P ) Flight Path Azimuth Considerations

23057page 14 Synthetic-Aperture Advantages Increased angle of collection on a target allows increased resolution – High resolution capability with short physical antenna Processed aperture size is easily increased as imaging distance increases – Azimuth resolution independent of slant range Increased angle of collection on a target allows increased resolution – High resolution capability with short physical antenna Processed aperture size is easily increased as imaging distance increases – Azimuth resolution independent of slant range