by Tony Freeman, Jet Propulsion Laboratory
An imaging radar works very like a flash camera in that it providesits own light to illuminate an area on the ground and take a snapshot picture,but at radio wavelengths. A flash camera sends out a pulse of light (theflash) and records on film the light that is reflected back at it throughthe camera lens. Instead of a camera lens and film, a radar uses an antennaand digital computer tapes to record its images. In a radar image, onecan see only the light that was reflected back towards the radar antenna.
A typical radar (RAdio Detection and Ranging) measures the strengthand round-trip time of the microwave signals that are emitted by a radarantenna and reflected off a distant surface or object. The radar antennaalternately transmits and receives pulses at particular microwave wavelengths(in the range 1 cm to 1 m, which corresponds to a frequency range of about300 MHz to 30 GHz) and polarizations (waves polarized in a single verticalor horizontal plane). For an imaging radar system, about 1500 high- powerpulses per second are transmitted toward the target or imaging area, witheach pulse having a pulse duration (pulse width) of typically 10-50microseconds (us). The pulse normally covers a small band of frequencies,centered on the frequency selected for the radar. Typical bandwidths foran imaging radar are in the range 10 to 200 MHz. At the Earth's surface,the energy in the radar pulse is scattered in all directions, with somereflected back toward the antenna. Thisbackscatter returns to the radaras a weaker radar echo and is received by the antenna in a specific polarization(horizontal or vertical, not necessarily the same as the transmitted pulse).These echoes are converted to digital data and passed to a data recorderfor later processing and display as an image. Given that the radar pulsetravels at the speed of light, it is relatively straightforward to usethe measured time for the roundtrip of a particular pulse to calculatethe distance or range to the reflecting object. The chosen pulse bandwidthdetermines the resolution in the range (cross-track) direction. Higherbandwidth means finer resolution in this dimension.
Radar transmits a pulse Measures reflected echo (backscatter )
In the case of imaging radar, the radar moves along a flight path andthe area illuminated by the radar, or footprint, is moved alongthe surface in a swath, building the image as it does so.
Building up a radar image using the motion of the platform
The length of the radar antenna determines the resolution in the azimuth(along-track) direction of the image: the longer the antenna, the finerthe resolution in this dimension. Synthetic Aperture Radar (SAR)refers to a technique used to synthesize a very long antenna by combiningsignals (echoes) received by the radar as it moves along its flight track.Aperture means the opening used to collect the reflected energy that isused to form an image. In the case of a camera, this would be the shutteropening; for radar it is the antenna. A synthetic aperture is constructedby moving a real aperture or antenna through a series of positions alongthe flight track.
Constructing a Synthetic Aperture
As the radar moves, a pulse is transmitted at each position; the returnechoes pass through the receiver and are recorded in an 'echo store.' Becausethe 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 allowsmany returned signals to be "focused" on a single point, effectively increasingthe length of the antenna that is imaging that particular point. This focusingoperation, commonly known as SAR processing, is now done digitally on fastcomputer systems. The trick in SAR processing is to correctly match thevariation in Doppler frequency for each point in the image: this requiresvery precise knowledge of the relative motion between the platform andthe imaged objects (which is the cause of the Doppler variation in thefirst place).
Synthetic aperture radar is now a mature technique used to generateradar images in which fine detail can be resolved. SARs provide uniquecapabilities as an imaging tool. Because they provide their own illumination(the radar pulses), they can image at any time of day or night, regardlessof sun illumination. And because the radar wavelengths are much longerthan those of visible or infrared light, SARs can also "see" through cloudyand dusty conditions that visible and infrared instruments cannot.
What is a radar image?
Radar images are composed of many dots, or picture elements. Each pixel(picture element) in the radar image represents the radar backscatter forthat area on the ground: darker areas in the image represent low backscatter,brighter areas represent high backscatter. Bright features mean that alarge fraction of the radar energy was reflected back to the radar, whiledark features imply that very little energy was reflected. Backscatterfor a target area at a particular wavelength will vary for a variety ofconditions: size of the scatterers in the target area, moisture contentof the target area, polarization of the pulses, and observation angles.Backscatter will also differ when different wavelengths are used.
Scientists measure backscatter, also known as radar cross section, inunits of area (such as square meters). The backscatter is often relatedto the size of an object, with objects approximately the size of the wavelength(or larger) appearing bright (i.e. rough) and objects smaller than thewavelength appearing dark (i.e. smooth). Radar scientists typically usea measure of backscatter called normalized radar cross section, which isindependent of the image resolution or pixel size. Normalized radar crosssection (sigma0.) is measured in decibels (dB). Typical values of sigma0.for natural surfaces range from +5dB (very bright) to -40dB (very dark).
A useful rule-of-thumb in analyzing radar images is that the higheror brighter the backscatter on the image, the rougher the surface beingimaged. Flat surfaces that reflect little or no microwave energy back towardsthe radar will always appear dark in radar images. Vegetation is usuallymoderately rough on the scale of most radar wavelengths and appears asgrey or light grey in a radar image. Surfaces inclined towards the radarwill have a stronger backscatter than surfaces which slope away from theradar and will tend to appear brighter in a radar image. Some areas notilluminated by the radar, like the back slope of mountains, are in shadow,and will appear dark. When city streets or buildings are lined up in sucha way that the incoming radar pulses are able to bounce off the streetsand then bounce again off the buildings (called a double- bounce) and directlyback towards the radar they appear very bright (white) in radar images.Roads and freeways are flat surfaces so appear dark. Buildings which donot line up so that the radar pulses are reflected straight back will appearlight grey, like very rough surfaces.
Imaging different types of surface with radar
Backscatter is also sensitive to the target's electrical properties,including water content. Wetter objects will appear bright, and drier targetswill appear dark. The exception to this is a smooth body of water, whichwill act as a flat surface and reflect incoming pulses away from a target;these bodies will appear dark.
Backscatter will also vary depending on the use of different polarization.Some SARs can transmit pulses in either horizontal (H) or vertical (V)polarization and receive in either H or V, with the resultant combinationsof HH (Horizontal transmit, Horizontal receive), VV, HV, or VH. Additionally,some SARs can measure the phase of the incoming pulse (one wavelength =2pi in phase) and therefore measure the phase difference (in degrees) inthe return of the HH and VV signals. This difference can be thought ofas a difference in the roundtrip times of HH and VV signals and is frequentlythe result of structural characteristics of the scatterers. These SARscan also measure the correlation coefficient for the HH and VV returns,which can be considered as a measure of how alike (between 0/not alikeand 1/alike) the HH and VV scatterers are.
Different observations angles also affect backscatter. Track angle willaffect backscatter from very linear features: urban areas, fences, rowsof crops, ocean waves, fault lines. The angle of the radar wave at theEarth's surface (called the incidence angle) will also cause a variationin the backscatter: low incidence angles (perpendicular to the surface)will result in high backscatter; backscatter will decrease with increasingincidence angles.
Radar backscatter is a function of incidence angle, (theta)i
NASA/JPL's Radar Program
NASA/JPL's radar program began with the SEASAT synthetic aperture radar(SAR) in 1978. SEASAT was a single frequency (L-band with lambda ~ 24 cmor 9.4 inches), single polarization, fixed-look angle radar. The ShuttleImaging Radar-A (SIR-A), flown on the Space Shuttle in 1981, was also anL- band radar with a fixed look angle. SIR-B (1984) added a multi-lookangle capability to the L-band, single polarization radar. SIR-C/X-SARis a joint venture of NASA, the German Space Agency (DARA), and the ItalianSpace Agency (ASI). SIR-C/X-SAR provided increased capability over Seasat,SIR-A, and SIR-B by acquiring images at three microwave wavelengths (lambda),L- band (lambda ~ 24 cm or 9.4 inches) quad-polarization; C-band (lambda~ 6 cm or 2.4 inches) quad- polarization; and X-band (lambda ~ 3 cm) withVV polarization. SIR-C/X-SAR also has a variable look angle, and can imageat incidence angles between 20 and 65 degrees. SIR-C/X-SAR flew on theshuttle in April and in October of 1994, providing radar data for two seasons.Typical image sizes for SIR-C data products are 50kmx100km, with resolutionbetween10 and 25 meters in both dimensions.
Parallel to the development of spaceborne imaging radars, NASA/JPL havebuilt and operated a series of airborne imaging radar systems. NASA/JPLcurrently maintain and operate an airborne SAR system, known as AIRSAR/TOPSAR,which flies on a NASA DC-8 jet. In one mode of operation, this system iscapable of simultaneously collecting all four polarizations (HH,HV, VHand VV) for three frequencies: L- band (lambda ~ 24 cm); C-band (lambda~ 6 cm) ; and P-band (lambda ~ 68 cm). In another mode of operation, theAIRSAR/TOPSAR system collects all four polarizations (HH,HV, VH and VV)for two frequencies: L- band (lambda ~ 24 cm); and P-band (lambda ~ 68cm), while operating as an interferometer at C-band to simultaneously generatetopographic height data. AIRSAR/TOPSAR also has an along-track interferometermode which is used to measure current speeds. Typical image sizes for AIRSAR/TOPSARproducts are 12kmx12km, with 10 meter resolution in both dimensions. Topographicmap products generated by the TOPSAR system have been shown to have a heightaccuracy of1 m in relatively flat areas, and 5 m height accuracy in mountainousareas.
JPL are studying designs for a free-flying multi- parameter imagingradar system like the one flown during the SIR-C/X-SAR missions. JPL arealso studying a global mapping mission (TOPSAT) which will use radar interferometryto generate high quality topographic maps over the whole world and monitorchanges in topography in areas prone to earthquakes and volcanic activity.
To inquire about the availability of imaging radar data from the SIR-C,SIR-B, SIR-A or Seasat missions, or the airborne AIRSAR/TOPSAR system,please contact:
Radar Data Center
Mail Stop 300 - 233
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
Fax: (818) 393 2640
e-mail: radar.data@jpl.nasa.gov
To learn more about NASA/JPL's Imaging Radar Program, if you are anInternet user, please refer to World Wide Web server site at URL: http://southport.jpl.nasa.gov/
