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Applications Lecture of the synthetic aperture radars: Sea ice mapping
Objectives: 1. Basics of imaging radars. 2. Basics of a synthetic aperture radar (SAR). 3. SARs in space: ERS-1 and ERS-2, JERS-1, RADARSAT-1 satellites
Additional/advanced reading:
Tutorials on SAR
http://southport.jpl.nasa.gov/scienceapps/dixon/index.html http://www.asf.alaska.edu/sitemap.html http://www.sandia.gov/RADAR/sar.html http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/stereosc/chap1/chapter1_1_e.html
1. Basics of imaging radars.
Recall lecture 20 in which non-imaging and imaging radars were introduced.
Two main types of imaging radars: SLAR (side-looking radar airborne radar) SAR (synthetic aperture radar)
SLAR is a real aperture radar. Side-looking viewing geometry of SLAR: Swath (C); Range (D) refers to the across-track dimension perpendicular to the flight direction; Azimuth (E) refers to the along-track dimension parallel to the flight direction.
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Slant range vs. Ground rage At all ranges the radar antenna measures the radial line of sight distance between the radar and each target on the surface. This is the slant range distance. The ground range distance (D) is the true horizontal distance along the ground corresponding to each point measured in slant range.
Unlike optical systems, a radar's spatial resolution is a function of the specific properties of the microwave radiation and geometrical effects. Azimuth resolution, Ra, is defined as R a =
H L cos
ct
p
[24.1]
Cross-track resolution, Rr, where
Rr =
2 sin
[24.2]
H is the height of the antenna (height of the airplane); is the wavelength at which the antenna emits with pulse duration tp, L is the length of the antenna, and is the incidence angle
Example: For the antenna with the following characteristics: = 1 cm, L= 5 m, tp = 30 ns, H=6000 m, and =590 (10 km from the ground track) => Ra = 23 m and Rr = 5.2 m
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Because Ra is proportional to H (see Eq.[24.1]), a long antenna (i.e., large L) would be requited to achieve a good resolution from a satellite => need for SAR
Recall the radar equation (Eq.[21.13])
Pr A2 r = Pt 4R 4 2
where b is the radar cross-section (or backscattering cross-section). Radar measures backscattering from surface (e.g., ocean, sea-ice, land, etc.) => cross-section is due to reflection from the surface and hence depends on the incident angle, polarization, and surface state, etc.
In remote sensing of the surfaces, it is common to express the radar equation in the differential form
dPr =
A2 1 0 dS 4R 4 2
[24.3]
where 0 (called sigma nought) is the dimensionless backscattering cross-section per unit surface area (also called normalized backscattering cross-section), and dS id the element of the surface area. NOTE: 0 depends on scattering characteristics of the surface, polarization, incident : angle among other factors.
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The backscatter response, and thus the appearance of an object on a radar image, strongly depend on surface roughness.
NOTE: Other factors affecting the radar backscatter: changes in the moisture content; size of the scatterers, polarization of the pulse, and observation angles.
Radar images consist of pixels which represent the radar backscatter for that area of the ground: darker areas represent low backscatter, while bright areas represent high backscatter.
Flat (smooth) surface => low backscattering => dark images Rough surface => high backscattering = bight images
2. Basics of a synthetic aperture radar (SAR)
SAR overcomes the problem of a decrease of the of the azimuthal resolution of SLAR (i.e., large Ra) with increasing altitude. SAR relies on the speed of the platform to achieve the higher resolution in the azimuth (along-track) direction and on greater sophistication in the processing of the return signal. Strategy: Store the radar returned signal, as amplitudes and phases, for the time period T => possible to reconstruct the signal which would have been obtained by an antenna of
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length VT, where V is the platform speed => making T large makes the synthetic aperture large and hence a higher resolution can be achieved.
How SAR works:
As a target (A) first enters the radar beam , the backscattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam some time later, determines the length of the simulated or synthesized (B). antenna The expanding beamwidth, combined with the increased time a target is within the beam as ground range increases, balance each other, such that the resolution remains constant across the entire swath.
The viewing geometry of a radar results in geometric distortions on the resultant image:
Slant-range distortion occurs because the radar is measuring the distance to features in slant-range rather than the true horizontal distance along the ground. This results in a varying image scale, moving from near to far range.
Foreshortening occurs when the radar beam reaches the base of a tall feature tilted towards the radar (e.g. a mountain) before it reaches the top. Because the radar
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measures distance in slant-range, the slope (a to b) will appear compressed and the length of the slope will be represented incorrectly (a' to b'). Layover occurs when the radar beam reaches the top of a tall feature (b) before it reaches the base (a). The return signal from the top of the feature will be received before the signal from the bottom. As a result, the top of the feature is displaced towards the radar from its true position on the ground, and "lays over" the base of the feature (b' to a'). The shadowing effect increases with greater look angles, just as our shadows lengthen as the sun sets.
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3. SARs in space: ERS-1 and ERS-2, JERS-1, RADARSAT-1 satellites
Examples of SAR applications (main focus in on the Earths surfaces): Sea ice monitoring Ocean wave spectra Monitoring disasters such as forest fires, floods, volcanic eruptions, and oil spills Cartography Surface deformation detection Glacier monitoring Crop production forecasting Forest cover mapping Urban planning Coastal surveillance (erosion)
SIR-C Almaz- ERS-1/2 JERS-1 RADARSAT- RADARSATAIRSAR (shuttle) 1 U.S.Seasat (aircraft) 1 2 Russian European Japanese Canadian SA Canadian SA NASA/JPL GermanSA SA SA Italian Mission Dates 1978 198867 (P) 23.5 (L) 5.7 (C) all 20-60 10-17 2-8 1990199192 1991-99 1992-98 19955.7 (C) VV 23 100 30 23.5 (L) HH 35 75 18 19955.6 (C) HH 20-59 50-500 10-100 2003 5.6 (C) all 20-60 10-500 3-100
Wavelength 23.5 (L) (cm) Polarization Incidence Angle Swath Width (km) Resolution (m) HH 23 100 25
23.5 (L) 5.7 (C) 10 (S) 3.2 (X) all 15-55 15-90 10-60 HH 30-60 350 10-30
RADARSAT 2
Viewing capability of RADARSAT -1
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Sea Ice characterization:
Principles: the physical and chemical properties of ice affect the ice dielectrical constant in microwave and smoothness of ice surfaces and hence the interaction of ice with microwave radiation Ice Type Identification with RADARSAT: Gulf of St Lawrence, Prince Edward Island, Canada March 6, 1996
Compare the RADARSAT data with ground photographs: (A) Nilas...