1. Monitoraggio Geodetico e Telerilevamento 5.2 Radar Imaging part 2
Carla Braitenberg
Dip. Matematica e Geoscienze
Universita’ di Trieste
Tel Tel. Assistente Dr. Nagy:

2. Range resolution of RADAR
Definitions of angles
Range resolution: first return from point B must be later than arrival of impulse-tail on A, in order to give distinct returns
Pulse length PL=c. Range resolution: PL/2. Ground resolution Rg=c/(2 cosd)
start 18 may 2017

3. Numerical example for ground resolution
Pulse of 0.1 microsec duration, depression angle of 45°.
Speed of light: c= km/sec
Rg=c/(2 cosd)=
=3*10^8 m/sec*1*10^-7sec/2/0.707=
=21m
Notice: at greater range, depression angle is shallower and ground resolution is finer.

4. Azimuth resolution for Radar
Azimuth resolution is fixed by the product of the angle aperture  and the range. The points A and B cannot be distinguished if they fall into the illuminated beam as in R2. Their distance must be greater than the beam as in R1.
R1=SR1 * 
Constructive limitations on the aperture angle are given by wavelength and the antenna length L
=/L
Consider radar with  =2 mrad beamwidth. Resolution at SR1=6 km and SR2=12 km slant ranges is:
R1= 6 10^3 m*2 10^-3 rad= 12m
R2= 12 10^3 m*2 10^-3 rad= 24m
At 5 cm-wavelength, to obtain 2 mrad beamwidth:
L= / = 5 10^-2 m /2 10^-3 rad = 25m
This is not feasible- it is a too large antenna.

5. Synthetic aperture Radar
Given the physical limitations of azimuthal resolution of RAR systems, the SAR was developped. The concept is to mimic a greater antenna through an array of receivers and modified recording and processing techniques. The velocity of the sesnor enters the measurement.

6. Doppler shift used to improve azimuth resolution
Near center returns are distinguished by the zero-shift in frequency of the return signal. Within the reflections of the wide beam, reflections behind the aircraft are down-shifted, reflections in front are up-shifted. Reflections from the narrow central stripe can be identified by the absence of freuqency shift.

7. From RAR to SAR Real aperture Radar to Synthetic Aperture Radar
First observation: one-to-one correspondence between along track coordinate of reflecting object and instantaneous doppler shift of signal reflected to the radar by the object
Frequency analysis of reflected signal enables finer along-track resolution than with the physical beam itself.
Real aperture radar (RAR) design used reflectors due to illumination of one radar pulse on beam
Doppler beam sharpening: doppler ferquency analysis at each resolvable picture element (pixel).
1974 study group formed at JPL and NOAA to develop methodology further and to find possible applications
SEASAT was the outcome- flew 1978
See table- H, V is horizontal and vertical polarization

10. RAR system RAR: transmit pulse of microwave energy
Pulse reflected from target is collected by reciever antenna
Measure time difference between transmitted and received pulse and determine distance of reflecting object (range or slant range)
For separated objects each is in different resolution cell and distinguishable. If not, radar return is combination of reflected energy from both objects.
Spatial resolution in range direction is function of proccessed pulse with: dx= dt c/2
c= speed of light, dt= pulse length
(see animation:
Slant range resolution is between 15 and 3.7 meters.

11. Cross range resolution
Cross range: direction orthogonal to radar beam, also called azimuth or along track)
There is no range difference as in range direction
For RAR cross range spatial resolution is proportional to wavelength, target range R and inversely proportional to antenna dimension D, as it depends on the angular aperture of emitted beam:
dx= R /D
Cross range resolution cannot be improved from space and is km or tens of km. SAR solves the problem. With the SAR methods resolution is determined by Doppler bandwidth of received signal, rather than along track width of beam.
Along track resolution of side looking radar is same order as the range resolution

12. SAR building blocks
SAR consists of: conventional radar with antenna, transmitter, receiver, data collection system providing Doppler shift and phase histories, advanced signal processor to make an image out of phase histories
SAR can produce image largely independent of wavelength and range

13. SAR principles (see fig. in next slide)
String of dots positions at which pulse is transmitted
Pulse travels to target area and return pulse is collected by antenna. Velocity of light is much faster than velocity of spacecraft.
SAR system saves phase histories of the responses at each position as the beam moves through scene and then weights, phase shifts, and sums them to focus on one point target at a time and suppress all others.
SAR imaging system performs weighting etc. On each point target in turn. It constructs an image by placing total energy response on the position in the image corresponding to that target.
High gain is achieved by coherent in-phase summation of range-correlated responses to the radar.
Thousands of pulses are summed for each resolution cell resulting in huge increase of reflected target signal

15. SAR principle- see previous figure
Antenna beam illuminates target when reaching position t1. Continues to illuminate target for distance LSA until it reaches t2. Time to translate beam through target is dwell time.
Spatial resolution in along track direction approaches length of antenna divided by 2.

18. Polarization filter

19. Polarization due to reflection

34. Tectonic applications of radar
Lineaments, joints, shear zones, changes in relief
Quaternary mapping
Minerals and Hydrocarbon exploration
Geologic hazard estimation
Lineaments should be non-parallel to look direction
Origin: anthropogenic, geomorphological, structural

35. Forested and Tropical Environments
Overburden is present on most terrains.
In many cases radar signal does not penetrate the overburden (soil, vegetation)
Lithological information can be obtained from erosional patterns and structures
In tropical regions the radar image maps the treetops surface
The local and regional geomorphology is reflected in the tree top surface
Geologist extracts: small scale geologic structures, erosional patterns, topographic features

37. Basic Principles on Radar imaging usage for geologic mapping 1
Geologic structure mapping: characteristic forms of geologic structures if located near surface, may be manifested in topography. Radar side-looking configuration highlights relief
Shallow incidence angles are ideal through shadowing. In high relief intermediate angles may be more suitable
Look direction should be considered in relation to orientation of geological structures

38. Basic Principles on Radar imaging usage for geologic mapping 2
Lineament identification: Lineaments, as folds and faults may be manifested as topographic relief.
Shallow ncidence angle ideal for subtle relief
Look direction perpendicular to direction of lineaments enhances detectability.
Acquisition of ascending and descending passes maximizes lineaments to be identified

39. Basic Principles on Radar imaging usage for geologic mapping 3
Seismic zone identification: seismic zones have presence of faults which may be manifested topographically
Side looking configuration highlights this topography
Ascending and descending passes maximize identification of lineaments

40. Basic Principles on Radar imaging usage for geologic mapping 4
Surficial bedrock geological mapping: depending on physical weathering, surficial bedrock may have fractures and fragment sizes
These depend on rock fabric, texture, mineral composition
This results in a unique backscatter to the rock type
Main parameter to characterize bedrock unit is surface roughness.
Shallow incidence angles maximize contrast in backscatter due to variance of roughness
Better acquire data when moisture level is low, so backscatter is correlated to roughness and not moisture content

41. Basic Principles on Radar imaging usage for geologic mapping 5
Sedimentology mapping: Unconsolidated sediments as those deposited by glaciers or fluvial systems are often manifested by topographic relief.
Sediments have different grain sizes with different moisture holding capabilities
Radar is sensitive to moisture and roughness-> contrasting backscatter between different sediments
Consolidated sediments have unique erosional patterns, as karsting in carbonates

42. Basic Principles on Radar imaging usage for geologic mapping 5 cont.
Incidence angle: for topographic relief and classification through surface roughness, shallow angles are preferred
For moisture differences steep angles are better, as they maximize contrast in surface roughness
Time: data should be acquired when vegetation is at minimum to not have vegetation dominate backscatter

43. Basic Principles on Radar imaging usage for geologic mapping 6
Landslides: landscape is changed through transportation of vegetation and soil.
Affected areas have different soil roughness and may have changes in vegetation
Backscatter variations may be present between affected and unaffected areas.

44. Basic Principles on Radar imaging usage for geologic mapping 7
Coastal erosion: smooth water is specular reflector-> low backscatter.
Land: diffuse backscatter.
Change in backscatter over time allows assessment of coastal erosion
Shallow incidence angle creates greatest contrast between water and land.