Monitoraggio Geodetico e Telerilevamento 5 Radar Imaging Carla Braitenberg Dip. Matematica e Geoscienze Universita’ di Trieste berg@units.it Tel. 339 8290713 Tel. Assistente Dr. Nagy: 040 5582257

RADAR Image interpretation Side looking radar image interpretation applied to: Map major rock units, geologic structure as folds, faults Map vegetation types Sea ice types Drainage features Imagery resembles aerial photography taken under low angle conditions Intensity of radar signal: feature characteristics and wavelength, incidence angle, polarization Most practical information relies on empirical relations, but theoretical studies for reflectivity and depolaristion are very important start 25 may 2017

Return strength depends on angle of reflecting surface

Rayleigh criterion to classify surfaces as rough or smooth Rough surface: acts as diffuse reflector Criterion: rough surface: root mean square (rms) height of surface variations > /(8 cos ) with  wavelength of Radar pulse  local incident angle Smooth surface: rms < /(8 cos ) A smooth surface acts a reflector and appears dark

Example of roughness for different Radar frequency bands

Rough and smooth is relative Amount of diffuse versus specular reflection given a surface varies with wavelength Example: a road appears rough in visible but a specular reflector at microwave In general radar images show more specular surfaces than do photographs Shape and orientation are also important: bright response is from corner reflector

Electrical characteristics of terrain Intensity of radar returns depends very strongly on surface geometry and on electrical characteristics of surface. Measured by dielectric constant Next page: definition of dielectric constant . It is a-dimensional and >1. Natural materials when dry:  = 3 to 8 water:  = 80 Reflectivity increases for higher dielectric constant

Dielectric constant - definition Dielectric constant, property of an electrical insulating material (a dielectric) equal to the ratio of the capacitance of a capacitor filled with the given material to the capacitance of an identical capacitor in a vacuum without the dielectric material. The insertion of a dielectric between the plates of, say, a parallel-plate capacitor always increases its capacitance, or ability to store opposite charges on each plate, compared with this ability when the plates are separated by a vacuum. If C is the value of the capacitance of a capacitor filled with a given dielectric and C0 is the capacitance of an identical capacitor in a vacuum, the dielectric constant, symbolized by the Greek letter kappa, κ, is simply expressed as κ = C/C0. The dielectric constant is a number without dimensions. It denotes a large-scale property of dielectrics without specifying the electrical behaviour on the atomic scale. From Encyclopedia Britannica

Capacitance definition Capacitance, property of an electric conductor, or set of conductors, that is measured by the amount of separated electric charge that can be stored on it per unit change in electrical potential. Capacitance also implies an associated storage of electrical energy. If electric charge is transferred between two initially uncharged conductors, both become equally charged, one positively, the other negatively, and a potential difference is established between them. The capacitance C is the ratio of the amount of charge q on either conductor to the potential difference V between the conductors, or simply C = q/V. 1 Farad = 1 Coulomb/Volt

Dielectric constants for some materials http://emgeo.sdsu.edu/emrockprop.html

Classifications of reflectivity Plant leaves: moist, therefore good reflectivity Metal bridges, vehicles, ships, silos, railoroad tracks, oil riggs: high reflectivity Soil moisture: dry and moist soil have very different dielectric constant and can be differentiated. Dry soil, or sand: L-band radar (23cm) of Landsat penetrates up to 2m below surface, and can reveal paleo rivers. Excersize: Sahara desert, Safsaf Oasis: compare visible and radar image.

Safsaf Oasis, Egypt. Dark braided patterns in right hand figure: drainage patterns of channels of ancient river valley. Age: tens of millions of years. Some patterns may be younger, at 0.5 million years, when area experienced wetter climate Optical band Composite Radar polarizations image https://www.nasa.gov/multimedia/imagegallery/image_feature_437.html

Sentinel 1 ESA The constellation will cover the entire world’s land masses on a bi-weekly basis, sea-ice zones, Europe's coastal zones and shipping routes on a daily basis and open ocean continuously by wave imagettes. Sentinel continues operations of C-band SAR Earth Observation of ESA’s ERS-1, ERS-2 and ENVISAT, and Canada’s RADARSAT-1 and RADARSAT-2. The satellite has been created by an industrial consortium led by Thales Alenia Space Italy as prime contractor

Sentinel 1 – the satellite shape The spacecraft is a three-axis, stabilised satellite, characterised by sun, star, gyro and magnetic field sensors, a set of four reaction wheels dedicated to orbit and attitude control and three torque rods as actuators to provide steering capabilities on each axis. The satellite is equipped with two solar array wings capable of producing 5 900 W (at end of life) to be stored in a modular battery.

Sentinel 1 orbit SENTINEL-1 will be in a near-polar, sun-synchronous orbit with a 12 day repeat cycle and 175 orbits per cycle for a single satellite. Both SENTINEL-1A and SENTINEL-1B share the same orbit plane with a 180° orbital phasing difference. With both satellites operating, the repeat cycle is 6 days. In particular for interferometry, SENTINEL-1 requires stringent orbit control. Satellite positioning along the orbit must be accurate, with pointing and timing/synchronisation between interferometric pairs. Orbit positioning control for SENTINEL-1 is defined using an orbital Earth fixed "tube", 50 m (RMS) wide in radius, around a nominal operational path. The satellite is kept inside this "tube" for most of its operational lifetime.

Geographical coverage A single SENTINEL-1 satellite will be able to map the entire world once every 12 days. The two-satellite constellation offers a 6 day exact repeat cycle. The constellation will have a repeat frequency (ascending/descending) of 3 days at the equator, less than 1 day at the Arctic and is expected to provide coverage over Europe, Canada and main shipping routes in 1-3 days, regardless of weather conditions. Radar data will be delivered to Copernicus services within an hour of acquisition.

Sentinel 1 ESA two-satellite constellation: Sentinel-1A launched April 2014, Sentinel-1B, launched April 2016. Orbiting 180° apart Orbit: Polar, Sun-synchronous at altitude of 693 km Revisit time: Six days (at equator) from two-satellite constellation Instrument: C-band synthetic aperture radar (SAR) at 5.405 GHz (wavelength 5.6 cm)

Satellite platform 3-axis stabilized, yaw/pitch/roll steering (zero Doppler) 0.01º attitude accuracy (each axis) Right looking flight attitude 10 m orbit knowledge (each axis, 3σ) using GPS Spacecraft availability: 0.998 Launch mass: 2 300 kg (incl. 130 kg fuel) Solar array power: 5 900 W Battery capacity: 324 Ah Science data storage capability: 1 410 Gb

Sentinel 1 Instrument Payload C-band Synthetic Aperture Radar Centre frequency: 5.405 GHz Polarisation: VV+VH,HH+HV,HH,VV Incidence angle: 20º - 45º Radiometric accuracy: 1 dB (3σ) End 25 may 2017

Operational modes Main modes: 1) Interferometric wide-swath mode at 250 km and 5×20 m spatial resolution 2) Wave-mode images of 20×20 km and 5×5 m spatial resolution (at 100 km intervals) Additional modes: Strip map mode at 80 km swath and 5×5 m spatial resolution Extra wide-swath mode of 400 km and 20×40 m spatial resolution

Interferometric Wide swath mode Interferometric Wide swath mode, the default mode over land, has a swath width of 250 km and a ground resolution of 5 x 20 m. This mode images in three sub-swaths using the Terrain Observation with Progressive Scans SAR – or TOPSAR. With this technique, the radar beam scans back and forth three times within a single swath (called sub-swaths), resulting in a higher quality and homogeneous image throughout the swath. 

Wave mode acquisition can help to determine the direction, wavelength and heights of waves on the open oceans – are 20 x 20 km, acquired alternately on two different incidence angles every 100 km. 

Additional modes: Stripmap and Extra Wide Swath. Stripmap mode provides a continuity of ERS and Envisat data, offering a 5 x 5 m resolution over a narrow swath width of 80 km.  Extra Wide Swath mode is intended for maritime, ice and polar zone operational services where wide coverage and short revisit times are demanded. This mode works similarly to the Interferometric Wide swath mode employs the TOPSAR technique using five sub-swaths instead of three, resulting in a lower resolution (20 x 40 m). Extra Wide Swath mode can also be used for interferometry.

Mode operation plan of Sentinel The high level Sentinel-1 observation strategy during full operations capacity is based on: optimum use of SAR duty cycle (25 min/orbit), taking into account the various constraints (e.g. limitation in the number of X-band RF switches, mode transition times, maximum downlink time per orbit and maximum consecutive downlink time) optimum use of single and dual polarisation acquisitions, in line with the available downlink capacity Wave Mode (WV) continuously operated over open oceans, with lower priority versus the high rate modes Interferometric Wide swath (IW) and Extra Wide swath (EW) modes operated over pre-defined geographical areas: over land: pre-defined mode is IW over seas and polar areas, and ocean relevant areas, pre-defined mode is either IW or EW.

Details on observational modes found in detailed description. https://sentinels.copernicus.eu/web/sentinel/missions/sentinel-1/observation-scenario Check also: https://scihub.copernicus.eu/userguide/2GraphicalUserInterface

Goals of sentinel 1 Sentinel-1 can image the surface of Earth through cloud and rain and regardless of whether it is day or night. monitoring polar regions, which are in darkness during the winter months monitoring tropical forests, which are typically shrouded by cloud Over oceans and seas: maps of sea-ice conditions for safe passage detect and track oil spills provide information on wind, waves and currents. Over land: track changes in the way the land is used monitor ground movement with exceptional accuracy. fast response to aid emergencies and disasters such as flooding and earthquakes.

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

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

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

Basic Principles on Radar imaging usage for geologic mapping 2 Lineament identification: Lineaments, as folds and faults may be manifested as topographic relief. Shallow incidence 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

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

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

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

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

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.

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.