Monitoraggio Geodetico e Telerilevamento 5. Radar Imaging From RAR to SAR, Sentinel1 part 2

1. Monitoraggio Geodetico e Telerilevamento 5. Radar ImagingFrom RAR to SAR, Sentinel1part 2 Carla Braitenberg Dip. Matematica e Geoscienze Universita’ di Trieste berg@units.it

2. 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 sensor enters the measurement.

3. 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 frequency shift.

4. From RAR to SARReal 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 frequency 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

5. End 29.05.2019

7. RAR system • RAR: transmit pulse of microwave energy • Pulse reflected from target is collected by receiver 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: http://www.radartutorial.eu/01.basics/Range%20Resolution.en.html ) Slant range resolution is between 15 and 3.7 meters.

8. 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

9. 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

10. 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

12. 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.

13. Sentinel 1 ESA The constellation covers 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

14. Sentinelsoverview • The Sentinel missions consist of: • Sentinel-1: High-resolution radar imaging • Sentinel-2: High-resolution multispectral imaging • Sentinel-3: Medium-resolution multispectral imaging and altimetry • Sentinel-4: Atmospheric composition monitoring from geostationary orbit • Sentinel-5, and Sentinel-5 Precursor: Atmospheric composition monitoring from lowEarth orbit • Sentinel-6 (Jason-CS): High precision radar altimeter mission

15. Sentinel 1 • The Sentinel-1 mission is based on a constellation of two satellites (A and B units). Sentinel-1 carries a C-band Synthetic Aperture Radar (SAR), and provides continuity of ERS and ENVISAT SAR types of missions. It allows all-weather and day/night imaging capability. SAR observations are key for operational applications over ocean, seas and polar areas (oil slick monitoring, sea-ice monitoring, ship traffic monitoring, ship routing, etc.). SAR observations are also used for land applications and provide data for emergency response and security, in particular under adverse weather conditions. SAR interferometry has proven scientific and operational value for terrain motion monitoring.

16. Sentinel-2 • The Sentinel-2 mission provides continuity to services relying on optical multi-spectral high spatial resolution observations over global terrestrial and coastal regions. The Sentinel-2 mission is based on a constellation of two satellites (A and B units). Sentinel-2 is used for land applications such as land cover, usage and change-detection maps, and to derive geophysical variable maps (e.g. leaf chlorophyll content, leaf water content, leaf area index). It also provides data for emergency response and security (based on the pre-defined observation plan), and may contribute also to coastal and inland waters monitoring.

17. Sentinel-3 and Sentinel 6 • The Sentinel-3 mission is based on a constellation of two satellites (A and B units). Sentinel-3 provides continuity of MERIS (ENVISAT), ATSR/AATSR (ERS/ENVISAT) and radar altimetry (ERS/ENVISAT/Cryosat) missions. The Sentinel-3 mission measures sea surface topography, sea and land surface temperature, and ocean and land surface colour. Sentinel-3 observations also support applications based on vegetation as well as fire, river and lake height and atmospheric products • Sentinel-6 (Jason-CS) Sentinel-6 and Sentinel-3 form a complementary pair in which both are needed to provide the necessary accuracy for Copernicus. The Sentinel-6 spacecraft, that ensures continuity of Jason series, will be based on a platform derived from CryoSat-2 adjusted to the specific requirements of the mission, including the much higher orbit. The instrument suite comprises a radar altimeter based on Sentinel-3 SRAL, a Microwave Radiometer (recurrent from Sentinel-3 but adapted to the higher orbit), a GPS device (recurrent from Sentinel-3), a DORIS receiver (recurrent from CryoSat-2) and a Laser Reflector (recurrent from CryoSat-2).

18. Sentinel-4 and Sentinel 5 • The Sentinel-4 mission is based on a payload to be embarked on EUMETSAT Meteosat Third Generation (MTG) satellites. Sentinel-4 instruments will be accommodated on board the two MTG-S satellites (sounding mission satellites). Sentinel-4 is used for atmospheric composition monitoring from the geostationary orbit. • The Sentinel-5 mission is based on a payload to be embarked on the EUMETSAT Polar System Second Generation (EPS-SG) satellites. Sentinel-5 is used for atmospheric composition monitoring from a low-Earth polar orbit. The Sentinel-5 Precursor satellite will extend data sets as provided by the SCIAMACHY/Envisat and NASA's OMI/Aura instruments, and will be followed by the Sentinel-5 mission..

19. 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.

20. Sentinel 1 orbit • SENTINEL-1 is 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.

21. 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.

22. 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)

23. 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

24. 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)

25. Sentinel-1 operation modesSentinel-1 instrument operation constraints Mode exclusivity: The Sentinel-1 SAR features four exclusive imaging modes of operations: • Interferometric Wide Swath (IW) • Extra Wide Swath (EW) • Strip Map (SM), with 6 possible incidence angles • Wave (WV). • The first three modes can be operated in 4 different schemes of polarisation (2 in single and 2 in double): HH, VV, HH+HV or VV+VH. The Wave mode can operate only in single polarisation, either in HH or VV. Overall this represents 34 possible sub-modes of operations. • Mode transition time: A transition time, in the order of 2.4 seconds (corresponding to roughly 17 km), is necessary to switch from a SAR measurement mode to another measurement mode, or to perform a change of polarisation. No data are acquired during this time interval.

26. 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

28. 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. 

29. 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. 

30. 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.

31. 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.

33. 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

34. 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.

35. From the Sentinel 1 special publication, the maingoals of sentinel 1 are described On demandfloodmappingthrough SAR. Floodextent information within 24 h of userrequest. Use: orientemergencyresponse teams on the ground. Highlightareasatrisk. Radar isallweathertool and thereforepreferable to opticalmonitoring.

36. Precise terraindeformationmapping (requiresinterferometric SAR, would be course 2, and isbeyond scope of presentcourse) • Terraindeformationmaps: importanttools to supportgeohazardriskassessment and mitigation. Maps are relevant for: hydrogeologicalriskassubsidence and landslides, flooddefence in coastal and lowlandareasdue to dykemonitoringsystems, tectonicsincludingseismicriskassessment. • Precise displacementmeasurementsmapgroundstablity and identifymotionpatterns. PersistentScattererInterferometrictechniqueinvolves the observation of radar phasechangesmeasured for repreatacquisitions. Large numbers of permanentscatterers are analyzed to detereminevelocity of movementalongsight. The methodallows to measureterrainmovements with an errorof 1.2 mm/year.

37. Subsidencemonitoring

38. Marine monitoringapplications • Oilspilldetection and polluteridentification. • Oilspillsvisible in SAR imageryascharacteristic dark features.

39. Sea-ice and iceberg monitoring • Quantities of interest on monitoringsea-ice are: iceconcentration, extent, type, thickness, driftvelocity. • Iceberg: location, size, drift • Services: sea-icecharting and ice-driftmonitoring. • Icediscriminationisdone with dualpolarisationobservation (HH+HV) thatallowsbettericediscriminationsthan with single-polarisation data.

40. Wind and wave information • Sea state and wind information important for maritimesafetly and rescue operations. • Offshore industry and marine engineeringoperations, shipping, pollutontracking, cleanupeoperationsrequirethis info. • Observedsea state isincorporatedintoEuropean Centre for Medium RangeWeatherForecasting (ECMWF). • Products from sentinel 1: wavespectra, significantwaveheight, oceancurrents, windspeed data.

41. Land Monitoringapplications:- land use mapping for forestry and agriculture- monitoring of snow, river and lakeice • Land use mapping: monitoring of the envronment. Land cover of forest: deforestation and forestdegradationmonitoring. Support for forest management and planning of timberharvestingworldwide. • Agriculturalmaps: estimates of cultivation in a country and growing season. Supportsefforts to ensurefood security in vulnerableareas. C-band Backscattercharacteristics of cropatdifferenstagesisused. • Primary source of information of agricultureis Sentinel-2 data, butathigherlatitudes and tropicssentinel 1 isexpected to be a valuabletool.

42. MonitoringSnow, river and lakeice • Topics of interest are: meltingsnow, river and lakeice, or converselyiceformation. Thesehaveimplications for a wide range of activitiesas: floodforecasting and warning, hydroelectricpower production, and freshwaterfishecology. • Timing of freeze-up and break-up of lakeice can be used to track the impacts of climatechange • The dualpolarisationsimprovesaccuracy of rivericeclassesand snowlinediscrimination

43. Security applications • Maritime surveillance: a) detection and tracking of vesselsnottransmittinganyothersignals or givingtheir positions. B) monitoring and control of allvessels in definedareas of interest. • Shippingvessels: strong reflections of radar signal due to scattering of shipstructures-> brightfeatures in SAR imagery. Non-metallic and smallervesselsmay be visiblebut with smallerbrightness. • Automaticdetection software: scan of SAR imagery to identifypossible vessel targets based on high backscatteringintensity. Analysis can provide information on as location, estimates of size, heading, and speed, for largervesselsalso vessel structure can be identified (e.g. container ship, oiltanker, ferry/cruise ship). Correlation with identification data from transittingships can indicate vesselsthat are nottransmitting appropriate information. SAR image (ENVISAT) of Mediterranean East of the Strait of Gibraltar.

44. Sentinel 1 VV date: 27 may 2019

45. Proposte studenti • - Murru: condizione di siccita’ in Sardegna: NDVI nel tempo, EVI nel tempo, eventualmente area di copertura di acqua del fiume Tirso, • Bernardi: Evoluzione Krakatoa 2018, indonesia, java island, effetti dello tsunami, evoluzione dell’isola, mutlispettrale, sar • Zanoli: Lago di Barcis- analisi riduzione dell’area del lago, multispettrale a falsi colori per aumentare contrasto terra-acqua, radar per evidenziare H20, differenza fra due periodi, • Maurizio: Osservazione correnti marine da satellkte, SAR, aree possibili: corrente del Golfo, • Bevilacqua: Ponte dello studente, frana del monte faggito, nicchia di distacco forse visibile, SAR interferometrico (corso numero 2), • Johnson: erosione costa golfo di guinea, da scegliere zona ben definita,