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GEOGG141 / GEOG3051Principles & Practice of Remote Sensing (PPRS) RADAR II: InterferometryDr. Mathias (Mat) DisneyUCL GeographyOffice: 113, Pearson BuildingTel: 7670 05921Email: mdisney@ucl.geog.ac.ukwww.geog.ucl.ac.uk/~mdisney

2References, Definitions/termsIfSAR or InSAR Interferometric SARDifSAR Differential Interferometric SARReferencesExcellent ESA tutorial on InSAR: http://www.esa.int/esapub/tm/tm19/TM-19_ptA.pdfOther ESA tutorials on InSAR: http://earth.esa.int/workshops/ers97/program-details/speeches/rocca-et-al/http://earth.esa.int/landtraining07/D1LB5-1-Rocca.pdfSentinel 1 Toolbox polarimetric InSAR tutorialhttp://sentinel1.s3.amazonaws.com/docs/S1TBX%20SAR%20Basics%20Tutorial.pdf

23References (cont.)Basics of InSAR: http://www.gi.alaska.edu/~rgens/teaching/asf_seminar/intro_insar.pdfhttp://www.jpl.nasa.gov/srtm/missionoverview.htmlhttp://www.jpl.nasa.gov/srtm/instrumentinterfmore.htmlhttp://southport.jpl.nasa.gov/scienceapps/dixon/report2.htmlhttp://southport.jpl.nasa.gov/nrc/index.html - good info but datedhttp://www-radar.jpl.nasa.gov/insar4crust/

34S1A: Nepal earthquake

https://earth.esa.int/web/guest/featured-image/-/article/sentinel-1-analyses-the-nepal-earthquake

45S1A: Ground movement, Mexico Cityhttps://earth.esa.int/web/guest/featured-image/-/article/sentinel-1-analyses-the-nepal-earthquake

Copernicus data (2014)/ESA/DLR Microwave and Radar InstituteSEOM InSARap study

56S1A: SAR image, Londonhttps://earth.esa.int/web/guest/featured-image/-/article/sentinel-1-analyses-the-nepal-earthquakeCopernicus data (2014)/ESA/DLR Microwave and Radar InstituteSEOM InSARap study

March 4, 2015 (image credit: Copernicus data, ESA)

67Mt Hokkaido, Japan

78INSARInterferometry is a technique for combining coherent measurementsEssentially looks at the difference in phase between two coherent measurements and deduces distance information from this

SAR interferometry needs at least (i) two radars or (ii) radar imaging from two places

89Principle of SAR Interferometry

A1 and A2 are known Positions determined From satellite orbit orGPS/INS

The range difference is determined from , the measured phase difference.

910Though ERS images are usually presented as amplitude (e.g. PRI images), the radar measurements are actually complex (e.g. SLC).

Strictly speaking we have two images, which encode amplitude and phase

To derive an amplitude image, we throw away the phase information

The phase image on its own may have no useful information

To derive an interferogram, we take the difference of two phase images

1011INSAR phase difference?phase is a measure of how far the wave has travelled

The relationship between phase and distance is (in general)f = 2pd / li.e. if we have travelled by a wavelength (d=l) then the phase has changed by 2p.

0 2p 4p 6p...

12INSARInterferometry depends on the fact that we are using waves (electric fields).Two waves can interact to give brighter light (constructive interference) but also darker radiation patterns (destructive interference) darkness.

+=

+=CONSTRUCTIVEDESTRUCTIVE

1213INSARPhase information is effectively random noise in a single SAR image (because the phases are randomised by all the scattering on the Earths surface)However, if we view from another position very close to the first, then the differences in phase tell us about the differences in distance.Then it is just a matter of geometry...

1314Difference between the two path lengths related to the difference in phase of the received electric fields,

Interferometry used to generate two sorts of products - a coherence image, and a phase image (called the interferogram)

1415Phase difference

1516InterferogramFor generating an interferogram, two co-registered SAR images covering the same area are multiplied in a complex fashion. The result of this complex multiplication is the average of the two SAR images and the difference of their corresponding phase values. The interference pattern, also called FRINGE, is stored in a range of [ - , ].

Interferograms show differences in phase. This phase difference is the result of a path length difference that can be caused by elevation differences, motion, or deformation. Hence, we can use interferograms to derive accurate elevation maps, monitor small motions, and detect tiny deformations.

1617RegistrationThe phase difference can only be determined from two images taken from slightly different positions both images are therefore almost identical

The phase difference is determined at pixel level on the two images, therefore pixels must correspond.

Registration is done by standard correlation and transformation techniques

1718Shuttle Radar Topography Mission SRTM

http://rst.gsfc.nasa.gov/Sect11/Sect11_10.html

1819INSARNote that with two images, we can create two products:-An entire image of the phase information is known as the interferogramAn image of the coherence (i.e. the correlation between the two images)coherence near 1 means the phase information is reliable (and the images have high degree of correlation)coherence < ~ 0.3 means the images have low correlation (noisy). In this case, the phase information is probably not useful.

1920Coherence

The coherence is a measure of the correlation of the phase information of two corresponding signals and varies in the range of 0 to 1.

The degree of coherence can be used as a quality measure because it significantly influences the accuracy of phase differences and height measurements.

Bright areas indicate regions of high coherence, whereas dark areas represent low coherence regions.

2021CoherenceThere are several factors decreasing the coherence. In approximate order: Local slope (steep slopes lead to low coherence) Properties of the surface being imaged (vegetated or moving surfaces have low coherence). Time difference between the passes in an interferogram (long time difference lead to low coherence) The baseline (large baselines lead to low coherence) Technical details of the generation of the interferogram (poor co-registration or resampling leads to low coherence) Atmosphere

2122Phase unwrapping

Phase can only be detected between - and -, but the actual phase shift between two waves is often more than this. Phase unwrapping is the process of reconstructing the original phase shift from this "wrapped" representation. It consists of adding or subtracting multiples of 2 in the appropriate places to make the phase image as smooth as possible. To convert interferometric phase into elevation, you must perform phase unwrapping.

2223MountVesuvius

ERSSAR

2324

Interferogram

2425

DEM

2526Mt Etna interferogramX-band(SIR-C/X-SAR mission)

2627

2728Data acquisitionConfiguration can come from:Repeat passERS-1, 2, Radarsat, ENVISAT ASAR, ALOS PALSARSingle PassShuttle Radar Topography Mission SRTMAircraft

2829Sources of IfSAR data - older

2930Sources of IfSAR data - newer

ASAR on ENVISAT (2002-?)Cosmo-Skymed 2007/8http://www.telespazio.it/cosmo.htmlTerraSAR-X (2007-), TANDEM-X (2010-)http://www.infoterra.de/tandem-x-satelliteRADARSAT 2 (2007- )http://www.asc-csa.gc.ca/eng/satellites/radarsat2/KOMPSAT-5 X-band, 2012?Smotr (Russia) TBD?

3031ProblemsPhysical changes between two acquisitions cause loss of coherence (eg rainfall, wind, field ploughing, vegetation growth)TEMPORAL DECOHERENCE (degradation in the quality of the phase measurement)Differential Interferometry - generate two interferograms and then take the difference

3132ERS Tandem MissionERS-1 and ERS-2 in same orbit with a repeat cycle of 35 daysERS-2 35 minutes behind ERS-1 this gives coincidence of ground track after 24 hours

3233SRTM - http://semana2.terra.com.co/imagesSemana/documentos/SRTM_Eos_vidamodmapanasa.doc

The Level-2 Terrain Height Data Sets contain the digital topography data processed from the C-Band data collected during the mission.

For data between the equator to 50 degrees latitude, the postings are spaced at 1" (one arcsecond) latitude by 1" longitude. At the equator, these are spacings of approximately 30 meters by 30 meters.

The absolute horizontal accuracy (90% Circular Error) is 20 meters. The absolute vertical accuracy (90% Linear Error) is 16 meters.

3334Processing of SRTM dataMeasurement of base length is criticalSRTM mast not stableTherefore movement of outboard antenna must be monitored and correction madeUnexpected movement of the shuttle can cause problems

3435Orientation and movement sensors

3536Accuracy of IfSARTheoretical accuracy very high sub wave lengthDependent onBase lengthTerrainAtmosphereCoherence

3637Global SRTM AccuracyVerification Data SetsKinematic GPS: Data collected by NIMA for SRTM validation using kinematic GPS data processing (estimated accuracy: