How to choose SAR satellite imagery for a good interferometric processing. summarized from many sources.

1. HOW TO CHOOSE SAR
SATELLITE IMAGERY
PAIR FOR GOOD
INTERFEROMETRIC
PROCESSING

2. WARNING!!!
YOU MUST HAVE AND UNDERSTAND BASIC KNOWLEDGE
AND THEORIES ABOUT REMOTE SENSING AND
RADAR/MICROWAVE REMOTE SENSING IN GENERAL.
THERE WILL BE SOME TECHNICAL TERMS THAT MAY BE
YOU ARE NOT YET UNDERSTAND WHAT THE MEANING
OF THOSE TERMS OR WHAT THOSE TERMS IS ABOUT.
SO READ AGAIN YOUR TEXTBOOKS OF USE GOOGLE
SEARCH ENGINE IF YOU FIND ANY UNFAMILIAR TERMS.
EITHER WAY COMMENTS BOX IS ALWAYS OPEN 24/7/365

3. THINGSNEED TO BE CONSIDEREDFOR INTERFEROMETRIC
IMAGE PAIR SELECTION
• View angle (ascending and descending passes)
• Geometrical baseline (important for Inferferometric DEM Generation!!)
• Temporal baseline
• Time of the acquisition
• ImageCoherence
• Meteorological conditions

4. Image Coherence
Coherence for the simplicity can be defined as the correlation of different acquisitionat same locationradar
backscatter(in single look complex values geometry).
Good coherence ( > 0,5) will guarantee accurateand precise interferometric processing of radar phase
information.
Because SAR radiowave targeting earth surface, the coherence will be affected greatly by
landcover type and behaviour.
Built-uparea and bare soil landcoverusuallygives a goodcoherence even afterseveral years because theyarequitesteady, if phase
differenceis detected from theselandcovers,it mustbe caused byvolcanic/seismic/terrainforce (good targetto measuredeformation,
subsidence, seismic properties)

5. Image Coherence • Low/sparsevegetationcover oragriculturalfieldsusuallymaintaingood
coherencein a shorttemporaldifference(1 to3 days,dependson thevegetation
dynamic)andit is graduallydecreased alongthe time.
• High vegetation cover likeforestis superhardtomaintaingood coherence(even
in hours,1daytimedifference)becauseit isfrequentlychangedin direction
(whooshedbywinds).
• RadarBandsis alsohavesomeeffectsto thecoherence,takeforestlandcoveras
example, shorterwavelength radarbandslike C-bandsandX-bands(used in
ERS,Envisat,Sentinel-1) usuallyfailstomaintaingoodcoherence,butlonger
radarbandslike L-bands(used in PALSAR,PALSAR-2)canmaintainbetter
coherenceforforestarea(in relativeshorttemporaldifference) becausethose
longer bandshavebetter abilitytopenetratethe earthsurfacetovegetation
stems andtrunksthatrelatively steady.

6. Viewangle
(ascending
and
descending)
• Selectingimageryfor interferometricpair should be in same directionof
viewing.SARsatellite usually recordsearthsurface inascending direction
or descending direction(eitherin same day acquisition or differentday).
• Whythe imageryshould be insame viewingdirection??? Because
ascending and descending acquisition willgenerateopposite angle of
foreshortening andlayovereffects onthe terrain,you willnot have any
coherence at allif you combine ascending and descending image as
interferometricpair.
• There arecertaincase/application that youstillcan use both of
ascending and descending images,BUT notina single pair(you can
mergethe interferogramresultsofascending and descending image to
geta richfeature composite interferogramfor certainanalysis)

7. Thingsto be
considered
aboutView
angle
• You must have deep knowledge about topography of you area of
interest.
• If your AOI is flat or gentle, consider yourself aslucky because
layover/foreshortening is not massive, so ascending or
descending image can be used without further problem.
• But if your AOI is mountainous, you should consider the
dominant aspect (slope angle) of the terrain. Why? It is because
area in foreshortening become non-coherent assoon asthe
perpendicular baseline is greater than a few metres, so if the area
of interest lies on a slope that ismainly oriented towards the
West, it wouldbe foreshortened on SAR ascending images. Thus,
descending orbits should be selected.

8. Geometrical Baseline
• There aretwo terms about geometrical baseline, (1)
interferometric baseline and (2) perpendicular baseline
• Interferograms with very small perpendicular baseline values (<
30 m)is easy and fast tounwrap, butusually useless because
they are prone to phase noise and atmospheric effects.
• Interferograms with normal/long baseline (values higher than
~450 m) are usually almost impossible to unwrap if it is not
assisted bya DEM and the topography of the areais notvery
smooth. Moreover the coherence is generally small, due to the
high geometrical and volume scattering decorrelation, although
the temporal difference of the pair relatively short!
FOR INTERFEROMETRIC DEM GENERATION

9. Geometrical Baseline
• Theoptimum perpendicular baseline is in the rangebetween 150
and 300 metres. However, the bestresult is achieved by using
more than oneinterferogram. Interferograms with small
baselines can be exploited to help unwrap interferograms with
high baselines. Moreover, different interferograms can be
combined in orderto reducethe atmospheric artefacts.
FOR INTERFEROMETRIC DEM GENERATION

10. Geometrical Baseline
• If you arelooking for interferometric phase caused by motion and
you dont have any DEM to assist the interferometry, usea single pair
of imagery (before and after the event) with close to zero
perpendicular baseline. Remember, You still have to consider other
criteria like meteorological condition, temporal baseline, and time of
acquisition to geta better coherence).Smaller baseline itself already
guarantee good coherence.
• If you cant find close to zero baselineinterferometric pair, you going
toneeds a DEM because the interferometric phase now is
contributed by motion andaltitude factor. Altitude contribution
can be removed using DEM (topographic phase removal)
FOR LAND SUBSIDENCE,VOLCANODEFORMATION,EARTHQUAKE INTERFEROMETRY

11. Temporal Baseline
Temporal baselinegenerally is
abouttime differences between
twoacquisitionofimages that
usedfor interferometricpair
(seconds, minutes,hours,days,
months)
01
Shorter temporal baseline
will guarantee better
coherence (without ignoring
other contributed factors)
02
Longer but consistent temporal
baseline are also possible to
generate good coherence (like
images taken from same season in
different years, or similar agricultural
growth stage in different months)
03

12. Time of
Acquisition
Night time acquisition usually less affected by
Atmospheric disturbation
Phasemeasurement of RADAR usually affected by atmosphere
dynamic.In theinteferogram it couldbe represented by a random
fringes, artefactsor non random phasestripes.
SAR Satellite usually operates in day timeor night time(obviously
because they are activeremote sensing system)

13. Meteorological Conditions
• Coherence values are affected by localweather. Avoid
acquisitions during rain, snow or strong wind. These
phenomena usuallycause loss of phase coherence.
• Discard images acquired during very hot days: hot air can
holdmuch more water vapour than cold air (a major
cause of atmospheric artefacts in SAR interferograms)

14. Further Reading
https://www.esa.int/esapub/tm/tm19/TM-19_ptB.pdf

16. THANKS FOR
WATCHING