INSAR Flat-earth Phase Removal Approach Based on DEM to

INSAR Flat-earth Phase Removal Approach Based on DEM to SettlementArea

Xing-Tong CHEN, Xiao-Feng ZHU, Xiao-Hong WANG, and Yang Cai

North China University of Science and Technology,Hebei,Tangshan,063009,[email protected]

Abstract. The difficulties of phase unwrapping was increased with the existence of flat-earth phase removal, at thesame time ,it covered up the real topographical conditions .In order to solve this problem ,the paper presented amethod of re-flattening. based on DEM to eliminate flat-earth phase, with the interferogram, and then chose somespecial control points to construct polynomial fitting, removed residual phase and noise. By analyzing theALOS/PALSAR at the mine subsidence area, the contrast experiment was carried out. The results showed that themethod can clearly reflect the earth′s surface changes, reduce the surface deformation error, get a more accurateinterferogram phase and improve the precision of the INSAR interferometry.

Synthetic Aperture Radar Interferometry(INSAR)measurement technique combined the syntheticaperture radar imaging theory and electromagneticinterference technology, using the observations of two ormore SAR image complex data in the same area toprocess coherence to obtain more precise digital elevationmodel and more accurate millimeter surface deformationinformation[1]. However, Flat-earth phase removal had avery important effect on the entire INSAR data. Due tothe using of synthetic aperture radar imaging with slantrange way. Flat-earth phase removal appeared, so thedistance was different when the radar echo signalpropagated, and phase position changed which shouldremain unchanged at the same height in the interferencefigure, the phenomenon also showed that theinterferogram presented periodically interferencefringes.The presence of flat-earth phase removal obscuredthe change of interference fringes, so the interferogramcan not reflect the real situation of geomorphology, andmade fringes intensive, so the difficulty of phaseunwrapping increased. There fore, before unwrapping theinterferogram, the flat-earth phase must be removed.Approaching to common flat-earth phase removal weremainly the following methods[2-3]: (1) A method offlat-earth phase removal based on orbit parameters was toeliminate flat-earth phase that the reference ellipsoidcaused by the position and orbital parameters of theimaging area. The principle of the method was simple andhigh precision theory but lowering efficiency, whichdepended on the accuracy of the orbit position.(2)Frequency shift method had a small amount of calculationand no other additional conditions merit. Tocalculate theflat phase by frequency interference fringes, eliminate theflat-earth phase. It was more effective for flat areas, But

for the settlement of the surface area, the fringe frequencythat some part of terrain changes was as same as peacefulfringes of the frequency, if it was removed. It was likelyto cause an error, and can notaccurately eliminate theinfluence of the flat phase. (3) Flat-earth phase removalbased on DEM method can achieve high elevationaccuracy under the conditions of little parameter data.However, this method was influenced by abnormal phaseand phase noise, made some difficult terrain of theinterferogram which was still not clearly reflected. Basedon DEM data to eliminate the flat-earth phase, This paperpresented a method of re-flattening to improve accuracy.

1 Geometric principles of flat-earthphase removalAccording to INSAR imaging techniques, as shown infigure 1, the figure had two points of the level terrain:P1and P2. the heights of points P1 and P2 are same. Inthe interference pattern the phase difference betweenP1and P2 was ΔΦ[4], as in

tan4

0rrBΦ s

(1)

Where B was expressed as the vertical baseline; Bwas expressed as the distance between the antennas; Δrswas expressed as the slope distance difference of P1 andP2 points; θ was expressed as the look-angle of radar;r0was expressed as the initial slope distance length.

As it can be seen from the equation (1) the groundsurface without elevation variation also generated linearvaried interference phase. so the specific geometry of

DOI: 10.1051/09004 (2016)4 7 7 , 607090047ITM Web of Conferences itmconf/201ITA 2016

© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).

INSAR interferometer system leaded to the existence ofthe flat-earth phase. For the the different range positionsand the exact same point, height of which was differentfrom the interferogram phase. Not only the flat-earthphase existed in the range, but the satellite orbit was notperfectly parallel, the earth was irregular ellipsoid , therealso existed in the azimuth direction. According to theequation (1),the common method of flat-earth phaseremoval calculated the relative change in the flat-earthphase of the scene to realize the purpose.

To eliminate the flat-earth phase of the range,multiplied the complex intensity of each pixel in theinterference pattern with the complex exponential relatedslant distance phase. it was possible to correct the impactof flat-earth phase[5]:

)tan

4exp(0 r

rBi s (2)

The interference phase difference became :

sin

4

0rzBΦ

(3)

Figure 1.The Diagram of Interferogram Phase Changes WithThe Slant range geometry

Where Δz was the actual height difference of theterrain. As can be seen from the above equation, afterphase compensation, the phase can directly reflect thetopology of the terrain height.

2 Flat-earth Phase Removal Based onDEM MethodFlat-earth phase removal based on DEM, the first stepwas to get DEM data in the study area. This paper used aglobal DEM data to eliminate flat-earth phase, it caneasily get DEM data in the study area, such as aresolution of about 90 m of the SRTM DEM, thereference coordinate system was WGS84 coordinatesystem. Specific methods using DEM data to eliminateflat-earth phase was as follows:

2.1 DEM Resampling

Read and calculate the interval of the midpoint betweenthe DEM, while calculating intervals of the elementpoints in the interference picture and their ratio.According to the ratio between the interference patternand the DEM, proceeded resample for the DEM point,and bilinear interpolation method was used to obtain itspixel elevation value. The expression for the bilinearinterpolation [6]:

)1,1(),1()1()1,()1(),()1)(1(),(

jiuvfjifvujivfujifvuvjuif

(4)

Where f (*, *) denoted a pixel gray values, u and vwere non-integers.

2.2 Flat-earth Phase Removal

According to the oblique projection equations of theDoppler equation, nonlinear equations and ellipsoidequation posed, by using of orbital parameters, the DEMdata was converted to geocentric coordinate system. Byorbit interpolation and bilinear interpolation, the DEMdata was converted into phase values, and then subtractedreference value calculated DEM data from theinterferogram fringe phase, it achieved the purpose offlat-earth phase removal.

2.3 Re-flattening

Since the interference patterns till existed abnormal phaseand phase noise caused by orbit parameters, thephenomena madeflat-earth phase removal not completely,to further Refinement and Re-flattening. After theinterferogram, selecting some correlation coefficientgreater than 0.7 control points from figure 2, whichfacilitated extraction of elevation, and control point wereselected relatively flat and stable place. The distributionof control points was not too scattered. The number ofcontrol points were less than 7, the error range was lessthan one pixel resolution. The selected control points datacan see in table 1.

By using the selected point setting orbit correctionparameters, recalculated the satellite position, and thencalculated the points on the datum to the distance of thesatellite, and then get the corresponding points of the flatphase. In the interferogram, by subtracting the datumfrom the interferogram fringe phase, it achieved thepurpose of Re-flattening


2

Table 1. Control Point Statistics

Number Correlation Coefficient

1 0.798

2 0.781

3 0.791

4 0.848

5 0.767

6 0.751

7 0.856

8 0.843

9 0.726

Figure 2. Control Points Distribution

The number of polynomial coefficients was used toremove residual phase, the degrees of polynomial wasused to estimate the phase slope, If the number ofcoefficients was higher, the control points inputting willautomatically decreased, The default value was 3, whichindicated phase ramp in the range and azimuth directionwhich added a constant phase to correct, its polynomialexpression was:

310

29

28

37

26

52

4321

ykxykyxkxkykxykxkykxkk

(5)

3 Experimental analysis

3.1 Experimental Area and the Experimental data

The area of surface subsidence was as the experimentalarea. ALOS/PALSAR images were chosen to processedinterference experiments. External DEM data wasobtained by using the EV-INSAR.

Since the surface subsidence area was complex, thebaseline data would be estimated before the experiment.the purpose was to determine whether data can beinterference from time baseline and space baseline. Thelong spatial baseline was better in INSAR data processing,and it was best if it was less than 1/3 of the criticalbaseline. Time baseline of master and slave images was90d in the experimental data area. space baseline was661.798m,and critical baseline was between -6196.108m

and 6196.108m, it conformed to interference conditionfor INSAR data.

3.2 Experimental Results and analysis

Two scene Spaceborne INSAR experimental data was get,by using the DEM data to remove flat-earth phase,selecting WGS84 model as a reference to removeflat-earth phase. Comparison of two methods ofextracting DEM image was given. After the master andslave images make coregistration in the study area, theplurality of master and slave images corresponding pixelsproceeds conjugate multiplier to obtain the interferogram,figure 3 was obtained interference phase. As can be seenfrom the figure 3,Regular interference fringes covered upthe real terrain, fringes were displayed, becausecorrelation coefficient was relatively poor. Interferencefringe spacing expressed the altitude difference, by meansof the baseline estimate, and its altitude difference was93.048m. Interference phase ranges from -ᴫ to ᴫ . theinterference fringe showed dense and diagonal. Itscharacteristics were not parallel to the fringe direction ofthe range and azimuth, the flat-earth phase existed mainlydirection of the range.It was mainly due to the twoparallel orbits. Therefore, the range and azimuth have toremove the flat-earth phase.

Figure 3.The Interference Phase Image

Figure 4 was the interference phase after removingflat-earth phase. It used the DEM data to remove theflat-earth phase. Interference fringe Substantiallyremoved in the figure, it showed the flat-earth phase hadbeen removed from the direction of range and azimuth.Interferogram can reflect changes in the terrain. However,due to the effects of noise and residual phase, Unwrappedphase do not correctly translate into deformation value.So it was necessary to re-flattening.


3

Figure 4. The Interference Image of Flat-earth phase Removal

Figure 5.The Interference Phase Image of Re-flattening

Figure 5 was the interference phase after re-flattening.Compared withfigure4, figure 5 interferogram phaseinformation was more obvious, the invisible levelsbecame more clear before re-flattening. Interferogram canclearly see the terrain change information.

In order to demonstrate consistent visual andquantitative evaluation of the results of the evaluationresults, this paper quantitatively analyzed from thesurface deformation and phase standarddeviation. It morefully and objectively showed that the method waseffective.

Figure 6 was the surface deformation image byflat-earth phaseremoval based on DEM method. Figure 7was the surface deformation image by the method ofre-flattening. Comparing figure 6 and figure 7,it can beseen, the surface deformation was generally lower in thesame area, maximum deformation reached 0.666146m.By refinement and noise removal, the highly data error bythis method slowly reduced ∆z approaches terrainchanged.

Comparing with flat-earth phase removal based onDEM method, re-flattening method removed better phasenoise. According to the indicator of quantitativeevaluation phase standard deviation, the size of thestandard deviation reflected dynamic range of phaseamplitude. As can be seen from Table 2, Phase standarddeviation became1.2878 to 1.4638,increasedby0.176. Itsuggested that standard deviation of the phase changedsmall, the method maintained the dynamic range of phaseamplitude.

Figure 6.The Surface Deformation of Flat-earth Phase RemovalImage(unit: m)

Figure 7.The Surface Deformation of Re-flattening(unit: m)

Table 2.Two Methods Have The Impact of Standard Deviation

SummaryBy analyzing the ALOS/PALSAR actual experimentaldata and the original DEM data, it showed that thepurpose of the removal re-flattening method can beachieved with DEM data. Constructing polynomialfitting can make DEM terrain’s change more obvious, theaccuracy of elevation improved effectively, and reducethe surface elevation error. Experimental results showedthis method was very effective. It had importantsignificance on improving the accuracy of DEMreconstruction and reducing the complexity level of theINSAR data processing. By using INSAR technology toextract DEM, it had all-weather, high accuracy ofextracting DEM data, fast speed and other characteristics,it can solve very difficult problem that the conventionalmethod while extracting DEM.INSAR technologyprovided more efficient, cost-effective new method forthe subsidence monitoring, and had good applicationprospects.

methods Standard deviation/rad

Flat-earth phase removalbased on DEM method 1.2878

Re-flattening method 1.4638


4

References1. Hanssen RF.Radar Interferometry Data Interpretation

and Error Analysis[M].New York: Kluwer AcademicPublishers(2002).

2. Hong-An W, Yi-Xian T, Hong Z, Chao W. Flat-PhaseRemoval for ALOS/PALSAR Interferometry Basedon Orbit State Vectors[J]. Geomatics and InformationScience of Wuhan University, 35(1):91-96(2010).

3. Zhi-Ming L, Xiao-Tao T. Baseline Estimation andFlat Earth Removal ofERS-l/2 Data Based on OrbitalParameter[J]. Geomatic Science and Engineering,24(1):26-28(2004).

4. Fen-Fen H, Ji-Xian Z, Ke-Zhong D. ComparativeStudy on Several Methods of Removing theFlat-earth Effect in Airborne InSAR System[J].Remote Sensing Information, 5:57-61(2010).

5. Lei Y, Yong-Jun Z, Zhi-Gang W. FrequencyShifted-based Approach for InSAR Flat Earth EffectRemoval[J]. Journal of Electronic Measurement andInstrument, 18(4):15-20.(2004).

6. Cai D. Research of Image Interpolation Algorithm.Ph D Thesis. Chongqing: ChongqingUniversity(2011).


5

Documents

INSAR Flat-earth Phase Removal Approach Based on DEM to