Ice Motion and Topography in the Siachen Glacier Area, Central Kashmir, derived withan operational processing system for INSAR-DEMs

B. Rabus and O. Lang

German Aerospace Center DLRGerman Remote Sensing Data CenterOberpfaffenhofen, D-82234 Wessling

phone: +49 8153 28 2895, fax: +49 8153 28 1445, e-mail: Bernhard.Rabus@dlr.de

ABSTRACTSiachen Glacier in the Karakoram is one of thelargest (> 70 km long) and highest glaciers outsidethe polar regions. The glacier’s near-tropicallocation makes it an interesting target forglaciological and climatological studies. In thispaper we use two pairs of interferograms fromascending and descending ERS-1/2 tandem passesto separate surface motion and topography ofSiachen Glacier. A fully automated differentialinterferometry technique based on DFD’s existingprocessing system for INSAR DEMs was tested.Results however were not satisfactory due to long-range phase-unwrapping errors in the ascendinginterferograms. In a less ambitious approach wecalculated the velocity field of Siachen Glacierassuming surface-parallel flow. Inputs are thedescending pair of interferograms plus a map ofhorizontal flow direction interpolated fromdigitized moraine features. Maximum velocities arearound 140 m a-1, in good agreement with valuesobtained with feature matching techniques.Additionally, we used an interactive procedure tomeasure 3D velocities near the centerline ofSiachen Glacier using 1D transverse profiles in theascending and descending interferograms. Forselected regions of the glacier this technique allowsto calculate deviations from surface-parallel flowdue to non-zero emergence velocity. Measureddeviations are compatible with the local massbalance regimes of the regions.

INTRODUCTIONOver 37 per cent of the Karakoram Himalaya iscovered by glaciers. [Bhutyani 1999]. Situated in asub-tropical location (centered around 35oN) andinfluenced by the monsoon, the glaciers in theregion are exposed to a climatic regime that isdistinct from those of mid-latitude and polarglaciers. Changes in glacier geometry and dynamicsassociated with advance and retreat can be used asreliable indicators of climate change at lowerlatitudes. Seasonal snow- and glacier melt is themain source of runoff for most rivers on the Indiansubcontinent. The glaciers act as natural waterstorage, releasing the water in the hot and dryseason and accumulating snow in winter [Vohra1981]. From this view point, the mid- and long-term behavior of the Karakoram glaciers hasdeciding influence on flood events as well as

agriculture production in Northern India andPakistan.

With a length of 74 km and an area of almost 1000km2 Siachen Glacier is the largest glacier in theKarakoram. It is located at 35.6oN, 77.3oE andcovers an elevation range of 4000 to over 7000 m.The main glacier and its largest side glacier, theTeram Sher branch, have south-westerly andwesterly aspect, respectively. Meltwater fromSiachen Glacier forms the main source of theNubra-River which belongs to the drainage of theGanges. Unfortunately the Siachen region is largelyinaccessible to scientific field studies since the1980s as it has become the main battle ground inthe on-going war between India and Pakistan overKashmir [Simons 1999]. Both countries installedarmy bases on the glacier, which can be consideredthe world's highest battle field. Radar remotesensing seems currently the only possibility tomeasure the ice flow of Siachen glacier. Due to thewar the validation of our results with ground data isunfeasible at the moment.

In this study we applied a new approach to separatethe topographic and motion contributions of theinterferometric phase. Glacier surface velocity isderived on a full scene basis using operationallyproduced digital elevation models (DEM). TheDEMs are generated from ERS-1/2 tandem sceneswith different baselines and look directions. The icemotion effects a height-anomaly in the DEM,depending on the respective baseline of theinterferogram. Ice motion in look direction can berecovered from this anomaly.

DATAWe used ERS-1/2 tandem data acquired betweenMarch and July 1999 at the German ground stationin Kitab/Uzbekistan. This campaign was the lasttandem-campaign of the two satellites before ERS-1 failed in spring 2000. The campaign’s main focuswas on applications of differential interferometry.Therefore the interferometric baselines are small formost tandem pairs. Four descending and threeascending scenes cover the test site. A marked dropin coherence after the onset of summer melt(around May according to [Bhutyani 1999])reduced the suitable data to two ascending and two

descending scenes. These four tandem scenes arepresented in Table 1 with one additional E2 sceneused in the feature matching later on. The rawscenes were processed with the operational DLR B-SAR processor and converted to interferogramswith the operational DLR GENESIS processor[Eineder and others, this issue]. The ground-resolution of the scenes is 25 m per pixel.

Orbit (E1/E2)Frame

flightdir. coherence Date B

[m]

1 40173/205002889

desc. excellent 22/23-Mar-99 114

2 40674/210012889

desc. decent 26/27-Apr-99 53

3 40123/20450693+711

asc. decent 18/19-Mar-99 65

4 40624/20951693+711

asc. bad 22/23-Apr-99 28

5 416762889 desc. - 05-Jul-99 -

Table 1: Specifications of used ERS-tandem pairs.Listed are Orbit and Frame, the flight direction,coherence quality flag, acquisition dates and theeffective baseline B. For the ascending scenes acombination of two frames was necessary. Theamplitude of E2-scene 5 was used for the featurematching method.

METHODThe usual way to separate motion and topographicphase is the differencing of coregistered phaseimages. Under the assumption of constant motion,the differential interferogram then only representstopography [e.g. Goldstein and others 1993,Joughin 1996a and b]. In the alternative approachpresented here we first unwrap and fully geocodethe interferograms and compare the resulting heightvalues.

DFD’s existing processing system for INSARDEMs is used to produce geocoded coordinates,elevation H(r, t), Easting E(r, t), Northing N(r, t)for each pixel (r,t) of the four slant rangeinterferograms. The resulting DEMs are stillcontaminated by motion to different amounts. Theyeither show increased height or a depression on theglaciers, depending on the angle between satellitelook and ice flow direction (see Figure 1a).Comparison of the interferograms with identicallook-direction (but different interferometricbaseline) recovers the true elevation Htrue (r, t) foreach pixel (and analogous for Easting andNorthing):

(1)

f1/2 are the height ambiguity factors that denote howmany meters of elevation correspond to one 2π-cycle. The line-of-sight motion VLOS can be

extracted for the ascending and descending pairsaccording to:

(2)

After this procedure the final resampling step of theINSAR/GEMOS processing system can be carriedout to obtain ascending/descending motioncomponents and DEMs on a regular grid of mapcoordinates.

Figure 1: INSAR DEM of Siachen Glacier. a) withmotion artifacts b) true topography according toEq. (1) .

PRECLUSIONThe motion corrected DEM produced from thedescending scenes 1 and 2 with the describedmethod is satisfactory. The glacier surface nowappears flat without the motion effects (see Fig.1b). Excellent coherence of scene 1 produced anunwrapped phase that is free from erroneous branchcuts for Siachen Glacier and most of itssurroundings. The unwrapped phase of scene 2 isalso of decent quality. There are no erroneousbranchcuts on the glacier except across thelowermost part which has unusually poor coherence

21

2111true

),(),(),(),(

fftrHtrH

ftrHtrH−−−=

21

211

),(),(2

),(),(ff

trHtrHtrHtrVLOS −

−−=

7811

7000

6000

5000

4000

3000

2129

Meters

in this scene. Unfortunately the unwrapping resultsfor the ascending interferograms proved to beunsatisfactory. Especially for scene 4 the averagecoherence was not high enough for automatic phaseunwrapping on a full scene basis. As a consequencethe cost-calculation for the used Minimum-Cost-Flow Unwrapper (MCF) [Constantini 1997]produced unrealistic branch-cuts through highcoherence areas on the glacier. These branch-cutsthen divide the image in several regions where theresulting INSAR DEM shows steps that correspondto the different 2π-phase levels. An attempt wasmade to use the automatically produced “regionmask”, which indicates the borders of theconsistently unwrapped regions, to flatten the phasesteps. However, the mask turned out to be far toooptimistic in that it often left sub-regionsundetected that then could not be corrected.

Figure 2: green: descending amplitude GTC frompasses 1 and 2, red: ascending amplitude GTCfrom passes 3 and 4.

In order to test whether ascending and descendingmotion phases can be coregistered onto each otherwe used the resulting ascending and descendingDEMs to create geocoded terrain correctedamplitude images (GTC). The result is shown inFigure 2. Because of the described problems theascending GTC is distorted and does not match thedescending GTC. As a consequence a combinationof the ascending and descending motion fields isnot possible. At the present standing we musttherefore abandon the idea of a fully automatedseparation of motion and topography on a full scenebasis. Whether the operational INSAR processingand geocoding system can be used for this task inthe future depends on the implementation of betteralgorithms for cost-generation in mountainous areasand a more reasonable region mask.

In the following we alternatively concentrate onless ambitious interactive techniques to calculatethe ice flow field of Siachen Glacier

VELOCITY FIELD FOR SURFACE PARALLELFLOW

In the analysis described in this section we use onlythe pair of interferograms from descending orbits (1and 2 in Table 1). These interferograms both havehigh coherence and no branch cuts on the mainglacier trunk and the Teram Sher branch.Intermediate steps of the automatic proceduredescribed in the previous paragraph have alreadyconverted these data into a DEM as well as thedescending motion phase in slant range geometry.These two data sets and the correspondingamplitude image were geocoded to a constantelevation above the WGS84 ellipsoid of 4900 m.As a third data set we use the flow angle, derivedfrom moraine directions in the geocoded amplitudeimage. the directions of medial moraines weredigitized at over 450 locations distributed over theentire ablation area. The point measurements offlow angle were then interpolated to the entire slantrange grid using a Delaunay triangulation method.

For a sub-area of the glacier Figure 3 shows theresulting flow direction grid as white arrows. Thedigitizing error of the individual measurements wasless than 1 degree; maximum errors at interpolatedgrid points should not exceed 2-3 degrees. What wehave derived is a long term average of the flowangle; the actual flow direction could depart fromthat of the moraine ridges due to non-steady stateeffects.

Figure 3: Digitized flow direction (white arrows).The colors indicate the surface slope in thisdirection, derived from the INSAR-DEM

The DEM was checked against a topographic map(US Army map 1:200,000, sheet Chulung). For thiswe identified in the corresponding amplitude imagefour ground control points on bedrock that wereindicated as reliable in the map. A weak ramp in theINSAR DEM of about 80-90 m over 50 km was

1o

- 2o

- 1o

0o

identified. We take this ramp as a measure of theglobal error in the INSAR DEM due to theuncertainty in the ERS precise orbits. Subsequentlywe use the DEM to extract surface slope along theflow direction. If it is real, the measured rampwould correspond to a slope error of less than 0.1degree. Atmospheric disturbances [e.g. Goldstein1995], which have typically spatial scales of 20 kmor more could have a stronger effect on the slopemeasurement. For a differential baseline of 60 mbetween interferograms 1 and 2 (Table 1) and anassumed disturbance of one fringe one wouldobtain a slope error of 0.5 degrees. Relative errorsin the slope depend on the phase noise and thespatial scale over which the slope is averaged. Wechose an averaging interval of 1.5 km in flowdirection, which should be on the order of two icethicknesses (coarsely estimated from the width ofthe glacier). At this averaging interval we estimatethis contribution to the relative slope error as lessthan 0.3 degrees.

The glacier slope in the direction of the flowdirection is shown in the color representation ofFig. 3. For most regions the slope is plausible beingdown-glacier and slowly varying at values around–1 to –1.5o. This smooth behavior of the slope inflow direction contrasts strongly with that of thecross-glacier slope (not shown). Cross-glacier slopeshows an extremely irregular pattern due to theoccurrence of numerous medial moraine ridges in

the middle and lower reaches of the glacier. Severalsmall regions in the upper side branches, close tothe ice edge and on the lowermost glacier (after thefinal bend) show unreasonably small or evenpositive surface slopes. These are artifacts due tophase noise and phase unwrapping errors in thevicinity of these regions.

The three input data sets, motion phase m− 0m,

flow angle G and slope angle G were converted tosurface parallel velocity v|| using the equations of[Rabus and Fatland 1999] for the case of steadystate and surface parallel flow. i.e. slope V andflow angle V of the velocity vector (with respect tothe radar look direction) are assumed equal to G

and G, respectively. We obtain

(3)

where λ = 0.0565 m is the radar wavelength, isthe radar incidence angle, and t is the temporalbase line (1day for ERS tandem data). The correctzero point of the motion phase, 0

m, was found andvalidated at several positions of the glacier margin,where the ice velocity is required to be zero. Theresulting map of the surface parallel velocity fieldof Siachen Glacier is shown in Figure 4. Maximumvelocities in the middle reaches of the main glaciertrunk and the Teram Sher branch range around 140m a-1 and 80 m a-1, respectively.

Figure 4: Ice flow velocity [m/yr] under the assumption of surface parallel flow. The dots and numbers indicatepoints of feature matching measurement numerated in Table 2.

( )( )GGG

mm

tv

sincoscoscossin4

0

| | −∆Φ−Φ

=

0

50

100

150

200

250

m a-1

COMPARISON WITH RESULTS FROMFEATURE MATCHING TECHNIQUES

To our knowledge there exist no terrestrialmeasurements of surface velocity for SiachenGlacier that could be used to validate our results.As an alternative, we do a comparison withvelocities calculated with feature matchingtechniques on two geocoded ERS-1 SAR amplitudeimages (processed at 25 m spatial resolution to4900 m above the WGS84 ellipsoid). The imagescover the period 22 March to 5 July 1999, i.e. theresulting velocities are time-averaged over 105days. The method minimizes the differencebetween two chips of the normalized amplitudes.The matching chip width was chosen as 60 pixels;they therefore represent a spatial average of 60 x25m= 1.5 km. The accuracy of the feature matchingis 1/8 pixel corresponding to an error of about ±15m a-1. The locations of the feature matchingmeasurements are shown as white dots labeled bynumbers in Fig.4. In Table 2 these measurementsare compared with interferometric velocities at thesame locations spatially averaged over the area ofthe feature matching chip.

We believe that, while less accurate than theinterferomery, the feature matching results can beconsidered bias-free. Overall the INSAR velocitiesagree with the feature matching to within about 10per cent or better. However there is a systematictrend towards higher INSAR velocities on the lowerglacier. There are several potential reasons for theobserved differences. One possibility is that theinterferometric velocity, which combines two 1 dayaverages in March and April was markedly higherthan the average March to July represented by thefeature matching. This is not expected however as

point

surface velocity [m a-1]feature INSARmatching

point

surface velocity [m a-1]feature INSARmatching

1 126 136 9 97 962 119 128 10 87 813 115 123 11 21 374 111 127 12 33 355 100 109 13 37 406 102 119 14 34 557 70 89 15 30 358 76 76 16 123 131

Table 2: Comparison of surface velocities derivedwith interferometric (surface parallel flowassumption) and feature matching methods. In bothcases values represent spatial averages over a 1.5 x1.5 km2 chip.

velocities should increase after the onset of surfacemelt. The melt season on Siachen Glacier startsbeginning of May and culminates in July [Bhuitani1999]. Another possibility is errors in theinterferometric DEM used in calculating the slopeangle of surface parallel flow. According to the

error estimation presented above this cannot explainthe observed magnitude of the differences. Mostlikely, the differences mark shortcomings in theparallel flow assumption. That the parallel flowassumption may introduce errors larger than 25 percent on a valley glacier was for exampledemonstrated by [Rabus and Fatland 1998] througha comparison with terrestrial measurements ofemergence velocity.

For Siachen Glacier we can perform a crudeanalysis of the effects of non-surface-parallel flowby estimating mean emergence velocity in theablation area from the runoff and equilibrium linealtitude measurements reported by [Bhutyani 1999].In Bhutyani, Fig.5, runoff r measured over theablation season May through September of fourdifferent years (1987 to 1991) amounts to about r =8×106 m3 d-1 on average. For an averageequilibrium line altitude of 5500 m Bhutiyani,Table 1 notes an ablation area size of A = 500 km2.The mean ablation is therefore b = r/A = 0.16 m d-

1. This would corresponds to a layer of about 6.5 mof ice that melts each year. If the mass balancegradient is assumed linear coarse estimates ofemergence velocity are 0 m a-1 at 5500 m (upperbowls of main glacier and Teram Sher branch), 6.5m a-1 at 4800 m (just down-glacier of the entry ofTeram Sher branch into the main glacier) and 13 ma-1 at 4100 m (near the snout of the main glacier).

For the region around 4800 m (points 1 and 17 inTable 2) we observe a down-glacier surface slopeof –0.9 degrees. Interferometric velocities arearound 135 m a-1. For an emergence velocity of 6.5m this would correspond to a slope of the velocityvector of about +2.7 degrees. Changing αG in Eq. 3from –0.9 to +2.7 degrees decreases theinterferometric velocity measurement by about 15per cent. This is close to the observed deviationsfrom the feature matching results.

MEASUREMENTS OF 3D CENTERLINEVELOCITY

In order to investigate the non-surface parallel flowof Siachen Glacier in more detail we developed aninteractive method to compare 1D profiles ofinterferometric velocity evaluated from ascendingand descending passes. In both cases we usedirectly the unwrapped phase (including bothmotion and topography contributions) in slant rangegeometry. Further inputs are the correspondingflow direction maps as well as the DEM describedbefore (descending slant range geometry). Figure 5shows a graphical interface that superimposes thephase unwrapping artifacts onto the amplitudeimage. Pixels affected by phase residuals and/orbranch cuts are colored in blue. This allows toselect 1D transverse profiles that are phase-error-free in both ascending and descending passes.Matching transverse profiles in ascending and

descending slant range geometry are found byconnecting identical features at opposite margins ofthe glacier. As the two profiles are not coregistered

pixel by pixel onto each other, we can onlycompare the velocity maxima near the centerline ofthe glacier.

Figure 5a: Resulting velocity of descending interferogram along shown profile. The image on the right shows theslant range amplitude (note that south is approximately up); the set of all branch cuts in scenes 1 and 2 is shownin blue. The chosen transverse profile is shown in red; flow direction is given by the blue arrow. In the leftdiagram the red and green lines are the profiles of scene 1 and 2, respectively

Figure5b: Same as Figure 5a for ascending scenes 3 and 4. The red profile in the right image connects the samepoints on the glacier sides as that in Figure 5a.

For the descending case we convert absolute phaseto velocity using the corresponding flow directionas well as the down-glacier slope angle from theDEM (both taken at the centerline). The zero pointsof the motion phase contribution at the glaciermargin can be identified to within 1/10 of a fringe(ca. 1 m a-1). For the indicated location at about5300 m on the main glacier Figure 5a shows theresulting velocity profile for descendinginterferograms 1and 2, acquired on 22/23 Marchand 26/27 April, respectively. The closecoincidence between the profiles shows that (i)cross-glacier topography can be neglected in thisanalysis and (ii) that velocity has been constantwithin the accuracy of measurement over this onemonth period.

We then use the same slope angle to convert theascending phase profiles. Figure 5b shows thecorresponding velocity profiles for ascendinginterferograms 3 and 4. Due to bad coherence,results for interferogram 4 are unreliable (branchcuts). The maximum of the velocity profile ofinterferogram 3 is about 8 m a-1 lower than that ofthe descending interferogram 1 or 2. By increasingthe slope of the velocity vector from –2.2 degrees to–1.3 degrees both velocity maxima match at a valueof 77 m a-1. This corresponds to an emergencevelocity of tan(2.2o-1.3o) × 77 m a -1 = + 1.3 m a-1 inapproximate accordance with the ablation rateestimated from the runoff measurements by[Butiyani 1999].

74 m a-1

82 m a-1

22/23-Mar26/27-Apr

18/19-Mar22/23-Apr

descending

ascending

CONCLUSIONS

We have tested a fully automated procedure toseparate motion and topography for full ERS scenesbased on DLRs processing system for InSARDEMs. Due to existing shortcomings of the phaseunwrapping for less than excellent coherence thisapproach is not operational to date. As a partialresult a motion-corrected DEM of Siachen Glacierwas generated .

Using the parallel flow assumption, we calculatedthe velocity field of Siachen Glacier. Inputs weretwo descending interferograms and map of flowangle, which can be inferred with high accuracyfrom the direction of medial moraines. Resultsagree with those obtained by feature matchingtechniques to within 15 per cent. The observedspatial trend in the observed deviations is explainedby an estimate of emergence velocity frompublished mass balance measurements.

An analysis of ascending and descending data along1D transverse profiles in both ascending anddescending data provides direct results on the 3Dvelocity vectors at the centerline of these profiles.Observed emergence velocities (evaluated asdeviations from surface parallel flow with theInSAR DEM) are again consistent with thoseinferred from the local mass balance estimates.

Our results on the velocity field should be a usefulbase datum for future glaciological field studies ofSiachen Glacier, should the war over it ever end.

ACKNOWLEDGEMENTS

We are grateful to the German Remote SensingCenter, which funded this study as an in-house-project. In person we want to thank K. Reiniger,and his group who acquired the ERS data in Kitabas well as A. Roth, W. Knöpfle and M. Eineder andother colleagues for interesting discussion andsoftware support

REFERENCES

Bhutiyani,M.R. 1999 Mass-balance studies on Sia-chen Glacier in the Nubra valley, Karakoram Hima-laya, India, Journal of Glaciology, Vol. 45, No.149,pp. 112-118

Constantini, M. 1997. A phase unwrapping methodbased on network programming. Proc. Fringe 96Worksh. on ERS SAR interferometry, Zürich, 30.Sept. - 2. Oct. 1996, ESA SP, 406, 261-272.

Goldstein, R.M. 1995. Atmospheric limitations torepeat-track radar interferometry, Geophys. Res.Lett., 22 (18), 2517-2520

Goldstein, R.M., H. Engelhardt, B. Kamb and R.M.Frolich. 1993. Satellite radar interferometry formonitoring ice sheet motion: Application to anAntarctic ice stream. Science 262, 1525-1530(1993)

Joughin, I., D. Winebrenner, M. Fahnestock, R.Kwok and W. Krabill. 1996. Measurement of ice-sheet topography using satellite-radarinterferometry. J. Glaciol. 42(140), 10-22

Joughin, I., R. Kwok, M. Fahnestock. 1996.Estimation of ice-sheet motion using satellite radarinterferometry: Method and error analysis withapplication to Humboldt Glacier, Greenland. J.Glaciol. 42(142), 564-575.Simmons,L.M. 1999 Kaschmir-Der ewige Konflikt,published in: National Geographic DeutschlandOkt. 1999, Gruner und Jahr, Hamburg

Vohra,C.P. 1981 Himalayan glaciers, in: Lal,J.S.and A.D. Moddie, ed.: The Himalaya: the aspects ofchange, Oxford, Oxford University Press, pp. 138-151

Rabus,B.T. and D.R. Fatland 2000 Comparison ofSAR-interferometric and surveyed velocities on amountain glacier: Black Rapids Glacier, Alaska,U.S.A., Journal of Glaciology, Vol. 46, No. 152,pp. 119-128