Ice Motion and Topography in the Siachen Glacier Area ... Motion and Topography in the Siachen Glacier

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  • 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 glaciers 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 DFDs 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 campaigns 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.

    DFDs 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 2-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 2-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