Pointing Angle Calibration of ZY3-02 Satellite Laser Altimeter using Terrain Matching

Transcription

1 Pontng Angle Calbraton of ZY3-02 Satellte Laser Altmeter usng Terran Matchng Guoyuan.L a,b,c, *, Xnmng.Tang a,b,c, Xaomng.Gao a,b,c, JaPeng Huang a,d, Jy Chen a,c,jng Lu a,c a Satellte Surveyng and Mappng Applcaton Center, NASG, Bejng, Chna - (lgy, txm, gaoxm,chenjy,luj)@sasmac.cn. b School of Resource and Envronmental Scences, Wuhan Unversty, Wuhan, Chna c Jangsu Center for Collaboratve Innovaton n Geographcal Informaton Resource Development and Applcaton, Nanjng, Chna d School of Geomatcs, Laonng Techncal Unversty, Laonng Fuxn ,Chna @163.com EuroCOW, WG I/4 KEY WORDS: ZY3-02; Satellte Laser Altmeter; Geometrc Calbraton; Terran Matchng; GLAS ABSTRACT: After GLAS (Geo-scence Laser Altmeter System) loaded on the ICESat (Ice Cloud and land Elevaton Satellte), satellte laser altmeter attracts more and more attenton. ZY3-02 equpped wth the Chnese frst satellte laser altmeter has been successfully launched on 30 th May, The geometrc calbraton s an mportant step for the laser data processng and applcaton. The method to calculate the laser pontng angle error based on exsted reference terran data s proposed n ths paper. The publc verson terran data, such as 90m-SRTM and 30m-AW3D30, can be used to estmate the pontng angle of laser altmeter. The GLAS data wth smulated pontng error and actual ZY3-02 laser altmetry data s expermented to valdate the algorthm. The concluson wll be useful for the future domestc satellte laser altmeter. 1. INTRODUCTION ZY3-02 satellte was successfully launched on 30 th May, 2016, whch has equpped wth the frst satellte laser altmeter of Chna for earth observng. The laser altmeter on the ZY3-02 satellte was desgned to valdate the acquston of the global hgh accuracy elevaton control ponts and the possblty of stereo mages accuracy mprovement wthout GCPs (Ground Control Ponts) (Tang, et al, 2016).. The on-orbt geometrc calbraton of satellte laser altmeter s a basc and ndspensble for the effectve applcaton of altmetry data, especally n order to be used as control ponts. Some researchers presented varety geometrc calbraton methods for GLAS (Geo-scence Laser Altmeter System) loaded on the ICESat (Ice Cloud and land Elevaton Satellte) (Rowlands et al, 2000; Luthcke et al, 2000, 2002; Schutz, 2001; Martn et al, 2005; Srota et al, 2005; Magruder et al, 2005), whch contaned maneuverng flght on the ocean, nfrared magng the ground footprnt of GLAS laser ponts durng the nght, layng the ground detectors to capture the laser sgnal or waveform matchng. Accordng to the above calbraton result, the accuracy of GLAS laser pontng angle could reach to better than 1.5", and the rangng accuracy was better than 10cm, whch ensured the absolute elevaton accuracy of 15cm on the flat terran fnally (Schutz et al, 2005; Wang et al, 2011). For ZY3-02 satellte laser altmeter, the pontng angle has some devaton between the truth and the measurement n the laboratory, whch can not meet the demand of hgh accuracy laser data processng and the feld experment of layng ground detectors whose poston should has lttle error calculated by the laser pontng angle value. So, the on-orbt geometrc calbraton to mprove the accuracy of the pontng angle s urgent and very mportant for the next step of data processng or feld experment. As early as a few years ago, scholars used the publc verson or the exsted verson of terran reference data to evaluate or compensate the satellte mage orentaton parameters. Jeong et al (2012) used 90m grd global DEM to orent the hgh resoluton satellte mages and mprove the mages geometrc accuracy. Km, et al (2011) used the 3 dmensonal smlarty transform to compensate the atttude of the satellte and the orbt parameter based DEM matchng and rgorous sensor model, and mproved the mappng accuracy of hgh resoluton satellte mage wthout GCPs. Zhou, et al (2016) ponted out that usng SRTM 90m data can effectvely mprove ZY3 mages wthout the control ponts, especally wth better elevaton accuracy. Moreover, matchng the exsted terran data for orentaton or poston n cruse mssle, underwater poston and other felds have also been wdely used( Rao et al, 2016). Affected by the measurement condtons and the precson of the nstrument, gravty envronment, vbraton and other factors, the pontng angle error of ZY3-02 satellte laser altmeter s non-gnorable, whch has dstnct effect on both the planmetry and the elevaton accuracy of the ground footprnt. Ths paper proposes the ZY3-02 satellte laser pontng angle coarse calbraton method based on the terran matchng, and the fne calbraton needs other method, such as the ground detectors wth hgh accuracy poston measured by RTK-GPS. The smulated experment derved from GLAS data and the true ZY3-02 satellte laser altmetry data s carred out and valdated. In Secton 2, the methodology s presented, and the experment and analyss s mplemented n Secton 3. At last, the concluson and future research work s remarked n Secton METHODOLOGY Consderng that the satellte laser altmeter footprnt ponts should be n agreement wth the actual terran profle, ths paper uses the terran matchng to estmate the laser pontng angle, acheve the systematc devaton of laser pontng angle wthout other data and predct the ntal footprnt poston for future feld experment effectvely. The satellte laser altmetry data s a seres of dscrete ponts located on the ground along the track of the satellte and the coordnates of laser footprnt ponts can be expressed as Eq.1, accordng to the referenced paper about ZY3-02 satellte (Tang, et al, 2016). p( x, y, z) f (, d,, ) (1) ( x, y, z) s the coordnate of the laser footprnt pnts. (, d,, ) s the laser rangng, rangng devaton and laser pontng angle. when the dstance from satellte to the ground s * Correspondng author: lgy@sasmac.cn do: /sprs-archves-xlii-1-w

2 fxed by the laser rangng value, then dfferent dstance devaton and pontng angle can obtan dfferent threedmensonal ponts groups. Among these ponts groups, there s only one group can ft the true terran for the best, and the laser pontng angle of ths ponts group s the best, whch s the closest value to the truth. If we defne the collecton of these dscrete ponts set as P, then there should be a set of actual terran Q, P=Q. P and Q are descrbed n Eq.(2) and Eq.(3), respectvely. It means that there s a group value of rangng devaton d and pontng angle (, ) whch s equal to the set of ponts set P and actual terran Q. P { p( x, y, z) ( x, y, z ) f (, d,, )} (2) Q= q(x,y,h) h = g(x, y ),(x,y )P (3) In theory, f all errors has been elmnated, there should be P=Q. But the random error of laser altmetry data, then the set of P has the unavodable error, moreover the nherent error of the actual terran dataset Q exsts, therefore P Q and the basc prncple s llustrated n Fgure 1. Pont p1 Pont q1 Pont p2 Pont q2 Pont p Pont q Flght track Ponts set P Ponts set Q Fgure 1. Satellte laser altmeter geometrc calbraton based on terran matchng How to measure the smlarty of set P and Q becomes the focus. In the aspect of smlarty measurement, t has the the correlaton coeffcent, the mnmum dstance, the mnmum square of the dfference and so on. The most common s the correlaton coeffcent, whch s the standardzed covarance functon (Zhang, et al., 2009), and t has been wdely used n the automatc matchng of photogrammetrc mages. But consderng that the correlaton coeffcent s related wth a lot of calculaton about the mean and varance of the data, ths paper selects the mnmum square of the elevaton dstance between the laser ponts set P and the actual terran set Q as the smlarty. Whch s descrbed n Eq.(4). n 2 (, ) mn ( ),{(,, ),(,, ) } 0 k z h x y z P x y h Q (4) The topography wthn the laser footprnt wll ntroduce error to the range and then the elevaton error of the laser pont (L et al,2017). Therefore, n order to ensure the relablty and consstency of the laser rangng accuracy n the calculaton of foot prntng, the footprnt terran of ZY3-02 satellte laser altmeter should be selected to ensure the heght accuracy. Accordng to the gven referenced terran data, the topographc features can be descrbed usng the slope S and the roughness, and the formula s as Eq. (5),(6). S S S (5) 2 2 arctan (tan X) (tan Y) ) 1 2 N HDEM _ H N (6) 1 Where, S, S s the slope angle of the laser footprnt along and X Y across the satellte track, respectvely. H s the elevaton values calculated accordng to the plane fttng equaton (7). H ax by c (7) Where, a tan S X, b tan S Y For ZY3-02 satellte, we select the laser footprnt ponts on the slope less than 2 º and the roughness less than 1.0m to calculate the elevaton dstance between ponts set P and Q. Once the pontng angle s, changed, we can get dfferent elevaton dstance value, and the mnmum value among them s the coarse pontng angle. Then, the mean value of the elevaton dfference between the laser pont and the reference terran can be regarded as the ntal value of d as the systematc error. It s clear that the nherent error and grd sze of the reference terran can nfluence the calculated result of the laser pontng angle, whch has been dscussed n the next secton. 3. EXPERIMENTAL AND ANALYSIS In order to verfy whether usng the dfferent pontng angle, then the sum of the elevaton dstance square between the laser footprnt ponts and reference terran ponts exst a mnmum value, and ZY3-02 satellte laser altmetry data wth dfferent pontng angle error s smulated, and the three-dmenson map s dsplayed n Fgure 2. Where, X, Y axs represents the laser pontng angle error, the Z axs represents the elevaton error of the laser footprnt ponts. Obvously, there s a mnmze error vertex n Fgure 2, whch can be further verfed n Fgure 3 whch means vewng Fgure 2 from the up, and the crcle contans the closest pontng angle error wth the actual value. Meanwhle, the square sum of the laser footprnt elevaton error s the mnmum. Therefore, by settng the change step value of the laser pontng angle, the pontng angle estmaton accordng to the Eq.(4) can be found. Error n the elevaton (Unt: m) Pontng angle error n the X axs (Unt: ) Pontng angle error n the Y axs (Unt: ) Fgure 2. Elevaton error dstrbuton of laser foot prnts wth dfferent pontng error Pontng angle error n the Y axs (Unt: ) Pontng angle error n the X axs(unt: ) Fgure 3. Elevaton error dstrbuton of laser foot prnts wth dfferent pontng error vewng from the Up ( Unt : m ) ( Unt : m ) do: /sprs-archves-xlii-1-w

3 The paper chooses the ICESat/GLAS actual data, smulates dfferent pontng angle error, and then estmates the error based on the terran matchng and compares the actual and smulated angle value to valdate the method. Durng the experment, the 30m ALOS-AW3D30 and 90m SRTM data are selected as pror reference terran data, and the actual GLAS altmetry data on 25 th March, 2009 derved from NSIDC (Natonal Snow and Ice Data Center) s vewed as expermental data, whch s llustrated n Fgure Lattude N GLAS Pont smulated data added pontng angle error. Obvously, the orgnal GLAS data s almost n accordence wth the terran data, and the smulated data derved from pontng error has large devaton wth the real terran n the elevaton drecton. Elevaton value(unt:m) Smulated data Reference terran Orgnal GLAS data Elevaton:m Longtude Fgure 4. The dstrbuton of GLAS laser footprnt ponts Durng the experment, the GLAS laser pontng angle n the X and Y axes s smulated to add 10 ", 20", 30 ", 50", 70 "and 120" error. And the orbt heght of ICESat s about 600km, then 1" pontng angle devaton can cause about 2.9m error on the ground, so the postonng error caused by the smulated angle error can be approxmately to 30m~360m. At the same tme, the pontng angle error wll cause the elevaton error, as shown n Fgure 5, the orgnal GLAS data s the red curve, the blue represents the terran data, and the green represents the GLAS A B C D E F G H The ID of ponts along the track Fgure 5. The profle of orgnal GLAS data, reference terran and smulated data derved from pontng error The grd sze of reference terran data AW3D30 s 30m, and SRTM s 90m, so the correspondng laser pontng angle s about 10" and 30". In the experment, when there are 10", 20", 30", 50", 75 "and 120" dfferent pontng angle error, whle the SRTM grd s approxmately equal to 30", so the pontng angle error less than 30" only usng AW3D30 30m terran data. And when the smulated error s more than 30", the two knds of terran data are compared, and the result s shown n Table 1. Table 1. Smulated GLAS laser pontng angle calculaton based on terran matchng Smulated 30m-AW3D30 90m-SRTM error value calculated value dfference calculated value dfference X / / Y / / X / / Y / / X / / Y / / X / / Y / / X Y X Y X Y X Y What s more, the laser repetton rate of GLAS s 40Hz whle the ZY3-02 satellte laser altmeter s only 2Hz, and the dstance between the neghbourng ponts s 170m and 3.5km for the two satelltes respectvely. In order to elmnate the calculaton error caused by dfferent densty, GLAS laser data s sparsed from 20 ponts to 2 ponts, makes the spacng of GLAS equal to the ZY3-02 satellte, and then smulates and calculates the pontng angle error, the result s shown n Table 2. It s clear that terran matchng for the smulaton data of GLAS stll can be used to estmate the laser pontng angle error even f they are sparsed, (unt: ) but the error s bgger than no-spared. Usng prmary data, the devaton value of the Y axs s about 5 when the AW3D30 s used, and the devaton value wll ncrease to 15 after the GLAS ponts are sparsed, t shows that the densty of laser ponts has an effect on the accuracy of the terran matchng calculaton. Based on terran matchng, we choose seven tracks of actual laser altmetry data from the ZY3-02 satellte to calculate the laser pontng error before the feld calbraton experment. And the expermental data of ZY3-02 laser data and terran reference do: /sprs-archves-xlii-1-w

4 data of the same area s descrbed n Table 3. There are SRTM wth 90m grd, ZY3-DSM wth 25m grd, and ALOS-AW3D30 wth 30m grd, respectvely. And the DSM data wth 5m grd sze along the 656th track s collected. The terran matchng Group A B C D E F G H usng the reference terran data s carred out to calculate the pontng angle error of these seven tracks laser altmetry data. The result s shown n Table 4. Table 2. Statstcal table of pontng angle error calculated by sparsed GLAS laser data axal Smulated 30m-AW3D30 90m-SRTM error value calculated value dfference calculated value dfference X / / Y / / X / / Y / / X / / Y / / X / / Y / / X Y X Y X Y X Y Table 3. The terran matchng expermental data of ZY3-02 satellte Track Reference DSM acqusted tme number 90m SRTM 30m AW3D30 25m ZY3-DSM 5m DSM th June th July th July th July th July st July st July Table 4. Pontng angle calbraton results of multple tracks ZY3-02 laser data Track No Reference terran Calculated laser pontng angle(unt:º) X axs Y axs Z axs 90m SRTM m ALOS-AW3D m DSM m SRTM m ALOS-AW3D m DEM m DSM m SRTM m ALOS-AW3D m DSM m SRTM m ALOS-AW3D m SRTM m ALOS-AW3D m DSM m SRTM m ALOS-AW3D m DSM m SRTM m ALOS-AW3D m DSM Orgnal laboratory measurements The mean of 25m ZY3-DSM The mean of 30m ALOS-AW3D The mean of 90m SRTM (unt: ) do: /sprs-archves-xlii-1-w

5 Based on the expermental result of the above, the dfference between the pontng angle and the average value of multple tracks data usng the same reference terran s calculated. The statstcal results are shown n Table 5. Table 5. The dfference between pontng angle and average value usng same reference terran Reference terran The dfference between the maxmum and the average of the pontng angle(unt: ) The dfference between the mnmum and the mean of the pontng angle(unt: ) X axs Y axs Z axs X axs Y axs Z axs 90m-SRTM m-AW3D m-DSM locaton. And the planmetrc error s llustrated n Table 6. Before the calbraton, the planmetrc error s about 8km usng the orgnal laser pontng angle, whch confrms the ndspensablty of the calbraton. After mplementng the pontng angle calbraton based on the terran matchng, the plane accuracy has mproved dramatcally, and when the reference terran wth smaller grd sze s used, the result can be better. For nstance, the accuracy s about 86m usng the pontng angle calbrated by the 90m-SRTM, whle the result can reach to 28.8m by the 5m-DSM, and the 28.8m s just equvalent to 10" error of the pontng angle. In order to verfy laser pontng angle calbraton accuracy based on the terran matchng, and the effect of reference terran data wth dfferent grd szes, we check the geo-locaton result of the Pont E2818 whch s the centre of footprnt pont captured by the ground detector n the feld experment. The poston of Pont E2818 has been surveyed by RTK-GPS wth hgh accuracy. In the experment, the geo-locaton result derved from orgnal pontng angle, the mean value usng SRTM wth 90m grd sze, the mean value usng AW3D30 wth 30m grd sze, the mean value usng DSM wth 30m grd sze, 5m-DSM and the total mean of all, are compared wth the true Table 6. The poston error of the ground detector usng the pontng angle derved from DSM wth dfferent grd The pont of E2818 Planmetry error (unt:m) The east The north The plane Orgnal pontng angle The mean of 90m-SRTM pontng angle The mean of 30m-AW3D30 pontng angle The mean of 25m-DSM pontng angle The mean of pontng angle The mean of 5m-DSM pontng angle Moreover, the elevaton result before and after the calbraton based on the terran matchng s compared usng the Track 382 of ZY3-02 satellte laser altmetry data. The profle along the track s llustrated n Fgure 6. The green and red ponts represent the result before and after the calbraton, respectvely. And the blue ponts represent the reference terran data. It s obvous that the laser footprnt pont elevaton profle after calbraton can acheve good results and be accordance wth the actual terran profle. Elevaton(Unt:m) ID of ponts along the track Laser Ponts before Calbraton Laser Ponts after Calbraton Reference Terran Data Fgure 6. Track 382 laser footprnt ponts and reference terran data before and after pontng angle calbraton terran matchng can effectvely estmate the laser pontng angle devaton of satellte laser altmeter. Accordng to GLAS data wth smulated error, the accuracy depends on the densty of laser ponts, grd sze and the elevaton accuracy of the pror terran data. The accuracy of pontng angle can reach to tmes grd sze of the terran data, and the result derved from AW3D30 wll be better than 90m-SRTM. And when the laser footprnt ponts are denser, the pontng angle calbrated result wll be better. For ZY3-02 satellte, the orgnal laser pontng angle has devaton wth the actual value, whch wll ntroduce geolocaton error about 8km. And the calbraton based on terran matchng can mprove the pontng and the postonng accuracy effectvely. After usng the 5m-DSM as reference terran, the accuracy of the laser footprnt ponts can reach to 28m, and t wll be useful and helpful for the next feld experment to lay the ground detectors for detal calbraton. And the method s stable for dfferent tracks laser data, whch can be reduced from Table 4, n whch the result s almost the same even f the track or the terran data s dfferent. It s also valdated by the GLAS data expermental result. In the next several years, the GF-7 satellte and Terrestral Ecosystem Bomass Montorng satellte wll be launched and loaded the laser altmeters, whch also need the calbraton for accurate pontng angle. The method based on terran matchng n ths paper can estmate the laser pontng and elmnate blunder effectvely. But detal calbraton about the laser altmeter stll needs further research for accurate mappng. 4. CONCLUSION In ths paper, the laser altmeter pontng angle calbraton method s proposed. And the smulated GLAS data and actual ZY3-02 laser altmeter data s mplemented. It can be seen from the expermental result, usng the reference terran data and ACKNOWLEDGEMENTS Ths work was supported by the Specal Fund for Hgh Resoluton Images Surveyng and Mappng Applcaton System ((Grant No.AH1601-8) and the Natonal Fund for Basc Surveyng and Mappng (Grant No. 2017KJ0204). The authors do: /sprs-archves-xlii-1-w

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