一種消除全極化微波輻射計順逆風向遙感模糊性的新方案――星上定標和掃描機制設計的有關考慮_圖文

1、Removing upwind/downwind ambiguity of ocean wind directionby polarimetric microwave radiome r:scheme design on on-orbit calibration and scanning geometryWANG Zhenzhan,LIU Jing-yi,YIN Xiao-bin,JIANG Jing-shanCenterfor Space Science and Applied Research,Chinese Academy ofSciences,Beijing100190,ChinaAb

2、stract:Polarimetric microwave radiometer,a new type of remote sensor,can measure not only the first two Stokes parameters,but also the third and the fourth Stokes parameter of ocean surface radiation.Thus it opens a new field of microwave remote sensing on wind vectors.There is wind direction ambigu

3、ity in actually retrieved oceanic wind field using fully polarimetric radiometer data because wind direction signals in L and瓦are very small comparing with othergeophysical parameters,while L and L are basically two order smaller than those of horizontal and vertical polarization, and meanwhile ther

4、e are calibration errors in each Stokes parameters.In this paper,we dedicate to improve the calibrationaccuracy by designing a fully onboard calibrator and reasonably allocating time for calibration and observationduring onescan cycle.The theory of polarimetricmicrowave radiometer calibration is giv

5、en and the actual considerations of the onboardfully polarimetric calibration standard aredemonstrated.An original idea of calibrating polarimetric radiometer on satellite is presented.The relation between scan mode and wind direction retrieval error is analyzed.A new scanning geometry to remove the

6、se wind direction ambiguous solutions is devised and modeled.Simulation results show this kindofscanning geometry canremove wind direction ambiguity effectively.The accuracy of wind direction retrieval can be improved to15o.On the other hand,due to the limited space on satellite,the swath of a singl

7、e side scan can reach above 760kilometers.Key words:polarimetric microwave radiometer,ocean wind direction ambiguity,onboard calibration,scanning geometry CLC number:TP732.1Document code:Al INTRODUCTIONSince Skylab and Seasat Satellite launched in1970s microwave radiometer has become a main sensor t

8、o measurePolarimetric microwave radiometer,a new type ofremotesensor,canmeasure not only the targets radiation, but also its polarizability.The correlation signals of the third and fourth stokes parameter in the ocean surface emissions vary periodly with relative wind direction.Thus they can be used

9、 to measure wind field more accurately.Polarimetric radiometer realizes utilization of all information about electromagnetic wave,including frequency,phase, amplitude and polarization.Therefore,in ocean remote sensing,it canmeasure many parameters,such as air cloud,liquid water,sea surface temperatu

10、re,wind vector, and so on.In January2003,the WindSat/Coriolis mission deployed the first spacebome fully polarimetric passive microwave radiometer.The purpose of the mission is to demonstrate the capacity to obtain nearsurface winds over the globaloceans frompassivemicrowave measurements(Gaiser. 200

11、4.Its practicability has been tested by a lot of satellite data(Bettenhausen,2006;Yueh,2006.Due to some improper configurations ofWindsatsscanning geometry, wind direction accuracycan notfully satisfy the measuring requirement of200(Laws,2006;Wentz,2005.A newReceived date:200711-05:Accepted date:200

12、805-13First author biography:WANG Zhenzhan(1969一,received PH.D in Aug.2005in polarimetric microwave remote sensing of oceanic windvectorprinciple,system designand application.His major interest is in microwave radiometer remote sensing of oceanic geophysical parameters,especially in its calibration

13、and validation,retrieval algorithm development,surface emission and scattering models,and other relatedapplicationstudies in ocean and atmosphere parameter.He has taken part in many space-borne sensorscalibration,validation and application study programs. such as SZ-4M3RS,FY一3MHS T/V calibration.and

14、 CE一1MRM moon retrieval study,as well as HY一1,FY1,FY-2and SZ-3optical sensors insitu ca“bration.WANG Zhenzhan et a1.:Removing upwind/downwind ambiguity of ocean wind direction by polarimetric microwaveradiometer:scheme design on onorbit calibration and scanning geometry191 scan geometry oftwice obse

15、rvation can remove winddirection ambiguity effectively,and improve the accuracyof wind direction retrieval up to150(Yin,2008.Calibration method and design scheme of polarimetricradiometer on satellite are presented.Time and space forcalibration and observation during one scan cycle arereasonably all

16、ocated.A new scanning geometry to removeupwind/down wind direction errors due to ambiguousso】utions is devjsed.2DESIGN oN0NORBIT CALIBRAToR OFFULLY POLARIMETRIC MICROWAVE RADloMETERDue to the crosstalk between polarimetric microwaveradiometers channels.channels unbalance exists.Two-pointmethod can o

17、nly calibrate every channel separatelyand can not reduce the unbalance error in the ortho.channel。which will weaken the calibration precision in thecrosspolorization channels.Although often neglected inconventional radiometric measurements,the inter.channelcross.talk has to be accounted for in polar

18、imetric radiometrydue to its potential influence.Here we device a new methodof end-to-end calibration for spacebome polarimetricradiometer.2.1Principle on fully polarimetric microwaveradiometer calibrationThe relation between the antenna temperature of afully polarimetric radiometer and its complete

19、 output canbe written as(Lahtinen,2003玨i g。h gn gnghh gh3gb493h93393494h943944;+元=蓄亍e+;+元r+(1where u is video output response vector;g and o consist of radiometer gain and offset parameters;and n is the instrument noise referred to the video outputs.The offdiagonal elements of g represent interchann

20、el crosstalk, which can be the result of one or more hardware limitations.In order to invert the antenna brightness temperature vector from”,the elements of g and o in (1are required.Estimates of these unknown gain and offset elements can be determined via fully polarimetric calibration,which requir

21、es the generation of at least five linearly independent brightness temperature vectors.111e fully polarimetric calibration standard can produce an infinite number of distinct linearly independent reference brightness vectors priorly with adequate precisionand timeliness.T11is standard is composed of

22、 two parts (Fig.1,one is a linearly polarized standard,which is formed by two blackbodies of different but precisely known emission temperatures along with a polarizationsplitting wire grid,the other is a microwave phase retardation plate which is put between the linearly polarized standard and the

23、radiometer antenna(Gasiewski,1993;Lahtinen,2003.blackbody targetFig.1Schematic diagram of a fullypolarimetric calibration standardThe thorough analysis of the function principle of this calibrator proceeds by first calculating the tripolarimetric Stokes vector of the linearly polarized standard。then

24、 multiplying thisvector by a transformation matrix describing the influence of the retardation plate.The grid wire orientation angle measured with respect to the antenna polarization basis is0.The rotation angle of the plate relative to the radiometer is咖.Changing0,咖Call produce many distinct calibr

25、ation data matrix rows for precise calibration provided that the variousmaterial and component parameters of the standard are well known(vliet.1981.2.2Design on on-orbit calibrator of fully polari-metric microwave radiometert瓦t民甌乳乳=192Journal of Remote Sensing遙感學報2009,13(2standard totally by adjusti

26、ng the mirrors reflection angle. The hot calibration load is made of wideband microwave absorber whose emissivity is close to1.Its temperature can be measured by Pt resistance.The antenna reflector,feed and receiver are installed on a rotary platform to scan.The five fully polarimetric calibration s

27、tandards are fixed between the reflector and feed.Five groups of gridsand retarding platesare fixed to the five aforementioned angle combinations.As0,咖,cold and hot temperatures are known,five groups of fully polarimetric brightness temperature are adequately known,which can be used to calibrate a s

28、paceborne fully polarimetric radiometer.Cold sky.I Reflector:佘.孽、傯:孽刈少季Retardplate0。90。450亙圈、/彳、,/1,I、"/爹l(薹。/、Fire grid 450Retardplate90。300闥閱Fig.2Fully polarimetric calibration standardonboard(aRetard plate is0。,wire grid is0。,90。and45。;(bRetard plate is90。,wire grid is45。and30。As the space i

29、s very limited on spaceship,the location of hot and cold calibration loads is very important.Since twice.scan needs low noise.calibrationstandards should be set on the side of the satellite.But this will reduce the swath width of observing the earth.For once-scan, calibration standards can be set on

30、 any position among scala area or at the back of scan area.Tllis can not affect the swath width.It is important for twice scan to designate where to observe the earth and where to calibrate during one scan cycle,and the swath must be wide enough at the sanle time.A new scan geometry will be given in

31、 section4.3RELATIoNSHIP BETWEEN WIND VECTOR RETRIEVALS AND SCANNlNG GEoMETRY OF SPACEBORNE POLARIMETRIC MICRoWAVE RADl0METERThe wind vector retrieval results under various scanning geometries are discussed(Yin,2008.They show that,it is difficult to remove errors of wind direction ambiguous solutions

32、 in0。,30。,150。, 200。and3300,360。with maximumlikelihood estimation(MLEand oncescan data,as shown in Fig. 3(a.But most of wind direction ambiguous solutions can be removed with oncescan optimization method,as shown in Fig.3(c,and the results are best with twice-scan optimization method,as shown in Fig

33、.3(e.The aforementioned conclusions are suitable for wind speedbelow20m/s,as shown in Fig.3(b,Fig.3(dand Fig.3(f.The3“and4“stokes parameters get stronger as wind gets harder.So the retrieval results are better in high wind speed.As true wind direction couldnt be known in practical retrieval,oncescan

34、 optimization and twicescan optimization methods couldnt be realized.However they confirm that when wind speed is up7m/s and the 3“and4stokes parametersinstrument and model errors are less than0.4K and0.25K separately.wind direction retrieval accuracycan be better than20。.The twice.scan can be reali

35、zed based on conical.scan similar to that of AMSR(AMSRE。2004.And the difference of scan angle in two scans will change from0。to 180。with different pixels of scan.as indicated by Fig.4.The relation between antenna scan azimuth angles西. of forwardlooking and咖2of backwardlooking for the same ground cel

36、l is咖l+咖2=180。.Suppose that咖l and 咖2are measured clockwise from the same ground cell, the difference of scan angles(otbetween for-look and aft.100k for the same ground cell is1802×西.The difference of scan angles(otcan also be calculated byWANG Zhenzhan et a1.:Removing upwind/downwind ambiguity

37、of ocean wind direction by polarimetric microwave radiometer:scheme design on on-orbit calibration and scanning geometry193e苫宕穹lWind spccd Ulo=10/(m/s706050403020107060True wind direction/(。量50o基40ol3020lOOTrue wind direction/(oTree wind direction/(。Fig.3Wind direction error based on various algorit

38、hm as a function of the true wind Wind direction error based on once.scan MLE(a,oncescan optimization(cand twicescan optimization(eas a function of the true wind direction Wind direction error based on once-scan MLE(b,oncescan optimization(dand twice.scan optimization(fas a function of the true wind

39、 direction and speedFig.4Operation principle of twice。scana=2刪“罷(2 where,',is velocity of subsatellite point,T is period of one scan。r is radius of ground swath,and療isthe number of scan.一?？凇Ｕ貿plJI享譬E-一o/矗b譬 194Journal of Remote Sensing遙感學報2009,13(2Cycle number overpassFig.5The difference of scan

40、 angles a versusthe number of scanThe same inter-angle of parallelly to the satellite orbit,twicescan pixels locate as shownin Fig.6.406080orbitdirectionInter-angle/(。Fig.6Scan angles a as scan widthThe algorithm based on closest solution of two scans shows different performances of removing upwind/

41、 downwind ambiguity as the angle Ot changes.When true wind direction is in the area of00一200and3400一360。,this algorithm is good for removing upwind/ downwind ambiguity as the angle d changes from200to 80。:When true wind direction is in the area of1600一200。.this algorithm performance is better for the angle a from800to140。.as indicated by Fig.7.Difference ofscan angle/。Fig.7Errors of retrieved winddirection versus arelative to the true wind directionFig.8(ashows the angle Ot of different forelook cell