Electronic Crosstalk in Aqua MODIS Long-Wave Infrared Photovoltaic Bands

remote sensing

Letter Electronic Crosstalk in Aqua MODIS Long-Wave Infrared Photovoltaic Bands

Junqiang Sun 1,2,* and Menghua Wang 1

1 NOAA National Environmental Satellite, Data, and Information Service, Center for Satellite Applications and Research, E/RA3, 5830 University Research Ct., College Park, MD 20740, USA; [email protected] 2 Global Science and Technology, 7855 Walker Drive, Suite 200, Greenbelt, MD 20770, USA * Correspondence: [email protected]; Tel.: +1-301-683-3338; Fax: +1-301-863-3301

Academic Editors: Gonzalo Pajares Martinsanz, Clement Atzberger, Richard Müller and Prasad S. Thenkabail Received: 13 July 2016; Accepted: 22 September 2016; Published: 28 September 2016

Abstract: Recent investigations have discovered that Terra MODerate-resolution Imaging Spectroradiometer (MODIS) long-wave infrared (LWIR) photovoltaic (PV) bands, bands 27–30, have strong crosstalk among themselves. The linear model developed to test the electronic crosstalk effect was instrumental in the first discovery of the effect in Terra MODIS band 27, and through subsequent investigations the model and the correction algorithm were tested further and established to be correct. It was shown that the correction algorithm successfully mitigated the anomalous features in the calibration coefficients as well as the severe striping and the long-term drift in the Earth view (EV) retrievals for the affected Terra bands. Here, the examination into Aqua MODIS using the established methodology confirms the existence of significant crosstalk contamination in its four LWIR PV, although the finding shows the overall effect to be of lesser degree. The crosstalk effect is characterized and the crosstalk correction coefficients are derived for all four Aqua LWIR PV bands via analysis of signal contamination in the lunar imagery. Sudden changes in the crosstalk contamination are clearly seen, as also in the Terra counterparts in previous investigations. These sudden changes are consistent with the sudden jumps observed in the linear calibration coefficients for many years, thus this latest finding provides an explanation to the long-standing but unexplained anomalies in the calibration coefficients of the four Aqua LWIR bands. It is also shown that the crosstalk contamination for these bands are of similar level for the two MODIS instruments in the early mission that can lead to as much as 2 K increase in brightness temperature for the affected bands, thus demonstrating significant impact on the science results already started at the early going. As Aqua MODIS is a legacy sensor, the crosstalk correction to its LWIR PV bands will be important to remove the impact of the crosstalk contamination from its calibration results and the associated science products.

Keywords: MODIS; Aqua; crosstalk; Thermal Emissive Bands; Long Wave Infrared (LWIR); Photovoltaic (PV)

1. Introduction The MODerate-resolution Imaging Spectroradiometer (MODIS) instruments on board the Terra and Aqua spacecrafts have successfully completed more than 16 and 14 years of on-orbit ﬂight, respectively [1,2]. MODIS is a cross-track whiskbroom scanning radiometer, which has a 360 degree rotating double-sided scan mirror, with several onboard calibrators (OBCs). Each MODIS instrument remotely collects information in 36 spectral bands, among which 14 are thermal emissive bands (TEBs) covering the spectral region from about 3.7 µm to 14.4 µm [3]. The calibration for a MODIS TEB applies a quadratic approximation to relate the at-sensor-aperture radiance and the background-subtracted instrument response. The calibration coefﬁcients, of the quadratic form, are calibrated on-orbit using

Remote Sens. 2016, 8, 806; doi:10.3390/rs8100806 www.mdpi.com/journal/remotesensing Remote Sens. 2016, 8, 806 2 of 11 an onboard black body (BB), one of the OBCs [4]. Among the TEBs, the four long-wave infrared (LWIR) photovoltaic (PV) bands, bands 27–30 (6.72–9.73 µm), are located on the LWIR focal plane assembly (FPA), which is one of the four FPAs [3]. These bands are primarily used in measuring water vapor distribution, volcanic cloud components, the cloud properties, stratospheric and tropospheric ozone concentrations, among others. Recent investigations into the four LWIR PV bands in Terra MODIS by Sun et al. [5–11] have discovered severe contamination due to crosstalk of signals among themselves, and that the crosstalk contamination has become more severe in recent years. Central to the discovery of the crosstalk effect is a linear model originally developed to test the existence of the effect prior to any confirmation of the existence of the effect [5,6]. A correction algorithm to characterize and mitigate the crosstalk effect also has been developed and implemented [5–11]. It is now understood that the crosstalk effect induces unexpected sudden jumps, rapid changes, and strong detector differences in the calibration coefficients derived from the BB calibration. The effect also induces strong striping artifacts and ghost images in the Earth view (EV) products and causes large long-term drifts in the EV brightness temperature (BT). A 6 K downward, 4 K downward, 1.5 K upward, and 2 K upward long-term drift in the EV BT have been determined for bands 27, 28, 29, and 30, respectively [7–10]. The crosstalk correction also has been applied to the quarterly BB warm-up–cool-down (WUCD) and the routine scan-based BB calibration as well as the MODIS Level-1B (L1B) EV retrievals for the four LWIR PV bands [11]. It is shown that the crosstalk correction can successfully remove the anomalous features in the calibration coefficients and restore the accuracy of both the WUCD and the routine BB calibration. In addition, the crosstalk correction can significantly diminish the striping and the ghost images in the EV retrievals and greatly reduce the long-term radiometric drifts, leading to a very stable lifetime trend and rectified MODIS L1B products. Overall, the extensive investigations of the crosstalk effect in Terra MODIS LWIR PV bands have established the linear model and the correction algorithm to be accurate over the entire dynamic range, and are applicable to all levels from instrument calibration to L1B retrieval. In fact, the best-mitigated result requires applying the crosstalk correction to all levels. The Aqua MODIS LWIR PV bands have also been observed for many years to have anomalous features in the calibration coefficients derived from the BB calibration even though they are relatively smaller compared to those observed in the corresponding Terra MODIS bands [12,13]. As similar behaviors observed in Terra MODIS LWIR PV bands have already been identified to be induced by the crosstalk effect, this work examines Aqua MODIS for the existence of its crosstalk contamination as well. Since Aqua MODIS similarly follows an approximate monthly roll maneuver to observe the Moon [14] in the same way as Terra MODIS, the lunar imagery analysis used to detect crosstalk signal in Terra MODIS [5,6] can be identically applied. In this paper, we will clearly show, from the lunar imagery analysis, that non-negligible crosstalk effect exists among the four Aqua LWIR PV bands and that the crosstalk contamination level changes with time. In addition, the effect has become more severe in recent few years. The crosstalk coefficients for these four bands will also be derived. The result will show that the crosstalk effect is significant enough to warrant a serious consideration for the mitigation of the impact in the four Aqua LWIR PV bands. The rest of the paper is organized as follows. In Section2, the crosstalk effect signals and their changes with time for Aqua MODIS LWIR PV bands are demonstrated using lunar images. In Section3, the crosstalk correction methodology and the crosstalk characterization algorithm are briefly reviewed. In Section4, the crosstalk coefficients are derived from the scheduled lunar observations and the results are discussed. The crosstalk coefficients for each of the LWIR PV bands are compared to those for the corresponding Terra MODIS LWIR PV band. The crosstalk impacts on the four Aqua MODIS LWIR PV bands are estimated. Section5 is a brief summary of this analysis.

2. Crosstalk Effect Electronic crosstalk in a remote sensor is a phenomenon causing electronic signals to be induced into a particular band from the detectors of neighboring bands on the same FPA at different times. Remote Sens. 2016, 8, 806 3 of 11

Remote Sens. 2016, 8, 806 3 of 11 The four MODIS LWIR PV bands, bands 27–30, are separately located on the LWIR FPA, and this is shownThe in four Figure MODIS1. As LWIR previously PV bands, mentioned, bands 27–30, among are se theparately artifacts located brought on the out LWIR by the FPA, crosstalk and this signals is are “ghostshown images” in Figure in1. As the previously imagery mentioned, of the receiving among band. the artifacts This isbrought due to out the by signal the crosstalk coming signals from the sendingare “ghost band thatimages” is at in a the different imagery location of the receiving on the same band. FPA.This is The due appearance to the signal ofcoming “ghost from images” the is sending band that is at a different location on the same FPA. The appearance of “ghost images” is a a unique and typical property of the electronic crosstalk that can be used to identify the crosstalk unique and typical property of the electronic crosstalk that can be used to identify the crosstalk effect. effect.Especially Especially in ina clear a clear scene scene with with sharp sharp edges, edges, a ghost a ghostimage imagecan be candistinctive be distinctive and pronounced. and pronounced. The The MoonMoon inin particularparticular has been been shown shown to to be be an an excellent excellent natural natural point-like point-like target target to exhibit to exhibit this effect this effect and hasand ahas similar a similar radiance radiance level level as as the the Earth’s Earth’s surface,surface, therefore, therefore, within within the the response response range range of the of the instrument,instrument, it enables it enables an an effective effective analysis analysis [ 14[14–17].–17]. From the the regularly regularly scheduled scheduled lunar lunar observations, observations, crosstalkcrosstalk effect effect in Aqua in Aqua MODIS MODIS can can be be identiﬁed identified andand its change over over time time can can be bequantified. quantiﬁed.

FigureFigure 1. MODerate-resolution 1. MODerate-resolution Imaging Imaging SpectroradiometerSpectroradiometer (MODIS) (MODIS) long-wave long-wave infrared infrared (LWIR) (LWIR) focal plane. focal plane. Figures 2 and 3 show the three-dimensional images of the integrated lunar observations for band 27 Figuresdetector 23, andband3 28show detector the 10, three-dimensional band 29 detector 6, imagesand band of 30 the detector integrated 3, as observed lunar observationson 19 July 2002 for bandand 27 detector11 March 3,2014, band respectively. 28 detector The 10,z-axis band labels 29 the detector magnitude 6, and of the band background-subtracted 30 detector 3, as lunar observed on 19signal July observed 2002 and through 11 Marchthe space 2014, view (SV) respectively. that is recorded The in z-axis the EV labelsdata sector the through magnitude a sector of the background-subtractedrotation during lunar lunar observation signal operation observed [14]. through The x-axis the space labels viewthe along-scan (SV) that direction is recorded (frames). in the EV Since the lunar size for each of the 1 km bands is about 7 × 7 pixels, the center 7 frames in the lunar data sector through a sector rotation during lunar observation operation [14]. The x-axis labels the images are the lunar signals of the detectors and the remaining frames will correspond to the signals along-scan direction (frames). Since the lunar size for each of the 1 km bands is about 7 × 7 pixels, from the neighboring bands. The y-axis labels the along-track direction (scans). The elongation effect the centerin the 7lunar frames images in theshown lunar in Figures images 2 areand the3 is lunardue to signalsthe oversampling of the detectors effect [14], and because the remaining each framespixel will in correspond the along-track to thedirection signals corresponds from the neighboring to multiple scans. bands. The The oversampling y-axis labels factor the and along-track the directionelongation (scans). effect The are elongation event-dependent. effect inFor the example, lunar imagesthe oversampling shown in factors Figures are2 1.89and 3and is due3.11 for to the oversamplingthe lunar effectobservations [14], because on 19 July each 2002 pixel and in 11 the March along-track 2014, respectively. direction correspondsExpectedly, the to elongation multiple scans. The oversamplingeffect in the lunar factor images and theof Figure elongation 3 is larger effect than are those event-dependent. of Figure 2. In For order example, to clearly the discern oversampling the factorscrosstalk are 1.89 signal, and the 3.11 lunar for theresponses lunar observationsare truncated to on 200 19 digital July 2002number and (dn) 11 Marchcounts. The 2014, maximum respectively. Expectedly,lunar view the elongationresponses for effect these in bands the lunar are asimages high as of4000 Figure dn. In3 fact,is larger all four than bands those are of saturated Figure2 .when In order they view the central area of the lunar surface. The portion of the signals shaped like a cylinder with to clearly discern the crosstalk signal, the lunar responses are truncated to 200 digital number (dn) magnitude 150 dn and above in each of the lunar images represents the moon surface. In an ideal counts. The maximum lunar view responses for these bands are as high as 4000 dn. In fact, all four case, without the crosstalk contamination, no signal will be present in the neighboring frames. Any bandsadditional are saturated response when in the they neighboring view the frames central ther areaefore of thecorresponds lunar surface. to the crosstalk The portion signal of from the the signals shapedneighboring like a cylinder bands, withthat is, magnitude the sending 150 bands dn or and the above interfering in each bands. of the lunar images represents the moon surface. In an ideal case, without the crosstalk contamination, no signal will be present in the neighboring frames. Any additional response in the neighboring frames therefore corresponds to the crosstalk signal from the neighboring bands, that is, the sending bands or the interfering bands. Remote Sens. 2016, 8, 806 4 of 11

Remote Sens. 2016, 8, 806 4 of 11 In Figures2 and3, hills and valleys can be seen near the central tall cylinders that represent the lunar responseIn Figures in the 2 and lunar 3, hills images. and valleys The hills can correspond be seen near tothe the central crosstalk tall cylinders contamination that represent signals the with positivelunar contributions response in tothe the lunar instrument images. The signals, hills correspond while the valleys to the crosstalk are those contamination with negative signals contributions. with Figurepositive2a demonstrates contributions that to Aquathe instrument band 27 signals, detector while 3 had the crosstalkvalleys are contamination those with negative on 19 contributions. July 2002, and Figure 2a demonstrates that Aqua band 27 detector 3 had crosstalk contamination on 19 July 2002, the crosstalk contamination has a positive contribution to the detector signal, indicated by the hills and the crosstalk contamination has a positive contribution to the detector signal, indicated by the aroundhills the around central the cylinder. central cylinder. Since Aqua Since MODIS Aqua MODIS was launched was launched on 4 on May 4 May 2002 2002 and and its its nadir nadir door door was openedwas on opened 24 June on 2002, 24 June this 2002, shows this that shows the crosstalkthat the crosstalk effect in effect Aqua in band Aqua 27 band was 27 already was already in existence in at theexistence very beginning at the very of thebeginning mission. of Figurethe mission.3a is theFigure lunar 3a is image the lunar observed image byobserved the same by the detector same on 11 Marchdetector 2014. on Besides11 March the 2014. mound Besides at the the mound left-back at th sidee left-back of the side lunar of the image, lunar a image, clear deepa clear valley deep on left-frontvalley side on is left-front seen, indicating side is seen, a negative indicating contribution a negative fromcontribution the crosstalk from the effect. crosstalk This showseffect. This that the crosstalkshows effect that may the crosstalk have positive effect andmay negativehave positive contamination and negative signals contamination at different signals parts at ofdifferent the image. As willparts be explainedof the image. later, As Aquawill be band explained 27 has later, crosstalk Aqua band contamination 27 has crosstalk from contamination all other three from LWIR all PV bandsother as the three sending LWIR bands,PV bands and as thethe sending contribution bands, per and sending the contribution band can per be sending either band positive can be or either negative. positive or negative. By comparing Figures 2a and 3a, it is seen that the crosstalk effect to the band By comparing Figures2a and3a, it is seen that the crosstalk effect to the band changes with time, which changes with time, which is similarly observed in the Terra LWIR PV bands. The same pattern of the is similarlycrosstalk observed effect change in the can Terra be LWIRseen from PV Figures bands. 2b,c The and same 3b,c pattern for bands of the 28 and crosstalk 29. In effectFigure change 2d, the can be seenvalley from besides Figure 2theb,c hill and is Figureseen in3 b,cthe forlunar bands image 28 andobserved 29. In by Figure band 230d, detector the valley 3 on besides 19 July the 2002, hill is seen inshowing the lunar also image that negative observed contributions by band from 30 detector the crosstalk 3 on effect 19 July to the 2002, detector showing already also started that negativeearly contributionsin the mission. from Figure the crosstalk 3d shows effect that negative to the detector contributions already to the started detector early became in the larger mission. on 11 March Figure 3d shows2014, that indicating negative contributionsthat for band 30 to the the crosstalk detector effect became changes larger with on time. 11 March 2014, indicating that for band 30 the crosstalk effect changes with time.

(a) (b)

Figure 2. Lunar images observed by Aqua MODIS LWIR PV bands on 19 July 2002: (a) band 27 Figure 2. Lunar images observed by Aqua MODIS LWIR PV bands on 19 July 2002: (a) band 27 detector 3; (b) band 28 detector 10; (c) band 29 detector 6; (d) band 30 detector 3. detector 3; (b) band 28 detector 10; (c) band 29 detector 6; (d) band 30 detector 3.

Remote Sens. 2016, 8, 806 5 of 11 Remote Sens. 2016, 8, 806 5 of 11

(a) (b)

FigureFigure 3. Lunar 3. Lunar images images observed observed by by Aqua Aqua MODIS MODIS LWIRLWIR PV bands bands on on 11 11 March March 2014: 2014: (a) (banda) band 27 27 detector 3; (b) band 28 detector 10; (c) band 29 detector 6; (d) band 30 detector 3. detector 3; (b) band 28 detector 10; (c) band 29 detector 6; (d) band 30 detector 3. 3. Crosstalk Correction Algorithm 3. Crosstalk Correction Algorithm Since the details of the circuitry in the electronic system for the FPA are not available, it is hardly Sincepossible the to details derive of the the relationship circuitry inbetween the electronic the crosstalk system contamination for the FPA signal are notand available, the sending it band is hardly possiblesignal to derivestarting the from relationship first principles between of electromagne the crosstalktic theory. contamination Even if the signalinformation and the is available, sending it band signalis starting still difficult from firstto describe principles the phenomenon of electromagnetic directly theory. since the Even crosstalk if the informationeffect has been is available,shown to it is still difficultchange towith describe time by the the phenomenon finding in this directly work sinceas well the as crosstalk in our previous effect has investigations been shown [5–11]. to change Nevertheless, it is both possible and essential to establish a relationship describing the crosstalk with time by the finding in this work as well as in our previous investigations [5–11]. Nevertheless, phenomena and subsequently prescribing its correction. As a first-order approximation, it is both possible and essential to establish a relationship describing the crosstalk phenomena and Sun et al. [5,6,15–17] use a linear relationship subsequently prescribing its correction. As a first-order approximation, Sun et al. [5,6,15–17] use xtalk msr dn (F) = c(B , D , B , D )dn (F + ΔF ) , a linear relationship Br Dr  r r s s Bs Ds rs (1) dnxtalk(F) = Bs ,Dsc(B D B D )dnmsr (F + F ) Br Dr ∑ r, r, s, s Bs Ds ∆ rs , (1) B ,D where Br, Dr, Bs, and Ds represent thes receivings band, receiving detector, sending band, and sending wheredetector,Br, Dr, B respectively;s, and Ds represent F is the pixel the receiving number along band, scan—which receiving detector, is also called sending frame, band, as mentioned and sending detector,previously—of respectively; bandF isBr; theΔFrs pixel is the numberframe shift along between scan—which the receiving is band also B calledr and the frame, sending as mentionedband Bs when viewing the same target due to location differences on the FPA as shown in Figure 1, and the previously—of band Br; ∆Frs is the frame shift between the receiving band Br and the sending actual values of the frame shifts among the MODIS LWIR PV bands are listed in Table 1; c(Br,Dr,Bs,Ds) band Bs when viewing the same target due to location differences on the FPA as shown in Figure1, is the crosstalk coefficient for the crosstalk from band Bs detector Ds to band Br detector Dr; and the actualmsr + valuesΔ of the frame shifts among the MODIS LWIR PV bands are listed in Table1; dnB D (F Frs ) is the measured background-subtracted instrument response of band Bs detector c(Br,Dr,Bss,Ds s) is the crosstalk coefficient for the crosstalk from band Bs detector Ds to band Br msr xtalk detectorDs atD frame; dn F +( FΔF+rs;∆ andF )dnisB theD (F measured) is the crosstalk background-subtracted signal from the sending instrument bands to response the receiving of band r Bs Ds rs r r B detectordetectorD ofat framethe receivingF + ∆F band.; and dnThextalk instrument(F) is the response crosstalk with signal the from removal the sending of the bandscrosstalk to the s s rs Br Dr receivingcontamination detector of can the be receivingwritten as band.[5,6,15–17] The instrument response with the removal of the crosstalk contamination can be written as [5,6,15–17] ( ) = msr ( ) − xtalk( ) dnBr Dr F dnBr Dr F dnBr Dr F . (2) Remote Sens. 2016, 8, 806 6 of 11

In addition to being applicable to TEBs, Equations (1) and (2) also work for reﬂective solar bands (RSB) [15–17]. To remove the crosstalk contamination in the receiving bands, Equations (1) and (2) need to be applied to solar diffuser (SD) calibration for the RSBs [15–17] and BB calibration for the TEBs [5,6,15], respectively, to correct the crosstalk effect in the calibration calculation and in the L1B code, which is used to retrieve the EV radiances.

Table 1. Frame shift, ∆Frs, between a receiving band (ﬁrst column) and a sending band (ﬁrst row).

B27 B28 B29 B30 B27 0 3 6 9 B28 −3 0 3 6 B29 −6 −3 0 3 B30 −9 −6 −3 0

Equations (1) and (2) have been applied to correct the crosstalk effect in Terra short-wave infrared (SWIR) bands [15] and the near-infrared (NIR) band 2 (865 nm) [16,17]. Additionally, the patterns of the artiﬁcial dots and the anomalous repeating dark and bright spots that were observed in the imagery of Terra band 2 have been successfully removed [16,17]. However, in practice, the implementation is highly computationally intensive, resulting in tremendous difﬁculty for the application of the correction in actual L1B EV retrievals. For a TEB, Equation (1) can further be approximated as [5,6]

dnxtalk(F) = C(B , D , B ) hdn (F + ∆F )i , (3) Br Dr ∑ r r s Bs Ds rs Ds Bs where the brackets indicate the average over the 10 detectors of the sending band Bs and C(Br,Dr,Bs) is the effective crosstalk coefficient from the sending band Bs to the receiving band Br detector Dr. In comparison to the implementation based on Equation (1), the computational demand using Equation (3) is reduced by a factor of ND, which is the number of detectors in a sending band. For MODIS, ND is 10, and thus this is a 10-fold reduction in computational time or power. The Earth surface scene temperature usually does not change significantly over the scale of 10 km and, thus, Equation (3) can be applied in MODIS TEBs for the purpose of catching the crosstalk signals. Sun et al. [5–11] have successfully applied Equations (2) and (3) to Terra LWIR PV bands to remove the crosstalk contamination in these bands. Since the computational demand is reduced by an order of magnitude using Equation (3) compared to Equation (1), the implementation of the crosstalk correction in operational L1B EV retrievals thus becomes possible to be done within a reasonable time frame. Most importantly, these previous efforts firmly established the correctness of the linear relationship between the crosstalk contribution of a sending band and the signal of the sending bands describing the crosstalk effect. Then, the key to the successful removal of the crosstalk contamination (in both the calibration coefficients and EV retrievals) is to accurately derive the crosstalk coefficients in either Equation (1) or (3). This is achieved via lunar-image analysis, as mentioned previously.

4. Crosstalk Coefficients Aqua MODIS is scheduled to view the Moon approximately every month. From these lunar observations, the crosstalk coefficients for the four Aqua LWIR PV bands can be derived via the established methodology [5,6]. As mentioned previously, a lunar image without crosstalk contamination appears as a clear cylinder; in fact, due to the oversampling effect, it appears as an oval cylinder [14]. The crosstalk contamination induces the anomalous hills and the valleys at the edges of the lunar images as shown in Figures2 and3. From these unexpected features at the edge of the lunar images, the crosstalk coefficients can be derived [5,6]. In this work, we focus on the sending-band detector-averaged crosstalk coefficients derived from the lunar observations. The derived coefficients are also compared with those for the Terra LWIR PV bands. Remote Sens. 2016, 8, 806 7 of 11

Figure4a–c gives the lifetime trending of the crosstalk coefficients for all detectors of Aqua Remote Sens. 2016, 8, 806 7 of 11 band 27, with each plot showing the result for one of the sending bands. It can be seen that the crosstalk effectFigure exists 4a–c gives from the the lifetime very beginning trending of of the the cro missionsstalk coefficients for all detectors for all detectors and that of the Aqua effect band comes from27, all with other each three plot PV showing bands, the i.e., result bands for 28–30.one of th Thee sending crosstalk bands. coefficients It can be areseen all that positive, the crosstalk resulting in positiveeffect exists contributions from the very to thebeginning signals of ofthe band mission 27 for in theall detectors early mission. and that Thisthe effect is consistent comes from with all the resultother shown three in PV Figure bands,2a, i.e., and bands the 2 coefficients8–30. The crosstalk remain coefficients positive forare theall positive, entire mission resulting for in positive most of the detectors.contributions However, to the sudden signals changes of band are 27 observedin the early for mission. detectors This 1 is at consistent around thewith beginning the result ofshown 2010 and in Figure 2a, and the coefficients remain positive for the entire mission for most of the detectors. for detector 3 at around the middle of 2004. The sudden changes of the crosstalk effect will definitely However, sudden changes are observed for detectors 1 at around the beginning of 2010 and for induce sudden jumps in the calibration coefficients and discontinuities in L1B EV retrievals. In fact, detector 3 at around the middle of 2004. The sudden changes of the crosstalk effect will definitely the suddeninduce changessudden jumps in the in Aqua the calibration MODIS bandcoefficients 27 linear and calibrationdiscontinuities coefficients in L1B EV of retrievals. the two detectorsIn fact, at the aforementionedthe sudden changes times in arethe Aqua known MODIS to exist band [12 27,13 linear], thus calibration this finding coefficients clarifies of the the cause. two detectors To compare the bandat the 27 aforementioned crosstalk effect times between are known Aqua to and exist Terra, [12,13], Figure thus4d this shows finding the clarifies crosstalk the coefficients cause. To for Terracompare band 27 the with band the 27 sending crosstalk band effect 29 between [5–7]. Comparison Aqua and Terra, of Figure Figure4b,c 4d shows shows that the thecrosstalk crosstalk effectcoefficients in Aqua band for Terra 27 is band much 27 smallerwith the thansending that band in Terra 29 [5–7]. band Comparison 27. Nevertheless, of Figure 4b,c the crosstalkshows that effect in Aquathe crosstalk band 27 iseffect not in negligible Aqua band and 27 should is much be smaller corrected, than consideringthat in Terra theband persistence 27. Nevertheless, of the the sudden jumpscrosstalk in the effect crosstalk in Aqua coefficients band 27 is andnot negligible in the linear and should calibration be corrected, coefficients. considering It is alsothe persistence worth noting of the sudden jumps in the crosstalk coefficients and in the linear calibration coefficients. It is also that the band 27 crosstalk coefficients are about the same level early in the mission for both Aqua and worth noting that the band 27 crosstalk coefficients are about the same level early in the mission for Terra, which can be seen from Figure4b,d. Since the Terra MODIS band 27 BT of an ocean site around both Aqua and Terra, which can be seen from Figure 4b,d. Since the Terra MODIS band 27 BT of an equatorocean in site Pacific around Ocean equator as well in Pacific as at Ocean the desert as well site as Libyaat the desert 1 has site been Libya shown 1 has to been be about shown 2 to K be higher due toabout the crosstalk2 K higher effectdue to earlythe crosstalk in the missioneffect early [7], in Aqua the mission MODIS [7], band Aqua 27MODIS BT at band these 27 two BT at sites these is also expectedtwo sites to be is2 also K higher expected due to tobe similar2 K higher contamination due to similar level. contamination level.

(a) (b)

FigureFigure 4. MODIS 4. MODIS band band 27 27 crosstalk crosstalk coefﬁcients: coefficients: (a) Aqua Aqua with with sending sending band band 28; 28;(b) (Aquab) Aqua sending sending band 29; (c) Aqua with sending band 30; (d) Terra with sending band 29. band 29; (c) Aqua with sending band 30; (d) Terra with sending band 29.

Remote Sens. 2016, 8, 806 8 of 11

Remote Sens. 2016, 8, 806 8 of 11 Figure5a–c shows the crosstalk coefficients for all detectors of Aqua band 28. As is the case with bandFigure 27, the 5a–c crosstalk shows the effect crosstalk exists coefficients from the for beginning all detectors of of the Aqua mission. band 28. ForAs is most the case of detectors,with band 27, the crosstalk effect exists from the beginning of the mission. For most of detectors, the the coefficients change slowly with time. For detectors 9 and 10, sudden jumps in 2007 and 2009 are coefficients change slowly with time. For detectors 9 and 10, sudden jumps in 2007 and 2009 are observed, and they match the sudden changes observed in the linear calibration coefficients of the observed, and they match the sudden changes observed in the linear calibration coefficients of the two detectorstwo detectors derived derived from from the the BB BB calibration calibration [[12,13].12,13]. From From Figure Figure 5a,5a, it itis isseen seen that that Aqua Aqua band band 28 28 detectordetector 1 has 1 has a larger a larger crosstalk crosstalk effect effect compared compared toto other detectors detectors inin the the band. band. For Forcomparison, comparison, FigureFigure5d gives 5d gives the crosstalkthe crosstalk coefficients coefficients for for Terra Terra band band 2828 with sending sending band band 29 29[8]. [ Comparison8]. Comparison of of FigureFigure5b,d 5b,d shows shows that that the the crosstalk crosstal effectk effect in in Aqua Aqua bandband 28 also also is is much much smaller smaller than than that that in Terra in Terra bandband 28. 28.

(a) (b)

Figure 5. MODIS band 28 crosstalk coefficients: (a) Aqua with sending band 27; (b) Aqua sending Figure 5. MODIS band 28 crosstalk coefficients: (a) Aqua with sending band 27; (b) Aqua sending band 29; (c) Aqua with sending band 30; (d) Terra with sending band 29. band 29; (c) Aqua with sending band 30; (d) Terra with sending band 29. Figure 6a–c displays the crosstalk coefficients for all detectors of Aqua band 29 showing the same Figurestart of6 thea–c effect displays in the theearly crosstalk mission. Again, coefficients for mo forst ofall the detectors detectors, the of Aquacoefficients band maintain 29 showing their the sameconstancy start of the for effect the entire in the mission, earlymission. thus the Again,effect al forso remains most of unchanged. the detectors, For thedetectors coefficients 2, 6, and maintain 9, theirsudden constancy changes for theare observed entire mission, in 2008, thus2010, and the effect2012, respectively. also remains These unchanged. changes, again, For detectorsmatch the 2, 6, and 9,sudden sudden jumps changes observed are in observed their calibration in 2008, coeffici 2010,ents and at 2012, the aforementioned respectively. These times changes,[12,13]. For again, comparison, Figure 6d shows Terra band 29 crosstalk coefficients with sending band 28 [9]. It is seen match the sudden jumps observed in their calibration coefficients at the aforementioned times [12,13]. from Figure 6b,d that the crosstalk effect in Terra band 29 changes much faster with time compared For comparison, Figure6d shows Terra band 29 crosstalk coefficients with sending band 28 [ 9]. It is with that in Aqua band 29. As with the band 27 comparison result, the crosstalk effects are about the seen fromsame Figurefor Aqua6b,d band that 29 the and crosstalk Terra band effect 29 inin Terrathe early band mission. 29 changes It is known much that faster Terra with MODIS time compared band with29 that BTin is about Aqua 1 band K lower 29. in As an withocean thesite bandaround 27 equator comparison in Pacific result, Ocean, the as crosstalkwell as at the effects desert are site about the sameLibya for 1, due Aqua to the band crosstalk 29 and effect Terra in bandthe early 29 mi inssion the early [9], and mission. therefore It the is knownimpact of that the Terra crosstalk MODIS bandcontamination 29 BT is about in Aqua 1 K lower MODIS in band an ocean 29 BT siteat these around two sites equator is estimated in Pacific also Ocean,to be 1 K as lower. well This as at the desert site Libya 1, due to the crosstalk effect in the early mission [9], and therefore the impact of the crosstalk contamination in Aqua MODIS band 29 BT at these two sites is estimated also to be 1 K Remote Sens. 2016, 8, 806 9 of 11 Remote Sens. 2016, 8, 806 9 of 11 lower.impacts This impacts MODIS MODIS science products science productsderived from derived Aqua from band Aqua 29 EV bandretrievals 29 EV and retrievals should be and corrected should be correctedin Aqua in Aqua MODIS MODIS L1B products. L1B products.

(a) (b)

Figure 6. MODIS band 29 crosstalk coefficients: (a) Aqua with sending band 27; (b) Aqua sending Figure 6. MODIS band 29 crosstalk coefficients: (a) Aqua with sending band 27; (b) Aqua sending band 28; (c) Aqua with sending band 30; (d) Terra with sending band 28. band 28; (c) Aqua with sending band 30; (d) Terra with sending band 28. Figure 7a–c represents the crosstalk coefficients for all detectors of Aqua band 30. Change with Figuretime for7a–c most represents of detectors the is crosstalkslow, and coefficientssudden jump fors are all observed detectors in ofthe Aqua coefficients band for 30. detectors Change 2 with time forand most 3 in of2009 detectors and 2013, is slow, respectively. and sudden Once jumps again, are the observed sudden inchanges the coefficients in the linear for detectorscalibration 2 and 3 in 2009coefficients and 2013, of Aqua respectively. band 30 detectors Once again, 2 andthe 3 have sudden been observ changesed [12,13] in the linearand the calibration timing matches coefficients the of Aquasudden band changes 30 detectors of the crosstalk 2 and 3 havecoefficients been observedshown in Figure [12,13 ]6a–c. and This the timingclearly matchesindicates that the suddenthe jumps in the calibration coefficients are induced by the crosstalk effect, same as the jumps in the changes of the crosstalk coefficients shown in Figure6a–c. This clearly indicates that the jumps in crosstalk coefficients of other PV bands. Figure 7d shows the crosstalk coefficients of Terra band 30 the calibration coefficients are induced by the crosstalk effect, same as the jumps in the crosstalk with sending band 29 for purpose of comparison [10]. Once again, the crosstalk coefficients of band coefficients30 are about of other same PV in bands. the early Figure mission7d showsfor the thetwo crosstalkinstruments. coefficients However, of the Terra crosstalk band coefficients 30 with sending of bandTerra 29 for band purpose 30 change of comparison dramatically [ 10with]. Oncetime, while again, those the crosstalkof Aqua band coefficients 30 change of slowly. band 30 are about same in the early mission for the two instruments. However, the crosstalk coefficients of Terra band 30 change dramatically with time, while those of Aqua band 30 change slowly.

Remote Sens. 2016, 8, 806 10 of 11 Remote Sens. 2016, 8, 806 10 of 11

(a) (b)

Figure 7. MODIS band 30 crosstalk coefficients: (a) Aqua with sending band 27; (b) Aqua sending Figure 7. MODIS band 30 crosstalk coefficients: (a) Aqua with sending band 27; (b) Aqua sending band 28; (c) Aqua with sending band 29; (d) Terra with sending band 29. band 28; (c) Aqua with sending band 29; (d) Terra with sending band 29. 5. Summary 5. Summary An examination of Aqua MODIS has confirmed the existence of significant crosstalk Ancontamination examination among of Aqua its four MODIS LWIR has PV confirmed bands. The the crosstalk existence effect of is significant characterized crosstalk and the contamination crosstalk amongcoefficients its four LWIR for the PV Aqua bands. LWIR The PV crosstalk bands are effect derived is characterized from the analysis and the of crosstalkthe scheduled coefficients lunar for the Aquaobservations. LWIR PV It is bands shown are that, derived for most from of the the detectors analysis of the of four the scheduledbands, the crosstalk lunar observations. effect is rather It is shownstable that, while for most a few of detectors the detectors per band of the show four dramatic bands, thechanges crosstalk of the effect crosstalk is rather effect stable at numerous while a few instances. In the early mission, the crosstalk contamination level in each of the four LWIR PV bands detectors per band show dramatic changes of the crosstalk effect at numerous instances. In the early is very similar between Terra MODIS and Aqua MODIS, although Aqua MODIS has a smaller overall mission, the crosstalk contamination level in each of the four LWIR PV bands is very similar between effect throughout the entire mission. The sudden jumps in the crosstalk coefficients match the Terradiscontinuities MODIS and in Aqua the calibration MODIS, coefficients although Aquathat ha MODISve been observed has a smaller for many overall years. Thus, effect this throughout work the entireidentifies, mission. for the The first sudden time, the jumps crosstalk in the effect crosstalk to be coefficientsthe root cause match of the the discontinuities discontinuities in the in the calibrationcalibration coefficients coefficients that for have these been four observedAqua MODIS for bands. many years.The impact Thus, of thisthe crosstalk work identifies, effect on the for the first time,EV BT the of crosstalkthese bands effect is also to beestimated. the root At cause a typical of the radiance, discontinuities the BT in in Aqua the calibration MODIS band coefficients 27 is for theseestimated four Aqua to be MODIS2 K higher, bands. while The for impactband 29 ofthe the result crosstalk is about effect 1 K onlower. the These EV BT differences of these bands are is also estimated.significant enough At a typical to affect radiance, the associated the BT scie in Aquance products MODIS derived band 27from is these estimated four Aqua to be MODIS 2 K higher, whilebands, for band and 29 therefore the result the is crosstalk about 1 correction K lower. Theseshould differences be applied areto the significant BB calibration enough and to Aqua affect the MODIS L1B code to improve the accuracy of the calibration coefficients and the EV retrievals. associated science products derived from these four Aqua MODIS bands, and therefore the crosstalk correctionAcknowledgments: should be applied We would to the like BB to calibrationthank Mike Chu and for Aqua valuable MODIS discussions. L1B code The to views, improve opinions, theaccuracy and of thefindings calibration contained coefficients in this paper and are the those EV of retrievals. the authors and should not be construed as an official NOAA or U.S. Government position, policy, or decision. Acknowledgments: We would like to thank Mike Chu for valuable discussions. The views, opinions, and findings contained in this paper are those of the authors and should not be construed as an official NOAA or U.S. Government position, policy, or decision. Author Contributions: Junqiang Sun is the chief investigator of this topic and is responsible for the formulation and the crosstalk analysis using Moon. Menghua Wang has provided vital support for this work. Remote Sens. 2016, 8, 806 11 of 11

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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