Research Paper Initial On-Orbit Modulation Transfer Function ... - KPubS

Research Paper

J. Astron. Space Sci. 29(2), 199-208 (2012) http://dx.doi.org/10.5140/JASS.2012.29.2.199

Technical Paper

J. Astron. Space Sci. 28(4), 345-354 (2011) http://dx.doi.org/10.5140/JASS.2011.28.4.345

Initial On-Orbit Modulation Transfer Function Performance Analysis for Geostationary Ocean Color Imager Implementation and Validation of Earth Acquisition Algorithm for

Eunsong Oh1, Sug-Whan Kim2†, Seongick Cho1,2, Joo-Hyung Ryu1, and Yu-Hwan Ahn1 Communication, Ocean and Meteorological Satellite 1

Korea Ocean Satellite Center, Korea Ocean Research & Development Institute, Ansan 425-600, Korea 1 2 Space Optics Laboratory, Department of Astronomy, Yonsei University, Seoul 129-749, Sang-wook Park1, Young-ran Lee , Byoung-Sun Lee2, Yoola HwangKorea , and Un-seob Lee1†

2

Ground Systems Division, Satrec Initiative, Daejeon 305-811, Korea Satellite System Research Team, Electronics and Telecommunications Research Institute, Daejeon 305-700, Korea

1 2

The world’s first geostationary ocean color imager (GOCI) is a three-mirror anastigmat optical system 140 mm in diameter. Designed for 500 m ground sampling distance, this paper deals with on-orbit modulation transfer function (MTF) Earth acquisition to solve when earthFirst, can be visible from satellite aftersource Sun acquisition during launchtoand opera-(865 nm) measurement andis analysis for GOCI. the knife-edge and point methods were applied theearly 8th band tion period or on-station anomaly. In this used paper, the algorithm and testKorean result ofpeninsula the Communication, image measured April 5th,satellite 2011. The target details are the coastlines of the and of Japan,Ocean and an island and meters Meteorological Satellite are0.34 presented case of on-station satellite anomalyWest status. The 400 in diameter. The (COMS) resultingEarth MTFsacquisition are 0.35 and for the in Korean East Coastline and Japanese Coastline edge targets, respectively, and 0.38 for the island target. The daily andparameters seasonal MTF variations at attitude the Nyquist frequency were also algorithms for the calculation of Earth-pointing attitude control including those direction vector, checked, and the result is 0.32time ± 0.04 average.are From these results, we confirm (Eurostar that the GOCI on-orbit MTF performance rotation matrix, and maneuver andon duration based on COMS configuration 3000 bus). The coordinate satisfies the design requirements of 0.32 for 865 nm wavelength. system uses the reference initial frame. The constraint calculating available time-slot to perform the earth acquisition considers eclipse, angular separation, solar local time, and infra-red earth sensor blinding conditions. The results of Elec-

Keywords geostationary ocean Research color imager, modulation transfer function, in-orbit knife method, tronics and: Telecommunications Institute (ETRI) are compared with that of theperformance, Astrium software toedge validate the point source methodETRI software. implemented Keywords: Earth acquisition, attitude control parameter, time-slot, communication, ocean and meteorological satellite

1. INTRODUCTION

direction and at East-West direction (Cho et al. 2009, 2010).

1. INTRODUCTION Geostationary ocean color imager (GOCI), launched June Communication, Oceanfirst andGeostationary Meteorological Satellite 27th, 2010, is the world’s Ocean remote (COMS, Chollian) was launched on July 26, 2010 sensing instrument with an effective aperture ofand 140ismm. now scientific in operation et al. 2011). The goalssuccessfully of GOCI are(Lee 1) monitoring of COMS the ocean Satellite Ground Control System (SGCS) is developed by environment around the Korean peninsula, 2) observation Electronics and Telecommunications Research Institute of oce an ecological system and 3) monitoring of climate (ETRI), and the algorithm of parameters and events for change. Fig. 1 shows the GOCI optical system consisting of COMS satellite configuration is developed according three mirrors that forms a three-mirror anastigmat (TMA)to the document provided from Astrium to ETRI (Laine type optical system to achieve a compact packaging volume. 2006). The ground sample distance (GSD) InGOCI the launch and early operation periodindicating (LEOP) orspatial in resolution is designed to be 500m, which the situation where the attitude of a satellite is is comparable not norto that low earth orbitis(i.e. satellites such as mal, theofsatellite attitude notpolar-orbit) known and thus it should MODIS, MERIS (Xiong et al. 2006). be fixed to a specific direction in order to acquire the norA attitude. single observation mosaic mal The positionsequence of the sungenerates is used asathe ref- of erences to fixobservation the satellite attitude in a aspecific direction. 16 snapshot tiles, using motorized pointing The sunwith acquisition refers to the process to fix mirror 2 degrees of freedom as shown in the Fig.satel2 below. lite attitude with reference to the solar position. Once the The GOCI image can cover 2,500km at North-South This is is an anOpen openAccess Accessarticle article distributed under the terms distributed under the terms of theof the Creative Commons Attribution Non-Commercial (http:// Creative Commons Attribution Non-Commercial LicenseLicense (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted creativecommons.org/licenses/by-nc/3.0/) which premits unrestricted non-commercial use, andand reproduction in any non-commercial use,distribution, distribution, reproduction in medium, any medium, provided the provided theoriginal originalwork workisisproperly properlycited. cited.

Copyright © The Korean Space Science Society

Copyright © The Korean Space Science Society

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image is data are takenin less than sun The acquisition completed, the position of 30 theminutes, earth is and it keptcan in the vision the of the satellite by adjusting the athreemeasure Korean peninsula 8 times day for high axis temporal attitude ofresolution. the satellite with reference to the relative position of the sun andmodulation the satellite. When the function initial at- (MTF) The on-orbit transfer titude of the satellite is stabilized, the normal attitude is measurement is a popular choice for high-resolution keptsatellite by obtaining the field of view (FOV) toward the earth performance analysis. Examples may include, but by means of the earth observation sensor. are not limited to, IKONOS (Cook et al. 2001, Choi 2002), This paper describes mainly the earth acquisition proOrbView (Crespi&Vendictis 2009), SPOT (Viallefont&Leger cess after the sun acquisition in case that the status of 2010) and KOMPSAT (Hwang et al. 2008). In particular, COMS changes from on-station to abnormal. Based on SPOT used the knife-edge method for its on-orbit MTF the orbit information for the COMS earth acquisition, the measurement analysis (Viallefont&Leger 2010) and article describes theand method to search the proper time SPOT3 used a spotlight as a point source for its periods considering the constraints for calculating theon-orbit MTF measurement andearth analysis (Leger can et al.be1994). appropriate time when the acquisition per- On the other hand, IKONOS used an array of mixed techniques for formed following the sun acquisition. In case of performits earth MTF acquisition performance, including the knife-edge ing the for COMS at the selected one ofmethod, the calculated time and period, thesource algorithm and simulation pulse method point method (Cook et al. 2001, result for attitude maneuver process,satellite maneuver time quality and Choi 2002). The KOMPSAT-2 image was duration is verified. The actually realized earth acquisiimproved with the MTF compensation technique utilizing Nov 15, 2011 Nov 23, 2011 Nov 25, May 2011 3, 2012 Received Revised Received Mar 5, 2012 Revised Apr 18,Accepted 2012 Accepted †

†Corresponding Corresponding Author Author

E-mail: [email protected] E-mail: [email protected] Tel: +82-42-365-7919 Fax: +82-42-365-7500 Tel: +82-2-2123-4247 Fax: +82-2-362-7891

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J. Astron. Space Sci. 29(2), 199-208 (2012)

Fig. 1. Geostationary ocean color imager (GOCI) instrument overview (Cho et al. 2009, 2010).

Fig. 2. Geostationary ocean color imager observational coverage area (Cho et al. 2009, 2010).

a variant of the knife-edge method (Hwang et al. 2008). In practice, these techniques requires for man-made edge targets or point sources such as lamps (Leger et al. 1994, Choi 2002, Hwang et al. 2008, Viallefont & Leger 2010). In comparison with other ocean color sensors, MODIS, having similar GSD, in-orbit MTF performance estimation used the Moon as a diffused uniform source with scanning observation concept, while not using an image-based ground target (Braga et al. 2004, Choi et al. 2008).

http://dx.doi.org/10.5140/JASS.2012.29.2.199

Despite the aforementioned techniques developed and used for other satellite performance investigation, the inorbit MTF performance of GOCI, the first ocean remote sensing instrument at the geostationary orbit, has never been studied with ground targets. This paper starts with a description of the GOCI overview and its image acquisition technique in Section 1. The methodology used to estimate the MTF performance using natural targets is described in Section 2. Section 3 describes the preliminary results of

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Table 1. Predicted GOCI MTF performance at Nyquist frequencybefore

for each spectral band was predicted in both E/W and S/N directions as shown in Table 1. The estimated MTF performance at Nyquist frequency was computed with Eq. (2) as follows,

launching (Herve et al. 2009).

E/W Spectral Estimated band Requiremet performance B1 0.40 0.45 B2 0.39 0.48 B3 0.38 0.46 B4 0.37 0.44 B5 0.35 0.44 B6 0.31 0.35 B7 0.32 0.38 B8 0.32 0.40

S/N Requirement 0.34 0.33 0.32 0.31 0.30 0.28 0.28 0.26

Estimated performance 0.42 0.41 0.42 0.39 0.36 0.32 0.33 0.32

(2-1)

(2-2) where MTFmeasured is measured as static MTF in the thermal vacuum environment, MTFsmearing is calculated from Eq. (2-2), kS = 1.2 and dLOS = 5.1μm. Straylightmargin to reduce the overall starylight level was determined to be about 6.5% (Herve et al. 2009).

GOCI: geostationary ocean color imager, MTF: modulation transfer function.

the GOCI on-orbit MTF measurements for the three study cases. The conclusions are presented in Section 4.

2.2 Knife Edge Method

2. METHODOLOGY

We used the knife-edge method,which is one of the widely used methods (Carnahan & Zhou 1986, Maeda et al. 1987, Choi 2002), and the point source method, as shown in Table 2. Edge spread function (ESF(x)) is defined by radiance (R) at pixel positions(x) in Eq. (3-1), line spread function (LSF(x)) of the system is computed by a simple discrete differentiation of the ESF as shown in Eq. (3-2). MTF (ξ) is obtained by Fourier transforming LSF.

2.1 MTF Definition Factors in the space environment, such as launch vibration and thermal loading, can change the resulting image quality (Holst 2000, Viallefont & Leger 2010). The on-orbit MTF performance assessment is a crucial step to determine whether the target optical system performance has been achieved in real instrument operation. The MTF performance can be estimated from the measured image products considering MTF degradation factors with the following equation (Manjunath 2003).

(3-1)

(3-2) (3-3)

(1)

In Fig. 3, considering the radiance recorded on the detector,the first step is to select the edge target. The target edges must have clear contrast differentiation in the edge boundary. When selecting such edges’ target area, nearby objects were trimmed out for noise reduction. The second step is to use the line profiles across the edge and to fit the

In Eq. (1), MTFtot refers to the final MTF from measured images, while MTFopt, MTFdet, MTFmot, MTFelec, MTFatm are MTF from the optical performance, detector performance, sensor operation effect, electronic noise and atmospheric effect, respectively. Prior to the GOCI launch, the on-orbit MTF performance

Table 2. Comparison of MTF assessment calculation methods. Method Edge method Point source Pulse input Radial target

Concept Computation LSF from edge profile Computation point spread in EW/NS direction Computation LSF directly from target Analyzes a series of “Pulses” lying on concentric path

Advantage Easy to implement Two-axis calculation (EW/NS) Less numerical errorfrom MTF Provides contiguous frequencies

Disadvantage Requires sufficient ground target contrast Requires confidence about location of point source center Requires knowledge of target width and resolution Difficult to implement

MTF: modulation transfer function, LSF: line spread function.

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Fourier Transform

Differentiation

MTF value

Frequency

Edge spread Function

Line spread Function

Modulation Transfer Funcdon

Fig. 3. Computation concept of on-orbit modulation transfer function performance (Hwang et al. 2008).

ESF as shown in Eq. (3-1). Its line profile data is the radiance read from each detector pixel position. Since the radiance at each pixel expresses the mixed radiance between land and sea, the interpolation technique was used to obtain 10 radiance data between adjacent pixel readings to eliminate the effects of the coastline environment from the measured radiance. The third step is to make the LSF by taking the numerical derivative of the equally spaced ESF samples as shown in Eq. (3-2). In the last step, MTF is obtained by performing a Fast Fourier transform (FFT) of the resulting LSF as shown in Eq. (3-3), and normalizing its magnitude to 1 at the zero spatial frequency. In the case of the GOCI detector, pixel size is 14.81 μm in E/W direction, and 11.53 μm in S/N direction, so the Nyquist frequency is calculated to 33.8mm-1 in E/W direction, and 43.4 mm-1 in S/N direction. We compare the case study results in terms of MTF at the Nyquist frequency.

in size are removed due to its signal being engulfed by the surrounding ocean signal. An island target of 400m in size, corresponding to 352.07m in E-W direction and 276.03m in N-S direction, was selected for the MTF evaluation as shown in Fig. 4. PSF of the target island imagewas calculated with the same procedure as the knife-edge method.

2.3 Point SourceMethod

3.1 MTF Evaluation for Three Target Areas

This method uses point spread function (PSF) defined by Eq. (4).

GOCI data processing uses the 8th band (865 nm), which serves as a reference wavelength for correction of atmospheric effects such as Rayleigh scattering, Mie scattering and aerosol. Therefore, using the 8th band image, the day and night time target images are selected with two conditions 1) i.e. smaller atmospheric effects (clear sky), 2) radiance standard deviation under 1% for average radiance of each target area (nearly uniform target). For East-West directional MTF evaluation, the knife-edge method uses shorelines with the clear linear edge occupying many pixel rows in order to obtain intensity profiles line by line. The point source method uses the aforementioned island and is capable of deriving MTF regardless of whether it is in N-S or E-W direction. Table 3 shows the selected target image coverage used for knife-edge and point source methods. Some example



3. CASE STUDIES First, the on-orbit GOCI MTF performance was computed for a clear image taken at UTC-03 April 5th, 2011. MTF for 8 images obtained from UTC-00 to UTC-07 April 5th, 2011 was then also derived to check the hourly MTF variation. The monthly MTF variation was estimated with 4-daysimages (in between Sep 15th, 2010 and May 17th, 2011) representing 3 seasons.

(4)

where PSF(i,j) is point spread function at an arbitrary pixel position (i, j), PSF (i,0) and PSF (0,j) are the 1-D LSF in horizontal and vertical direction respectively, and PSF (0,0) is obtained at the center position (Hwang et al. 2008). Using a powerful light source or lamp on the ground or a bright star source for a space telescope (Leger et al. 1994, Lee 2010), PSF can be acquired from the image with Eq. (4) (Hwang et al. 2008). The basic concept of the point source method is to use the FFT of blurred PSF that is similar to the LSF in the knife-edge method. We used GOCI L1B level data where small islands tens of metersor a hundred meters

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profiles for pixel lines and the best fit line to the data. LSF was computed from differentiation of ESF as shown in Figs. 7c and 8c. The MTF estimation was then performed with the FFT, as shown in Fig. 7d and 8d. The resulting MTF from the Korea East Coastline edge target in Fig.7 is 0.35 at the Nyquist Frequency, and the result from the west coastline of Japan is 0.34. The MTF estimation with the point source

coastline targets are illustrated in Fig. 5, whereas the point source target image of the GOCI 8th band is illustrated in Fig. 6. In addition, Figs. 7 and 8 show interim results in the MTF computation process with knife-edge method using the image taken at 03:15 UTC April 5th, 2011. In particular, Figs. 7a and 8a show edge targets used in the knife-edge method, and Figs. 7b and 8b show the combined edge

Table 3. MTF test target determination. Method

Korea coastline

Japan coastline

Point source

Latitude (°) Longitude (°) Line Column Number of pixels

37.71~37.82 128.87~129.01 2,294~2,318 2,586~2,610 25×25

39.29~39.60 140.00~140.12 1,810~1,834 4,488~4,512 25×25

34.20~34.24 126.99~127.04 3,082~3,091 2,231~2,240 10×10

MTF: modulation transfer function.

Fig. 4. Target island for point source method.

Fig. 5. Modulation transfer function natural target area for knife-edge method. 203

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8 band (865nm) image of GOCI

Pixel (N/S direction)

1000

2000

3000

Point Target of MTF 3

4000

Pixel (N/S direction)

4

5000 1000

2000

3000

Pixel (E/W direction)

4000

5000

5

6

7

3

4

5

Pixel (E/W direction)

6

7

Fig. 6. Modulation transfer function (MTF) target for point source method. GOCI: geostationary ocean color imager.

Target for Kinfe Edge Method

(a)

(b)

3

Edge Spread Fuction

x 108

2 2.5

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(d)

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Modulation Transfer Function 1

0.9

value at Nyquist Frequincy = 0.35

0.9 0.7

Normalized value

Normalized value

0.8

0.6 0.5 0.4 0.3

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Fig. 7. Calculation steps of knife-edge method for Korea coastline natural target (a) target area (b) edge spread function (c) line spread function (d) modulation transfer function curve.

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target is shown in Fig. 9a, and the computed MTF turns out to be 0.38 at Nyquist frequency as in Fig. 9b.

and 07, using the same knife edge method, cannot satisfy the requirement, and may be affected by the atmospheric radiance noise from cloud and aerosol. We also note that the MTF results from the point source method using the images at UTC-04, 06, and 07 are contaminated by the pixel spacing noise (Leger et al. 1994).

3.2 Trend Analysis for Daily MTF Variation From the 8th spectral band images taken sequentially from UTC-00 to UTC-07 on April 5th, 2011, 8 target images used in the knife-edge method are listed in Fig. 10. The images show the variation in radiance according to the solar zenith angle, and UTC-03 image is the brightest, as shownin Fig. 11. In Fig. 12, the estimated MTF using the Korean coastline shows that MTF from the UTC-03 image has the best performance, with the maximum radiance contrast between land and sea surfaces, as the sun elevation angle is the highest. We note that the MTF results from three different targets are slightly different but satisfy the requirement of 0.32, as in Table 1. In the meantime, the radiance difference between land and sea may affect the daily MTF variation, as shown in Fig. 12. The measured MTF using Japan’s coastline at UTC-06,

(a)

3.3 Seasonal Trend Analysis We compiled clear images taken over the time period of about 8 months from Sep 15th, 2010 to May 17th, 2011. The knife-edge method was then applied for every 8th spectral band image and we calculated the MTF at the Nyquist frequencyrespectively. The resulting MTF is shown in Table 4 and in Fig. 13.We note that there are some seasonal MTF variations in excess of the design MTF requirement. These can be interpreted as environmental (i.e. most likely atmospheric effects) degradation of the target image,rather than the optical system MTF variation. We also recognize that most results satisfy the MTF design requirement. Based

(b)

Target for Kinfe Edge Method 2

2

6

Radiance (Wm-2sr-1)

Pixel (N/S dinection)

4

Edge Spread Fuction

x 108

2.5

8 10 12 14 16

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Modulation Transfer Function 1

value at Nyquist Frequincy = 0.34

0.9 0.8

0.7

Normalized value

Normalized value

0.8

0.6 0.5 0.4 0.3

0.7 0.6 0.5

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Fig. 8. Calculation step of knife-edge method for Japan coastline natural target (a) target area (b) edge spread function (c) line spread function (d) modulation transfer function curve.

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point Target of MTF

(a)

point Spread Function

(b) 15 x 10

7

1 2

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3 4 5 6 7

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value at Nyquist Frequincy Measured MTF (PSF method) = 0.3771 0.6

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0.5

Fig. 9.

Modulation transfer function (MTF) assessment results with point source method at the image of April 5th UTC-03, 2011 (a) target area (b, c) point spread function (PSF) at each direction (d) MTF curve.

Fig. 10.

The geostationary ocean color imager8th spectral image and the modulation transfer function edge target image variation accordivng to the Sun elevation angle.

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Daily Variation

Table 4. MTF test target determination. UTC-00 UTC-01 UTC-02 UTC-03 UTC-04 UTC-05 UTC-06 UTC-07

2010.09.15

2011.01.31

2011.03.10

2011.05.17

0.34 0.34 0.34 0.34 0.37 0.35 0.31 0.19

0.21 0.27 0.32 0.37 0.37 0.36 0.36 0.28

0.28 0.33 0.36 0.29 0.37 0.34 0.29 0.29

0.32 0.32 0.31 0.31 0.33 0.36 0.30 0.32

MTF: modulation transfer function.

on these results and interpretation, we are confident that the current GOCI operational MTF performance satisfactorily meets the resign requirement.

Fig. 11. Modulation transfer function target signal variation with the sun elevation angle.

4. CONCLUSIONS

Daily Variation

For the GOCI L1B level data, we have established the computational methods to monitor seasonal MTF trend for GOCI image products, and report based on the observational evidence of successful GOCI operation that the GOCI on-orbit MTF performance is well within the requirements. We used the coastline edge target and a small island as a point source for the 8th spectral band image obtained after geometric correction in the data pipeline procedure. Our on-orbit MTF estimation results show 0.32±0.04 on average, which satisfies the design requirement of 0.32. In addition, daily and seasonal variation test results illustrate that the GOCI MTF variation is stable within or close to the requirement. We believe that some MTF variations in excess of the requirement are caused by the target object condition or images rather than the optical system MTF.

Fig. 12. Daily variation of on-orbit modulation transfer function (MTF)

performance from the image of April 5th, 2011, using three different natural targets.

Seasona Variation

ACKNOWLEDGMENTS This work was supported by Support for research and applications of Geostationary Ocean Color Satellite (PM56890) of the Ministr y of Land, Transport and Maritime Affairs in the Republic of Korea, and the Functional Improvement of Korea Ocean Satellite Center project (PE98781) funded by the Korea Ocean Research & Development Institute (KORDI). The manuscript preparation was supported, in part, by grants 20110020672 and 2010-0029390 from the National Research Foundation of Korea. We acknowledge partial support from Korea Research Fund grant no. 2011-0030055, and the support of Yonsei University - Korea Astronomy and Space

Fig. 13. Seasonal variation of on-orbit modulation transfer function (MTF) performance of using knife-edge method for the Korean coastline target.

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Science Institute (KASI) Joint Research for the Frontiers of Astronomy and Space Science program (2011), funded by the KASI.

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