Radiometric calibration of DMSP-OLS Sensor using ...

Radiometric calibration of DMSP-OLS Sensor using VIIRS Day/Night Band Xi Shao1, Changyong Cao2, Bin Zhang1, Shi Qiu1, Christopher Elvidge3, and Michael Von Hendy3 1

University of Maryland, College Park, MD, USA NOAA/NESDIS/STAR, College Park, MD, USA 3 Earth Observation Group, NOAA National Geophysical Data Center, Boulder, Colorado, USA 2

ABSTRACT Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) has been collecting global night light imaging data for more than 40 years. With the launch of Suomi-NPP satellite in 2011, the Day/Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) represents a major advancement in night time imaging capabilities because it surpasses DMSP-OLS in having broader radiometric measurement range, more accurate radiometric calibration, finer spatial resolution, and better geometric quality. DMSP-OLS sensor does not have on-board calibration and data is recorded as digital number (DN). Therefore, VIIRS-DNB provides opportunities to perform quantitative radiometric calibration of DMSP-OLS sensor. In this paper, vicarious radiometric calibration of DMSP-OLS at night under lunar illumination is performed. Events were selected when satellite flies above Dome C in Antarctic at night and the moon illuminates the site with lunar phase being more than quarter moon. Additional event selection criteria to limit solar and lunar zenith angle range have been applied to ensure no influence of stray light effects and adequate lunar illumination. The data from DMSP-OLS and VIIRS-DNB were analyzed to derive the characteristic radiance or DN for the region of interest. The scaling coefficient for converting DMSP-OLS DN values into radiance is determined to optimally merge the observation of DMSP-OLS into VIIRS-DNB radiance data as a function of lunar phases. Calibrating the nighttime light data collected by the DMSP-OLS sensors into radiance unit can enable applications of using both sensor data and advance the applications of night time imagery data. Keywords: DMSP-OLS, VIIRS-DNB, Dome C, lunar vicarious calibration, night time imaging, night light sensor calibration

1. INTRODUCTION Satellite sensors such as Operational Linescan System (OLS) onboard Defense Meteorological Satellite Program (DMSP) have been acquiring night images since early 70’s. The remote sensing of nighttime light with DMSP-OLS were studied and shown to be an economical, and straightforward way for applications such as military surveillance, estimating population, monitoring social-economic development and power consumption, and providing weather and climate related data [1-9]. With the launch of Suomi National Polar-orbiting Partnership (Suomi-NPP) satellite in October 2011, the Day/Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard SuomiNPP represents a major advancement in night time imaging capabilities [10-16]. DNB serves primarily to provide imagery of clouds and other Earth features over illumination levels ranging from full sunlight to quarter moon. Other applications of using DNB such as light outage detections during major storms have been recently demonstrated [17]. Comparison of design specifications for DMSP-OLS and Suomi-NPP VIIRS-DNB is shown in Table 1 [18]. DMSP satellites operate in sun-synchronous orbits with nighttime overpassing at local time around 7:30 pm. And the SuomiNPP satellite was placed into the sun-synchronous orbit with local equatorial crossing times at ~1:30 am during nighttime. In general, DNB of VIIRS surpasses DMSP-OLS in having broader radiometric measurement range, more accurate radiometric calibration, finer spatial resolution, and better geometric quality. The DNB is a de facto radiometer because it uses an onboard calibration system to generate the radiances for Earth observations, compared to the DMSPEarth Observing Missions and Sensors: Development, Implementation, and Characterization III, edited by Xiaoxiong Xiong, Haruhisa Shimoda, Proc. of SPIE Vol. 9264, 92640A © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2068999 Proc. of SPIE Vol. 9264 92640A-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/21/2015 Terms of Use: http://spiedl.org/terms

OLS which is an imager and has no onboard calibration. The DNB of the VIIRS sensor utilizes a backside-illuminated charge coupled device (CCD) focal plane array (FPA) for sensing of radiances spanning 7 orders of magnitude in one panchromatic (0.5-0.9 μm) reflective solar band (RSB). In order to cover this extremely broad measurement range, the DNB employs four imaging arrays that comprise three gain stages. The low gain stage (LGS) gain values are determined by solar diffuser data. In operations, the medium and high gain stage values are determined by multiplying the LGS gains by the medium gain stage (MGS)/LGS and high gain stage (HGS)/LGS gain ratios determined from data collected along solar terminator region, respectively [21]. The radiometric signals observed by DNB sensor are digitized using 14 bits for the HGS and 13 bits for the MGS and LGS. The fine quantization of HGS enhances the appearance of terrestrial light emissions, including faint city lights. By applying gain coefficients and offsets, raw data from DNB observation is converted into radiometric units, i.e. W/cm2-sr [12, 14, 16]. The DNB relies on collocation with multispectral measurements on VIIRS and other Suomi-NPP sensors for accurate geolocation. The spatial resolution of the DNB is approximately 742 m across the entire swath. This is achieved by performing on-chip aggregation of the CCD detector elements that form pixels, which results in 32 aggregation zones through each half of the instrument swath on either side of nadir. The aggregation zones near the end of scan (EOS) have fewer pixels than the zones near nadir, as the footprint of a single CCD detector element on the ground is much larger at EOS. These improvements, coupled with the multispectral complementary information from other collocated VIIRS channels, enables the use of Suomi-NPP to pursue quantitative applications heretofore restricted to daytime measurements - a true paradigm shift in nighttime remote sensing capability. Table 1: Comparison of design specifications for DMSP-OLS and Suomi-NPP VIIRS-DNB Suomi-NPP VIIRS DNB

DMSP-OLS Spectral Band Number of Bits in A/D: Saturation Dynamic Range (W/cm2-sr) Additional spectral bands Calibration

0.5 -0.9 um

0.5 – 0.9μm

6 bits Saturates on urban lights at night

14 bits (16,384 levels) for HGS; 13 bits (8,192 levels) for MGS and LGS Does not saturate even with full solar illumination

Not in radiance unit Thermal infrared (10 um) None for low imaging band

light

3×10-9 to 0.0209 21 additional bands of VIIRS spanning 0.4 to 12 um. Solar diffuser is used to calibrate LGS. MGS and HGS are calibrated with gain ratio derived from data collected along solar terminator region.

Calibration Uncertainty (HGS)

Not normally calibrated

15% (1σ) [Liao et al., 2014]

Spatial Resolution:

5 km× 5 km at nadir

Nominally 742 m x 742 m

Swath

3000 km

3040 km

~19:30

~01:30

Local overpass

nighttime

Recent work in [19] demonstrated the use of lunar illumination to perform vicarious radiometric calibration of DNB at night. This is performed by selecting events when Suomi-NPP flies above the vicarious sites such as Dome C in Antarctic and Greenland in northern hemisphere at night and the moon illuminates the site with lunar phase being more than quarter moon. Dome C is one of the CEOS endorsed vicarious calibration sites with minimal atmospheric effect and has been recommended to be used as a community reference standard for calibration/validation of visible and nearinfrared (VNIR) channels [20]. The DMSP-OLS sensor does not have on-board calibration and the data is recorded as digital number (DN). In the past, empirical formula has been used to convert these DN to radiance unit. VIIRS-DNB provides opportunities to perform quantitative radiometric calibration of DMSP-OLS sensor. In this paper, we perform vicarious radiometric calibration of DMSP-OLS at night under lunar illumination using VIIRS-DNB. In section 2,

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events were selected when satellite flies above Dome C in Antarctic at night and the moon illuminates the site with lunar phase being more than quarter moon. Additional event selection criteria to limit solar and lunar zenith angle range have been applied to ensure no influence of stray light effects and adequate lunar illumination. In Section 3, the data from DMSP-OLS and VIIRS-DNB were analyzed to derive the characteristic radiance for the region of interest (ROI). The scaling coefficient for converting DMSP-OLS DN values into radiance is determined to optimally merge the observation of DMSP-OLS into VIIRS-DNB radiance data as a function of lunar phases.

2. METHOLOGY The antarctic Dome C site (75.1⁰ S, 123.35⁰ E) has been selected as the vicarious site in this study to perform radiometric calibration of DMSP-OLS sensor using VIIRS-DNB. Dome C is an extremely stable calibration site for VIS/NIR channels and provides climate quality calibration for VIS/NIR radiometers [20-23]. Dome C is a highly uniform (better than 1%) snow site located at high latitude under a very cold and dry climate. It has high reflectance with very low atmospheric absorption, low aerosol and dust content and a high percentage (greater than 75%) of cloud-free time. Since our scheme of calibrating DMSP-OLS requires lunar illumination over the Dome C site at night without solar light contamination, the selected cases for analysis occur at perpetual nights. For Antarctic Dom C, this occurs during May to August each year. In this study, we focus on analyzing data collected during 2012 by VIIRS-DNB and DMSP-OLS. Following [19], we first select observations and then derive characteristic radiance or DN from DMSP-OLS and DNB observations as a function of lunar phase. The guideline for event selection is to identify events with overpassing of Dome C region by VIIRS or DMSP-OLS, sufficient lunar illumination and no solar light contamination. In selecting observations, attentions need to be paid to stray light effect on the instrument due to solar illumination after the satellite passes through the Day/Night terminator projected on Earth’s surface. This effect is more significant during solstice. Detailed selection criteria are as following. o

Solar zenith angle >118.4⁰. This assures that the overpass of NPP occurs at night and there are no influences of stray light effects present at the selected region of interest (ROI) during the observations.

o

Lunar phase angle is less than 90 degree, i.e. lunar phase is larger than the quarter moon.

o

Lunar zenith angle < 80 degree. Both latter conditions ensure that adequate lunar light illuminates over the vicarious sites.

To select VIIRS-DNB observations, the distance of Suomi-NPP nadir to the center of ROI is set to be < 50 km to ensure the overpassing of Dome C site by Suomi-NPP. This reduces the uncertainty due to the variations in the satellite-view angle toward the ROI. Figure 1a showed an example of DNB observation when Suomi-NPP flew over Dome C. In total, 13 observations of Dome C region by VIIRS DNB are selected in 2012. Figure 1b shows the time series of reflectance for Dome C as derived from DNB observations from May to August in 2012 [19]. The selection criterion for the overpassing of Dome C by DMSP-OLS is relaxed so that we can collect enough events for analysis. The distance of DMSP nadir to the center of DOME-C is set to be < 300 km. In total, 8 observations are selected from DMSP data from May to July in 2012. Figure 2 shows the observation of Dome C by DMSP-OLS under moon light with different lunar phases. Dome C is marked as red ‘+’ on the plot. Here, the map from DMSP-OLS data is shown in the scale of DN and the pixel value of data is in 6-bit format with a value between 0 and 63. The maximum DN is around 37 during lunar phase = -3.86o. It can be clearly seen that the Dome C region varies from bright to dark as the lunar phase changes from nearly full moon to close to half moon. Top of atmosphere (TOA) radiance data are then extracted by collecting radiance data from DNB observation for selected cases in the region within 50 km in radius to the calibration site, i.e. ROI. Once all the radiance data within an . is regarded as ROI is extracted, radiance data within the ROI are processed to derive mean TOA radiance the characteristic radiance for the ROI. To extract the characteristic DN for the ROI from DMSP-OLS data, we note that the observation of Dome C by DMSPOLS shows strong scan angle dependence with the peak DN at the nadir and decays fast away from the nadir (see Figure 3). To make our analysis consistent and not affected by the scan angle, we focus on the nadir region of DMSP-OLS. In

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other words, for events with Dome C being within 300 km to the nadir of DMSP-OLS, the DN data is selected from a region within 10 km in radius around the DMSP-OLS nadir. Dome C 1 0.95 0.9

Reflectance

0.85 0.8 0.75 0.7 0.65

t

0.6 0.55 0.5 120 140 160 180 200 220 Day of Year (Since 2012)

 

Figure 1: (Left) DNB observations of Dome C on 07/04/2012; (Right) Time series of reflectance for Dome C as derived from DNB observations in 2012. DN 30

-64 -

-64

66

Lunar Phase = -3.86 deg.

-66

25

Lunar Phase = 24.52 deg.

-68 -68 15

Dome C

70

-8

-8

20

Dome C

-70

-8

2 -72-

15

-72

-74-74

-76 -

5

-76

-78

zIrSigY

78

60

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150

Longitude (deg)

Longitude (deg)

30

30

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25

-66

-64 -66

-68

-68 -53

25

20

20

-70

-70 15

! -72

-72

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o

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1

o

Longitude (deg)

Longitude (deg)

Figure 2: DMSP-OLS F18 Observation of Dome C under moon light with different lunar phases. The grey scale map is of scale of DN. Negative lunar phase angle indicates waxing lunar phase.

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40 F18201207031313, Phase = -3.86 deg. F18201207041301, Phase = 11.66 deg.

35

30

DN

25

20

15

10

5

0

0

500

1000

1500

Pixel Along Scan Line

Figure 3：Scan-line-dependent DN data from two DMSP-OLS observations in the Dome C region.

3. RESULTS Figure 4 shows the characteristic TOA radiance derived from DNB observations of Dome C as a function of lunar phase for the selected events in 2012. Figure 4 also shows the predicted TOA radiance from lunar spectral irradiance model [24] (MT2009). The MT2009 model is developed in preparation of calibrating nighttime low-light measurements from DNB sensors to enable quantitative nighttime multispectral applications. The model uses solar source observations, lunar spectral albedo data, and accounts for the time-varying Sun/Earth/Moon geometry and lunar phase. It produces 1-nm resolution irradiance spectra over the interval [0.3, 1.2 um] for any given lunar phase. In this model, the scattering property of lunar surface is assumed to be Lambertian. The model has been benchmarked against lunar observations such as with SeaWiFs and Modis-Aqua satellites and the ROLO lunar irradiance model. In predicting the TOA radiance for the ROI, the modeled lunar radiance has been multiplied with the reflectance (ρ = 0.886) of Dome C derived from the Hyperion observations. The consistency between DNB observation and lunar model confirms that lunar illumination on the Earth can be used as light source for effective vicarious calibration of DNB. DNB Observation MT2009 Model

35

2

Radiance aroud Dome-C (nW/cm -sr)

40

30 25 20 15 10 5 0 0

10

20

30 40 50 60 |Lunar Phase| (deg.)

70

80

90

  Figure 4: Radiance from DNB measurement (+ sign) and from prediction of MT2009 lunar model (red line) vs. lunar phase. All of these radiances have been scaled to mean Sun-Earth and Earth-Moon distance.

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The pixel value of DMSP-OLS data is of DN in 6-bit format with a value between 0 and 63. In [4], a formula was given to convert from DN of DMSP-OLS to radiance unit (W/cm2-sr) as following. /



(1)

200

35

180 around Dome-C

40

30 25

1.5

20

Mean DN

Mean DN around Dome-C

In this paper, we use DNB lunar observation together with MT2009 model to determine the conversion factor α. Figure 5 shows the mean DN (left panel) and (Mean DN)1.5 (right panel) within 10 km around DMSP-OLS nadir in the Dome C region as a function of lunar phase for the eight events we have identified in 2012. The decrease of mean DN in the Dome C region as the lunar phase changes from nearly full moon (~ 0 degree) to about half moon (~ 90 degree) can be clearly identified. The corresponding variation range of mean DN in the Dome C region is between 10 and 31.

15 10 5 0

160 140 120 100 80 60 40

0

20

20 40 60 80 |Lunar Phase| (deg.)

0

20 40 60 80 |Lunar Phase| (deg.)

Figure 5: Mean DN (left panel) and (Mean DN)1.5 (right panel) around DMSP-OLS nadir near Dome C vs. lunar phase. 40

20

Radiance aroud Dome-C (nW/cm -sr)

Converted DMSP Radiance vs. Model from DNB Converted DMSP Radiance vs. MT2009 Model

DNB Observation Model derived from DNB observation Converted DMSP Obsevration

35

2

2

Residual Difference in Radiance (nW/cm -sr)

22

18 16 14 12 10 8 6 4

0

0.5

1 1.5 2 DMSP Conversion Coefficient 

2.5

3 x 10

30 25 20 15 10 5 0

0

10

20

-10

30 40 50 60 |Lunar Phase| (deg.)

70

80

90

Figure 6: (Left) Residual difference between converted DMSP-OLS radiance and predicted radiance from model derived from DNB observation (blue line) or MT2009 (red line) as a function of conversion coefficient α; (Right) Radiances from DNB measurement, from model derived from DNB observation (black line) and from converted DMSPOLS measurement vs. lunar phase. The DMSP-OLS data is scaled with mean Sun-Moon and Moon-Earth distance and the conversion coefficient α is determined by optimally merging the observation of DMSP-OLS into VIIRS-DNB radiance data as a function of lunar phases. Left panel in Figure 6 shows the residual difference between converted DMSP radiance using Eq. 1 and prediction from DNB observation-derived model (blue line) or MT2009 (red line) as a function of conversion coefficient α. The minimum difference between converted DMSP radiance and prediction from DNB observation-derived model

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occurs at α = 1.65×10-10 with residual difference (RMSE) = 4.05 nW/cm2-sr. The minimum difference between converted DMSP radiance and MT2009-derived radiance occurs at α = 1.59×10-10 with residual difference (RMSE) = 4.15 nW/cm2-sr. By choosing conversion coefficient α = 1.65×10-10, Right panel in Figure 6 shows the converted DMSP-OLS measurement in radiance together with the radiance measurements from DNB w.r.t. lunar phase angle during 2012. We note that since the dynamic range of DN is limited for the events we have identified with lunar illumination, the conversion coefficient we have determined is valid for a limited range of DN, i.e. between 10 and 31.

4. SUMMARY In comparison with DMSP-OLS, VIIRS-DNB has broader radiometric measurement range, not subject to saturation at night side, more accurate radiometric calibration, finer spatial resolution, and better geometric quality. This enables us to calibrate DMSP-OLS sensor and quantify its DN into radiance. This paper demonstrates a scheme to use vicarious calibration under moon light to calibrate night-time imager such as DMSP-OLS with both lunar irradiance model and radiometricly calibrated VIIRS-DNB data. Vicarious calibration is one of the important and increasingly accepted postlaunch calibration techniques to independently monitor and verify the instrument performance over time, identify instrument anomalies, and maintain consistent radiometric calibration between multiple satellite instruments. The use of lunar illumination to perform vicarious radiometric calibration extends such kind of calibration to night time imager. The conversion coefficient to convert DN1.5 of DMSP-OLS to radiance unit is determined to be ~ 1.65 ×10-10 and matches the DNB observation the best. Calibrating the nighttime light data collected by the DMSP-OLS sensors into radiance unit can enable applications of using both sensor data and advance the applications of night time imagery data. The calibrated long term DMSP-OLS observations can in turn assist the calibration of DNB through inter-satellite comparison. Further work will be performed to incorporate difference between the spectral response functions of DNB and OLS, incorporate the scan-angle dependence of DN of DMSP-OLS, investigate the offset in DMSP-OLS data, extend the analysis to other period for Dome- C, and improve the determination of conversion factor for DMSP-OLS.

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