=============================================================================== Global Radiance Calibrated Nighttime Lights Product Readme =============================================================================== Distrubutor: Earth Observation Group, Payne Institute for Public Policy, Colorado School of Mines Author: Feng-Chi, Hsu (fengchihsu@mines.edu) Contact: Dr. Chris D. Elvidge (celvidge@mines.edu) Date: 01/15/2015 Modified: 09/09/2020 1. PURPOSE This product is aimed to solve the "saturation" problem which heppens in ordinary DMSP-OLS nighttime light product. DMSP-OLS has a very limited dynamic range, thus could not accomodate bright sources (i.e. city centers) within its dynamic range, Those area become highest value in the dataand all details lost, a.k.a saturated. 2. METHODOLOGY 2-1. MergingFixed-Gain Images and Blend Stable Light The radiance calibrated product fixes the gain setting of DMSP satellite into 3 stages to record nighttime lights of different strength of brightess. Ususally thehighest gain is about 100 times more sensitive than the lowest. These special images are called "fixed-gain images". By merging images taken under each gain setting, extra large dynamic range that can fully accomodate bright city lights is achieved. These fixed-gain images are further blended with operational stable light product to recover more detail in dim area. In order to matchch the operational stable light, which is taken by sensors set at variable but highest level of gain, the merging process of fixed-gain images takes the image with highest gain setting as reference and apply weighting factors to other images. Note that data from different satellites are being merged and blended into final product in order to gain maximum coverage. The sensor sensitivity difference is addressed in following section. (See Table 1. for date ranges of data used in each product.) 2-2. Inter-Satellite Calibration DMSP-OLS sensers on each satellite have different sensitivity. This difference needs to be addressed and calibrated when merging and blending data from different satellites into a single product. Although gain setting when acquiring data for operational stable light is not recorded, it is assumed to be gain 55. Moreover, the sensor difference between each product is also calibrated with the same approach with "F16 gain 55" as reference. The multipliers for inter-satellite calibration is calculated from pre-fllight sensor calibration of DMSP-OLS and shown in Table 2. 3. NAMING CONVENTION The file names of radiance calibrated products are defined as: Fxx_yyyymmdd-yyyymmdd_rad_v4.[avg_vis|count(cf_cvg)] Fxx : Satellite name, Fxx-Fyy for multiple satellites. yyyymmdd-yyyymmdd: Earliest date and latest date of fixed-gain data that was used. rad_v4 : Product identifire. Radiance calibrated produce version 4. avg_vis : Averaged visible light. count : Count of times that data went into the final product, (cf_cvg) including fixed gain images and operational stable light product. (cf_cvg stands for cloud free coverage) 4. SPATIAL SPECIFICATION Datum : WGS-84 Projection : Geographic Latitude/Longitude (Platte Carree Projection) Resolution : 30 arc seconds Boundary : 75N/65S/180E/180W 5. KNOWN ISSUE 5-1. Unit The radiance calibrated product is deemed to be unitless. The fundamental reason is the lacking of on-board calibration system for all DMSP-OLS. Therefore even though data used in each product is calibrated to the same reference, it is still not enough to address the degradation of sensors over time. Moreover, the uncertainty of operational stable light products which were taken under variable gain settings make it unlikely to derive a unit for radiance calibrated product. As a result, it is suggested that the radiance calibrated product is only suitable for analysis which does not require actual radiance. The conversion factor from values in radiance calibrated product to radiance is listed in Table 2 for reference. Note all products are inter-satellite calibrated to "F16 gain 55". 5-2. Inter-annual Calibration To perform comparative study between radiance calibrated products, users need to apply inter-annual calibration. The NGDC version of coefficient is shown in Table 3 (Read the note!). 5-3. Dual Version of 2010 Product There are two versions of 2010 product. Namely F16_20100111-20101209, and F16_20100111-20110731. The first one takes only observations done in year 2010, while the second also takes some sparce observation from 2011. Users prefer "pure" 2010 data should use the first one, while those prefer better visual presentation might want to use the second. Their detailed date coverage is shown in Table. 1. 6. TABLES ------------------------------------------------------------------------------- Table 1. Date Ranges for Data Used in Radiance Calibrated Product *OP: Operational stable light product (b): Base gain for sensitivity weighting reference. F12_19960316-19970212_rad_v4 Gain Satellite/Period 24 F12/19960316-19960324 F12/19970105-19970212 40 F12/19960316-19960324 (b) 50 F12/19970105-19970212 OP* F12/1997 F12_19990119-19991211_rad_v4 Gain Satellite/Period 15 F12/19990119-19991211 35 F12/19990119-19991211 (b) 55 F12/19990119-19991211 OP F14/1999 F12-F15_20000103-20001229_rad_v4 Gain Satellite/Period 15 F12/20000103-20001229 F15/20000303-20000508 35 F12/20000103-20001229 F15/20000303-20000508 (b) 55 F12/20000103-20001229 F12/20000303-20000508 OP F15/2000 F14-F15_20021230-20031127_rad_v4 Gain Satellite/Period 15 F14/20021230-20031127 F15/20021230-20030205 35 F14/20021230-20031127 F15/20021230-20030205 (b) 55 F14/20021230-20031127 F15/20021230-20030205 OP F15/2003 F14_20040118-20041216_rad_v4 Gain Satellite/Period 15 F14/20041118-20041216 35 F14/20041118-20041216 (b) 55 F14/20041118-20041216 OP F15/2004 F16_20051128-20061224_rad_v4 Gain Satellite/Period 15 F16/20051128-20061224 35 F16/20051128-20061224 (b) 55 F16/20051128-20061224 OP F16/2006 F16_20100111-20101209_rad_v4 Gain Satellite/Period 15 F16/20100111-20101209 35 F16/20100111-20101209 (b) 55 F16/20100111-20101209 OP F18/2010 F16_20100111-20110731_rad_v4 Gain Satellite/Period 15 F16/20100111-20101209 F16/20110129-20110731 35 F16/20100111-20101209 F16/20110129-20110731 (b) 55 F16/20100111-20101209 F16/20110129-20110731 OP F18/2010 ------------------------------------------------------------------------------- Table 2. Inter-satellite Calibration Multiplier Satellite Base_gain Multiplier Radiance@DN1 (w/cm2/sr) F12 50 1.71 2.56e-10 55 0.96 1.44e-10 F14 55 0.82 1.23e-10 F15 55 0.90 1.35e-10 F16 50 1.77 2.66e-10 55 1.00 1.50e-10 ------------------------------------------------------------------------------- Table 3. Inter-annual Calibration Coefficients NOTE: (1) This table is preliminary, and needs to be updated. (2) There is no data for "F16_20051128-20061224_rad_v4" for it being the reference dataset. (3) F16_20100111-20101209_rad_v4 and F16_20100111-20110731_rad_v4 share the same data for there is onyl small diferrence from adding images from year 2011. Equation Y=Coeff0+Coeff1*X Y-File F16_20051128-20061224_rad_v4 X-File Pow Coeff0 Coeff1 R2 N_Pt F12_19960316-19970212_rad_v4 1 4.336 0.915 0.971 20540 F12_19990119-19991211_rad_v4 1 1.423 0.780 0.980 20846 F12-F15_20000103-20001229_rad_v4 1 3.658 0.710 0.980 20866 F14-F15_20021230-20031127_rad_v4 1 3.736 0.797 0.980 20733 F14_20040118-20041216_rad_v4 1 1.062 0.761 0.984 20844 F16_20100111-20101209_rad_v4 1 2.196 1.195 0.981 20848 F16_20100111-20110731_rad_v4 1 -1.987 1.246 0.981 20848