The Intercalibration of the Night Lights Dataset

The NOAA National Geophysical Data Center (NGDC) has archived approximately 17 years of data from the Operational Linescan System (OLS) aboard the Defense Meteorological Satellite Program (DMSP) spanning from 1992 to the present flown on 5 different satellites. However, this extraordinary record of night lights lacks an onboard calibration system so the radiometric value of the instruments’ data numbers vary due to changes in orbital parameters, sensor degradation, and internal gain settings in addition to changes in signal strength. Without having all the information needed to calibrate the data numbers, definitive measurements of change have been elusive. We have modeled reflected moonlight from high-albedo locations (e.g. White Sands NM) to estimate the calibrated radiance the sensor experienced. By comparing the sensors’ reported uncalibrated radiance to the modeled received radiance we obtain an estimate of the sensors’ efficiency. Then each satellite and year can be calibrated based on the practically invariant geophysical properties of moonlight and desert albedo. After applying this calibration, the time series varies in a more predictable fashion with more agreement between co-incident observations than we were previously able to achieve. See Figure 1 for an example of prior intercalibration (Elvidge et al, 2009). Note that the prior method failed to converge on complete agreement between the observations and there are features in the time series that were probably introduced by an imperfect intercalibration procedure. This paper will present the intercalibration based on an improved methodology.