Quantitative Analysis of VIIRS DNB Nightlight Point Source for Light Power Estimation and Stability Monitoring

Changyong Cao,Yan Bai

doi:10.3390/rs61211915

Abstract

The high sensitivity and advanced onboard calibration on the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB) enables accurate measurements of low light radiances which leads to enhanced quantitative applications at night. The finer spatial resolution of DNB also allows users to examine social economic activities at urban scales. Given the growing interest in the use of the DNB data, there is a pressing need for better understanding of the calibration stability and absolute accuracy of the DNB at low radiances. The low light calibration accuracy was previously estimated at a moderate 15% using extended sources while the long-term stability has yet to be characterized. There are also several science related questions to be answered, for example, how the Earth’s atmosphere and surface variability contribute to the stability of the DNB measured radiances; how to separate them from instrument calibration stability; whether or not SI (International System of Units) traceable active light sources can be designed and installed at selected sites to monitor the calibration stability, radiometric and geolocation accuracy, and point spread functions of the DNB; furthermore, whether or not such active light sources can be used for detecting environmental changes, such as aerosols. This paper explores the quantitative analysis of nightlight point sources, such as those from fishing vessels, bridges, and cities, using fundamental radiometry and radiative transfer, which would be useful for a number of applications including search and rescue in severe weather events, as well as calibration/validation of the DNB. Time series of the bridge light data are used to assess the stability of the light measurements and the calibration of VIIRS DNB. It was found that the light radiant power computed from the VIIRS DNB data matched relatively well with independent assessments based on the in situ light installations, although estimates have to be made due to limited ground truth data and lack of suitable radiative transfer models. Results from time series analysis are encouraging in potentially being able to detect anomalies in the DNB calibration. The study also suggests that accurate ground based active lights, when properly designed and installed, can be used to monitor the stability of the VIIRS DNB calibration at near the specified minimum radiances (3 nW∙cm−2∙sr−1), and potentially can be used to monitor the environmental changes as well.

Highlights

Among the twenty-two bands of the Visible Infrared Imaging Radiometer Suite) (VIIRS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite, the Day/Night Band (DNB) represents an unprecedented night observation capability
34 meters × 10.4 meters) is much smaller than one VIIRS DNB pixel, there are 24 pixels illuminated in the DNB for this vessel, with a Gaussian like distribution (Figure 5; Table 1), and a maximum pixel radiance value of 3.652 nW·cm−2·sr−1, which is at the level of specified minimum radiance for the VIIRS DNB instrument
This study demonstrates for the first time that the VIIRS DNB radiances from point sources such as those from fishing vessels, bridges, and city light bulbs at radiance levels near 3 nW·cm−2·sr−1 can potentially be used to estimate relatively accurately the electrical power used based first principle physics

Summary

Introduction

Among the twenty-two bands of the Visible Infrared Imaging Radiometer Suite) (VIIRS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite, the Day/Night Band (DNB) represents an unprecedented night observation capability. The onboard calibration uses the solar diffuser to determine the low gain, which is transferred to the mid and high gain stages using gain ratios determined near the terminator transient zones where all three gain stages have valid values. The point source needs to be convolved with the spatial response of the VIIRS DNB or the line spread function (LSF), both along- and cross-track. In this study we only consider the near nadir observations in aggregation zone 1, which has an approximately rectangular cross-track LSF (Figure 3) [9]. It is known that for the low light measurements (in high gain stage), the VIIRS DNB design is rather sophisticated, consisting of two redundant Charge-Coupled Device (CCD) arrays, each with 250 × 672 subpixles. We focus on the DNB measurements at the pixel-level near nadir without involving discussions of the subpixel or CCD level [10]

Methods

Results

Conclusion