Abstract
The LuoJia1-01 satellite can acquire high-resolution, high-sensitivity nighttime light data for night remote sensing applications. LuoJia1-01 is equipped with a 4-megapixel CMOS sensor composed of 2048 × 2048 unique detectors that record weak nighttime light on Earth. Owing to minute variations in manufacturing and temporal degradation, each detector’s behavior varies when exposed to uniform radiance, resulting in noticeable detector-level errors in the acquired imagery. Radiometric calibration helps to eliminate these detector-level errors. For the nighttime sensor of LuoJia1-01, it is difficult to directly use the nighttime light data to calibrate the detector-level errors, because at night there is no large-area uniform light source. This paper reports an on-orbit radiometric calibration method that uses daytime data to estimate the relative calibration coefficients for each detector in the LuoJia1-01 nighttime sensor, and uses the calibrated data to correct nighttime data. The image sensor has a high dynamic range (HDR) mode, which is optimized for day/night imaging applications. An HDR image can be constructed using low- and high-gain HDR images captured in HDR mode. Hence, a day-to-night radiometric reference transfer model, which uses daytime uniform calibration to calibrate the detector non-uniformity of the nighttime sensor, is herein built for LuoJia1-01. Owing to the lack of calibration equipment on-board LuoJia1-01, the dark current of the nighttime sensor is calibrated by collecting no-light desert images at new moon. The results show that in HDR mode (1) the root mean square of mean for each detector in low-gain (high-gain) images is better than 0.04 (0.07) in digital number (DN) after dark current correction; (2) the DN relationship between low- and high-gain images conforms to the quadratic polynomial mode; (3) streaking metrics are better than 0.2% after relative calibration; and (4) the nighttime sensor has the same relative correction parameters at different exposure times for the same gain parameters.
Highlights
Nocturnal lighting is a primary method that enables to study human activity from space and is used extensively worldwide in residential, commercial, industrial, and public facilities and roadways [1]
The remote sensing of night-time lighting has been studied and shown to be economical and straightforward for applications [2] such as extracting, assessing, and monitoring urbanization dynamics [3,4,5], power consumption [6], gas flaring volume [7], CO2 emissions [8], material stocks [9], analyzing urban activities based on street lights [10,11,12,13,14], recognition of fishing boats based on light used for fishing [15], investigating artificial light pollution [16,17,18,19], armed conflicts [20] and mapping urban extents [21]
The difficulty in the on-orbit relative radiometric calibration of the nighttime sensor of LJ1-01 without a light calibration reference is overcome by constructing a day-night radiometric reference transfer model
Summary
Nocturnal lighting is a primary method that enables to study human activity from space and is used extensively worldwide in residential, commercial, industrial, and public facilities and roadways [1]. Satellite observation of the location and intensity of nocturnal lighting provides a unique view of humanity’s presence and can be used as a Sensors 2018, 18, 4225; doi:10.3390/s18124225 www.mdpi.com/journal/sensors. The remote sensing of night-time lighting has been studied and shown to be economical and straightforward for applications [2] such as extracting, assessing, and monitoring urbanization dynamics [3,4,5], power consumption [6], gas flaring volume [7], CO2 emissions [8], material stocks [9], analyzing urban activities based on street lights [10,11,12,13,14], recognition of fishing boats based on light used for fishing [15], investigating artificial light pollution [16,17,18,19], armed conflicts [20] and mapping urban extents [21].
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