Robotic Lunar Observatory Model Research Articles

Evaluation of Nocturnal Aerosol Optical Depth Determining From a Lunar Photometry in Lanzhou of Northwest China

AbstractA Cimel Sun‐sky‐lunar photometer (CE318‐T) is designed to perform daytime and nighttime photometric measurements and calculate diurnal cycle of aerosol optical depth (AOD). Nevertheless, the determination of nocturnal AOD from CE318‐T requires a precise knowledge of extraterrestrial lunar irradiance, which significantly changes with moon's phase angle (MPA) and lunar libration in a single night. This study evaluated the 1‐year nocturnal AODs at Lanzhou by using three different methods, which were validated by collocated measurements of DIAL Lidar (as a reference) and Cimel software (as a proxy). The results indicated that three independent approaches could implement a similar performance to compute nocturnal AOD near full moon phase (i.e., MPA = 0°) under moderate aerosol loading. The spectral AOD values at nighttime calculated by ROLO Lunar Langley (Robotic Lunar Observatory model) and Sun‐moon Gain Factor (SMGF) methods are significantly underestimated under low moon's illumination or high MPA (MPA < −47° or MPA > 47°) and distinctly dependent on MPA. The RIMO correction factor (RCF) (ROLO Implementation for Moon photometry Observation correction factor) method could compensate the underestimated extraterrestrial lunar irradiances of ROLO model for about 6.76%–9.78%, and thus greatly improve the calculation accuracy of nighttime AOD. The day/night coherence transition test has demonstrated that we would obtain a good diurnal variation of AODs at Lanzhou after RCF correction. The overall averages of nocturnal AOD440nm differences between Cimel software and ROLO model and SMGF method are 0.064 ± 0.024 and 0.052 ± 0.022, respectively, while the corresponding difference of RCF is less than 0.021 ± 0.014 for all wavelengths, falling within uncertainty range of AERONET AOD products (∼±0.02). The diurnal variations of AODs determined from RCF method agree well with synchronous results of DIAL Lidar, with total mean AOD532nm differences of 0.038 ± 0.024 and 0.023 ± 0.017 in daytime and nighttime, respectively. The spectral AODs computed from RCF method are well consistent with Cimel software, although there are some discrepancies under low AOD cases (AOD440nm < 0.30), and the overall average of AOD440nm differences are less than −0.0053 ± 0.002 and −0.0185 ± 0.013 in daytime and nighttime, respectively. Our results confirmed that the CE318‐T photometer can be reasonably calculated nighttime AOD and Ångström exponent (AE440–870nm) at urban Lanzhou by using three independent methods, although the former two need to be greatly improved under low moon's illumination. The RCF method was proved to reliably calculate nocturnal AOD from moonlight irradiance, which didn't rely on MPA. A more accurate lunar irradiance model needs to be developed to improve the underestimation of current ROLO model. Long‐term climatological information of nocturnal AOD is crucial for comprehensively characterizing the diurnal variations of aerosol optical properties and atmospheric boundary layer structure during the winter at typical northern city of China, which deserves to be further investigated in the future.

LIME: Lunar Irradiance Model of ESA, a new tool for absolute radiometric calibration using the Moon

Abstract. Absolute calibration of Earth observation (EO) sensors is key to ensuring long-term stability and interoperability, and it is essential for long-term global climate records and forecasts. The Moon provides a photometrically stable calibration source within the range of the Earth's radiometric levels and is free from atmospheric interference. However, to use this ideal calibration source, one must model the variation in its disc-integrated irradiance resulting from changes in Sun–Earth–Moon geometries. The Lunar Irradiance Model of the European Space Agency (LIME) is a new lunar irradiance model developed from ground-based observations acquired using a lunar radiometer operating from the Izaña Atmospheric Observatory near Mount Teide, located in Tenerife, Spain. Nightly top-of-atmosphere (TOA) irradiance is determined using the Langley plot method, and each observation is traceable to the international system of units (SI) through the radiometer calibration performed at the National Physical Laboratory (NPL). Approximately 590 lunar observations acquired between March 2018 and December 2022 currently contribute to the model parameter derivation, which builds on the widely used ROLO (Robotic Lunar Observatory) model analytical formulation. This paper presents the strategy used to derive LIME parameters: the characterisation of the lunar radiometer, the derivation of nightly top-of-atmosphere lunar irradiance and a description of the model parameter derivation, along with the associated metrologically rigorous uncertainty. The model output has been compared to PROBA-V, Pléiades and Sentinel-3B, as well as to the VITO implementation of the ROLO model. Initial results indicate that LIME predicts 3 %–5 % higher lunar-disc-integrated irradiance than the ROLO model for the visible and near-infrared channels. The model output has an expanded (k=2) radiometric uncertainty of ∼ 2 % at the lunar radiometer wavelengths, and it is expected that planned observations until at least 2024 further constrain the model parameters in subsequent updates.

MRRT: Modeling of Multiple Reflections of Radiation Between Terrains Applied on the Lunar Surface Region

The lunar surface has a stable luminosity. To use the Moon as a calibration standard, the Robotic Lunar Observatory (ROLO) program models the integration of the radiance of the entire lunar surface. However, the albedos of the mare and the highlands are very different. The modeling based on the lunar global irradiance/reflected radiance is bound to result in higher uncertainty. In contrast, if the local calibration of the lunar surface is adopted, the lunar complex topography effect cannot be ignored. This paper presents a new model for quantifying multiple reflections of radiation between terrains (MRRT). The relationship between the bidirectional reflectance factor (BRF) of the observed pixel and the true microtopography reflectance is established, which shows that the BRF is mainly influenced by the true topography reflectance, the terrain undulation, the incident irradiance on the topography surface, and the masking in the observation direction. The new model applied on the lunar surface obtains clearer terrain details. The inversion reflectance of the Chang’e-3 landing area is closer to the reflectance measured in situ, and the reflectance curves of the Apollo 16 landing area are almost consistent under different illumination observation geometries. This shows that the MRRT model can effectively eliminate the topographic effect. Compared with the ROLO model, the MRRT model does not restrict the specific selection, so it can select a region with a uniform material distribution, small albedo difference, and low topography undulation to establish the lunar surface radiometric calibration field with the advantage of providing stable radiation characteristics.

Improved Lunar Irradiance Model Using Multiyear MODIS Lunar Observations

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the Terra and Aqua spacecrafts were launched on December 18, 1999, and May 4, 2002, respectively. One of the features of the MODIS instruments is the ability to perform observations of the lunar surface from its space view (SV) port. This event is scheduled approximately once a month via a spacecraft roll maneuver, which enables the lunar phase to be confined to within 1° for each instrument. The Moon is considered to be an extremely stable reference to monitor the long-term radiometric stability of the reflective solar bands (RSB). Each MODIS instrument can also view the Moon for about four months in a year without a roll maneuver. This is caused by the intrusion of the Moon in the SV. The lunar phase angles of these unscheduled lunar observations are distributed over a wide range varying from approximately 50° to 80° for Terra MODIS and from about -80° to -50° for Aqua MODIS, where the positive phase angle refers to a waning Moon, while the negative phase angle corresponds to a waxing Moon. Together, the scheduled and unscheduled lunar observations are used to monitor the long-term radiometric stability of the RSB. Of the several challenges involved in the modeling of the lunar optical properties, such as its absolute brightness, a number of optical and view geometry effects need to be considered. These effects are much easier to characterize for the scheduled observations due to confinement of the lunar phase angles compared to those for the unscheduled intrusions of the Moon in the SV. Nevertheless, it is still a challenge to remove the view geometry effect in the calibration coefficients derived from the scheduled lunar observations and even more challenging for the unscheduled lunar intrusions. In this work, the lunar absolute irradiance is modeled using known attributes and from the lunar observations by the two MODIS instruments from the time period between the years 2005 and 2012. The model developed here attempts to mitigate for the deficiencies in the lunar irradiance measurements by the RObotic Lunar Observatory (ROLO) model, developed by the United States Geological Survey. Overall, the relative uncertainty of the ROLO model for MODIS calibration has been assessed to be about 4% for the shortest wavelength (a center wavelength of 412 nm) in the phase angle range mentioned above. With our new established lunar model, the calibration coefficients derived from the lunar observations, especially those from the unscheduled lunar observations, for the RSB of the two MODIS instruments are significantly improved for the entire mission. A good agreement is observed between the calibration coefficients derived from the scheduled and unscheduled lunar observations. Both the absolute uncertainty of the new lunar irradiance model and its relative uncertainty due to view geometry variation are much smaller than those of the current ROLO model which has been widely used for most remote sensors' lunar calibrations. Our newly developed lunar irradiance model can be applied to other remote sensors for their lunar calibrations as well. Finally, the significant improvement in the measurement of the lunar irradiance led to a polarization effect in the Moon response for MODIS to be identified. In this article, the impact of the polarization of the moonlight for the MODIS RSB is quantified. This is extremely vital as polarization effect for remote sensors such as MODIS and the follow-on suite of the Joint Polar Satellite System Visible Infrared Imaging Radiometer Suite has been found to significantly increase the calibration uncertainty, especially at short wavelengths.

Comparison of the Lunar Models Using the Hyper-Spectral Imager Observations in Lijiang, China

A lunar observation campaign was conducted using a hyper-spectral imaging spectrometer in Lijiang, China from December 2015 to February 2016. The lunar hyper-spectral images in the visible to near-infrared region (VNIR) have been obtained in different lunar phases with absolute scale established by the National Institute of Metrology (NIM), China using the lamp–plate calibration system. At the same time, the aerosol optical depth (AOD) is measured regularly by a lidar and a lunar CE318U for atmospheric characterization to provide nightly atmosphere extinction correction of lunar observations. This paper addressed the complicated data processing procedure in detail from raw images of the spectrometer into the spectral lunar irradiance in different lunar phases. The result of measurement shows that the imaging spectrometer can provide lunar irradiance with uncertainties less than 3.30% except for absorption bands. Except for strong atmosphere absorption region, the mean spectral irradiance difference between the measured irradiance and the ROLO (Robotic Lunar Observatory) model is 8.6 ± 2% over the course of the lunar observation mission. The ROLO model performs more reliable to clarify absolute and relative accuracy of lunar irradiance than that of the MT2009 model in different Sun–Moon–Earth geometry. The spectral ratio analysis of lunar irradiance shows that band-to-band variability in the ROLO model is consistent within 2%, and the consistency of the models in the lunar phase and spectrum is well analyzed and evaluated from phase dependence and phase reddening analysis respectively.

Radiometric Model and Inter-Comparison Results of the SGLI-VNR On-Board Calibration

The Second Generation Global Imager (SGLI) on Global Change Observation Mission–Climate (GCOM-C) satellite empowers surface and atmospheric measurements related to the carbon cycle and radiation budget, with two radiometers of Visible and Near Infrared Radiometer (SGLI-VNR) and Infrared Scanning Radiometer (SGLI-IRS) that perform a wide-band (380 nm–12 µm) optical observation not only with as wide as a 1150–1400 km field of view (FOV), but also with as high as 0.25–0.5 km resolution. Additionally, polarization and along-track slant view observations are quite characteristic of SGLI. It is important to calibrate radiometers to provide the sensor data records for more than 28 standard products and 23 research products including clouds, aerosols, ocean color, vegetation, snow and ice, and other applications. In this paper, the radiometric model and the first results of on-board calibrations on the SGLI-VNR, which include weekly solar and light-emitting diode (LED) calibration and monthly lunar calibration, will be described. Each calibration data was obtained with corrections, where beta angle correction and avoidance of reflection from multilayer insulation (MLI) were applied for solar calibration; LED temperature correction was performed for LED calibration; and the GIRO (GSICS (Global Space-based Inter-Calibration System) Implementation of the ROLO (RObotic Lunar Observatory) model) model was used for lunar calibration. Results show that the inter-comparison of the relative degradation amount between these three calibrations agreed to within 1% or less.

Nocturnal aerosol optical depth measurements with modified sky radiometer POM-02 using the moon as a light source

Abstract. The majority of aerosol data are obtained from daytime measurements, and there are few datasets available for studying nighttime aerosol characteristics. In order to estimate the aerosol optical depth (AOD) and the precipitable water vapor (PWV) during the nighttime using the moon as a light source, a sky radiometer (POM-02, Prede Ltd., Japan) was modified. The amplifier was adjusted so that POM-02 could measure lower levels of input irradiance. In order to track the moon based on the calculated values, a simplified formula was incorporated into the firmware. A new position sensor with a four-quadrant detector to adjust the tracking of the Sun and moon was also developed. The calibration constant, which is the sensor output for the extraterrestrial solar and lunar irradiance at the mean Earth–Sun distance, was determined by using the Langley method. The measurements for the Langley calibration were conducted at the National Oceanic and Atmospheric Administration/Mauna Loa Observatory (NOAA/MLO) from 28 September 2017 to 7 November 2017. By assuming that the correct reflectance is proportional to the reflectance estimated by the Robotic Lunar Observatory (ROLO) irradiance model, the calibration constant for the lunar direct irradiance was successfully determined using the Langley method. The ratio of the calibration constant for the moon to that of the Sun was often greater than 1; the value of the ratio was 0.95 to 1.18 in the visible and near-infrared wavelength regions. This indicates that the ROLO model often underestimates the reflectance. In addition, this ratio depended on the phase angle. In this study, this ratio was approximated by a quadratic equation of the phase angle. By using this approximation, the reflectance of the moon can be calculated to within an accuracy of 1 % or less. In order to validate the estimates of the AOD and PWV, continuous measurements with POM-02 were conducted at the Japan Meteorological Agency/Meteorological Research Institute (JMA/MRI) from January 2018 to May 2018, and the AOD and PWV were estimated. The results were compared with the AOD and PWV obtained by independent methods. The AOD was compared with that estimated by the National Institute for Environmental Studies (NIES) High Spectral Resolution Lidar measurements (wavelength: 532 nm), and the PWV was compared with the PWV obtained from a radiosonde and the Global Positioning System. In addition, the continuity of the AOD (PWV) before and after sunrise and sunset in Tsukuba was examined, and the AOD (PWV) of AERONET and that of POM-02 at MLO were compared. In the results, the daytime and nighttime AOD (PWV) measurements are shown to be statistically almost equivalent. The AODs (PWVs) during the daytime and nighttime for POM-02 are presumed to have the same degree of precision and accuracy within the measurement uncertainty.

Radiometric Cross-Calibration for Multiple Sensors with the Moon as an Intermediate Reference

The instrument cross-calibration is an effective way to assess the quality of satellite data. In this study, a new me-thod is proposed to cross-calibrate the sensors among satellite instruments by using a RObotic Lunar Observatory (ROLO) model and Apollo sample reflectance in reflective solar bands (RSBs). The ROLO model acts as a transfer radiometer to bridge between the instruments. The reflective spectrum of the Apollo sample is used to compensate for the difference in the instrument’s relative spectral responses (RSRs). In addition, the double ratio between the observed lunar irradiance and the simulated lunar irradiance is used to reduce the difference in instrument lunar viewing and illumining geometry. This approach is applied to the Moderate Resolution Imaging Spectroradiometer (MODIS), the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), and the Advanced Land Imager (ALI) on board three satellites, respectively. The mean difference between MODIS and SeaWiFS is less than 3.14%, and the difference between MODIS and ALI is less than 4.75%. These results indicate that the proposed cross-calibration method not only compensates for the RSR mismatches but also reduces the differences in lunar observation geometry. Thus, radiance calibration of any satellite instrument can be validated with a reference instrument bridged by the moon.

On-Orbit Radiance Calibration of Nighttime Sensor of LuoJia1-01 Satellite Based on Lunar Observations

The high-resolution nighttime light (NTL) data of the LuoJia1-01 NTL remote sensing satellite has enriched the available data of NTL remote sensing applications. The radiance calibration used as a reference to convert the digital number (DN) recorded by the nighttime sensor into the radiance of the corresponding ground object is the basic premise to the effective application of the NTL data. Owing to the lack of on-board calibration equipment and the absence of an absolute radiometric calibration light source at night, it is difficult for LuoJia1-01 to carry out on-orbit radiance calibration. The moon, as an exoatmospheric stable radiation source, is widely used for the radiometric calibration of remote sensing satellite sensors and to monitor the stability of the visible and near-infrared sensors. This study, based on lunar observation of the LuoJia1-01 NTL sensor, focused on on-orbit radiometric calibration and included monitoring changes in the nighttime sensor radiometric response for nearly a year by using the Robotic Lunar Observatory (ROLO) lunar irradiance model (Version 311 g). The results showed that: (1) the consistency of the radiometric calibration results based on the ROLO model and the laboratory calibration results of LuoJia1-01 exceeded 90%; (2) the nighttime sensor of LuoJia1-01 radiometric response underwent approximately 6% degradation during the observation period of nearly one year (353 days).

COMS Visible Channel Calibration Using Moon Observation Data

COMS (Communication, Ocean, and Meteorological Satellite) has been in operation since April 2011. The COMS MI (Meteorological Imager) has one visible and four infrared channels. Through the use of the GSICS (Global Space-based Inter-Calibration System), the KMA (Korea Meteorological Administration) has monitored the radiometric performance of the COMS visible channel through its own calibration system and the use of four different earth targets: ocean, desert, water cloud, and DCC (Deep Convective Cloud). To use the invariant target of the Moon, the KMA adopted GIRO (GSICS Implementation of the ROLO (Robotic Lunar Observatory) model), which was released by EUMETSAT. In this study, a total of 146 lunar observation data (Level 1A) with various phase angles obtained between April 2011 and December 2017 are analyzed. A degradation rate of approximately −1.52 ± 0.10% per year occurred using the GIRO method, which was a degradation of approximately 12% over the eight years (from 2011 to 2018) during COMS MI’s operational period. The lunar calibration result was in good agreement with results provided by other calibration methods such as the DCC using the RTM (Radiative Transfer Model), which was approximately −1.43 ± 0.13% per year. Selection of on-Moon pixels provided an uncertainty of approximately 0.01% per year, while that related to the phase angle (Sun-Moon-Satellite) was approximately 0.06% per year.

A novel hyperspectral lunar irradiance model based on ROLO and mean equigonal albedo

Lunar observations are an attractive candidate for radiometric calibration of satellites in reflective solar bands (RSBs). However, our inadequate knowledge of the lunar surface’s reflectance restricts the use of lunar as a standard for calibration. The uncertainty of a well-known lunar irradiance model for calibration, the RObotic Lunar Observatory (ROLO) model, is 5–10%. One of the uncertainty sources is due to the fact that the model is based on samples from the Apollo missions. To remedy this problem, we develop a novel method to improve the current ROLO lunar hyper-spectral irradiance model, which uses a mean equigonal albedo to replace the reflectance of Apollo soil samples. It is demonstrated that the 2009 Miller-Turner (MT2009) model predicts values, at wavelengths of 350–2550nm, that are larger than the new model. Further, the irradiance of the new model is closer to that of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) than the ROLO model in the visible and near-infrared (VNIR). The mean discrepancy between MODIS measured irradiance and the ROLO values is 7.49%, between MODIS measured and the new model is 4.20%; between SeaWiFS measured values and the ROLO model is 3.93%; and between SeaWiFS measured and the new model is 2.90%.

Cross calibration of SeaWiFS and MODIS using on-orbit observations of the Moon

Observations of the Moon provide a primary technique for the on-orbit cross calibration of Earth remote sensing instruments. Monthly lunar observations are major components of the on-orbit calibration strategies of SeaWiFS and MODIS. SeaWiFS has collected more than 132 low phase angle and 59 high phase angle lunar observations over 12 years, Terra MODIS has collected more than 82 scheduled and 297 unscheduled lunar observations over nine years, and Aqua MODIS has collected more than 61 scheduled and 171 unscheduled lunar observations over seven years. The NASA Ocean Biology Processing Group Calibration and Validation Team and the NASA MODIS Characterization Support Team use the USGS RObotic Lunar Observatory (ROLO) photometric model of the Moon to compare these time series of lunar observations over time and varying observing geometries. The cross-calibration results show that Terra MODIS and Aqua MODIS agree, band to band, at the 1%-3% level, while SeaWiFS and either MODIS instrument agree at the 3%-8% level. The combined uncertainties of these comparisons are 1.3% for Terra and Aqua MODIS, 1.4% for SeaWiFS and Terra MODIS, and 1.3% for SeaWiFS and Aqua MODIS. Any residual phase dependence in the ROLO model, based on these observations, is less than 1.7% over the phase angle range of -80° to -6° and +5° to +82°. The lunar cross calibration of SeaWiFS, Terra MODIS, and Aqua MODIS is consistent with the vicarious calibration of ocean color products for these instruments, with the vicarious gains mitigating the calibration biases for the ocean color bands.