Lunar Calibration

Introduction

Lunar radiation is highly stable in the microwave spectrum because of the stable geophysical properties of the Moon’s surface. Most space-borne microwave-sounding instruments can collect lunar radiation data from space-view observations during so-called lunar intrusion events that usually occur several days a month. For some miniaturized microwave instruments, a special Moon observation mode can be designed to enable lunar irradiance to be measured as an important microwave source for on-orbit calibration. For example, the NASA Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission is a follow-on constellation of six 3U cube satellites based on the Micro-sized Microwave Atmospheric Satellite 2 (μMAS2) design that is scheduled for launch in 2020. Each Space Vehicle (SV) includes a 12-channel scanning passive microwave radiometer in the W-band, F-band, and G-band. TROPICS mission dual-spinner configuration provides a unique opportunity for solar and lunar calibration, as solar and lunar intrusions are periodic and occur every orbit [1].

In the future, if an SI-traceable microwave source is available and can be transferred to an instrument like μMAS2, the Moon can therefore be used as an SI traceable calibration reference for microwave instruments to evaluate the calibration accuracy and assess the long-term calibration stability. This brief overview establishes a general frame work for microwave lunar calibration, based on lunar observations from current operational microwave radiometers. The method and results developed here could be made further improvement when more reliable and accurate observations are available from an advanced SI-traceable microwave radiometer in future.

Lunar Measurements

Major challenges of using the Moon as a radiometric standard source for microwave sensors include the reliability of measurements of the microwave brightness temperature (Tb) spectrum of the lunar surface, and knowledge of the lunar phase lag because of penetration depths at different detection frequencies. Microwave radiation can penetrate the lunar sub-surface, so microwave thermal emission in terms of Tb can be different from the skin temperature. Previous studies have shown that due to the special structure of the Moon’s surface and the penetration characteristics of microwave radiation, the maximum microwave emission from the Moon typically lags behind the maximum temperature when the Moon is full. The lag angle depends on the ratio of the physical thickness of the emission layer to the penetration depth [2]. Satellite-based lunar observations are valuable because of the high accuracy and stability of space-borne microwave instruments. Unlike ground-based observations, spaceborne microwave radiometers can cover a wide range of frequencies and experience less contamination from their surroundings.

General Description of Satellite Lunar Microwave Tb Retrieval Algorithms

When the Moon appears in the satellite observation field of view, the effective microwave radiance of the Moon’s disk, , can be derived from the receiver output counts difference between the clean space view and the space view with lunar intrustion (LI) [3], which is given in Equation 1 as follows:

where G is the instrument calibration gain, and is the difference between and the cosmic background microwave radiance , which is given below in Equation 2

.

For most weather satellites with an orbital altitude of ~830 km, the apparent angle of the lunar disk is ~0.55^o, which is much smaller than the antenna beam width. In this case, the effective lunar disk radiance can be expressed as a function of the antenna response , the normalized solid angle of the Moon , and the average disk-integrated lunar radiance as follows in Equation 3:

The antenna solid angle and antenna gain term on the right side of Equation 3 are instrument-related parameters and will not change over time after the instrument is in orbit.

Retrieval Results of the Lunar Microwave Tb Spectrum

Lunar Microwave Tb Spectrum from NOAA-20 ATMS

NOAA-20 was successfully launched on 18 November 2017, and is the follow-on satellite program of the Suomi National Polar-orbiting Partnership (SNPP) operated by NOAA. The Advanced Technology Microwave Sounder (ATMS) onboard NOAA-20 is a 22-channel passive microwave radiometer with frequencies ranging from 23 GHz to 183 GHz. Soon after launch, a spacecraft pitch maneuver operation was carried out, during which a perfect 2D lunar disk in a full-moon phase was captured by the ATMS deep-space scan. Based on well-calibrated lunar antenna temperature datasets, the disk-integrated lunar surface microwave Tb spectrum with frequencies from 23 to 183 GHz can be derived. Table 1 lists the microwave Tb retrieval results at the full-moon phase for each ATMS frequency. The retrieved Moon-disk-averaged Tb spectrum shows a strong frequency-dependent feature, i.e., increasing from 239 K at 23 GHz to 293 K at 183 GHz at the full-moon phase.

Table 1: Microwave Tb retrieval results at the full-moon phase for each ATMS frequency

Moon Phase Lag Angle Observed from the Drifting Satellite Orbits

For a sun-synchronous, polar-orbiting Earth satellite, the Right Ascension of the Ascending Node (RAAN), equivalent to the local time of the ascending node (LTAN), determines the orientation of its orbital plane. For most current cross-track-scanning microwave-sounding instruments, the scan plane of instrument is perpendicular to the orbital plane. When the orbital LTAN is kept stable, lunar radiation can only be collected within a narrow range of Moon phase angles. With the rotation of the orbital plane around its orbital axis, Moon phase angles collected in the LI events of the scanning instruments change consistently with the orbit LTAN.

Taking an example of AMSU-A onboard NOAA-18 satellite, the mean Moon phase angle captured at each LI event changed from around -80^o in 2005 to about 45^o in 2019. Lunar radiation data samples collected over a large range of Moon phase angles enables the study of phase-angle-dependent features of lunar radiation, and more importantly, increasing knowledge about Moon phase angle lags at microwave frequencies. To obtain information about Moon phase angle lags, all space-view data samples from different satellite orbits were collected. For each LI event, only those LI samples collected when the Moon passed through the antenna beam center were kept. Clean deep space view counts were then identified and used to calculate the instrument calibration gain and do the internal warm calibration. Table 2 lists the lunar phase lags at different frequencies from low to high derived from AMSU/MHS.

Table 2. Moon phase lag angles and the maximum lunar disk Tb at microwave frequencies.

Conclusions and Future Work

The microwave Tb and phase-angle-dependent features of the Moon’s disk are studied based on NOAA-20 ATMS and NOAA-16, -18, and -19 and Metop AMSU-A and AMSU-B/MHS Moon observations. Presented are retrievals of the Tb spectrum from 23 to 183 GHz. A strong frequency-dependent feature of the Moon’s surface Tb was found. By examining lunar radiation samples collected from the drifting orbits of NOAA/MetOp satellites, the phase lag feature of the Moon’s surface microwave Tb was also studied at different frequencies. In the most recent studies by Yang et al. [3,4] and Burgdorf et al. [5,6], well-calibrated satellite observations were used to derive the lunar microwave Tb spectrum from 23 GHz to 183 GHz. Their results show that the maximum lunar disk-integrated Tb varies from 239 K at 23 GHz to 293 K at 183 GHz. The phase lag also changes from 40^o at 23 GHz to 12^o at 183 GHz. Current work shows that based on accurate spaceborne microwave radiometer observations, taking the Moon’s disk as an SI traceable calibration reference is possible and promising by building up a reliable microwave lunar radiation model.

Future work will focus on studying the surface temperature conductivity and microwave emission properties of the Moon’s surface and developing a comprehensive, physically based microwave Tb model for microwave calibration.In the future, if an SI-traceable microwave source is available and can be transferred to instrument like MICROMAS, the Moon can therefore be used as an SI traceable calibration reference for microwave instruments to evaluate the calibration accuracy and assess the long-term calibration stability. The method and results developed here could have further improvements when more reliable and accurate observations are available from an advanced SI-traceable microwave radiometer in future.

Hu Yang, Cooperative Institute for Satellite Earth System Studies (CISESS), University of Maryland, 5825 University Research Court, Suite 4001, College Park, MD, USA, 20740-3823; huyang@umd.edu

Martin Burgdorf, Universität Hamburg, Faculty of Mathematics, Informatics and Natural Sciences, Department of Earth Sciences, Meteorological Institute, Bundesstraße 55, 20146 Hamburg, Germany; martin.burgdorf@uni-hamburg.de

Key Scientific Papers

1. Angela Crews ; Kerri Cahoy ; William Blackwell ; R. Vincent Leslie ; Michael Grant, 2019, Solar and Lunar Calibration for Miniaturized Microwave Radiometers, IEEE Aerospace Conference, DOI:10.1109/AERO.2019.8741641
2. Krotikov, V.D.; Troitskii, V.S. Radio emission and nature of the Moon. Sov. Phys. Uspekhi 1964, 6, 841–871.
3. Yang, H.; Weng, F. On-orbit ATMS lunar contamination corrections. IEEE Geosci. Remote S. 2016, 54, 1–7.
4. Yang, H.; Zhou, J.; Weng, F.; Sun, N.; Anderson, K.; Liu, Q.; Kim, E.J. Developing vicarious calibration for microwave sounding instruments by using lunar radiation. IEEE Geosci. Remote S. 2018, 56, 6723–6733.
5. Burgdorf, M.J.; Buehler, S.A.; Lang, T.; Michel, S.; Hans, I. The Moon as a photometric calibration standard for microwave sensors. Atmos. Meas. Tech. 2016, 9, 3467–3475.
6. Burgdorf, M.J.; Buehler, S.A.; Hans, I.; Prange, M. Disk-integrated lunar brightness temperatures between 89 and 190 GHz. Adv. Astron. 2019, 2019, doi:10.1155/2019/2350476.
7. Keihm, S.J.; Cutts, J.A. Vertical-structure effects on planetary microwave brightness temperature measurements: applications to the lunar regolith. Icarus 1981, 48, 201–229.
8. Keihm, S.J. Interpretation of the lunar microwave brightness temperature spectrum: feasibility of orbital heat flow mapping. Icarus 1984, 60, 568–589.
9. Bonsignori, R. In-orbit verification of microwave humidity sounder spectral channels coregistration using the moon. J. Appl. Remote Sensing 2018, 12, doi:10.1117/1.JRS.12.025013.
10. Yang, Hu, Ninghai Sun, Kent Anderson, Quanhua Liu, Ed Kim, 2018, “Developing vicarious calibration for microwave sounding instruments using lunar radiation”, IEEE Transactions on Geoscience and Remote Sensing, 56 (11), 6723-6733.

-- RobbieIacovazzi - 14 Apr 2020