Intrusions of the Moon in Deep Space View
Working Paper #00436 (Martin Burgdorf - February 2020)
Microwave sounders in polar orbits around the Earth employ a two-point calibration with the cold reference being deep space, i. e. the cosmic microwave background (CMB). The region of the sky chosen for this measurement is always close to the orbital axis of the satellite and more than 90° away from the Sun. As a consequence, the Moon appears occasionally in the deep space view (DSV) and increases the amount of radiation entering the instrument, thereby altering the flux from the cold reference. Hence models of the brightness temperature of the Moon were developed that make it possible to correct for its contribution in the calibration process (Mo & Kigawa, 2007, Yang & Weng 2016).
The light curve of a passage of the Moon through the DSV, however, contains also information about properties of the instrument in flight that is difficult to obtain otherwise.
2 Instrument Properties Relevant to Moon Intrusions
- Beam pattern: The shape of the light curve of the Moon moving through the DSV contains information about the beam pattern in the along-track direction. The height of the maxima of these light curves in the different DSVs contains information, albeit less accurate, about the beam pattern in the across-track direction.
- Pointing accuracy: AAPP (ATOVS and AVHRR Pre-processing Package) calculates the time of the closest approach between DSV and the Moon. Multiplying its difference to the time of the maximum of the light curve with the angular velocity of the DSV as it moves in the sky, one can get the pointing error for the DSV in the along-track direction (Bonsignori, 2018). From a comparison of the light curves from the different DSVs it is possible to derive the pointing error in the across-track direction as well.
- Photometric stability: The light curve of a Moon intrusion can mostly be fitted with a well-defined Gaussian, but uncertainties remain in comparing the signal strength from different events. The Moon will cross the DSV in different distances from the centre of the beam, and its brightness changes with phase angle and, to a much lesser extent, with the distance of the Moon from the Sun. It is therefore desirable to compare only intrusions with similar characteristics.
The light curve of a Moon intrusion in MHS follows very closely a Gaussian fit to the counts. In the figure below, the Gaussian is represented by a red line, and the measured counts in the DSV are represented by blue points for channel H1 of MHS. The light curve of some Moon intrusions, however, can show deviations from the Gaussian shape, because the antenna patterns are not quite symmetric  . This is more of a problem for MHS on METOP than on NOAA-satellites.
- The values for q 3dB given by MATRA MARCONI SPACE in the END ITEM DATA PACKAGE (MHS-DP-JA291-MMP) are too small by up to 0.2° for MHS on both NOAA-18 and NOAA-19. This error is twenty times the accuracy claimed for the ground tests and means non compliant, where compliance was declared (Burgdorf et al., 2019).
- The uncertainty in calculating the pointing direction of the DSV with AAPP is 0.3° according to the MHS Level 1 PGS. When a Moon intrusion happens in the DSV, however, it is possible to determine the pointing direction of the instrument in the along-track direction very accurately, namely from the exact moment in time, when the light curve reaches its maximum. When the signal from the Moon is the same in two different positions of the DSV (pixels), then one gets also a very accurate pointing direction in the across-track direction, because the Moon must be exactly in between these two pixels, and as a consequence the DSV position in the sky is known much more accurately than from AAPP.
- Channels 18 -20 of AMSU-B and H34 of MHS have the same central frequency, i. e. 183.31 GHz, and the same quasi-optical path. This means that these channels, on a given instrument, must measure the same radiance of the Moon, even when beam efficiency, size, etc. are only poorly known. This fact enables the direct measurement of biases between sounding channels in flight and is useful to check their relative stability (Burgdorf et al., 2018).
4 Suggested Future Tasks
Work will progress in the following steps:
- Make a list of Moon intrusions that are particularly useful for characterising instrument properties, which contains the fit parameters of the light curve and the exact position of the satellite.
- Confirm or correct the assumptions about beam pattern and pointing accuracy made until now by analysing Moon intrusions with high signal-to-noise ratio.
- Create an accurate model of the lunar radiance in the microwave range by combining the results from Moon intrusions observed with instruments on several satellites (AMSU-B and MHS). Such a model will be useful to check the photometric stability over long time periods, because the surface of the Moon does not change, and it will even enable inter-calibration of instruments that were operational at different times. Such measurements will be particularly sensitive with future instruments, whose smaller beams will be filled almost completely by the Moon.
- Extend this method to instruments operating in the thermal infrared.
- Bonsignori, R., “In-orbit verification of microwave humidity sounder spectral channels coregistration using the moon”, Journal of Applied Remote Sensing, 12 , 025013, doi: 10.1117/1.JRS.12.025013, 2018.
- Burgdorf, M., et al., “Inter-channel uniformity of a microwave sounder in space”, Atmospheric Measurement Techniques, 11, 4005-4014, doi: 10.5194/amt-11-4005-2018, 2018.
- Burgdorf, M., et al., “Disk-Integrated Lunar Brightness Temperatures between 89 and 190 GHz”, Advances in Astronomy, 2350476, doi: 10.1155/2019/2350476, 2019.
- Mo, T., & Kigawa, S., “A study of lunar contamination and on-orbit performance of the NOAA 18 Advanced Microwave Sounding Unit-A”, Journal of Geophysical Research, 112, D20124, doi: 10.1029/2007JD008765, 2007.
- Yang, H., & Weng, F., “Corrections for On-Orbit ATMS Lunar Contamination”, IEEE Transactions on Geoscience and Remote Sensing, 54, 1918-1924, doi: 10.1109/TGRS.2015.2490198, 2015
- Yang, Hu and Fuzhong Weng, 2016, “On-Orbit ATMS Lunar Contamination Corrections”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 54 Issue: 4, page(s): 1-7
_  https://www.eumetsat.int/website/home/Satellites/CurrentSatellites/Metop/MetopDesign/MHS/index.html_
- 14 Apr 2020