The Moon as a photometric calibration standard for microwave sensors

Abstract

Abstract. Instruments on satellites for Earth observation on polar orbits usually employ a two-point calibration technique, in which deep space and an onboard calibration target provide two reference flux levels. As the direction of the deep-space view is in general close to the celestial equator, the Moon sometimes moves through the field of view and introduces an unwelcome additional signal. One can take advantage of this intrusion, however, by using the Moon as a third flux standard, and this has actually been done for checking the lifetime stability of sensors operating at visible wavelengths. As the disk-integrated thermal emission of the Moon is less well known than its reflected sunlight, this concept can in the microwave range only be used for stability checks and intercalibration. An estimate of the frequency of appearances of the Moon in the deep-space view, a description of the limiting factors of the measurement accuracy and models of the Moon's brightness, and a discussion of the benefits from complementing the naturally occurring appearances of the Moon with dedicated spacecraft maneuvers show that it would be possible to detect photometric lifetime drifts of a few percent with just two measurements. The pointing accuracy is the most crucial factor for the value of this method. Planning such observations in advance would be particularly beneficial, because it allows observing the Moon at welldefined phase angles and putting it at the center of the field of view. A constant phase angle eliminates the need for a model of the Moon's brightness when checking the stability of an instrument. With increasing spatial resolution of future microwave sensors another question arises, viz. to what extent foreground emission from objects other than the Moon will contaminate the flux entering the deep-space view, which is supposed to originate exclusively in the cosmic microwave background. We conclude that even the brightest discreet sources have flux densities below the detection limit of microwave sensors in a single scan.

A Study of Lunar Microwave Radiation Based on Satellite Observations

“…For most climate applications, the detection frequency needs to be extended to as high as 183 GHz, making the observations from Chang'E less useful. In recent years, Yang et al [20,21] and Burgdorf et al [22,23] have explored a method of retrieving lunar-disk-integrated microwave T b s from the space-view observations of microwave-sounding instruments onboard weather satellites. The basic idea is that during lunar intrusion (LI) events, when the Moon appears in the satellite observation field of view, the effective microwave radiance of the Moon's disk, R e f f moon , can be derived from the receiver output counts difference between the clean space view and the space view with LI [20]:…”

“…Unlike ground-based observations, spaceborne microwave radiometers can cover a wide range of frequencies and experience less contamination from their surroundings. In the most recent studies by Yang et al [20,21] and Burgdorf et al [22,23], well-calibrated satellite observations were used to derive the lunar microwave T b spectrum from 23 GHz to 183 GHz. Their results show that the maximum lunar disk-integrated T b varies from 270 K at 23 GHz to 300 K at 183 GHz.…”

Disk-Integrated Lunar Brightness Temperatures between 89 and 190 GHz

“…Considering those intrusions, where the Moon gave the maximum signal in the DSV pixel with number n = 2 or 3, and identifying those instances, where it gave almost the same signal in the neighbouring DSV pixels n-1 and n+1, we obtained a collection of events, where the Moon came closer than 0.1 ∘ to the centre of DSV n. This way we made sure that the maximum signal was measured with The time is given in seconds after 17:23 UT, the time of the first scan in the raw data file (level 1b). The angles calculated with AAPP are not quite accurate, and this is why DSV 2 gives a slightly larger signal than DSV 4 [7].…”

“…The DSV points some 75 ∘ away from nadir-angle in (3)-and describes a circle in the sky during an orbit of the satellite. When the Moon touches this circle, it appears in the DSV [3]. reach its maximum, however, again at full Moon.…”

A study of a two-dimensional scanned lunar image for Advanced Technology Microwave Sounder (ATMS) geometric calibration

“…However, the possibility of using the Moon for geolocation validation and correction of microwave sensors has not been widely discussed. Attempts have been made to utilize the lunar intrusion data to assess the boresight pointing error of the Microwave Humidity Sounder (MHS) (Burgdorf et al, 2016) and ATMS (Zhou et al, 2017) and the mutual alignment of channels of MHS (Bonsignori, 2018). The advantage of using lunar intrusion data to assess the beam misalignment is that it will not interrupt the routine operations of the satellite.…”