## 1 Introduction
Recent CubeSat ERB missions, such as radiometer assessment using vertically aligned nanotubes (RAVAN) [12] and ultraviolet and infrared sensors at high quantum efficiency onboard a small satellite (UVSQ-SAT) [13], have the potential to improve the temporal coverage of OLR and OSR observations with a constellation of substantial numbers of low-cost compact satellites. Similarly, a MERO would also provide an opportunity to measure the Earth's OLR and OSR variations at a small temporal scale. A MERO could continuously observe a certain region 12 h per day on average, thus producing near-real-time TOA OLR and OSR measurement every 15 min under the state of the art (15 min is the sampling interval of the in-orbit GERB). Compared to the CubeSat ERB mission, a MERO could provide a simultaneous half-globe TOA OLR and OSR fluxes at an accepted spatial resolution; such data could help to reveal the regional and global radiation imbalance with accepted accuracy without the bias introduced by the non-simultaneous sampling of spatial and temporal variability of TOA OSR and OLR. The keys to limiting the temporal sampling coverage of the CubeSat mission and MERO are different. For CubeSat ERB missions, the primary factor to restrict the temporal sampling coverage is the number of satellites in the constellation. However, this challenges the maintenance and calibration of the system. Numerous small satellites need an effective constellation exit-and-replacement launch procedure; the calibration and inter-calibration of a constellation with such a small number of satellites are still a technique challenge [14]. For a MERO system, the major limit to the temporal sampling frequency is the state-of-art of the instrument, which is closely related to the detector response time, the thermal noise, the data storage and transmit capacity and other related systematic parameters. With the progressive implementation of the Chinese lunar exploration program (the Change-E missions), a MERO is likely to be deployed in the near future. A MERO system is designed as a non-scanner multi-pixel spectroradiometer with two bands: 0.2-4 \(\upmu\)m for the measurement of the Earth top of atmosphere (TOA) OSR flux and 4-100 \(\upmu\)m for the Earth TOA OLR flux. The typical observing scenario of a MERO system is shown in Figure 1. Nearly half of the globe is observed by a MERO sensor simultaneously. A detector unit of the focal plane array (FPA) just focuses on a certain region; OSR/OLR radiation coming from the detector-viewed region (pixel) on the Earth TOA (hollow green rectangle in Figure 1) passes through the MERO optical system, then arrives at the detector unit (solid green rectangle in Figure 1) of the FPA. Thereafter the radiation is transformed to an electrical signal by this detector unit. Subsequently, this electrical signal would be processed by the readout circuit and finally be outputted as the digital number (DN) value. However, there are several disadvantages of the MERO platform. The orbiting of the Moon around Earth would result in inconsistency in the number of samples at different latitudes on Earth. For example, the regions with latitude >70\({}^{\circ}\) or <-70\({}^{\circ}\) can only be viewed by a MERO over half of the Moon's orbital period, whereas other regions can be viewed for a whole orbital period. Due to the long Earth-Moon distance, the MERO signal is substantially less powerful than current low Earth orbit (LEO) and geostationary Earth orbit (GEO) satellites, which means that the MERO instrument should have much less noise than current satellite-based instrument to cope with the weak signal. Because of the weak magnetic field of the Moon, there will be a large number of high-energy particles on the Moon, which would affect the operation of the MERO instrument. Therefore, a robust protection system is crucially needed. As for the instrument design, it is necessary to determine the spatial resolution and temporal sampling scheme (sampling interval and sampling temporal sequence).