Chang'e Microwave Radiometer (MRM) instrument

ODE:Moon

Mission: Chang'e

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The microwave radiometer (MRM) is a radiometer that measures energy emitted at one millimeter-to-metre wavelengths (frequencies of 0.3–300 GHz) known as microwaves. The Chang'e (CE) MRM channels fall primarily into the super high frequency (SHF) or "centimeter" band (the 37 GHZ channel technically edges into the extremely high frequency (EHF) band). A handful of other planetary science investigations have taken SHF data, including Juno MWR, Rosetta MIRO, and Cassini RADAR. However, only the CE-1 and CE-2 MRMs have systematically imaged a large body's surface in this band (not counting coarse observations by ground-based radiotelescopes).

SHF technologies are robust, cheap, and extremely mature; terrestrial applications range from aviation radar to short-range high-bandwidth communications (like WiFi) to ground-based weather radar to kitchen appliances. It is also extensively used in radioastronomy. Because water absorbs SHF very effectively, SHF radiometry has limited use in terrestrial geology. However, in the absence of water, passive SHF radiometers can essentially “see” into the top few meters of a body’s surface. Emissions in this band are principally due to blackbody radiation from variations in physical temperature, and because the depth of this “vision” is frequency-dependent (lower frequencies see deeper into the surface), a multiband radiometer can effectively measure subsurface temperature gradients. Effective measurement depth is affected by dielectric material properties as well as frequency, so regional variations in heating and cooling over a diurnal cycle can also give insight into compositional properties, particularly metallicity. See Siegler et al., 2020 and Siegler et al., 2023 for further discussion and examples of application.

The MRM has 4 channels, centered respectively at 3, 7.8, 19.35, and 37 GHz, with respective bandwidths of 100, 200, 500, and 500 MHz. Nominal radiometric resolution is 0.5 K for all channels. Angular full width at half maximum (FWHM) is ~13 degrees for channel 1 and ~10 degrees for the higher channels, although the main beams are platykurtic and slightly asymmetrical. The first side lobes are larger and highly asymmetric. Outer side lobes are not characterized in the literature. Please refer to Wang et al., 2010b for amore complete description of antenna patterns.

Orbital height varies significantly within each data set. CE-1 mean orbital height was ~185 km with a standard deviation of ~32 km; CE-2’s was ~100 km with a standard deviation of ~10 km. This means that surface footprint size is not constant within either data set. However, at mean CE-2 orbital height (~100 km), the diameter of the surface footprint of the main beam at 50% of maximum response (i.e. FWHM) is ~23 km for channel 1 and ~17 km for the higher channels; at 10%, diameter is ~40 km for channel 1 and ~25 km for the higher channels. At mean CE-1 orbital height (~185 km), FWHM is ~45 km for channel 1 and ~30 km for the higher channels; diameter at 10% is ~70 km for channel 1 and ~45 km for the higher channels. (All of these figures assume perfect nadir pointing.)

Various values for the MRM’s “spatial resolution” are quoted in the literature without derivation; they are usually close to FWHM at mean orbital height. Note, however, that it is not really meaningful to assign a single “spatial resolution” value to the MRM data. The effective resolution of the data set is much higher than the “resolution” of a sample considered in isolation, and is determined as much by sample density as by footprint size. Furthermore, both mean sample density and mean footprint size vary greatly between regions.

ODE supports the search, retrieval, and download of MRM calibrated and derived data products. Information about MRM products can be found in Primary Documentation for chang_e_microwave_processed.

 

Reference

  • Siegler, M.A., Feng, J., Lucey, P.G., Ghent, R.R., Hayne, P.O., White, M.N., 2020. Lunar Titanium and Frequency‐Dependent Microwave Loss Tangent as Constrained by the Chang’E‐2 MRM and LRO Diviner Lunar Radiometers. J. Geophys. Res. Planets 125, e2020JE006405. https://doi.org/10.1029/2020JE006405

  • Siegler, M.A., Feng, J., Lehman-Franco, K., Andrews-Hanna, J.C., Economos, R.C., Clair, M.St., Million, C., Head, J.W., Glotch, T.D., White, M.N., 2023. Remote detection of a lunar granitic batholith at Compton–Belkovich. Nature 620, 116–121. https://doi.org/10.1038/s41586-023-06183-5

  • Wang, Z., Li, Y., Zhang, X., JingShan, J., Xu, C., Zhang, D., Zhang, W., 2010b. Calibration and brightness temperature algorithm of CE-1 Lunar Microwave Sounder (CELMS). Sci. China Earth Sci. 53, 1392–1406. https://doi.org/10.1007/s11430-010-4008-x