H. K. Vedantham et al., "Lunar occultation of the diffuse radio
sky: LOFAR measurements between 35 and 80 MHz," Monthly Notices of
the Royal Astronomical Society, V450, no. 3, pp2291-2305, July
2015.
abstract:
We present radio observations of the Moon between 35 and 80 MHz to
demonstrate a novel technique of interferometrically measuring
large-scale diffuse emission extending far beyond the primary beam
(global signal) for the first time. In particular, we show that (i)
the Moon appears as a negative-flux source at frequencies 35 < nu < 80
MHz since it is "colder" than the diffuse Galactic background it
occults, (ii) using the (negative) flux of the lunar disc, we can
reconstruct the spectrum of the diffuse Galactic emission with the
lunar thermal emission as a reference, and (iii) that reflected RFI
(radio-frequency interference) is concentrated at the centre of the
lunar disc due to specular nature of reflection, and can be
independently measured. Our RFI measurements show that (i) Moon-based
Cosmic Dawn experiments must design for an Earth-isolation of better
than 80 dB to achieve an RFI temperature <1 mK, (ii) Moon-reflected
RFI contributes to a dipole temperature less than 20 mK for
Earth-based Cosmic Dawn experiments, (iii) man-made
satellite-reflected RFI temperature exceeds 20 mK if the aggregate
scattering cross-section of visible satellites exceeds 175
m2 at 800 km height, or 15 m2 at 400 km
height. Currently, our diffuse background spectrum is limited by
sidelobe confusion on short baselines (10-15 per cent level). Further
refinement of our technique may yield constraints on the redshifted
global 21 cm signal from Cosmic Dawn (40 > z > 12) and the Epoch of
Reionization (12 > z > 5).
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