An Imaging Algorithm for a Lunar Orbit Interferometer Array

Abstract

Radio astronomical observation below 30 MHz is hampered by the refraction and absorption of the ionosphere, and the radio frequency interference (RFI), so far high angular resolution sky intensity map is not available. An interferometer array on lunar orbit provides a perfect observatory in this frequency band: it is out of the ionosphere and the Moon helps to block the RFIs from the Earth. The satellites can make observations on the far side of the Moon and then send back the data on the near side part of the orbit. However, for such array the traditional imaging algorithm is not applicable: the field of view is very wide (almost whole sky), and for baselines distributed on a plane, there is a mirror symmetry between the two sides of the plane. A further complication is that for each baseline, the Moon blocks part of the sky, but as the satellites orbit the Moon, both the direction of the baseline and the blocked sky change, so even imaging algorithms which can deal with non-coplanar baseline may not work in this case. Here we present an imaging algorithm based on solving the linear mapping equations relating the sky intensity to the visibilities. We show that the mirror symmetry can be broken by the three dimensional baseline distribution generated naturally by the precession of the orbital plane of the satellites. The algorithm is applicable and good maps could be reconstructed, even though for each baseline the sky blocking by the Moon is different. We also investigate how the map-making is affected by inhomogeneous baseline distributions.

Imaging sensitivity of a linear interferometer array on lunar orbit

“…In our imaging simulation, we first generate time-ordered interferometric visibility data by assuming an input sky map and a beam model, taking into account the orbital motion, the Moon blockage, the antenna response, and with an additive random noise. This mock data is then processed to reconstruct the sky map by applying the generic imaging algorithm developed by Huang et al (2018). In this section we shall describe our model setup, while the imaging algorithm and array configurations are discussed in the next section.…”

“…In choosing the orbit parameters, we keep in mind three aspects of the problem: (1) the stability of the orbit; (2) the good observation time during which the Earth (and the Sun) is shielded. (3) the nodal precession of the orbit which generates a three dimensional distribution of baselines, which is important for breaking the mirror symmetry in image synthesis (Huang et al 2018). Some of these may conflict with each other, for example, a lower orbit will allow a larger part of the sky being blocked, thus increasing the good observation time, but the lower orbit is also less stable, which would require more orbit maintenance to ensure the safety of the array.…”

“…First, there is no ground to shield half of the sky, and by using electrically short dipole antennas, nearly the whole sky is within the FOV. For data taken during one orbit, where the baselines are distributed on a circular ring on the orbital plane, there is a mirror symmetry problem: a source located on one side of the plane produce exactly the same visibility as a source located at the mirror image position on the other side of the plane, the two can not be distinguished (Huang et al 2018). Second, the precession of the orbital plane produces a three-dimensional distribution of baselines.…”

“…Despite of these complications, as long as the interferometric visibilities are linearly related to the sky temperature, and this linear relation is precisely known, in principle it can be inverted mathematically, and a general formalism of lunar orbiting array imaging synthesis has been developed (Huang et al 2018). This approach can accommodate various complications faced by the DSL mission, including the all-sky FOV, 3-D moving baselines, the problem of mirror symmetry with respect to a single plane of baselines, and the varying sky blockage by the Moon.…”

System Equivalent Flux Density of a Polarimetric Tripole Radio Interferometer

“…In planetary and solar science, we find tripole antennas as space probes mounted on satellites (Rucker et al 1996;Cecconi & Zarka 2005;Bale et al 2008;Zarka et al 2012;Fischer et al 2021). More recently in radio astronomy, there have been increasing interests in tripole antennas as elements for radio astronomy imaging (Carozzi & Woan 2009), moon-based radio astronomy (Klein Wolt et al 2012), and in particular, for a lunar orbiting radio interferometer (Boonstra et al 2016;Bentum et al 2020;Chen et al 2018;Chen et al 2021;Huang et al 2018;Rajan et al 2016;Arts et al 2019). Space missions are costly, and hence a space mission plan requires a reliable model-based prediction of key system performance such as sensitivity.…”