Doppler radar imaging of spherical planetary surfaces

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Abstract
A new approach to making radar reflectivity images of spherical planetary objects uses echo spectra acquired as a function of rotational phase and at an arbitrary number of subradar latitudes. If only equatorial views are used then the image will have a north‐south ambiguity. If non‐equatorial views are used then unambiguous images are possible. The technique is tailored for depolarized or diffuse (nonspecular) polarized backscatter and works best when the limb darkening is minimal. In developing the Doppler‐radar imaging system, the target's reflectivity distribution is expanded in a truncated spherical harmonic series and the distribution of echo power in rotational phase and Doppler frequency is obtained as a linear, analytic function of the series coefficients. To estimate the coefficients from an observed phase‐Doppler distribution, the inversion is cast as a least‐squares problem and solved using singular value decomposition. The result is a linear imaging system whose capabilities and sensitivity to such factors as subradar latitude coverage and signal‐to‐noise ratio are easily explored with simulations. Doppler‐radar imaging can be used with existing radar telescopes to map the diffuse component of echoes from the inner planets and to make north‐south ambiguous reflectivity maps of the icy Galilean satellites. SNRs needed for Doppler‐radar imaging of the largest asteroids, Io, and Titan would be accessible upon implementation of upgrades proposed for the Arecibo telescope.