Antenna Pattern Compensation in Radar Image Reconstruction

There are currently several missions in cruise phase and planned, operating a radar for sub-surface imaging to celestial bodies (e.g. RIME on JUICE or JURA on HERA/Juventas). The radar has a much wider field-of-view than e.g. a laser altimeter, which is also influenced by the radiation pattern of the antenna. Said pattern varies with angular direction as well as over frequency, both in amplitude and phase. Additionally, the trajectory of the spacecraft carrying the radar may cause imaging under sub-optimal conditions with nadir pointing off-axis w.r.t. the line spacecraft to center of observed body.

Forward simulations indicate an influence of the radiation pattern of the transmit and receive antenna on the results. In simulations, a theoretical reference of an isotropic radiation pattern can be assumed, enabling comparison with processing of radar data with the radiation pattern accounted for. Since the radiation pattern adds an additional phase shift, which changes over frequency, the inverse Fourier transform places reflections at slightly different distances than they actually are. This causes imperfect additions in e.g. back-projection image forming algorithms and consequently less contrast or even false targets.

The naive approach of compensating the antenna radiation pattern is to determine the direction between the antenna and the point of reconstruction, retrieving an (interpolated) complex-valued spectrum of the radiation pattern in that direction and dividing the received spectrum by the antenna spectrum. In case of a bi-static radar, both the transmit and receive antenna radiation patterns are different and need to be accounted for. For a mono-static radar, both patterns and directions are identical, slightly reducing complexity. Besides computational complexity, adequate knowledge of the radiation pattern and antenna’s (or spacecraft’s) attitude are required inputs for the compensation.

Using simulations (antenna radiation pattern, forward radar simulations of a simplified scenario), we will show the benefit of compensating the radiation pattern of a radar’s antenna radiation pattern on the quality of reconstructed image. Furthermore, we will try to include actual results from SHARAD, pending availability of a sufficiently accurate radiation pattern.