Characterizing the antenna and gate of RIMFAX

Abstract

RIMFAX is a Ground Penetrating Radar (GPR) created for the Mars Perseverance rover mission. RIMFAX consists of an antenna and an electronic box which contains the transmitter/receiver of the radar. The objective of RIMFAX is to gather information about the subsurface of Mars. One piece of this information is the dielectric constant, which defines the electric properties of a material. In order to estimate the dielectric constant, it is important to have accurate models of the waves that are transmitted from the antenna. Thus, we have attempted to characterize the antenna of RIMFAX. Measured radiation patterns have been interpolated with Spherical Harmonic functions. This method yielded good results at frequencies 150-220 MHz and 465-570 MHz. A Spectral Domain Fitting Model (SDFM) was used to interpolate the frequency behavior of the antenna. Due to difficulties in estimating the radiation efficiency of the antenna and inaccurate phase-information in the measured radiation patterns, this method was deemed unreliable. Far-field considerations led to two suggested frequency bands for estimating the dielectric constant; 465-570 MHz and 465-780 MHz. The electronic box of RIMFAX separates the receiver, transmitter and antenna with a gate. This gate switches access to the antenna between the transmitter and receiver periodically. If the gate is set to transmit while a wave reflects back to the antenna, some proportion of the reflected wave will not be received. To correct for this, we have characterized the behavior of the gate. The lost energy due to gating results in an an amplitude change in the radar traces. We call this amplitude change the gate-function. The gate-function was estimated from measurements and showed similarities to its theoretical counterpart. It also seemed to have features that were not present in the theoretical function, indicating that the amplitude could be corrected more accurately with the estimated function.