Performance Characterization of RIME aboard JUICE with Instrument Transfer Function

The JUICE (Jupiter ICy Moon Explorer) mission aims to explore Jupiter and three of its Galilean moons. The spacecraft is set to arrive in the Jovian system in 2031, utilizing its onboard instruments, including the Radar for Icy Moons Exploration (RIME), to study these celestial bodies. RIME operates as a subsurface radar at a center frequency of 9 MHz. The chirp signal has a bandwidth of 2.8 MHz, allowing it to penetrate icy surfaces up to a depth of approximately 9 km with a vertical resolution of about 30 m. The radar sounder uses a 16.6 m dipole consisting of two 8.3 m rods, which are made of a carbon fiber reinforced plastic structure.

To replicate the properties of the radar sounder on Earth, an antenna simulator (AS) was developed. This simulator mimics the behavior of the RIME antenna system in space and is essential for calibrating the radar by estimating and minimizing distortions in the instrument’s performance. The calibration process involves calculating the instrument transfer function (ITF) across the entire transmission path, including the transmitter and receiver paths.

Since the low operating frequency limits direct antenna measurements, simulations are used to approximate the antenna's behavior with negligible discrepancies. However, measurements were made for other components such as the central matching network (CMN), terminal matching network (TMN), dummy loads (DL), and combiner (CMB), across the full frequency range. Therefore, it is possible to accurately reproduce the behavior of the AS.

The transmission characteristics of the TX and RX paths were determined using the radar system's original flight electronics and the AS. Calibration requires de-embedding the AS within the respective path to account for load-dependent behaviors, enabling simulations to incorporate frequency-dependent factors of the antenna system. For these simulations, an ideal chirp signal is assumed as input. Using this assumption, transfer functions are calculated for the power amplifier and the Receiver and Digital Subsystem.

Combining the individual transmission functions results in the ITF. In addition, an average reflection factor can be assumed to approximate the impact of the surface. This makes it possible to estimate the losses occurring along the entire transmission path.