Alignment calibration of the GALA laser altimeter through laser ranging to Earth

Abstract

As part of the ESA mission to Jupiter and Ganymede carried out by the Jupiter Icy Moons Explorer (JUICE) spacecraft, the Ganymede Laser Altimeter (GALA) will determine the topography and detect Ganymede’s tidal degree-2 signal in order to investigate the existence of a subsurface ocean and constrain its thickness. To deliver successful measurements GALA is to be calibrated for its misalignment with an accuracy of at least 14 arcsec. This misalignment cannot be measured on ground since it will be subject to vibrations, microgravity and settling after launch. Thus this calibration is to be performed while JUICE is on its interplanetary cruise to Jupiter.

This thesis aims to answer the question how this calibration can be performed using laser ranging to an Earth-based ground station. This calibration procedure consists of a scan performed by JUICE with a size as large as the maximum expected misalignment found at 333 arcsec. A model for the attitude during laser ranging was developed to be used to simulate the distribution and intensities of the laser pulses transmitted by GALA. During this scan of several hours and with a shot frequency of 30 Hz, the ground station will be able to detect 500 to 1500 pulses. This result was validated with the real life MLA and Hayabusa experiments already performed. The spatial distribution of these detected pulses is used to reconstruct the Gaussian pulse shape of the laser pulses. By determining its peak intensity, the ground station location can be estimated which is used to measure the misalignment. This estimation procedure was found to be robust and reliable enough to be incorporated in the simulation tool that can be run many times to achieve a statistically significant result on the estimation error. Since the estimation is based on the spatial distribution, the main drivers for the accuracy are the slew rate, scan pattern, distance, detection threshold and attitude model. These were investigated for their influence on the calibration and used to find their optimal values.

The trajectory of JUICE was analysed for the most optimal opportunities given various constrains to perform a laser ranging campaign leading to a selection of options to be analysed. Using the simulation it was found that interplanetary laser ranging can be used to calibrate GALA by employing 8-10 hours of scanning at a distance no further than around 0.5 AU by using ground station characteristics such as found at Wettzell with a telescope diameter of 0.75 metres and optical and quantum efficiencies of 0.5 and 0.2 respectively. For the most optimal opportunity, laser ranging will be performed at a distance of 0.39 AU for 10 hours with a slew rate of 0.0055 deg/s. This leads to a 2 sigma calibration residual of 3.62 arcsec resulting in a total calibration error of 11 arcsec. This confirms the assumption that a calibration accuracy of 14 arcsec is possible and thus a successful laser altimetry mission can be performed by GALA.