Miniaturized Laser Altimeter for Small Satellite Applications

Abstract

Laser Altimetry is a powerful tool to create absolutely calibrated digital terrain maps of planetary surfaces, to analyze their surface geology, and to get insight into the interior structure of planetary bodies by measuring tidal elevations and libration amplitudes and frequencies. The recent ESA missions BepiColombo and the Jupiter Icy Moons Explorer (JUICE) carry the first European laser altimeter instruments, i.e., the BepiColombo Laser Altimeter (BELA) and the Ganymede Laser Altimeter (GALA), the latter of which has a strong contribution from JAXA teams. The measurement principle of a laser altimeter is very simple. It is based on the time-of-flight measurement of an optical pulse. BELA, which is now on the way to Mercury orbit, applies a diode-laser pumped Nd:YAG laser sending pulses with an energy of 50 mJ, a width of about 5 ns, and a repetition rate of 10 Hz. Over typical ranging distances of 400 km to more than 1000 km, the BELA telescope collects pulses with a few hundred photons and a width of about 25 ns where the time of arrival gives the mean topographic altitude of the area illuminated by the 5 to 40 m diameter laser beam. The return pulse width further gives information on slope and roughness within this area. GALA is a similar instrument with 17 mJ pulse energy but 30 Hz repetition rate and was launched in April 2023 to enter the Jovian system after a eight-year cruise to fly-by at Europa and Callisto and finally orbit the Jovian moon Ganymede at an altitude of about 500 km above its icy surface. BELA and GALA are instruments that consume about 50 W and have a mass of close to 15 kg and 25 kg, respectively. The instrument dimensions are largely determined by the telescope diameter of about 30 cm. In order to enable the use of these instruments on small satellites the size, weight and power (SWaP) budgets need to be drastically reduced. This can be achieved by deriving the time-of-flight information from just a single return photon. The reduction factor of about 100 in the detected photon number can be shared by a reduction in laser energy and a reduction of telescope aperture diameter. Our aim is to reduce laser pulse energy from 17 mJ to 1 mJ and telescope diameter from 22 cm (for GALA) to 8 cm which implies in total a reduction factor about 130. GALA typically detects 700 photons per pulse at an altitude of 500 km which leads to about 5 photons to be analyzed per event by a single photon detection laser altimeter. The major challenges for a single photon detection laser altimeter are the reduction of the background photon rate by reducing the field-of-view of the telescope as well as better spectral filtering. We present first results from a conceptual experimental study of such a system designed for use on small satellites applying a newly developed detection scheme using a Single Photon Avalanche Diode (SPAD) and a diode-laser pumped microchip Nd:YAG laser emitting 1 mJ pulses with a pulse width of 1 ns. The reductions in dimension, mass, and power consumption of this instrument are discussed, and the scientific performance is simulated based on first experimental results. The feasibility of accommodating the instrument on the modular TUBiX20 microsatellite platform developed by Technische Universität Berlin is explored and the necessary requirements for attitude and orbit determination and control as well as SWaP budgets are derived.