Design and Testing of a Miniaturized Photon Counting Laser Altimeter for Topographic Mapping

Kurzfassung

Since their first use during the Apollo 15, 16, and 17 missions, laser altimeters have become indispensable for planetary exploration, enabling topographic mapping of rocky bodies throughout the solar system. The latest European contributions in this field are represented by the BepiColombo LaserAltimeter (BELA) and the Ganymede Laser Altimeter (GALA), developed by the DLR Institute of Planetary Research and currently en route to Mercury and the Jovian moons, respectively. However, to date, such instruments have only been deployed on large satellites, failing to meet the SWaP (Size, Weight, and Power) requirements of miniaturized systems. This thesis investigates the adaptation of laser altimeter technology for smaller platforms, focusing on the NLA (New Laser Altimeter) developed for the SER3NE (Selene’s Explorer for Roughness, Regolith, Resources, Neutrons, and Elements) mission proposal. The instrument aims to improve the precision of Lunar topographic data to support the characterization of future landing sites for crewed missions from a 12 U microsatellite. To meet the stringent SWaP constraints, the design will feature a transceiver and a single-photon detection system - an approach never previously applied to topographic altimeters. This thesis aims to develop an optical design that fits the instrument within a 3 U volume. Given the innovative nature of the design, a trade-off analysis was conducted to evaluate several configurations based on their compactness, cross-coupling between the laser source and detector, footprint size at the target, co-alignment between the transmitter and receiver, and transmission losses. The selected design stood out in providing a footprint size in the range allowed by the requirements, reducing the probability of damage to the detector due to internal reflections from the laser, and guaranteeing a more stable co-alignment between the transmitter and receiver paths. It includes a transmitter with two 45° folding mirrors, a borehole mirror that allows the laser to pass while deflecting the returning signal, and a shared telescope. A test campaign is then conducted on a dedicated optomechanical design to assess 1) its compliance with some trade-off criteria and 2) its functionality. The expansion performance is demonstrated as predictable and reliable, guaranteeing the desired divergence and footprint at the target. On the other hand, the transmittance is increased above the constraints by varying the angle of incidence of the light on the band-pass filter. For the functional tests, the quality of the wavefront is first evaluated, resulting in an aberration-free transmitter and a slightly worse receiver, but still acceptable for altimetry applications. Finally, the prototype is aligned thoroughly and tested for timing measurements with single-pixel detectors, successfully providing the range to the target. In the next instrument development phase, throughout the pre-phase A studies in collaboration with ESA (European Space Agency) and relevant companies, the prototype will be enhanced to verify the performance in the other criteria and test its functionality in a relevant environment with a flight campaign.