Miniaturized Laser Altimeter for Small Satellite Applications

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

Initial Tracking, Fast Identification in a Swarm and Combined SLR and GNSS Orbit Determination of the TUBIN Small Satellite

Flight dynamics is a topic often overlooked by operators of small satellites without propulsion systems, as two-line elements (TLE) are easily accessible and accurate enough for most ground segment needs. However, the advent of cheap and miniaturized global navigation satellite system (GNSS) receivers and laser retroreflectors as well as modern, easy-to-use, open-source software tools have made it easier to accurately determine an orbit or to identify a spacecraft in a swarm, which helps with improving the space situational awareness in orbits that are more and more crowded. In this paper, we present tools for small satellite missions to generate orbit predictions for the launch and early orbit phase (LEOP), identify spacecraft in a swarm after a rideshare launch, and carry out routine orbit determination from multiple sources of tracking data. The TUBIN mission’s LEOP phase set a new standard at Technische Universität Berlin: the first global positioning system (GPS) data were downloaded less than four hours after separation, orbit predictions allowed successful tracking by the ground stations, and the spacecraft could be identified in the swarm as soon as the TLE were released by Space-Track. Routine orbit determination from GPS and satellite laser ranging (SLR) tracking data was carried out over several months, and the quality of the orbit predictions was analyzed. The range residuals and prediction errors were found to be larger than those of most SLR missions, which was due to the difficulty of modeling the atmospheric drag of a tumbling, non-spherical spacecraft at low orbital altitudes

Initial results of the TUBIN small satellite mission for wildfire detection

TUBIN is a microsatellite mission tasked with the demonstration of microbolometer technology for the detection of high-temperature events. To this end, the spacecraft employs a set of two microbolometers sensitive in the thermal range of the electromagnetic spectrum as well as a complementary CMOS-based imager for the visible spectrum. Additionally, the mission serves to demonstrate the upgraded capabilities of the TUBiX20 microsatellite platform after its initial implementation within the TechnoSat mission, launched in 2017. Here, the platform was upgraded with star trackers for enhanced attitude determination to enable the pointing accuracy required by the mission. Launched in June 2021, TUBIN has already successfully detected wildfires and volcanic activity in different biomes across the globe. Additionally, the sensor suite of the TUBIN mission was applied to examine its capabilities in the assessment of light-pollution in dedicated night time imaging campaigns. In this paper, we present and analyze data sets captured in the first six months of operation to illustrate capabilities, accomplishments, and challenges regarding payload operation, calibration, and data analysis for the TUBIN mission. Finally, the plans for the second half of TUBIN’s nominal lifetime are outlined, including an on-orbit calibration campaign and increased automation in payload operations

A Last-Minute Upgrade: Rapid Integration of an Opportunity Payload into the TUBIN Mission

Small satellites have established themselves as pathfinders for a broad range of applications. The ever-increasing capabilities of both platforms and payloads enable a variety of scientific and commercial applications in fields like communications, remote sensing or even interplanetary exploration based on this satellite class. TUBIN is a microsatellite mission in the 20 kg class aiming at the detection of high-temperature events in the thermal infrared using microbolometer technology. To this end, the satellite carries a payload consisting of two infrared sensors in conjunction with a medium-resolution CMOS imager for the visible spectrum. The TUBIN mission implements the flight proven modular TUBiX20 microsatellite platform of Technische Universität Berlin that was first demonstrated on orbit in 2017. XLink is an X-band transceiver developed by IQ wireless GmbH from Berlin, Germany in cooperation with Technische Universität Berlin. It bases on a modular software-defined radio platform providing two uplink and two downlink channels that can be configured to different frequency bands and provide downlink data rates of up to 100 Mbps. The transceiver was qualified for LEO applications in 2019, at which time TUBIN was already in production phase. However, given the significant advantages a that faster downlink channel would provide for the TUBIN mission, a study was conducted to assess the expenditure of a late integration of XLink into TUBIN. The changes in hardware could be limited to minor additions to two electronic boards and an update to an outer structure element to include the transceiver itself and necessary additional antennas. This paper presents the integration of the XLink transceiver into the TUBIN satellite, while highlighting how the modular systems architecture of the TUBiX20 platform minimised the changes to the spacecraft required for this task. Furthermore, operational scenarios for the demonstration of XLink are presented, highlighting the potential advantages the transceiver may bring for the TUBIN mission. The ground segment of Technische Universität Berlin with the already existing three-meter dish has been extended to support X-band downlink and will be used for TM/TC as well as for broadband payload data reception