Noda et al. Earth, Planets and Space (2017) 69:2 DOI 10.1186/s40623-016-0589-8 FULL PAPER Open Access Laser link experiment with the Hayabusa2 laser altimeter for in‑flight alignment measurement Hirotomo Noda1* , Hiroo Kunimori2, Takahide Mizuno3, Hiroki Senshu4, Naoko Ogawa5, Hiroshi Takeuchi3, Chris Moore6, Alex Pollard6, Tomohiro Yamaguchi5, Noriyuki Namiki1, Teiji Kase7, Takanao Saiki5 and Yuichi Tsuda5 Abstract We report results of a laser link experiment between a laser altimeter called light detection and ranging (LIDAR) aboard Hayabusa2 and ground-based satellite laser ranging stations conducted when the spacecraft was near the Earth before and after the gravity assist operation. Uplink laser pulses from a ground station were successfully detected at a distance of 6.6 million km, and the field of view direction of the receiving telescope of the LIDAR was determined in the spacecraft frame. The intensities of the received signals were measured, and the link budget from the ground to the LIDAR was confirmed. By detecting two successive pulses, the pulse intervals from the ground- based station were transferred to the LIDAR, and the clock frequency offset was thus successfully calibrated based on the pulse intervals. The laser link experiment, which includes alignment measurement of the telescopes, has proven to be an excellent method to confirm the performance of laser altimeters before they arrive at their target bodies, especially for deep space missions. Keywords: Hayabusa2, LIDAR, Laser altimeter, Laser link, SLR Introduction telescope, which emits laser pulses, and a receiving tel- In recent years, laser altimeters have been installed on escope, which detects the photons reflected from the sur- lunar and planetary exploration missions (e.g., Clem- face of the planet. The boresight of both telescopes must entine, NEAR, Mars Global Surveyor, Hayabusa, MES- be co-aligned to enable detection of laser footprints on SENGER, Kaguya, Chang’E-1, Chandrayaan-1, and the the surface of the planet by the receiving telescope. The Lunar Reconnaissance Orbiter) and have contributed sig- direction of the fields of view of telescopes in the space- nificantly to the measurement of planetary topography. craft frame (here called the FOV axes) is also important Laser altimeters also play an important role in spacecraft for retrieving the planetary topography data because the navigation as bus instruments. For example, in the Haya- telescope pointing direction is determined using space- busa asteroid sample return mission, a laser altimeter craft attitude data. During integration testing before a was used for autonomous navigation by determining the launch, alignment is established using alignment mir- spacecraft’s absolute position with respect to the asteroid rors. However, it is possible that this alignment may be during the touchdown sequence. Almost all laser altim- compromised by the shock of the launch and subsequent eters aboard planetary missions consist of a transmitting thermal environment changes after the launch. There- fore, in situ measurements of alignment are essential. It may be possible to estimate alignment using prominent surface features such as boulders by comparing altim- *Correspondence: [email protected] 1 National Astronomical Observatory of Japan, Mitaka, Tokyo 181‑8588, eter data to images taken with cameras, the FOV axes of Japan which are typically well-determined based on calibration Full list of author information is available at the end of the article with images of stars or planets. © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Noda et al. Earth, Planets and Space (2017) 69:2 Page 2 of 14 However, surface features on asteroids are difficult the spacecraft was near the Earth before and after the to recognize from ground-based observations, and it gravity assist operation. is often unclear whether such prominent features really A transponder mode was added to the LIDAR system exist. Therefore, other means of estimating the FOV axes so that it would wait for a laser pulse from the ground are highly desirable. For example, the laser altimeter on station that, once detected, would trigger transmission Hayabusa was used to determine the FOV axes by detect- of a new laser pulse back in the same direction; two-way ing the footprints of laser shot with the Near-Infrared laser ranging can thus be carried out at planetary dis- Spectrometer (NIRS) during the exposure time because tances. In addition, this mode allows two laser pulses it was sensitive to the 1 μm wavelength of the laser altim- to be detected within a given waiting period such that eter (Abe et al. 2006). Laser link experiments between measurements of the time intervals between the two ground-based satellite laser ranging (SLR) stations and pulses can be returned to the ground as telemetry data laser altimeters may also provide an alternative method. via the standard microwave link. This allows the known Such experiments have been conducted at lunar distance pulse interval on the ground to be used to calibrate the by the Lunar Reconnaissance Orbiter (LRO) (Sun et al. onboard clock. 2014), at 23 million km by the MESSENGER spacecraft The purposes of this laser link experiment can be sum- (Smith et al. 2006), and at 80 million km by the Mars marized as follows: Global Surveyor (Abshire et al. 2006). Laser link experiments may be useful not only for 1. Estimating the LIDAR FOV axis and checking the alignment measurement but also for performance alignment between the transmitting and receiving checks, demonstrations of optical communications, and telescopes if a two-way laser link is established. time transfer to spacecraft from well-calibrated clocks on 2. In-flight testing of the laser link budget. the ground. Notably, time transfer has recently become 3. Synchronous two-way ranging at planetary distances, an active area of research in the SLR community. This i.e., farther than the lunar distance, as a technological field was first studied by a French research group in the demonstration. 1980s; the first experiments were undertaken using the 4. Testing the new time transfer technique for onboard laser synchronization from a stationary orbit (LASSO) clock calibration as a technological demonstration. instrument aboard MeteoSat-P2 in 1992 (Fridelance and Veillet 1995). Since those early experiments, global Next, we describe the alignment requirement from the navigation satellite system (GNSS) experiments have perspective of asteroid remote sensing. been conducted using a Compass satellite (Yang et al. Asteroids are a category of solar system objects that are 2008), and an experiment called time transfer by laser considered the remains of the building blocks of planets link (T2L2) onboard the JASON-2 spacecraft launched in the solar system. Porosity is a key parameter for aster- in 2008 was very successful; many ground stations par- oids; this parameter can be used to determine whether an ticipated in the time transfer experiment (Samain et al. asteroid is monolithic or rubble-pile after a long history 2010). These experiments are used to compare clocks of collision and aggregation. It is therefore a major fac- not only between ground stations and spacecraft but also tor in controlling asteroid evolution. Global porosity can between different ground stations via spacecraft. Such be estimated from mean density which is calculated from comparisons can be used to reduce errors of various the volume and mass, and density of the rocks that form parameters used in precise orbit determination, includ- the asteroid. In the case of Hayabusa2, optical images ing precise estimation of atmospheric delay (Prochazka taken with a camera are scaled by the LIDAR range data, et al. 2011). Typically, once a two-way link is established and these images are used to create a global shape model. between a ground station and the laser reflector onboard Range data to the asteroid during free-fall toward it are the spacecraft, an onboard photon detector is used to used to estimate its total mass and the regional gravity time-tag detected laser pulses from the ground, which field. Rock types can be estimated based on remote sens- allows the satellite clock to be calibrated against the clock ing data from instruments such as optical and infrared on the ground (Fridelance et al. 1997). spectrometers and from samples to be returned in 2020. The Hayabusa2 spacecraft was launched on December All of these data are combined to estimate porosity. If the 3, 2014, as the second Japanese explorer to the asteroid porosity can be estimated with error <10%, the asteroid 162173 Ryugu. It is equipped with a laser altimeter called can be identified as monolithic or fractured. This error light detection and ranging (LIDAR) for navigation, sci- corresponds to 5% error in volume estimation or the entific observations (Mizuno et al. 2016), and laser link shape model, which is the required level of accuracy for experiments. Such experiments were conducted, while asteroid science (Namiki et al. 2014). Noda et al. Earth, Planets and Space (2017) 69:2 Page 3 of 14 Here, we estimate the impact of uncertainty in deter- Table 1 Specifications of the LIDAR mining the FOV axis. If the angular uncertainty of its Item Value direction is Δθ, the distance to the asteroid is D, and the surface slope is α, the error of the LIDAR range Measurement range 30 m–>25 km associated with the shift in the laser footprint position Range resolution 0.5 m from the position without uncertainty is approximately Laser D(�θ) sin α.