Search

An altimeter is a critical instrument in planetary missions, for both safe operations and science activities. We present required specifications and link budget calculations for light detection and ranging (LIDAR) onboard the Martian Moons Exploration (MMX) spacecraft. During the mission phase, this LIDAR will continuously measure the distance between the spacecraft and its target. The time-series distance provides important diagnostic information for safe spacecraft operations and important information for geomorphological studies. Because MMX is a sample return mission, its LIDAR must accommodate physical disturbances on the Martian satellite surface. This resulted in changes to the optical system design. Graphical abstract

In this study, we determined the alignment of the laser altimeter aboard Hayabusa2 with respect to the spacecraft using in-flight data. Since the laser altimeter data were used to estimate the trajectory of the Hayabusa2 spacecraft, the pointing direction of the altimeter needed to be accurately determined. The boresight direction of the receiving telescope was estimated by comparing elevations of the laser altimeter data and camera images, and was confirmed by identifying prominent terrains of other datasets. The estimated boresight direction obtained by the laser link experiment in the winter of 2015, during the Earth’s gravity assist operation period, differed from the direction estimated in this study, which fell on another part of the candidate direction; this was not selected in a previous study. Assuming that the uncertainty of alignment determination of the laser altimeter boresight was 4.6 pixels in the camera image, the trajectory error of the spacecraft in the cross- and/or along-track directions was determined to be 0.4, 2.1, or 8.6 m for altitudes of 1, 5, or 20 km, respectively.

A PZT type telescope for observations of gravity gradient and lunar rotation was developed, and a Bread Board Model (BBM) for ground experiments was completed. Some developments were made for the BBM such as a tripod with attitude control system, a stable mercury pool and a method for collecting the effects of vibrations. Laboratory experiments and field observations were performed from August to September of 2014, in order to check the entire system of the telescope and the software, and the results were compared to the centroid experiments which pursue the best accuracy of determination of the center of star images with a simple optical system. It was also investigated how the vibrations of mercury surface affect the centroid position on Charge Coupled Device (CCD). The results of the experiments showed that the effects of vibrations are almost common to stars in the same view, and they can be corrected by removing mean variation of the stars; and that the vibration of mercury surface can cause errors in centroid as large as 0.2 arcsec; and that there is a strong correlation between the Standard Deviation (SD) of variation of the centroid position and signal to noise ratio (SNR) of star images. It is likely that the accuracy of one (1) milli arcsecond is possible if SNR is high enough and the effects of vibrations are corrected.

The Ganymede Laser Altimeter (GALA) is one of the ten scientific instruments selected for the Jupiter Icy Moons Explorer (JUICE) mission currently implemented under responsibility of the European Space Agency (ESA). JUICE is scheduled for launch in mid 2022; arrival at Jupiter will be by end of 2029 with the nominal science mission—including close flybys at Ganymede, Europa, and Callisto and a Ganymede orbit phase—ending by mid 2033. GALA’s main objective is to obtain topographic data of the icy satellites of Jupiter: Europa, Ganymede, and Callisto. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the surface ice layer from the deep interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean in a 500 km orbit around the satellite and will provide constraints on Ganymede’s ice shell thickness. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also surface roughness and albedo (at 1064 nm) measurements. GALA is a single-beam laser altimeter operating with up to 50 Hz (nominal 30 Hz) shot frequency at a wavelength of 1064 nm and pulse lengths of 5.5 ± 2.5 ns using a Nd:YAG laser. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and is digitized at a sampling rate of 200 MHz providing range measurements with a subsample resolution of 0.1 m and surface roughness measurements from pulse-shape analysis on the scale of the footprint size of about 50 m at 500 km altitude. The instrument is developed in collaboration of institutes and industry from Germany, Japan, Switzerland, and Spain. © 2019, CEAS., Financial support was provided under grant 50 QJ 1401 on behalf of the DLR Space Administration by the German Bundesministerium fur Wirtschaft und Energie. This research has been supported by the Spanish Ministerio de Economia y Competitividad under Contract ESP 2016-76076-R.

著者人数: 12名, Accepted: 2016-02-11, 資料番号: SA1150345000

著者人数: 12名, Accepted: 2015-12-23, 資料番号: SA1150343000

In this paper we review the lunar laser ranging conducted by the laser altimeter (LALT) on board the KAGUYA lunar explorer (2007–2009). Five aspects of LALT measurements are described: (1) General operational history, (2) Laser shot and data statistics, (3) Revisions to LALT topographic data, (4) Variations in laser output energy, and (5) Peak height analysis of laser echo pulses. LALT was able to range to the lunar surface despite some troubles with respect to laser output energy in the middle of the KAGUYA mission. The time series topographic data set was revised (Ver. 2) by incorporating new lunar gravity model based on KAGUYA and other historical lunar satellite’s orbit data, along with other improvements, for example by incorporating the accurate position of the laser collimator on board the KAGUYA; however, more than half of the acquired range data could not be converted properly due to problems with orbit accuracy during the extended phase of the mission. The spherical harmonic coefficients and the basic lunar figure parameters derived from LALT_LGT_TS agree very well with LRO-LOLA and the Chang’E-1 LAM model. It is possible that partial failure to the laser diode was responsible for the gradual degradation of laser power (0.835 mJ per million shots) and the rapid decrease that occurred over April 9–14, 2008. The laser power also proved to be extremely sensitive to the temperature of the laser oscillator. The peak height ratio – that is peak height telemetry data divided by calculated ratio – is about 19% on average using the mean slope and albedo data from LALT and Spectral Profiler on KAGUYA space craft, respectively, which suggests the performance of peak height measurement is more than 1/5 for more than 70 km altitude, if compared with calculated one. The peak height ratio may be better if we take the effect of small scale topography within a footprint into account.

A single aperture and hollow retroreflector [corner-cube mirror (CCM)] that in principle has no internal optical path difference is a key instrument for achieving lunar laser ranging one order or more accurate than the current level (~2 cm). We are developing CCM whose aperture is 20 cm with optimized dihedral angles. The 20-cm CCM yields two times peak height for returned laser pulse compared with Apollo 15’s retroreflector. Two investigations were conducted to confirm the feasibility of the 20-cm aperture CCM. The first is thermo-optical simulation and evaluation of the 20-cm CCM in the lunar thermal environment. Through this simulation, it has turned out for the first time that 20-cm aperture CCM made of single-crystal Si or “ultra-low expansion glass–ceramics” such as CCZ-EX® (OHARA Inc.) can be used for CCM with no thermal control, if the perfectly fixed point of CCM is limited to one. The second is annealing and shear loading experiments of single-crystal silicon (Si) samples. Through these experiments, high-temperature annealing from 100 to 1000 °C is confirmed to be effective for the enhancement of the adhesive strength between optically contacted surfaces with no optical damage in roughness and accuracy, indicating that this annealing process would enhance the rigidity of CCM fabricated by the optically contacted plates.

The measurement of the rotation of the Moon is one of the key techniques to get the information of the internal structure. For this purpose, we proposed a small telescope experiment on the surface of the Moon in which motion of stars are utilized for the estimation of the rotation parameter. This paper describes results of simulation of observation, in which star trajectories observed are decomposed to librations, polar motion, and the precession and the amplitude and phase of each component are estimated. The standard deviation of the parameter estimation becomes nearly 1 ms of arc, which will be better than the Lunar Laser Ranging observation. From the viewpoints of accuracy of observation, thermal condition, and electric power generation, the instrument should be placed where the much sunshine is achieved on the lunar polar region.

The SELENE Laser Altimeter (LALT) is designed to map the Moon’s topography and will be launched in summer 2007. LALT incorporates Q-switched Cr doped Nd:YAG laser (1064 nm) with an output energy of 100 mJ and 1 Hz repetition frequency for about one year mission period. The laser pulse travels to the Moon’s surface and reflections from the surface are detected by a silicon avalanche photo-diode. The ranging distance is 50–150 km with about 5 m accuracy. Several corrections for accurate ranging data are investigated. The flight hardware has been qualified and passed all the integration tests. A principal goal of the LALT instrument is to obtain a much more detailed lunar topographic map which is superior in global coverage, measurement accuracy and number of data points to previous observations and models. The overall science objectives of LALT are (1) determination of lunar global figure, (2) internal structure and surface processes, (3) exploration of the lunar pole regions, and (4) reduction of lunar occultation data.

We report the current status of Japanese lunar exploration SELENE (SELenological and ENgineering Explorer). As of the end of 2004, scientific instruments onboard the Main Orbiter are under final checkout before they are provided to the proto-flight-model (PFM) integration test. Also, we present the future perspectives of the lunar based instruments and facilities. `In-situ Lunar Orientation Mea-surement (ILOM)` experiment measures the lunar rotation with high accuracy by tracking stars on the Moon with a small photo-zenith-tube type optical telescope. A basic idea of a radio telescope array of very low frequency range on the lunar far-side is also mentioned.

SELENE (Selenological and Engineering Explorer) is a Japan's lunar probe which was launched and injected into the lunar polar orbit in 2007. The Main Orbiter of SELENE named Kaguya has separated the Relay Satellite: Rstar (Okina). We have executed four-way Doppler measurements which determined the orbit of Kaguya aviating above the lunar far side. The ground station up-links ranging signals, and the relay satellite transponder on Rstar (RSAT-1) relays the carrier waves to Kaguya. Then the transponder on Kaguya (RSAT-2) receives the signals and returns to Rstar, and down-linked to UDSC. Three of four receivers through four-way links acquires signals and tracks Doppler frequency shift with phased lock loops. For that purpose, the receivers should be locked sequentially under the condition of large Doppler shift due to mutual velocities of three moving bodies. The receivers were designed in consideration of the above condition and system operability. Eventually, our system has realized the first case to track two fully moving links between the lunar orbiters and carried out Doppler measurements. Results of the orbit determinations for Kaguya above the lunar far side have shown the anomaly distributions of the gravity fields which could be scarcely found by the conventional two-way RARR methods.

[1] The sunlit conditions of the lunar polar regions were estimated by using a digital elevation model made by the laser altimeter data onboard the Japanese lunar orbiter KAGUYA (SELENE). The result shows that i) there are no peaks of eternal light in either north or south polar regions, ii) most continuously lit surfaces are 89% for north and 86% for south, iii) there are permanently shadowed regions. These information will be useful for the long-durable landing experiments on the Moon and possibly for the human activity on the Moon in the near future.

In Japan’s lunar mission called SELENE (SE-Lenological and ENgineering Explorer), detailed measurements of the lunar gravity field will be made by using two methods. The first is the differential VLBI measurement with two sub-satellites, and the second is the four-way Doppler measurement by using a sub-satellite which relays radio waves from the main lunar orbiter. The lunar far-side gravity field will be directly measured for the first time by using the second method, which will give us new information about the inner structure of the Moon. For the four-way Doppler measurement, the confirmation of the link establishment is an important issue because there is no telemetry relay from the main satellite to the sub-satellite in SELENE. The instruments are now under development, and a compatibility test of the instruments to the ground antenna station was carried out in March 2003. We have applied recently proposed algorithms for the confirmation of the link establishment and confirmed that they work successfully for data obtained during this test. The accuracy of the 2- and 4-way Doppler measurement are also measured and confirmed to be the expected level.

著者人数：17名, Accepted: 2005-03-23, 資料番号: SA1002567000

著者人数：25名, 資料番号: SA1000935000

The Laser Altimeter (LALT) on board the main orbiter of KAGUYA (SELENE) started nominal observation on December 30, 2007 for mapping the lunar topography. As of March 31, 2008, LALT has obtained about 6.7 million topographic data and the return rate is 97%. Several qualifications of the data to produce reliable topography are now in progress. The present status and preliminary results are reported in this article.

We are proposing a selenodetic mission, e.g. In situ Lunar Orientation Measurement (ILOM) to study lunar rotational dynamics by direct observations of the lunar physical liberation and the free librations from the lunar surface with an accuracy of 1 millisecond of arc in the post-SELENE project. Year-long trajectories of the stars provide information on various components of the physical librations and they can also be used to possibly detect the lunar free librations in order to investigate the lunar mantle and the liquid core. The PZT on the moon is similar to that used for latitude observations of the Earth except the half mirror above the objective, a CCD with high well capacity, and the heater beneath the mercury pool. Although a star position on the focal plane does not change even if the telescope inclines in principle, the tilt of the telescope affects the star position due to aberrations of the objective in the actual case. We obtained the relation between the deviation of the star position in CCD and the tilt of the telescope by ray tracing method and found that the effect of the tilt less than 100 arc seconds upon the star position does not exceed 1 mas. Thermal test of a tube made of CFRP showed that the tube did not incline by more than 100 seconds of arc even in the conditions of the lunar surface. We have a prospect to attain an accuracy of positioning of better than 1 mas from simulated experiments in laboratory using a CCD.