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In laser-based space technologies where laser beams are transmitted from a ground station, it is required for the ground station to avoid irradiating aircraft with high energy lasers. A simple, inexpensive acoustic positioning technique using four microphones is developed and its performance is evaluated. It is demonstrated to be sensitive to and effective in detecting aircraft flying at short ranges. Aircraft closer than 5 km can easily be detected and the maximum range is about 10 km. The time series of two-dimensional directional position estimates are harmonic with those of ADS-B (Automatic Dependent Surveillance-Broadcast), and the precision reaches 1 degree (standard deviation) in the best case. The processing time for data collection and computation is short enough to establish a realtime feedback mechanism to stop firing laser. With its advantages and disadvantages, effective combinations with existing methods are discussed to avoid injury to the eyes, in which the acoustic approach is found to be useful to enhance safety measures in particular of low-altitude aircraft with a large angular velocity. [ABSTRACT FROM AUTHOR]

The precise orbit of the Hayabusa2 spacecraft with respect to asteroid Ryugu is dynamically determined using the data sets collected by the spacecraft's onboard laser altimeter (LIght Detection And Ranging, LIDAR) and automated image tracking (AIT). The LIDAR range data and the AIT angular data play complementary roles because LIDAR is sensitive to the line-of-sight direction from Hayabusa2 to Ryugu, while the AIT is sensitive to the directions perpendicular to it. Using LIDAR and AIT, all six components of the initial state vector can be derived stably, which is difficult to achieve using only LIDAR or AIT. The coefficient of solar radiation pressure (SRP) of the Hayabusa2 spacecraft and standard gravitational parameter (GM) of Ryugu can also be estimated in the orbit determination process, by combining multiple orbit arcs at various altitudes. In the process of orbit determination, the Ryugu-fixed coordinate of the center of the LIDAR spot is determined by fitting the range data geometrically to the topography of Ryugu using the Markov Chain Monte Carlo method. Such an approach is effective for realizing the rapid convergence of the solution. The root mean squares of the residuals of the observed minus computed values of the range and brightness-centroid direction of the image are 1.36 m and 0.0270°, respectively. The estimated values of the GM of Ryugu and a correction factor to our initial SRP model are 29.8 ± 0.3 m3/s2 and 1.13 ± 0.16, respectively. [ABSTRACT FROM AUTHOR]

Quality control systems for satellite laser ranging (SLR) observations have been developed at a number of analysis institutes worldwide, using various software packages of precise orbit determination and data analysis. Satellite laser range observations, primarily from the two LAGEOS satellites but also from other satellites in low-Earth orbits and up to GNSS altitude, are being processed on a sub-daily to weekly basis. The generated quality control reports are widely used to detect various kinds of problems and quickly provide anomalous information to laser ranging stations. They have been effective in shortening the time to return to normal operations when anomalous data are detected and in quantifying the performance of laser ranging stations. Consequently, a rapid feedback loop has now been incorporated in the modern SLR operation. [ABSTRACT FROM AUTHOR]

Abstract This study aims to investigate solar radiation pressure acting on the spherical geodetic satellites, Ajisai, LAGEOS-1, and LAGEOS-2. The solar radiation pressure coefficients ( C R ) are derived in two independent ways: (a) through precise orbit determination (POD) using satellite laser ranging (SLR) data, and (b) through modeling using the optical properties of the satellite surface material. The average C R value of Ajisai (1.039), as calculated from the time series of C R POD estimates every 15 days, is consistently smaller than those of LAGEOS-1 (1.140) and LAGEOS-2 (1.103). This difference can be explained by the fact that the surface of Ajisai is mostly covered by mirrors. The Ajisai C R values estimated by POD show remarkable semi-annual variation, which disagrees with the results of a previous study (Sengoku et al., 1995) predicting that the C R of Ajisai varies almost annually. We attribute this semi-annual variation to two physical reasons: the non-spherical additional cross-sectional area due to the "attached fitting ring" and the low reflectivity of the surface material in the polar regions. Furthermore, the solar radiation pressure acting on Ajisai varies annually in a direction perpendicular to the sun-satellite vector. Finally, the two independent C R values of Ajisai agree more when we assume a total solar irradiance of 1361 W/m2, whereas the value 1367 W/m2 has been commonly used in POD. [ABSTRACT FROM AUTHOR]

The aim of this study is to find an effective way to expand the ground tracking network of satellite laser ranging on the assumption that a new station is added to the existing network. Realistic numbers of observations for a new station are numerically simulated, based on the actual data acquisition statistics of the existing stations. The estimated errors are compared between the cases with and without a new station after the covariance matrices are created from a simulation run that contains six-satellite-combined orbit determination. While a station placed in the southern hemisphere is found to be useful in general, it is revealed that the most effective place differs according to the geodetic parameter. The X and Y components of the geocenter and the sectoral terms of the Earth's gravity field are largely improved by a station in the polar regions. A middle latitude station best contributes to the tesseral gravity terms, and, to a lesser extent, a low latitude station best performs for the Z component of the geocenter and the zonal gravity terms. [ABSTRACT FROM AUTHOR]

To realize the full potential of satellite laser ranging for accurate geodesy, it is crucial that all systematic effects in the measurements are taken into account. This paper derives new values for the so-called center-of-mass corrections for three geodetic satellites that are regularly tracked and used in geodetic studies. Optical responses of the twin satellites, Starlette and Stella, and the LARES satellite are retrieved from kHz single-photon laser-ranging data observed at Herstmonceux and Potsdam. The detection timing inside single-photon systems, C-SPAD-based systems and photomultiplier-based systems is numerically simulated, and the center-of-mass corrections are derived to be in the range of 74 to 82 mm for Starlette and Stella, and 127-135 mm for LARES. The system dependence is below 1 cm, but should not be ignored for millimeter accuracy. The longtime standard center-of-mass correction 75 mm of Starlette and Stella is revealed to be too small for the current laser-ranging stations on average, which is considered to have resulted in a non-negligible systematic error in geodetic products. [ABSTRACT FROM AUTHOR]

Multi-technique space geodetic analysis software has been developed which allows to combine data on the observation level. In addition to local tie information, site-wise common parameters, i.e., troposphere and clocks, can be estimated with this software. Thus, it will be discussed how common parameters have to be estimated and where biases/offsets need to be taken into account. To test such a novel concept, Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) data from the CONT11 campaign are being utilized. Since the VLBI baselines of this campaign extend over several thousands of kilometers, GPS data are processed in precise-point positioning mode and satellite orbits and clocks are kept fixed to the IGS final products. From the obtained results, it can be shown that the combination of space geodetic data on the observation level leads to a consistent improvement of station position repeatability as well as nuisance parameters like troposphere estimates. Furthermore, estimation of common parameters (troposphere or clocks) at co-located sites helps to improve the solution further and derive an utmost physically consistent model of the concerned parameters. [ABSTRACT FROM AUTHOR]

Abstract: A multi-technique space geodetic analysis software named c5++ has been developed and allows one to combine data on the observation level. With SLR and VLBI modules being ready and tested, this software has been used to compute coordinate time series of the geodetic fundamental station TIGO, located near Concepción, Chile. It can be shown that the combination of space geodetic data on the observation level leads to a significant improvement of station position repeatability, which is an important measure for the stability of a station in the terrestrial reference frame. Moreover, it could be demonstrated that the geophysical signal of the post-seismic tectonic plate movement is usually more complete than detected by any of the two single-technique solutions. In addition, it has been confirmed that so-called nuisance parameters, which are relying on data from a single technique, are not biased when combing observations from different space geodetic techniques. [Copyright &y& Elsevier]

• Synchronous two-way laser ranging in deep space was successful for the first time. • A laser altimeter aboard spacecraft Hayabusa2 was used as an optical transponder. • The longest distance was 6.46 million km. In December 2020, synchronous two-way laser ranging in deep space was successful for the first time over distances up to 6.46 million km. A laser altimeter aboard spacecraft Hayabusa2 was used as an optical transponder, detecting laser pulses emitted from the ground stations on Earth and retransmitting the pulses from the spacecraft to the ground. These retransmitted pulses were successfully detected at the ground station. The experiment was conducted as a demonstration of the deep-space laser ranging. The repetition frequency of the onboard instrument was limited to 0.5 Hz, and there was a lot of background noise because the experiments were carried out in daytime on the ground. Nevertheless, laser detection on the ground was still possible due to the high time-correlation between the detected and the predicted in the presence of random noise. Procedures similar to satellite laser ranging were applied to create residuals by subtracting the orbit predictions of the spacecraft from the observed round-trip time. Furthermore, some of the transponder return pulses were uniquely identified, where the time coherence of the measurements had been recovered from the telemetry data of the laser altimeter. [ABSTRACT FROM AUTHOR]

Abstract: We numerically examined various retroreflectors as laser ranging targets for future missions to the Moon. The geometric conditions, such as the angle of incidence and velocity aberration, with lunar targets are much more restricted than those with most of the earth-orbiting artificial satellites. The numerical optical response simulation carried out in this study indicates that a single retroreflector with a diameter of 150–250mm performs similar to the existing Apollo retroreflector arrays. Further, no dihedral angle is required for small retroreflectors with diameters below 150mm for uncoated ones and below 100mm for coated and hollow ones. Retroreflectors with larger diameters require dihedral angles of 0.20, 0.25, and 0.35arcsec for coated, uncoated and hollow types, respectively. The objective of this fundamental study is to underlie the development of future laser ranging targets that are to be placed on the Moon. [Copyright &y& Elsevier]

The spin rate and spin axis orientation of the Laser Geodynamics Satellite 2 (LAGEOS-2) satellite were determined by analysis of photometric observation data. A photometer system developed at the Space Geodesy Facility at Herstmonceux, U.K., was used to time solar glints from the rotating satellite. It was found that the spin rate of LAGEOS-2 slowed by 38% per year between March 2000 and November 2002 and that the spin axis undergoes a slow, complex precession with respect to the earth's rotation axis. Moreover, the analysis revealed that the direction of the spin was the reverse of that of the earth's rotation and that the optical alignment of a reflector surface on LAGEOS-2 can deviate from its position vector by up to 15 arcmin. [ABSTRACT FROM AUTHOR]

Presents a study which derived the spin rate of the geodetic satellite AJISAI through a spectral analysis of laser range data obtained by precise high-return-rate tracking stations. Target signature effect dependent on the satellite orientation; Spectral analysis of AJISAI full-rate data; Slowdown of spin rate; Conclusions.

Presents information on a study which modeled the detection timing of laser pulse returned from the satellite AJISAI for a single-photon laser ranging system. Modeling of center-of-mass correction for AJISAI; Range bias estimation; Conclusions.

The conventional detection of micrometeoroid and orbital debris (MMOD) impacts on satellites is based on in-situ sensing or direct, visual inspection of the retrieved surface elements exposed to the particle flux and thus requires active in-orbit operations. We propose Quanta Photogrammetry (QPM) as an optical method for remote detection of surface structural anomalies of passive satellites by measuring solar photon flux reflected off the satellite surface towards the ground detection system.The hypertemporal light curves of the Experimental Geodetic Satellite - Ajisai (NORAD 16908, altitude of 1490 km) are collected with the state-of-the-art photon counting system designed and operated by the Graz Satellite Laser Ranging Observatory (Austria). QPM utilizes the inertial attitude model of the satellite to project the high-rate photometric samples onto the spacecraft body-fixed frame. The single-photon light curves collected from Oct. 2015 until Jan. 2018 are used to map reflectivity of 149 mirror panels on-board Ajisai (approx. size of 20 × 20 cm each) and reveal relatively small, irregularly shaped and spot-like anomalies that can indicate surface degradation due to the long-term environmental interactions and MMOD hypervelocity impacts. • Quanta Photogrammetry remotely detects reflective anomalies of Ajisai mirrors. • The high-resolution photograms of Ajisai mirrors reveal location of the anomalies. • Detected anomalies are most likely caused by the aging effects or MMOD impacts. [ABSTRACT FROM AUTHOR]

We present improved modelling to compute centre of mass (CoM) offsets for spherical geodetic satellites and derive new values for the spacecraft LAGEOS, LAGEOS-2, Etalon-1/2, Starlette, Stella, LARES, and Ajisai for all system configurations of the laser ranging network, past and present. The updated CoM values, i.e. the average distances between the centres of mass and the optical reflection points, reveal that a major contributor to the error budget in satellite laser ranging (SLR) for reference frame realisation has been a limited accuracy of the CoM corrections hitherto employed to model some of the systems of the SLR network. Our modelling strategy takes into account target characteristics, detection hardware, and mode of operation. For the optical modelling, we have employed high precision data from the single-photon, high-repetition rate SLR system at Herstmonceux (UK), performing the relevant computations for all laser frequencies ever used for tracking operations in the SLR network. For stations that do not operate in single-photon mode, we performed Monte Carlo simulations to estimate the CoM offsets, in this way taking into consideration previously neglected aspects of the detection process. The newly derived values reduce by up to 50% the biases estimated for the two biggest targets, Etalon and Ajisai, and eliminate the generally positive trend in range biases previously observed across the network. These CoM values, mostly lower than the previous ones, imply higher estimated station heights, with a consequent increase in the scale of the terrestrial reference frame that is realised by SLR on the basis of LAGEOS and Etalon observations. This increase in scale, of about 0.65 ppb, is confirmed by geodetic solutions reported here using nineteen years of observations from the ILRS network of tracking stations. We also examine the consequences for the determination of the geocentric gravitational constant, GM, that result from using the new CoM corrections. [ABSTRACT FROM AUTHOR]

The hypertemporal light curves of the sunlit Ajisai satellite allow for reflectivity measurement of the individual on-board mirror panels. The photon counting technology developed at Graz Satellite Laser Ranging (SLR) station makes it possible to distinguish between the solar flux diffused and specularly reflected off the spinning Ajisai. The flux intensities measured at 10 kHz sampling rate during the period from Oct. 2015 until Jan. 2018 are analyzed through the spacecraft micro-model link budget equation and indicate reflectivity of the 149 mirrors of between 82.3% and 88.2% with the mean value of 85.3% and the RMS of 1.2%. It is predicted that this high specular reflectivity of the satellite will allow for the establishment of a laser link between the distant ground locations with the individual mirrors acting as a zero-latency, passive optical relay. Simulations of the laser link between the Matera (Italy) and Graz (Austria) SLR systems via spaceborne mirror reflections indicate that such a channel can be operated at mean signal strength of 3.46 photoelectrons per laser pulse. The predicted mean number of the laser link intervals per pass is 874.6 with a mean interval duration of 9.15 ms. [ABSTRACT FROM AUTHOR]

The high repetition rate Satellite Laser Ranging system Graz delivers the millimeter precision range measurements to the corner cube reflector panels of Ajisai. The analysis of 4599 passes measured from October 2003 until November 2014 reveals the secular precession and nutation of Ajisai spin axis due to the gravitational forces as predicted by Kubo (1987) with the periods of 35.6 years and 116.5 days respectively. The observed precession cone is oriented at RA = 88.9°, Dec = −88.85° (J2000) and has a radius of 1.08°. The radius of the nutation cone increases from 1.32° to 1.57° over the 11 years of the measurements. We also detect a draconitic wobbling of Ajisai orientation due to the ‘motion’ of the Sun about the satellite’s orbit. The observed spin period of Ajisai increases exponentially over the investigated time span according to the trend function: T = 1.492277·exp(0.0148388· Y ) [s], where Y is in years since launch (1986.6133), RMS = 0.412 ms. The physical simulation model fitted to the observed spin parameters proves a very low interaction between Ajisai and the Earth’s magnetic field, what assures that the satellite’s angular momentum vector will remain in the vicinity of the south celestial pole for the coming decades. The developed empirical model of the spin axis orientation can improve the accuracy of the range determination between the ground SLR systems and the satellite’s center-of-mass ( Kucharski et al., 2015 ) and enable the accurate attitude prediction of Ajisai for the laser time-transfer experiments ( Kunimori et al., 1992 ). [ABSTRACT FROM AUTHOR]

The Graz kHz Satellite Laser Ranging (SLR) system is the first system operating with a 2-kHz-repetition-rate laser. Using Graz 2-kHz SLR data only, we applied a new analytical approach to determine the spin period of the passive satellite AJISAI. This method analyzes the range measurements to the single corner-cube-reflector panels of AJISAI, allowing accurate determination of an actual attitude of this satellite during day and night. Using Graz kHz SLR data of more than five years, we processed 877 passes of AJISAI (October 9, 2003-December 22, 2008) and calculated its spin period (∼2 s) with an accuracy of 0.0042% (84 μs). This spin period (T ) is increasing, following an exponential trend: T = 1.9028 · Exp (0.014859 · (Year - 2003.0)) s. This slow down is mainly caused by the gravitational andmagnetic fields of the Earth. The high accuracy allows, for the first time, the detection of small perturbations of the spin period caused by nongravitational effects related to the solar energy flux to which the satellite is exposed. [ABSTRACT FROM AUTHOR]

The Graz kHz Satellite Laser Ranging (SLR) system is the first system operating with a 2-kHz-repetition-rate laser. Using Graz 2-kHz SLR data only, we applied a new analytical approach to determine the spin period of the passive satellite AJISAI. This method analyzes the range measurements to the single corner-cube-reflector panels of AJISAI, allowing accurate determination of an actual attitude of this satellite during day and night. Using Graz kHz SLR data of more than five years, we processed 877 passes of AJISAI (October 9, 2003-December 22, 2008) and calculated its spin period (~2 s) with an accuracy of 0.0042% (84 µs). This spin period (T) is increasing, following an exponential trend: T = 1.9028 ⋅ Exp (0.014859 ⋅ (Year - 2003.0)) s. This slow down is mainly caused by the gravitational and magnetic fields of the Earth. The high accuracy allows, for the first time, the detection of small perturbations of the spin period caused by nongravitational effects related to the solar energy flux to which the satellite is exposed. [ABSTRACT FROM AUTHOR]

Laser technology has revolutionized scientific research and applications since the very early years after its invention (Maiman [1]). Earth and space science were the prime beneficiaries of this cutting edge research used to range to satellites and the moon within the first decade since its invention. This second special issue on Laser Ranging is intended to present the reader with the current status of the technique applied to earth and space science and engineering today. [Extracted from the article]

Recent efforts of tracking low Earth orbit and medium Earth orbit (MEO) satellites using geodetic very long baseline interferometry (VLBI) raise questions on the potential of this novel observation concept for space geodesy. Therefore, we carry out extensive Monte Carlo simulations in order to investigate the feasibility of geodetic VLBI for precise orbit determination (POD) of MEO satellites and assess the impact of quality and quantity of satellite observations on the derived geodetic parameters. The MEO satellites are represented in our study by LAGEOS-1/-2 and a set of Galileo satellites. The concept is studied on the basis of 3-day solutions in which satellite observations are included into real schedules of the continuous geodetic VLBI campaign 2017 (CONT17) as well as simulated schedules concerning the next-generation VLBI system, known as the VLBI Global Observing System (VGOS). Our results indicate that geodetic VLBI can perform on a comparable level as other space-geodetic techniques concerning POD of MEO satellites. For an assumed satellite observation precision better than 14.1 mm (47 ps), an average 3D orbit precision of 2.0 cm and 6.3 cm is found for schedules including LAGEOS-1/-2 and Galileo satellites, respectively. Moreover, geocenter offsets, which were so far out of scope for the geodetic VLBI analysis, are close to the detection limit for the simulations concerning VGOS observations of Galileo satellites, with the potential to further enhance the results. Concerning the estimated satellite orbits, VGOS leads to an average precision improvement of 80% with respect to legacy VLBI. In absolute terms and for satellite observation precision of 14.1 mm (47 ps), this corresponds to an average value of 17 mm and 7 mm concerning the 3D orbit scatter and precision of geocenter components, respectively. As shown in this study, a poor satellite geometry can degrade the derived Earth rotation parameters and VLBI station positions, compared to the quasar-only reference schedules. Therefore, careful scheduling of both quasar and satellite observations should be performed in order to fully benefit from this novel observation concept. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Precise information of spacecraft position with respect to target body is of importance in terms of scientific interpretation of remote sensing data. In case of Hayabusa2, a sample return mission from asteroid Ryugu, such information is also necessary for landing site selection activity. We propose a quick method to improve the spacecraft trajectory when laser altimeter range measurements and a shape model are provided together with crude initial trajectory, spacecraft attitude information, and asteroid spin information. We compared topographic features contained in the altimeter data with those expressed by the reference shape model, and estimated long-period trajectory correction so that discrepancy between the two topographic profiles was minimized. The improved spacecraft positions are consistent with those determined by image-based stereophotoclinometry method within a few tens of meters. With such improved trajectory, the altimeter ranges can be converted to Ryugu's topographic profiles that are appropriate for geophysical interpretation. We present a geophysical application that invokes possibility of impact-induced formation of the Ryugu's western bulge. • Hayabusa2 LIDAR data contribute to quick improvement of the spacecraft trajectory • With this trajectory, topography of Ryugu is delineated by scan observations. • Formation of Ryugu's western bulge may have been triggered by an oblique impact. [ABSTRACT FROM AUTHOR]