CryoSat Ice Baseline-D validation and evolutions

The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 to monitor the precise changes in the thickness of terrestrial ice sheets and marine floating ice. To do that, CryoSat orbits the planet at an altitude of around 720 km with a retrograde orbit inclination of 92∘ and a quasi repeat cycle of 369 d (30 d subcycle). To reach the mission goals, the CryoSat products have to meet the highest quality standards to date, achieved through continual improvements of the operational processing chains. The new CryoSat Ice Baseline-D, in operation since 27 May 2019, represents a major processor upgrade with respect to the previous Ice Baseline-C. Over land ice the new Baseline-D provides better results with respect to the previous baseline when comparing the data to a reference elevation model over the Austfonna ice cap region, improving the ascending and descending crossover statistics from 1.9 to 0.1 m. The improved processing of the star tracker measurements implemented in Baseline-D has led to a reduction in the standard deviation of the point-to-point comparison with the previous star tracker processing method implemented in Baseline-C from 3.8 to 3.7 m. Over sea ice, Baseline-D improves the quality of the retrieved heights inside and at the boundaries of the synthetic aperture radar interferometric (SARIn or SIN) acquisition mask, removing the negative freeboard pattern which is beneficial not only for freeboard retrieval but also for any application that exploits the phase information from SARIn Level 1B (L1B) products. In addition, scatter comparisons with the Beaufort Gyre Exploration Project (BGEP; https://www.whoi.edu/beaufortgyre, last access: October 2019) and Operation IceBridge (OIB; Kurtz et al., 2013) in situ measurements confirm the improvements in the Baseline-D freeboard product quality. Relative to OIB, the Baseline-D freeboard mean bias is reduced by about 8 cm, which roughly corresponds to a 60 % decrease with respect to Baseline-C. The BGEP data indicate a similar tendency with a mean draft bias lowered from 0.85 to −0.14 m. For the two in situ datasets, the root mean square deviation (RMSD) is also well reduced from 14 to 11 cm for OIB and by a factor of 2 for the BGEP. Observations over inland waters show a slight increase in the percentage of good observations in Baseline-D, generally around 5 %–10 % for most lakes. This paper provides an overview of the new Level 1 and Level 2 (L2) CryoSat Ice Baseline-D evolutions and related data quality assessment, based on results obtained from analyzing the 6-month Baseline-D test dataset released to CryoSat expert users prior to the final transfer to operations.

Discovery of rotation axis alignments in Milky Way globular clusters

There is an increasing number of recent observational results that show that some globular clusters exhibit internal rotation while they travel along their orbital trajectories around the Milky Way center. Based on these findings, we searched for any relationship between the inclination angles of the globular cluster orbits with respect to the Milky Way plane and those of their rotation. We discovered that the relative inclination, in the sense of inclination of the rotation axis to orbit axis, is a function of the orbit inclination of the globular cluster. Rotation and orbit axes are aligned for an inclination of ∼56°, while the rotation axis inclination is far from the orbit inclination between ∼20° and −20° when the latter increases from 0° up to 90°. We further investigated the origin of this linear relationship and found no correlation with the semimajor axes and eccentricities of the globular cluster orbits, nor with the internal rotation strength, the globular cluster sizes, actual and tidally disrupted masses, or half-mass relaxation times, among others. The uncovered relationship will affect the development of numerical simulations of the internal rotation of globular clusters, our understanding of the interaction of globular clusters with the gravitational field of the Milky Way, and the observational campaigns made to increase the number of globular clusters with detected internal rotation.

Long-term activity of Comet C/2007 N3 (Lulin) as monitored by the SLT at the Lulin Observatory

The green comet C/2007 N3 (Lulin) is a new Oort cloud comet that has a retrograde orbit (inclination of $178^{\circ }$). It reached its perihelion on 2009 January 10, and its closest distance to Earth was 0.411 astronomical units (au) on February 24. Soon after its discovery on 2007 July 11, the coma activity of Comet Lulin was monitored closely by an Super Light Telescope 41 cm telescope until 2009 April. After long-term monitoring of Comet Lulin, the dust production rate [A(θ)fρ] was estimated. An unexpected increase in the ${A(0)f\rho}$ near the perigee appears to indicate an opposition effect. By investigating the surface brightness profiles, dust-to-gas ratios, and magnitudes, we ruled out the influences of gas and ion contamination and the outburst phenomenon. We discovered the anti-tail in late December 2008 but were unsure of the composition. We found that this abnormal tail lasted for a considerable time because of the effect of the orbital geometry. We also found that the jet activity coincided with the peak ${A(\theta)f\rho}$ values, and this clue helped us realize what was happening in the dust coma of Comet Lulin.

An Analysis of Flight Dynamics of a Space Debris Collector Transferring from its Orbital Plane to the Orbital Plane of a Debris Fragment

A space debris collector and a debris fragment move along random noncoplanar orbits ranging from 400 to 2000 km in height. The space collector leaves the base station, transfers into the orbital plane of a debris fragment, aligns itself with and approaches the fragment, grabs it and returns to the base station. The execution time of the flight mission is 24 hours. This paper examines only the stage when the debris collector transfers from its orbital plane to the fragment’s orbital plane. Dampening is provided by repeated activation of a cruise propulsion unit with the thrust of no less than 20000 N and the fuel specific impulse of no less than 15000 m/s. An analysis of dynamics of the orbital flight is performed by numerically integrating the equations of orbital movement of the debris collector and the debris fragment using the fourth order Runge-Kutta methods. The change of the scalar product sign of the vector of the orbit area integral of the debris fragment and the radius-vector of the debris collector is the criterion for intersecting the final orbit plane. Fuel depletion and the nonsphericity of the Earth’s gravitational field in the second zonal harmonic are taken into account, and an example of the calculations is given. Convergence estimates for the integration procedure with regard to the final orbit inclination relative to the orbit’s eccentricity are provided.

CryoSat Ice Baseline-D Validation and Evolutions

The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 to monitor the precise changes in the thickness of terrestrial ice sheets and marine floating ice. For that, CryoSat orbits the planet at an altitude of around 720 km with a retrograde orbit inclination of 92° and a quasi repeat cycle of 369 days (30 days sub-cycle). To reach the mission goals, the CryoSat products have to meet the highest quality standards to date, achieved through continual improvements of the operational processing chains. The new CryoSat Ice Baseline-D, in operation since 27th May 2019, represents a major processor upgrade with respect to the previous Ice Baseline-C. Over land ice the new Baseline-D provides better results with respect to previous baseline when comparing the data to a reference elevation model over the Austfonna ice cap region, improving the ascending and descending crossover statistics from 1.9 m to 0.1 m. The improved processing of the star tracker measurements implemented in Baseline-D has led to a reduction of the standard deviation of the point-to-point comparison with the previous star tracker processing method implemented in Baseline-C from 3.8 m to 3.7 m. Over sea ice, the Baseline-D improves the quality of the retrieved heights in areas up to ~ 12 km inside the Synthetic Aperture Radar Interferometric (SARIn or SIN) acquisition mask, which is beneficial not only for freeboard retrieval, but for any application that exploits the phase information from SARIn Level-1 (L1) products. In addition, scatter comparisons with the Beaufort Gyre Exploration Project (BGEP, https://www.whoi.edu/beaufortgyre) and Operation IceBridge (OIB, Kurtz et al., 2013) in-situ measurements confirm the improvements in the Baseline-D freeboard product quality. Relative to OIB, the Baseline-D freeboard mean bias is reduced by about 8 cm, which roughly corresponds to a 60 % decrease with respect to Baseline-C. The BGEP data indicate a similar tendency with a mean draft bias lowered from 0.85 m to −0.14 m. For the two in-situ datasets, the Root Mean Square Deviation (RMSD) is also well reduced from 14 cm to 11 cm for OIB and with a factor 2 for BGEP. Observations over inland waters, show a slight increase in the percentage of good observations in Baseline-D, generally around 5–10 % for most lakes. This paper provides an overview of the new Level-1 and Level-2 (L2) CryoSat ice Baseline-D evolutions and related data quality assessment, based on results obtained from analysing the 6-month Baseline-D test dataset released to CryoSat expert users prior the final transfer to operations.

Periodic transit timing variations and refined system parameters of the exoplanet XO-6b

Only a few exoplanets are known to orbit around fast rotating stars. One of them is XO-6b, which orbits an F5V-type star. Shortly after the discovery, we started multicolor photometric and radial-velocity follow-up observations of XO-6b, using the telescopes of Astronomical Institute of the Slovak Academy of Sciences. Our main scientific goals were to better characterize the planetary system and to search for transit timing variations. We refined several planetary and orbital parameters. Based on our measurements, the planet XO-6b seems to be about 10% larger, which is, however, only about 2σ difference, but its orbit inclination angle, with respect to the plane of the sky, seems to be significantly smaller, than it was determined originally by the discoverers. In this case we found about 9.5σ difference. Moreover, we observed periodic transit timing variations of XO-6b with a semi-amplitude of about 14 min and with a period of about 450 days. There are two plausible explanations of such transit timing variations: (1) a third object in the system XO-6 causing light-time effect, or (2) resonant perturbations between the transiting planet XO-6b and another unknown low-mass planet in this system. From the O-C diagram we derived that the assumed third object in the system should have a stellar mass, therefore significant variations are expected in the radial-velocity measurements of XO-6. Since this is not the case, and since all attempts to fit radial velocities and O-C data simultaneously failed to provide a consistent solution, more realistic is the second explanation.

Thermal Numerical Analysis of the Primary Composite Structure of a CubeSat

A thermal computational analysis for the composite structure of a CubeSat is presented. The main purpose of this investigation is to study the thermal performance of carbon fibre/epoxy resin composite materials with Zinc Oxide nanoparticles in order to be used in the panels of the primary structure of a CubeSat. The radiative heat fluxes over each composite panel are computed according to the orbit trajectory and they are utilized as boundary conditions for the analysis. The direct solar, albedo and Earth infrared radiation fluxes are considered in this study. The model implementation, including the computation of the orthotropic thermal conductivity of the composite material is presented. The thermal simulations were performed for three different orbit inclination angles: the selected mission ( β = 57 ∘ ), the worst hot ( β = 90 ∘ ) and the worst cold ( β = 0 ∘ ). The temperature ranges in the electronic boards are analyzed in order to show that are into the operating limits of each electronic component.

Combination Analysis of Future Polar-Type Gravity Mission and GRACE Follow-On

Thanks to the unprecedented success of Gravity Recovery and Climate Experiment (GRACE), its successive mission GRACE Follow-On (GFO) has been in orbit since May 2018 to continue measuring the Earth’s mass transport. In order to possibly enhance GFO in terms of mass transport estimates, four orbit configurations of future polar-type gravity mission (FPG) (with the same payload accuracy and orbit parameters as GRACE, but differing in orbit inclination) are investigated by full-scale simulations in both standalone and jointly with GFO. The results demonstrate that the retrograde orbit modes used in FPG are generally superior to prograde in terms of gravity field estimation in the case of a joint GFO configuration. Considering the FPG’s independent capability, the orbit configurations with 89- and 91-degree inclinations (namely FPG-89 and FPG-91) are further analyzed by joint GFO monthly gravity field models over the period of one-year. Our analyses show that the FPG-91 basically outperforms the FPG-89 in mass change estimates, especially at the medium- and low-latitude regions. Compared to GFO & FPG-89, about 22% noise reduction over the ocean area and 17% over land areas are achieved by the GFO & FPG-91 combined model. Therefore, the FPG-91 is worthy to be recommended for the further orbit design of FPGs.

Forecasting the orbital-climate diagram dynamics based on the wavelet analysis and fuzzy neural networks

Based on the new concept of the V. Bolshakov's orbital theory of paleoclimate, as well as the multiscale time-series wavelet decomposition method, and neural network fuzzy inference rules, the paper derives a predicted curve for the so-called orbital-climate diagram (OCD) in the ratio of (eccentricity, orbit inclination, precession) within the time interval from -1000 kyr in the past to 100 kyr in the future since modern times. This diagram features the Earth climate change caused by an insolation change to be the principal factor of the climate change, driven by Earth's orbital elements changes. Efficiency of the time-series forecast method is proved by the obtained predicted OCD trajectory verification within the past 100 kyr period and other paleoclimate data with a correlation coefficient 0.93.