Monitoring absolute vertical land motions and absolute sea-level changes from GPS and tide gauges data over French Polynesia

In this study,we estimate the absolute vertical land motions at three tidal stations with collocated Global Navigation Satellite System(GNSS)receivers over French Polynesia during the period 2007-2020,and obtain,as ancillary results,estimates of the absolute changes in sea level at the same locations.To verify our processing approach to determining vertical motion,we first modeled vertical motion at the International GNSS Service(IGS)THTI station located in the capital island of Tahiti and compared our estimate with previous independent determinations,with a good agreement.We obtained the following estimates for the vertical land motions at the tide gauges:Tubuai island,Austral Archipelago-0.92±0.17 mm/yr,Vairao village,Tahiti Iti:-0.49±0.39 mm/yr,Rikitea,Gambier Archipelago-0.43±0.17 mm/yr.The absolute variations of the sea level are:Tubuai island,Austral Archipelago 5.25±0.60 mm/yr,Vairao village,Tahiti Iti:3.62±0.52 mm/yr,Rikitea,Gambier Archipelago 1.52±0.23 mm/yr.We discuss these absolute values in light of the values obtained from altimetric measurements and other means in French Polynesia.

An Independent Degree-eight Mars Gravity Field Model and the Expected Results from the Tianwen-1 Mission

Tianwen-1 is China's first independent interplanetary exploration mission,targeting Mars,and includes orbiting,landing,and rover phases.Similar to previous Mars missions,the Tianwen-1 orbiter was designed for polar orbits during the scientific mission period but has an exceptional eccentricity of approximately 0.59.We provide the first independent eight-degree Martian gravity field model in this paper,which was developed exclusively by a team working in China with our independent software as well,based on about two months of radiometric Doppler and range data from only the Tianwen-1 mission.This model is independent from the models created by the groups at NASA Jet Propulsion Laboratory and Goddard Space Flight Center in the United States,as well as the Centre National d'Etudes Spatiales in France.Furthermore,in order to optimize the engineering and scientific benefits,we proposed a number of potential orbits for the extended Tianwen-1 mission.In order to solve a higher-degree independent Mars gravity field model,the viability of modifying the perigee height was investigated,with the priority considerations of fuel savings and implementation hazards being controlled.

Combination of Tsoft and ET34-ANA-V80 software for the preprocessing and analysis of tide gauge data in French Polynesia

Since 2008 a network of five sea-level monitoring stations was progressively installed in French Polynesia.The stations are autonomous and data,collected at a sampling rate of 1 or 2 min,are not only recorded locally,but also transferred in real time by a radio-link to the NOAA through the GOES satellite.The new ET34-ANA-V80 version of ETERNA,initially developed for Earth Tides analysis,is now able to analyze ocean tides records.Through a two-step validation scheme,we took advantage of the flexibility of this new version,operated in conjunction with the preprocessing facilities of the Tsoft software,to recover co rrected data series able to model sea-level variations after elimination of the ocean tides signal.We performed the tidal analysis of the tide gauge data with the highest possible selectivity(optimal wave grouping)and a maximum of additional terms(shallow water constituents).Our goal was to provide corrected data series and modelled ocean tides signal to compute tide-free sea-level variations as well as tidal prediction models with centimeter precision.We also present in this study the characteristics of the ocean tides in French Polynesia and preliminary results concerning the non-tidal variations of the sea level concerning the tide gauge setting.

月球重力场对月球内部结构研究的作用

月球是人类深空探测的首要目标,而月球的内部结构则是我们了解月球与地月系形成与演化的重要窗口.相比于其他物理场,月球重力场模型具有全球均匀覆盖、精度和分辨率持续提高的特点,在月球内部结构研究中扮演着重要作用.本文对月球重力场在月球内部结构研究中发挥的作用进行了论述,包括月壳、月幔以及月核,其中重点介绍了高阶次重力场模型对浅层结构的研究贡献.进而,讨论了利用月球重力数据反演内部结构中面临的若干问题.最后,对借助重力场模型开展月球内部结构研究的应用前景进行了总结与展望.

Simulated Gravity Field Estimation for the Main Belt Comet 133P/Elst-Pizarro Based on a Satellite-to-satellite Tracking Mode

Most asteroids and comets are formed in the early stages of the solar system and therefore contain a wealth of information about their birth.The asteroid exploration mission planned in the coming years by China will likely target the celestial body named 133P/Elst-Pizarro(estimated diameter of about 4 km).The orbit of this asteroid stays within the asteroid belt,but nevertheless,it displays a comet-like dust tail.In this study,we used differential tracking data between two simulated probes and the data from an Earth station to estimate 133P gravity field model.This observation mode is similar to how the gravity field was estimated for large celestial objects in the GRAIL and GRACE missions,but here the object is the very small 133P asteroid.We compared the estimated gravity fields obtained for 133P from the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode,with only the Earth-based two-way range rate mode.The results show that the accuracy of the low-degree(4 degree and order)estimate of the gravity field is improved by one order of magnitude by using the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode with respect to the Earthonly tracking.Furthermore,another order of magnitude improvement in the gravity field solution is gained by decreasing the orbit altitude from 12 to 8 km.

Monitoring Zenithal Total Delays over the three different climatic zones from IGS GPS final products:A comparison between the use of the VMF1 and GMF mapping functions

The International GNSS Service(IGS) final products(ephemeris and clocks-correction) have made the GNSS an indispensable low-cost tool for scientific research, for example sub-daily atmospheric water vapor monitoring. In this study, we investigate if there is a systematic difference coming from the choice between the Vienna Mapping Function 1(VMF1) and the Global Mapping Function(GMF) for the modeling of Zenith Total Delay(ZTD) estimates, as well as the Integrated Precipitable Water Vapor(IPWV) estimates that are deduced from them. As ZTD estimates cannot be fully separated from coordinate estimates, we also investigated the coordinate repeatability between subsequent measurements.For this purpose, we monitored twelve GNSS stations on a global scale, for each of the three climatic zones(polar, mid-latitudes and tropical), with four stations on each zone. We used an automated processing based on the Bernese GNSS Software Version 5.2 by applying the Precise Point Positioning(PPP)approach, L3 Ionosphere-free linear combination, 7 cutoff elevation angle and 2 h sampling. We noticed an excellent agreement with the ZTD estimates and coordinate repeatability for all the stations w.r.t to CODE(the Center for Orbit Determination in Europe) and USNO(US Naval Observatory) products, except for the Antarctic station(Davis) which shows systematic biases for the GMF related results. As a final step, we investigated the effect of using two mapping functions(VMF1 and GMF) to estimate the IPWV,w.r.t the IPWV estimates provided by the Integrated Global Radiosonde Archive(IGRA). The GPS-derived IPWV estimates are very close to the radiosonde-derived IPWV estimates, except for one station in the tropics(Tahiti).

Inversion of Venus internal structure based on geodetic data

Understanding the internal structure of Venus promotes the exploration of the evolutionary history of this planet.However,the existing research concerning the internal structure of Venus has not used any inversion methods.In this work we employed an inversion method to determine the internal structure of Venus using observational or hypothetical geodetic data;these data include mass,mean radius,mean moment of inertia and second degree tidal Love number k2.To determine the core state of Venus,we created two models of Venus,an isotropic 3-layer model with entire liquid core and an isotropic 4-layer model with liquid outer core and a solid inner core,assuming that the interior of Venus is spherically symmetric and in hydrostatic equilibrium.A series of the sensitivity analysis of interior structure parameters to the geodetic data considered in here shows that not all of the parameters can be constrained by the geodetic data from Venus.On this basis,a Markov Chain Monte Carlo algorithm was used to determine the posterior probability distribution and the optimal values of the internal structure parameters of Venus with the geodetic data.We found that the 3-layer model is more credible than the 4-layer model via currently geodetic data.For the assumption of the 3-layer model with the k2=0.295±0.066,I/MR^2=0.33±0.0165,andρ=5242.7±2.6 kg m^-3,the liquid iron-rich core of Venus has a radius of 3294+215-261km,which suggests a larger core than previous research has indicated.The average density of the mantle and liquid core of Venus are 4101+325-375and 11885+955-1242kg m~^-3,respectively.

A new method to determine the gravity field of small bodies from line-of-sight acceleration data

We present a new method to derive line-of-sight acceleration observables from spacecraft radio tracking data. The observables can be used to estimate the mass and gravity of a natural satellite as a spacecraft flyby. The corresponding observation model adapts to one-way and two/three-way tracking modes. As a test case for method validation and application, we estimated the mass and degree two gravity field for the Martian moon Phobos using simulated tracking data when the spacecraft Mars Express flew by Phobos on 2013 December 29. We have a few real tracking data during flyby and they will be used to confirm raw data simulation. The main purpose of this paper is to demonstrate the method of line-of-sight acceleration reduction from raw tracking data and the feasibility to estimate mass and gravity of a natural satellite using this type of observable. This novel method is potentially applicable to planet and asteroid gravity field studies combined with Doppler tracking data.

Strong Spatial Aggregation of Martian Surface Temperature Shaped by Spatial and Seasonal Variations in Meteorological and Environmental Factors

Spatio-temporal variation in the Martian surface temperature(MST)is an indicator of ground level thermal processes and hence a building block for climate models.However,the distribution of MST exhibits different levels of spatial aggregation or heterogeneity,and varies in space and time.Furthermore,the effect of regional differences in meteorological or environmental factors on the MST is not well understood.Thus,we investigated the degree of spatial autocorrelation of MST across the surface of Mars globally by Moran’s I,and identified the hot spots by GetisOrd G;*.We also estimated the regional differences in the influence of seasonally dominant factors including thermal inertia(TI),albedo,surface pressure,latitude,dust and slope on MST by a geographically weighted regression model.The results indicate(1)that MST is spatially aggregated and hot and cold spots varied over time and space.(2)Hemispheric differences in topography,surface TI and albedo were primarily responsible for the hemispheric asymmetry of hot spots.(3)The dominant factors varied by geographical locations and seasons.For example,the seasonal Hadley circulation dominates at the low-latitudes and CO;circulation at the high-latitudes.(4)Regions with extreme variations in topography and low TI were sensitive to meteorological and environmental factors such as dust and CO_(2)ice.We conclude that the spatial autocorrelation of MST and the spatial and seasonal heterogeneity of influencing factors must be considered when simulating Martian climate models.This work provides a reference for further exploration of Martian climatic processes.

A simulation experiment on the GM estimation for Comet 133P/Elst-Pizarro

In China’s asteroid mission to be launched around 2025,(7968)133 P/Elst-Pizarro(hereafter 133 P)will be the second target,after a visit to asteroid(469219)Kamo’oalewa.This paper describes a simulation of precise orbit determination for the spacecraft around comet 133 P,as well as estimation of its gravitational parameter(GM)value and the solar radiation pressure coefficient Cr for the spacecraft.Different cometocentric distances of 200,150 and 100 km orbits are considered,as well as two tracking modes:exclusive two-way range-rate mode(Earth station to spacecraft)and combinations of two-way range-rate and local spacecraft onboard ranging to the comet.Compared to exclusive two-way range-rate,the introduction of local ranging observables improves the final GM uncertainties by up to one order of magnitude.An ephemeris error in the orbit of 133 P is also considered,and we show that,to obtain a reliable estimate of the GM for 133 P,this error cannot exceed a one km range.

Reducing loss of significance in the computation of Earth-based two-way Doppler observables for small body missions

Two-way Doppler measurement is a typical Earth-based radiometric technique for interplanetary spacecraft navigation and gravity science investigation.The most widely used model for the computation of two-way Doppler observables is Moyer’s differenced-range Doppler(DRD) formula,which is based on a Schwarzschild approximation of the Solar-System space-time.However,the computation of range difference in DRD formula is sensitive to round-off errors due to approximate numbers defined by the norm IEEE754 in all PCs.This paper presented two updated models and their corresponding detailed instructions for the computation of the two-way Doppler observables so as to impair the effects of this type of numerical error.These two models were validated by two case studies related to the Rosetta mission—asteroid Lutetia flyby and comet 67 P/Churyumov-Gerasimenko orbiting case.In these two cases,the numerical noise from the updated models can be reduced by two orders-of-magnitude in the computed two-way Doppler observables.The results showed an accuracy from better than 6 × 10^(-3) mm s^(-1) at 1 s counting time interval to better than 3 × 10^(-5) mm s^(-1) at 60 s counting time interval.

Computation of the atmosphere-less light intensity curve during a total solar eclipse by using Lunar Reconnaissance Orbiter topography data and the DE430 astronomical ephemeris

Observations of the sky irradiation intensity in the visible wavelengths during a solar eclipse permit to model the Sun diameter,a key number to constrain the internal structure of our star.In this paper,we present an algorithm that takes advantage of the precise Moon topography from Lunar Reconnaissance Orbiter to compute,with a high resolution in time,the geometrical part(i.e.top-of-atmosphere,and for a given wavelength)of the sky irradiation at any given location on the Earth during these events.The algorithm is also able to model the Baily’s beads.We give as an application the theoretical computation of the light curve corresponding to the solar eclipse observed at Lakeland(Queensland,North Australia)on 2012 November 13.The application to real data,with the introduction of atmospheric and instrumental passbands,will be considered in a forthcoming paper.

中国后续月球探测候选着陆区沙克尔顿撞击坑的光照及其风化层温度

为探讨中国后续月球探测候选着陆区沙克尔顿撞击坑的光照和温度,利用NASA的SPICE系统估算了考虑地形遮挡的实时光照.结果表明,实时光照分布图与早期研究相似,证明本文算法合理.另外,文章还估算了巡视器可能移动期间的累积光照分布,发现沙克尔顿撞击坑的内壁光照占整个内壁的27%,表明巡视器可以在其移动期间接收充足的太阳辐射能量.为了稳定初始温度,利用估算的实时光照和通用的一维热扩散模型,连续估算了近20年的风化层温度,并以最后时刻的温度值作为后续研究时间段的初始值,后续研究时间从2023年7月20日至2027年5月8日.结果表明,永久阴影区的温度几乎保持不变,表明长时段连续估算温度,以末时刻的温度值作为后续研究时段的温度初始值,这样的温度稳定算法是合理的.结果同时表明,月表温度对瞬时光照非常敏感,而地底温度却更多地与累积光照有关,这种差异间接地反映了风化层低效的热传导特性.太阳辐射充足区的温度远大于水冰稳定的阈值(约112K),而永久阴影区的温度低至25K,该极端低温对巡视器具有一定的危害性.尽管如此,研究区域仍然存在温度适宜区域,其特点是表层较暖和而底部较冷,较冷的底部有助于水冰的保存.为了进一步确保月球车的正常移动,文章给出了这类温度适宜区域的分布图,发现这类区域不仅存在于撞击坑内壁,还存在于撞击坑的外部.考虑到适宜的温度和平缓的坡度,提出4个样品采集的优选位置.

基于改进彗星星历的67P/Churyumov-Gerasimenko的三阶重力场模型

本文解算了Rosetta彗星探测任务的主要目标——彗星67P/Churyumov-Gerasimenko的重力场模型.与以往模型的主要区别是,本文使用了JPL团队最新解算的67P星历模型,该彗星星历在很大程度上得到了改进.67P的改进星历能够对其GM值的解算产生一定的影响,这一影响与67P通过近日点由于彗星喷发造成的质量损失处于同一水平.新的彗星重力场解算结果表明,67P重力异常比过去的模型更加显著.从2.65 km的参考球上看,新的重力场模型最大重力异常值约为0.9±0.03 mGal,较大的异常值表明了该彗星内部质量分布不均一的特性,这可能是由于高孔隙率和分层的彗核结构导致的.

Illumination and regolith temperature at China's next candidate lunar landing site Shackleton crater

This study focuses on the illumination and temperature at China’s next lunar candidate landing site Shackleton crater.We used the NASA’s SPICE system to evaluate the terrain obscuration effect on real-time illumination;the resulting illumination map resembles previous studies,validating the methodologies used in our study.In addition,we estimated an accumulated illumination map for the period of likely rover movement.The map indicates the illuminated inner wall of the Shackleton crater is close to 27%of the whole,meaning that the rover will likely receive solar radiation during its movement.Using the real-time illumination and the distributed 1-D thermal diffusion model,we continuously evaluated the regolith temperature for more than 20 years to stabilize the temperature,and selected the temperature of the end time as the initial value used in a thermal study set for July 20,2023 and May 8,2027.Our results indicate the temperature in the permanent shadow region remains nearly constant,thus validating the stability of our estimated initial temperature.Our results also indicate that the surface temperature is more sensitive to transient illumination,but the subsurface temperature is more likely to be associated with the accumulated illumination.This difference indirectly implies that the conductivity of the lunar regolith is inefficient.The locations receiving more solar radiation show a temperature larger than the threshold(~112 K)of ice stability.The permanently shadowed regions can be as cold as 25 K,and such extreme coldness is a hazard to the rover.There are suitable temperature locations which have a warm surface but cold subsurface to preserve water ice.To further ensure normal rover movement,we provided a map of suitable temperature sites and found that these locations exist not only in the Shackleton crater’s inner wall,but also outside the crater.We suggested four trade-off sampling sites with suitable temperatures and gradual slopes.

Reconstructing the cruise-phase trajectory of deep-space probes in a general relativistic framework:An application to the Cassini gravitational wave experiment

Einstein’s theory of general relativity is playing an increasingly important role in fields such as interplanetary navigation,astrometry,and metrology.Modern spacecraft and interplanetary probe prediction and estimation platforms employ a perturbed Newtonian framework,supplemented with the Einstein-Infeld-Hoffmann n-body equations of motion.While time in Newtonian mechanics is formally universal,the accuracy of modern radiometric tracking systems necessitate linear corrections via increasingly complex and error-prone post-Newtonian techniques—to account for light deflection due to the solar system bodies.With flagship projects such as the ESA/JAXA BepiColombo mission now operating at unprecedented levels of accuracy,we believe the standard corrected Newtonian paradigm is approaching its limits in terms of complexity.In this paper,we employ a novel prototype software,General Relativistic Accelerometer-based Propagation Environment,to reconstruct the Cassini cruise-phase trajectory during its first gravitational wave experiment in a fully relativistic framework.The results presented herein agree with post-processed trajectory information obtained from NASA’s SPICE kernels at the order of centimetres.

小行星433-Eros土壤厚度分布和表面强度估计

根据Near探测器飞跃433-Eros小行星时观测得到的重力数据分析,在433-Eros小行星外侧边缘处存在轻微的负布格异常现象(Garmier et al.,2002).本文借鉴Geissler在Ida小行星上的结论,认为该异常是由其表面撞击过程中高速溅射物聚集效应产生的.文中计算表明:在合理范围内调整模型Z参数和强度P参数,Eros上高速溅射物将聚集在其表面边缘外侧,这与Geissler实验结论是一致的.计算结果同时表明433-Eros行星表面的土壤层厚度以及布格异常数值与433-Eros表面物质的强度相关,通过这一相关性,可以约束433-Eros表面的强度参数P及其土壤层分布.取Z模型中Z参数为2.5时,我们估计出Eros表面物质的强度值大约为106Pa,土壤层的堆积情况是全球平均堆积厚度约为30 m,Eros边缘末端处厚度为50 m.

岁差章动极移对轨道根数的影响

基于岁差章动和极移的理论模型,以及卫星受摄运动的高斯方程,本文研究了岁差章动和极移对开普勒轨道根数的影响.除了定性分析的理论推导外,本文还根据定性理论公式给出了定量的计算分析,从而获得了卫星轨道解析理论中处理岁差章动极移所需要的理论依据.岁差章动对开普勒轨道根数的影响是比摄动力低一阶的,极移的影响是低二阶的.因此对于一阶轨道解析理论,岁差章动和极移的影响全部是二阶的从而可以忽略不计.对于卫星轨道二阶解析理论,仅一阶摄动力需要考虑岁差章动的影响,其余可以全部忽略不计.由于只有地球扁率摄动是一阶的,所以只有地球扁率摄动J2项需要考虑岁差章动的影响.在定轨时间段小于三天时,岁差章动在这期间的变化量是二阶,所以可以把岁差章动的影响看成是不变的.本文结论对解析定轨具有指导意义.