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Variability and accuracy of Zenith Total Delay over the East African tropical region

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Richard Cliffe Ssenyunzi, Bosco Oruru, Florence Mutonyi D’ujanga, Eugenio Realini, Stefano Barindelli, Giulio Tagliaferro, Nick van de Giesen

Abstract

The Global Navigation Satellite System (GNSS) can be used to derive accurately the Zenith Tropospheric Delay (ZTD) under all-weather conditions. The derived ZTDs play a vital role in climate studies, weather forecasting and are operationally assimilated into numerical weather prediction models. In this study, variations and statistical analysis of GNSS-derived ZTD over the East African tropical region are analysed. The data is collected from 13 geodetic permanent stations for the period of 4 years from 2013 to 2016. The 13 stations consist of 5 International GNSS Service (IGS) stations plus 8 stations as follows: 4 Africa Array stations and 4 Malawi Rifting stations from Uganda, Kenya, Tanzania and Rwanda. The ZTD time series were processed using goGPS software version 1.0 beta1, a MATLAB based GNSS processing software, originally developed for kinematic applications but recently re-engineered for quasi static applications. The annual variation of the ZTD time series was investigated using Lomb Scargle periodograms. The semi-annual frequency has the dominant power in subregion 1 (latitudes 4 °S and 4 °N) and the annual frequency has the dominant power in subregion 2 (latitudes 12 °S to 4 °S). The highest ZTD estimates occur during the rainy seasons, at all stations, and the lowest estimates occur during the dry seasons. The results also show that the ZTD estimates are largest at stations located at low elevation (regions close to the Indian Ocean). The derived ZTDs are compared to the values derived from the GIPSY-OASIS via Jet Propulsion Laboratory (JPL) online Automatic Precise Positioning Service (APPS) and the Unified Environmental Modelling System (UEMS) numerical weather prediction (NWP) model. The comparison of goGPS and APPS ZTD at the 13 stations shows an overall average bias, Root Mean Square (RMS) and standard deviation (stdev) of −0.9 mm, 3.2 mm and 3.0 mm respectively, with correlation coefficients ranging from 0.974 to 0.999. The comparison of goGPS ZTD against UEMS NWP ZTD at 8 selected stations shows average bias, RMS and stdev of −12.4 mm, 22.0 mm and 17.6 mm respectively, with correlation coefficients ranging from 0.802 to 0.974. The agreement between the GPS ZTD and the NWP ZTD indicates that goGPS ZTD can be assimilated into NWP models in the East African region.

Variation of ionospheric range errors for L1 frequency GPS users during the 23rd solar cycle over BAHR IGS station

Publication date: Available online 4 June 2019

Source: Advances in Space Research

Author(s): Abdollah Masoud Darya, Muhammad Mubasshir Shaikh, Ilias Fernini

Abstract

Errors induced by the ionosphere on global navigation satellite systems (GNSS) signal propagation significantly affect the positioning calculation done by ground receivers. These ionospheric errors may end up reaching tens of meters in the final positioning calculation. In this study, the ionospheric range error (IRE) was monitored over the local ionosphere of BAHR, Bahrain (26.209N, 50.608E) during the period of the 23rd solar cycle. IRE values were obtained through observation data derived from RINEX files and compared with NeQuick 2 (NQ2) model calculations. It was found that, for the region of study, NQ2 overestimated the total electron content (TEC) values as compared to observation data, resulting in higher IRE values of up to 12 m. However, IRE derived using GNSS observations and NQ2 follow similar trends over the course of the solar cycle. IRE values were also compared to the smoothed sunspot number (SSN) and F10.7 indexes which resulted in significant correlation between the seasonal calculation of IRE and solar activity. Throughout the 23rd solar cycle, the highest IRE values were found during the equinoxes and the lowest during solstices. The largest IRE value was observed in the vernal equinox of 2000 (19.13 m), while the lowest IRE value was observed in the winter solstice of 1998 (0.276 m).

Cut-off features in interplanetary solar radio type IV emission

Publication date: Available online 4 June 2019

Source: Advances in Space Research

Author(s): Silja Pohjolainen, Nasrin Talebpour Sheshvan

Abstract

Solar radio type IV bursts can sometimes show directivity, so that no burst is observed when the source region in located far from the solar disk center. This has recently been verified also from space observations, at decameter wavelengths, using a 3D-view to the Sun with STEREO and Wind satellites. It is unclear whether the directivity is caused by the emission mechanism, by reduced radio wave formation toward certain directions, or by absorption/blocking of radio waves along the line of sight. We present here observations of three type IV burst events that occurred on 23, 25, and 29 July 2004, and originated from the same active region. The source location of the first event was near the solar disk center and in the third event near the west limb. Our analysis shows that in the last two events the type IV bursts experienced partial cut-offs in their emission, that coincided with the appearance of shock-related type II bursts. The type II bursts were formed at the flanks and leading fronts of propagating coronal mass ejections (CMEs). These events support the suggestion of absorption toward directions where the type II shock regions are located.

Advanced meteor radar observations of mesospheric dynamics during 2017 minor SSW over the tropical region

Publication date: Available online 4 June 2019

Source: Advances in Space Research

Author(s): S. Eswaraiah, M. Venkat Ratnam, Yong Ha Kim, Kondapalli Niranjan Kumar, G. Venkata Chalapathi, L. Ramanajaneyulu, Jaewook Lee, P. Vishnu Prasanth, K. Thyagarajan, S.V.B. Rao

Abstract

The observations of the advanced meteor radar at the Indian tropical station Tirupati (13.63°N, 79.4°E) have been used to investigate the effects of the 2017 minor Sudden Stratospheric Warming (SSW) on the tropical mesosphere and lower thermosphere (MLT). The episodic minor warmings were observed in February 2017 with the gain of ∼40 K in the polar stratospheric temperature (PST), followed by a weakening of the eastward wind by 27 m/s. The observations show the large temporal variations in the zonal wind at 75–80 km during the SSW. We report the occurrence of 14–16 day waves in the MLT zonal wind during SSW and the secondary waves (2–7 days) in the meridional wind after SSW. A large enhancement (∼30 m/s) is observed in the amplitude of the semi-diurnal tide (SDT) during the SSW. The present results are similar to those observed during the major SSW. Therefore, the present results stress that the studies of minor SSW on the tropical MLT dynamics must be important.

A novel adaptive beamforming algorithm against impulsive noise with alpha-stable process for satellite navigation signal acquisition

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Haichuan Zhang, Fangling Zeng, Daqian Lv, Huishu Wu

Abstract

Adaptive beamforming is an effective spatial filtering technique for overcoming the vulnerability to interference of global navigation satellite system (GNSS) receivers. Although beamforming-based satellite system has the capability of nulling electronic interference sources, the distortions to GNSS receiver induced by impulsive noises are always neglected. This paper addresses the satellite navigation signal acquisition problem in the presence of impulsive noises with alpha-stable noise using the maximum correntropy criterion in framework of the GNSS system. In addition, in order to decrease the number of active elements for avoiding overmuch energy consumption, a sparse regularization is introduced to the constraints of the novel criterion. From the analysis, the novel constraint sparse maximum correntropy (CSMC) beamforming technique that can achieve robustness against impulsive noises which uses less power is developed in this manuscript for satellite signal acquisition. The proposed CSMC, maintains the robustness against impulsive outliers and achieve better performance in conjunction with less power consumption. A mean square analysis of the CSMC algorithm is presented to verify the validity of our theory. Simulation results demonstrate the superiority of the proposed methods over other previously developed beamforming techniques in GNSS.

Experimental study on drilling basalt with small diameter drilling tools

Publication date: Available online 1 June 2019

Source: Advances in Space Research

Author(s): Xiaogan Peng, Ting Zeng, Zhongwang Yin, Zeng Zhao, Liang Li

Abstract

Basalt is one of the potential hard rock targets for drilling sampling in asteroid exploration, and low sampling reaction forces are required in drilling sampling activities of an asteroid. In this paper, the experimental study of drilling basalt with small diameter cemented carbide triangular bit and diamond trepanning drill was carried out, and the drilling thrust force models of basalt drilled by cemented carbide triangular bit and diamond trepanning drill were established, respectively. The ratio of volume removal rate to drilling thrust force was proposed as the evaluation index for the primary selection of drilling tools. The test results showed that the Φ4 mm cemented carbide triangular bit was the preferred small diameter tool for drilling basalt when the minimum drilling thrust force is 40 N. The Φ8 mm electroplated diamond trepanning drill was also the preferred small diameter tool for drilling basalt, Its minimum critical drilling thrust is between 100 and 110 N. Further analysis of the drilling thrust model of the two types of drilling tools showed that the drilling thrust force of cemented carbide triangular bit drilling basalt increased with the feed rate, and it increased with the increase of the apex angle of cemented carbide triangular bit. There was minimum and maximum critical drilling thrust force in drilling basalt with diamond trepanning drill. When the drilling thrust force was between the minimum and maximum critical drilling thrust force, the volume removal rate can be improved by increasing the rotating speed of the diamond trepanning drill. This experimental study helps to optimize the drilling tools and set the drilling thrust force and rotating speed when further studying of hard rock sampling for asteroid drilling.

Optimal transfer between elliptic orbits with three tangential impulses

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Andrea Caruso, Alessandro A. Quarta, Giovanni Mengali

Abstract

This paper introduces a mathematical model that can be used to evaluate the total velocity variation required to accomplish a given two-dimensional orbit transfer, using up to three tangential impulsive maneuvers. The problem is addressed in an optimal framework, by looking for the transfer trajectory that minimizes the total velocity variation. In particular, by exploiting the boundary nonlinear constraint equations, the total velocity variation can be calculated as a function only of the spacecraft angular position at which the impulses are applied. The small number of control variables involved in the algorithm allows the optimization problem to be solved in a simple and robust way, with a small computational effort. The algorithm is able to find the optimal transfer strategy in a generic ellipse-to-ellipse, two-dimensional, mission scenario.

Adaptive anti-windup control of post-capture combination via tethered space robot

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Yingbo Lu, Panfeng Huang, Zhongjie Meng

Abstract

Stabilization control is an essential mission for the tethered space robot-target combination during the post-capture phase of tethered space robot (TSR). With the consideration of the space tether and the three dimensional attitude of the post-capture combination, dynamic model of the combination is derived by using Lagrange method. Considering the unknown dynamic parameters (the tether attachment point, unknown target inertia parameters) of the post-capture combination, an adaptive anti-windup control approach is presented to overcome the problem of dynamic uncertainty and control input saturation. Complete stability and performance analyses are presented and illustrative simulation results of application to the post-capture combination system verify the effectiveness of the proposed algorithm.

Safe deployment of cluster-flying nano-satellites using relative E/I vector separation

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Pengfei Liu, Xiaoqian Chen, Yong Zhao

Abstract

Nano-satellites cluster flight has attracted an increasing interest in the domain of distributed spacecraft system in recent years. As the first phase of a cluster flight mission, on-orbit deployment process constitutes a great technical challenge, since not only safe relative trajectories, but also practical operational constraints must be considered. To deal with these issues, the concept of relative Eccentricity/Inclination (E/I) vector separation was utilized in the safety concept design and the sequent release parameters solving, as it provides direct insight into the safety characteristics of relative motion. Accordingly, a novel operational methodology for the safe deployment of cluster-flying nano-satellites is provided. It can deterministically generate deployment sequences that ensure safe relative trajectories between released satellite (RS) pairs, as well as that between the launch vehicle (LV) and RSs, for enough long time interval. Particularly, according to our methodology, no maneuver efforts are required for the LV and RSs either during the deployment process or after deployment. Moreover, the proposed methodology adheres to practical constraints from either the LV or ground station. A typical simulation scenario was setup for the deployment process of the pioneering cluster flight mission - Satellite Mission for Swarming and Geolocation (SAMSON). Results demonstrate the feasibility and efficiency of our methodology.

Equatorial and low-latitude positive ionospheric phases due to moderate geomagnetic storm during high solar activity in January 2013

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): B.A.G. Ribeiro, P.R. Fagundes, K. Venkatesh, A. Tardelli, V.G. Pillat, G.K. Seemala

Abstract

The day-to-day variability of the equatorial and low-latitude ionosphere during quiet and disturbed periods is one of the ionospheric highlighted Space Weather research topics, particularly the ionospheric electrodynamics during geomagnetic storms. This study investigates the response of ionospheric F-region from the equatorial region to beyond the Equatorial Ionization Anomaly (EIA) crest during moderate geomagnetic storm (minimum Dst = −53 nT) that took place on January 17 to 18, 2013, during the high solar activity period of solar cycle 24. The Total Electron Content (TEC) obtained through a network of 82 dual frequency GPS receivers, spanning over an area of 30° × 30° in latitude and longitude are used. Also the F-layer virtual height (h’F) and critical frequency (foF2) observations from 3 ionosondes, in the South American sector are used. Specifically, these GPS-TEC receivers and ionosondes are used to investigate how the F-layer was disturbed by two positive ionospheric phases occurred during the aforementioned disturbed period. The first positive ionospheric phase was probably due to a travelling ionospheric disturbance (TID). When this TID reached the Brazilian coast at low-latitude, the EIA crest was in the growth phase and makes it challenging to separate the spatial-temporal evolution of both phenomena. The second positive ionospheric phase was caused by an anomalous nighttime equatorial positive ionospheric cloud travelling from the east sector towards the west sector. In addition, how the EIA was disturbed by these two positive ionospheric phases in the eastern and western Brazilian sectors is also investigated.

Hypertemporal photometric measurement of spaceborne mirrors specular reflectivity for Laser Time Transfer link model

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Daniel Kucharski, Georg Kirchner, Toshimichi Otsubo, Hiroo Kunimori, Moriba K. Jah, Franz Koidl, James C. Bennett, Hyung-Chul Lim, Peiyuan Wang, Michael Steindorfer, Krzysztof Sośnica

Abstract

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.

Orbit and tracking data evaluation of Chang’E-4 relay satellite

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Songhe Qin, Yong Huang, Peijia Li, Quan Shan, Min Fan, Xiaogong Hu, Guangli Wang

Abstract

A lander and a rover were placed on the Moon in China’s Chang’E-4 Lunar exploration mission, which is the first lander on the far side of the Moon. To provide communication and telemetry between the Earth and the lander, the CE-4 relay satellite (Queqiao) was launched firstly to the Earth-Moon L2 region in May 2018. The orbit precision of the Chang’E-4 relay satellite is evaluated in this paper. The ground tracking data including Ranging, Doppler, VLBI delay and rate are processed using a batch least squares method. The results show that the residual RMS of ranging, Doppler, VLBI delay and VLBI delay rate based on S-band are about 0.53 m, 0.37 mm/s, 1.16 ns and 0.67 ps/s, respectively. Compared with the previous missions (CE-1/CE-2/CE-5T1) using S-band tracking systems, the measurement precision of the CE-4 relay satellite is significantly improved. Furthermore, the analysis shows that VLBI data can improve the precision of orbit determination and prediction of the halo orbit around the Earth-Moon L2 point: the position precision is better than 100 m and the velocity precision is better than 2 mm/s. Since the CE-4 relay satellite is less than 300 km away from the lunar surface when it flies by the Moon, the lunar non-spherical gravitation has a significant influence on the orbit determination, and the lunar gravity field need to be considered when the Earth is chosen as the central body.

Ionospheric scintillation impact on ambiguity resolution using ADOP in closed form

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): C.M. Silva, D.B.M. Alves, E.M. Souza, P.T. Setti Junior

Abstract

The Ambiguity Dilution of Precision (ADOP) is a well-known scalar measure that can be used to infer the strength of the Global Navigation Satellite System (GNSS) model of the carrier phase ambiguities involved in precise relative GNSS positioning. Odijk and Teunissen (2008a) derived closed-form expressions for single-baseline GNSS models that allow verifying the factors affecting the ambiguity resolution as well as the probability of its correct resolution as integer values. However, this weighted-ionosphere ADOP closed form assumes that the standard deviation of the ionosphere delay is dependent on a function associated with the baseline length. This means that a baseline of the same length at different locations around the world and in different moments in time would have the same standard deviation, which in practice is not true. This becomes even worse in regions like Brazil, where the ionospheric anomalies are more intense and frequent, especially in periods of high solar activity. In this work, a new method for calculating the ionospheric delay standard deviation that considers the ionospheric scintillation S4 index is proposed to improve the closed-form ADOP performance. Experiments with a baseline of 280 m located in Presidente Prudente, Brazil (magnetic latitude of around −13°), were carried out. The results showed that in periods of both weak and strong ionospheric scintillation, the introduction of S4 into the ionospheric delay standard deviation was beneficial for ADOP. The average improvement of the ADOP closed form was of around 72% in the analyzed period of weak scintillation and of 35% in the period of strong scintillation. In addition, the results showed that in periods of strong scintillation, relative positioning accuracy is around 100 times worse, up to 30 m, when compared to periods of weak ionospheric scintillation.

Altimetry-based sea level trends along the coasts of Western Africa

Publication date: Available online 25 May 2019

Source: Advances in Space Research

Author(s): Florence Marti, Anny Cazenave, Florence Birol, Marcello Passaro, Fabien Léger, Fernando Niño, Rafael Almar, Jérôme Benveniste, Jean François Legeais

Abstract

We present results of contemporary coastal sea level changes along the coasts of Western Africa, obtained from a dedicated reprocessing of satellite altimetry data done in the context of the ESA ‘Climate Change Initiative’ sea level project. High sampling rate (20 Hz) sea level data from the Jason-1 and Jason-2 missions over a 14-year-long time span (July 2002 to June 2016) are considered. The data were first retracked using the ALES adaptative leading edge subwaveform retracker. The X-TRACK processing system developed to optimize the completeness and accuracy of the corrected sea level time series in coastal ocean areas was then applied. From the 14-year long sea level time series finally obtained, we estimate sea level trends along the Jason-1 & 2 tracks covering the study region. We analyze regional variations in sea level trends, with a focus on the changes observed between the open ocean to the coastal zone. Compared to the conventional 1 Hz sea level products dedicated to open ocean applications, the retracked 20 Hz measurements used in this study allow us to retrieve valid sea level information much closer to the coast (less than 3–4 km to the coast, depending on the satellite track location). The main objective of this study is twofold: (1) provide sea level products in the coastal areas from reprocessed altimetry data and (2) check whether sea level changes at the coast differ from that reported in the open ocean with conventional altimetry products. In the selected region, results show that over the study period, sea level trends observed near the coast of Western Africa are significantly different than offshore trends. In order to assess the robustness of the results, detailed analyses are performed at several locations to discriminate between possible drifts in the geophysical corrections and physical processes potentially able to explain the sea level changes observed close to the coast.

Comparative study of charged particle precipitation from Van Allen radiation belts as observed by NOAA satellites during a land earthquake and an ocean earthquake

Publication date: 1 August 2019

Source: Advances in Space Research, Volume 64, Issue 3

Author(s): Suman Chakraborty, Sudipta Sasmal, Tamal Basak, Sandip K. Chakrabarti

Abstract

Ionospheric-magnetospheric transition region and the time correlation of particle rate fluctuation and earthquake has been a subject of interest for various authors for the last few decades. Van Allen Radiation Belt (VAB) is a zone of energetic charged particles originated from solar wind that are captured by Earth’s magnetic field lines. There are several Low Earth Orbital (LEO) satellites to observe count rate of energetic particles in the VAB. In the present study, we used Medium Energy Proton and Electron Detector (MEPED) instrument data which is on-board the NOAA-15 satellite. This study presents a comparison of the effects of a land earthquake and an ocean earthquake on the observed particle bursts (PBs). The transport of electromagnetic emissions of seismic origin from the ground to the magnetosphere through the ionosphere depends on various land and atmospheric variables. One of the primary parameters is the ground conductivity. The conductivity of ocean surface is much higher compared to that of the land surface and thus, an ocean earthquake is expected to impact the ionosphere-magnetosphere region more than a land earthquake. With this aim, we considered one land earthquake on January 3, 2016 and an ocean earthquake on December 6, 2016. The data were taken from the NOAA website and analyzed for the entire months of January and December, 2016. For the January 3 (land) earthquake, PBs were found to be accumulated only around the earthquake day, being maximum on the day of the earthquake and with complete absence of such events on days away from the event day. For the December 6 (ocean) earthquake, the effects were found to be post-seismic in nature and existed for days away from the earthquake day. Also the maximum particle count rate recorded on the day of the December 6 earthquake was found to be significantly high compared to the January 3 earthquake. We conclude that the difference in ground conductivity of land and ocean surface is the decisive factor behind such differences in the observed effects.

GRACE accelerometer data transplant

Publication date: 1 August 2019

Source: Advances in Space Research, Volume 64, Issue 3

Author(s): Tamara Bandikova, Christopher McCullough, Gerhard L. Kruizinga, Himanshu Save, Bruno Christophe

Abstract

In the last few months of GRACE (Gravity Recovery and Climate Experiment) mission operation, the accelerometer (ACC) aboard GRACE-B was turned off due to battery problems. In order to compute a gravity field model for these months, a data processing strategy was developed to retrieve the missing information, the so called Accelerometer data transplant. The ACC data transplant uses linear accelerations as measured by the GRACE-A accelerometer to generate the missing GRACE-B ACC data. A simple method of ACC data transplant was presented by Save et al. (2006), which only applies attitude and time correction due to orbit separation. In this paper we present an improved method of the ACC data transplant, which also includes thruster spike correction. The thruster spikes are residual linear accelerations occurring at each thruster firing caused by thruster imperfections and misbalance. We provide detailed description of the thruster spike model, which was derived analytically based on the ACC transfer function and 10 Hz ACC data. Using the transplanted ACC data we computed the gravity field models for the months with missing GRACE-B ACC data (November 2016–June 2017). We show that the gravity field model based on in the improved ACC data transplant has significantly reduced noise, especially for degrees beyond the first orbital resonance, when compared to the simple transplant method. The improved ACC transplant became therefore a part of the processing standards for the upcoming JPL RL06 gravity solution.

Operations dynamics analysis of solar thermal propulsion for CubeSats

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Zaynulla S. Zhumaev, Georgy A. Shcheglov

Abstract

This article considers operational dynamics of a solar thermal propulsion unit (STPU) with a solar energy concentrator for CubeSat nanosatellites. It was shown that for a 6U CubeSat with a 1U propulsion system, a resulting ΔV of more than 35 m/s in less than 24 h is possible. This ΔV is enough to move apart two satellites to opposite points of a 600 km circular orbit in less than 5 days. An original scheme of the engine with a capillary feeder system and a Fresnel lens as the solar energy concentrator is proposed. A simplified mathematical model of the engine was created to make it possible to calculate hundreds of thruster operating cycles with different parameters in the chamber. The ODEs were numerically integrated using the Matlab-Simulink software package.

Calculating risk and payoff in planetary exploration and life detection missions

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Ralph D. Lorenz

Abstract

A framework for quantitative assessment of different mission architectures is described using historical data and formal (Bayesian) information value measures. The science value of the result is argued for binary questions (e.g. 'is there life on Europa?) to be proportional to the logarithm of the posterior likelihood ratio of the answers, and can be derived from estimates of the false positive rates of instrumentation and of the presence (PD) of biosignatures at a given site. The expectation payoff is the product of the sought result with Markovian success probabilities of the required steps of launch, landing, sample acquisition etc., and historical planetary mission data are reviewed to derive (sometimes dismaying) estimates of these probabilities, e.g. historical landing successes rates are of the order of 66% and when landing is successful, the conditional rates of individual sample acquisition/analysis/return have similar values. The history of seafloor exploration on Earth is used as an analog, and indicates that in the absence of close reconnaissance data, PD may have rather low values of the order of 1% or less. The data acquisition success framework is demonstrated on the value of single versus multiple landers, on the choice of flyby altitudes for multiple plume fly-through missions, and on the value of surface mobility, which for small values of PD multiplies the science return by the number of sites visited. Bayesian reasoning requires encapsulation of prior information: while such estimates (of biosignature presence, false alarm rates, etc.) are inevitably subjective, the decomposition of that information onto specific factors affords transparency into their contribution to the final result and provides a basis for rational mission evaluation.

Continuous precise ephemerides for Beidou-2 maneuvered satellites based on a new thrust force model

Publication date: 1 August 2019

Source: Advances in Space Research, Volume 64, Issue 3

Author(s): Jing Qiao, Wu Chen

Abstract

Beidou geostationary earth orbit (GEO) and inclined geosynchronous orbit (IGSO) satellites are frequently maneuvered to keep them in the designed orbits by thrust forces. As the thrust forces are generally unknown, precise satellite orbits are difficult to be determined during maneuver periods. At present, precise ephemerides are not available or not complete for the satellite on the day of maneuver, and sometimes even the day after. This study aims to provide continuous precise orbit for Beidou-2 satellites during the maneuver period through better modeling of the thrust forces. We firstly present a thrust model based on the thrust behavior of Beidou-2 satellites detected in our previous study. Then, for the maneuvered satellite, the precise orbit is determined by estimating the extra thrust model parameters, together with other orbit parameters of initial satellite state and solar radiation pressure (SRP). Using observations from the Multi-GNSS Experiment (MGEX) network in Sep and Oct 2017, we have detected 10 GEO in-plane, 1 GEO and 3 IGSO out-of-plane maneuvers among the 5 GEO and 6 IGSO satellites; precise orbit determination (POD) is conducted for all the GEO in-plane and IGSO maneuvered satellites. The accuracy of our recovered orbit has been evaluated by comparing to a concatenation of the dynamic orbit before/after the maneuver and kinematic orbit during the maneuver. The RMSs of orbit differences before/after maneuvers are about 0.30, 2.34 and 0.42 m in the radial, along-track, and cross-track (RAC) directions for GEOs, and 0.33, 0.62 and 0.26 m for IGSOs. A similar level of accuracy has also been achieved during maneuvers as shown by the orbit differences and the stable POD residuals during the whole maneuver day. The recovered orbit accuracy is comparable to that of the normal Beidou-2 precise ephemerides from International GNSS Service (IGS) analysis centers (WHU/GFZ).

Analysis of pre-telescopic sunspots and auroras from 8th to 16th century

Publication date: 15 August 2019

Source: Advances in Space Research, Volume 64, Issue 4

Author(s): Mohamed Reda Bekli, Ilhem Chadou

Abstract

In this paper, we use non-parametric kernel approach to estimate the probability density function (pdf) of auroral-night and naked-eye-sunspot series over multi-century timescale. We selected the events observed in medieval epoch, and auroral records from low geographical latitude (<45°). These astronomical phenomena are recorded mainly in the Oriental historical sources and much less in Occidental sources. The collected events are published in many recent catalogues. The density function are calculated using the Gaussian kernel, including 95% CI obtained by bootstrapping.

Results confirm the existence of strong connection between auroras and sunspot activity, especially from 11th to 16th century. Indeed, a high correlation degree r = 0.72 is obtained for the period AD 800–1500 between the two pdf of sunspots (pdfS) and auroras (pdfA) for bandwidth h = 10 years, and exceed 0.8 for h > 43 years. However, we notice a time-shift between the two density functions. To calculate it, we use the cross-correlation technique, and we find τ = −5.9 years, that is strangely close to the half of solar cycle period. It is very important to note that our results revealed that the high frequency of low latitude auroras occurs before the maximum of naked-eye sunspots activity, and not afterward, on contrary to what is expected.

The pdf curves show multiple peaks occurring at quasi-periodic times and show clearly the three grand minima of solar activity: Oort, Wolf and Spörer. In auroral-night data, the mean period at 95% CI obtained is:T‾=61±7years. Using power spectrum analysis, we observe clearly a strong signal of 60 years period that is a new evidence of the existence of such cycle. More importantly, we identified a cycle of 262 years period in the sunspot data, and a new oscillation of 310 years in auroral-night data. These two cycles appear clearly in the pdf curves using optimized bandwidth obtained by the direct plug-in approach of Sheather and Jones.

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