Pair Of Satellites Research Articles

Comparación de modelos de altura de la vegetación para estimación de biomasa en un bosque de manglar en el Caribe Colombiano

Este trabajo integra satélite-LIDAR para comparar la evaluación de stock de carbono en alta resolución en un bosque de manglar (Rincón Mosquito) en la zona costera del mar Caribe. Se comparó la altura de vegetación resultante de la extracción de un modelo de altura de la vegetación satelital EROS-B y un LIDAR aerotransportado vs datos en campo. Los satélites ofrecen una oportunidad de monitorear cambios en el carbono forestal causados por la deforestación y la degradación, igual que nuevos métodos aéreos, especialmente el método LIDAR, ofrecen una forma de estimar la densidad del carbono forestal, lo cual ayuda en el desarrollo de líneas base para inventarios de carbono. Luego de calcular la biomasa total desde los datos de satélite y LIDAR, se comparó la medida de biomasa tomada en campo. En términos de escalas detalladas se puede concluir que el sensor con mayor precisión es el LIDAR al tener un nivel de resolución espacial que permite gran detalle de la masa arbórea., El uso de estéreo pares satelitales es viable en grandes extensiones y no en pequeñas áreas, donde particularmente en los últimos años, los datos tomados con vehículos aéreos no tripulados parecen ser una alternativa más viable en disponibilidad y costo.

Mapping Retrogressive Thaw Slumps Using Single-Pass TanDEM-X Observations

Vast areas of the Arctic host ice-rich permafrost, which is becoming increasingly vulnerable to terrain-altering thermokarst in a warming climate. Among the most rapid and dramatic changes are retrogressive thaw slumps. These slumps evolve by a retreat of the slump headwall during the summer months, making them detectable by comparing digital elevation models over time using the volumetric change as an indicator. Here, we present and assess a method to detect and monitor thaw slumps using time series of elevation models applied on two contrasting study areas in Northern Canada. Our two-step method is tailored to single-pass InSAR observations from the TanDEM-X satellite pair, which have been acquired since 2011. For each acquisition, we derive a digital elevation model and uncertainty estimates. In the first step, we difference digital elevation models and detect the significant elevation changes using a blob-detection algorithm. In the second step, we classify the detections into those due to thaw slumps and other causes using a simple thresholding method (accuracy: 78%), a random forest classifier (87%), and a support vector machine (86%). When our method is applied to other areas, the classifiers should be trained with data from part of the study area or with data obtained from similar areas in terms of topography, vegetation, and thaw slump characteristics to achieve the best performance. The obtained locations of thaw slumps can be used as a starting point to extract important slump properties, such as the headwall height and the volumetric change, which are currently not available on regional scales.

Chromosomal Karyotype Differences of the Dark-Purple Tea Cultivar ‘Ziyan’ and Its Natural Hybrid Progenies

‘Ziyan’ is a new variety of dark purple tender shoots tea tree which is rich in anthocyanins. ZY-1 and ZY-3 were two natural hybridized offspring of ‘Ziyan’, which were cultivated by single plant selection. ‘Ziyan’ is as the female parent, whereas the male parent has yet to be determined. In this study, we try to investigate cytological parameters of the three kinds of tea plants, and analyze the chromosomal karyotype differences among them. The results showed that the max arm ratio of ‘Ziyan’ was determined 2.50, and relative length was between 3.56% and 5.39%. The karyotype asymmetry index was 59.920%, and the karyotype formula was 2n=2x=30=22m (2SAT)+8sm. The max arm ratio of ZY-1 was 2.50, and relative length was between 4.80% and 8.69%. The karyotype asymmetry index was 60.307%, and the karyotype formula was 2n=2x=30=20m+10sm. The max arm ratio of ZY-3 was 2.77, and relative length was between 3.95% and 8.70%. The karyotype asymmetry index was 58.679%, which was lower than ‘Ziyan’ and ZY-3, and the karyotype formula was 2n=2x=30=24m+6sm. Although ‘Ziyan’ was significantly different from its two offspring, the karyotype characteristics of three varieties of tea plants were all type 2B. ‘Ziyan’ had a pair of satellites, but neither ZY-1 nor ZY-3 had a satellite. According to the karyotype asymmetry index and average arm ratio, the hybrid progeny ZY-3 was the most primitive, followed by the female parent ‘Ziyan’, while the hybrid progeny ZY-1 was the most evolutive.

Satellite Formation Flight Simulation Using Multi-Constellation GNSS and Applications to Ionospheric Remote Sensing

The Virginia Tech Formation Flying Testbed (VTFFTB) is a global navigation satellite system (GNSS)-based hardware-in-the-loop (HIL) simulation testbed for spacecraft formation flying with ionospheric remote sensing applications. Past applications considered only the Global Positioning System (GPS) constellation. The rapid GNSS modernization offers more signals from other constellations, including the growing European system—Galileo. This study presents an upgrade of VTFFTB with the incorporation of Galileo and the associated enhanced capabilities. By simulating an ionospheric plasma bubble scenario with a pair of LEO satellites flying in formation, the GPS-based simulations are compared to multi-constellation GNSS simulations including the Galileo constellation. A comparison between multi-constellation (GPS and Galileo) and single-constellation (GPS) shows the absolute mean and standard deviation of vertical electron density measurement errors for a specific Equatorial Spread F (ESF) scenario are decreased by 32.83% and 46.12% with the additional Galileo constellation using the 13 July 2018 almanac. Another comparison based on a simulation using the 8 March 2019 almanac shows the mean and standard deviation of vertical electron density measurement errors were decreased further to 43.34% and 49.92% by combining both GPS and Galileo data. A sensitivity study shows that the Galileo electron density measurements are correlated with the vertical separation of the formation configuration. Lower C/N 0 level increases the measurement errors and scattering level of vertical electron density retrieval. Relative state estimation errors are decreased, as well by utilizing GPS L1 plus Galileo E1 carrier phase instead of GPS L1 only. Overall, superior performance on both remote sensing and relative navigation applications is observed by adding Galileo to the VTFFTB.

DENSE MULTI-VIEW IMAGE MATCHING FOR DSM GENERATION FROM SATELLITE IMAGES

Abstract. Digital Surface Model (DSM) can be generated from stereo pairs of satellite or aerial images. Among the most state-of-the-art matching algorithms, Semi-Global Matching (SGM) has widely been used for generating DSM from both satellite and aerial images. This paper presents an approach to improve the accuracy of DSM generated by SGM from multi-view satellite images using a novel technique including several filters. The filters are used for deleting mismatches between very tall buildings in urban areas and removing the sea regions. The technique, in contrast to the recent multi-view matching approaches, considers some of the points generated with only a pair of images in the final DSM. The approach is implemented on five sequential high resolution images acquired by the Worldview-2 satellite. The results are locally evaluated in shape and quantitative terms in comparison with commercial software to reveal the capability of the approach to generate a reliable and dense point cloud. Experiments show that the proposed method can achieve below half-pixel accuracy.

Estimating the Azimuthal Mode Structure of ULF Waves Based on Multiple GOES Satellite Observations

AbstractCharacterizing the azimuthal mode number, m , of ultralow‐frequency (ULF) waves is necessary for calculating radial diffusion of radiation belt electrons. A cross‐spectral technique is applied to the compressional Pc5 ULF waves observed by multiple pairs of GOES satellites to estimate the azimuthal mode structure during the 28‐31 May 2010 storm. We find that allowing for both positive and negative m is important to achieve a more realistic distribution of mode numbers and to resolve wave propagation direction. During the storm commencement when the solar wind dynamic pressure is high, ULF wave power is found to dominate at low‐mode numbers. An interesting change of sign in m occurred around noon, which is consistent with the driving of ULF waves by solar wind buffeting around noon, creating antisunward wave propagation. The low‐mode ULF waves are also found to have a less global coverage in magnetic local time than previously assumed. In contrast, during the storm main phase and early recovery phase when the solar wind dynamic pressure is low and the auroral electrojet index is high, wave power is shown to be distributed over all modes from low to high. The high‐mode waves are found to cover a wider range of magnetic local time than what was previously assumed. Furthermore, to reduce the 2nπ ambiguity in resolving m , a cross‐pair analysis is performed on satellite field measurements for the first time, which is demonstrated to be effective in generating more reliable mode structure of ULF waves during high auroral electrojet periods.

AGN-driven quenching of satellite galaxies

ABSTRACT We explore the effect of active galactic nucleus (AGN) feedback from central galaxies on their satellites by comparing two sets of cosmological zoom-in runs of 27 haloes with masses ranging from 1012 to 1013.4$\rm {M}_{\odot}$ at z = 0, with (wAGN) and without (noAGN) AGN feedback. Both simulations include stellar feedback from multiple processes, including powerful winds from supernovae, stellar winds from young massive stars, AGB stars, radiative heating within Strömgren spheres and photoelectric heating. Our wAGN model is identical to the noAGN model except that it also includes a model for black hole seeding and accretion, as well as AGN feedback via high-velocity broad absorption line winds and Compton/photoionization heating. We show that the inclusion of AGN feedback from the central galaxy significantly affects the star formation history and the gas content of the satellite galaxies. AGN feedback starts to affect the gas content and the star formation of the satellites as early as z = 2. The mean gas-rich fraction of satellites at z = 0 decreases from 15 per cent in the noAGN simulation to 5 per cent in the wAGN simulation. The difference between the two sets extends as far out as five times the virial radius of the central galaxy at z = 1. We investigate the quenching mechanism by studying the physical conditions in the surroundings of pairs of satellites matched across the wAGN and noAGN simulations and find an increase in the temperature and relative velocity of the intergalactic gas.

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

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.

Intercomparisons of Long-Term Atmospheric Temperature and Humidity Profile Retrievals

This study builds upon a framework to develop a climate data record of temperature and humidity profiles from high-resolution infrared radiation sounder (HIRS) clear-sky measurements. The resultant time series is a unique, long-term dataset (1978–2017). To validate this long-term dataset, evaluation of the stability of the intersatellite time series is coupled with intercomparisons with independent observation platforms as available in more recent years. Eleven pairs of satellites carrying the HIRS instrument with time periods that overlap are examined. Correlation coefficients were calculated for the retrieval of each atmospheric pressure level and for each satellite pair. More than 90% of the cases examining both temperature and humidity have correlation coefficients greater than 0.7. Very high correlation is demonstrated at the surface and two meter levels for both temperature (>0.99) and specific humidity (>0.93). For the period of 2006–2017, intercomparisons are performed with four independent observations platforms: radiosonde (RS92), constellation observing system for meteorology ionosphere and climate (COSMIC), global climate observing system (GCOS) reference upper-air network (GRUAN), and infrared atmospheric sounding interferometer (IASI). Very close matching of surface and two meter temperatures over a wide domain of values is depicted in all presented intercomparisons: intersatellite matches of HIRS retrievals, HIRS vs. GRUAN, and HIRS vs. IASI.

High-dynamic baseline determination for the Swarm constellation

Baseline determination for the European Space Agency Swarm magnetic field mission is investigated. Swarm consists of three identical satellites -A, -B and -C. The Swarm-A and -C form a pendulum formation whose baseline length varies between about 30 and 180 km. Swarm-B flies in a higher orbit, causing its orbital plane to slowly rotate with respect to those of Swarm-A and -C. This special geometry results in short periods when the Swarm-B satellite is adjacent to the other Swarm satellites. Ten 24-hr periods around such close encounters have been selected, with baseline lengths varying between 50 and 3500 km. All Swarm satellites carry high-quality, dual-frequency and identical Global Positioning System receivers not only allowing precise orbit determination of the single Swarm satellites, but also allowing a rigorous assessment of the capability of precise baseline determination between the three satellites. These baselines include the high-dynamic baselines between Swarm-B and the other two Swarm satellites.For all orbit determinations, use was made of an Iterative Extended Kalman Filter approach, which could run in single-, dual-, and triple-satellite mode. Results showed that resolving the issue of half-cycle carrier phase ambiguities (present in original release of GPS RINEX data) and reducing the code observation noise by the German Space Operations Center converter improved the consistency of reduced-dynamic and kinematic baseline solutions for both the Swarm-A/C pendulum pair and other combinations of Swarm satellites. All modes led to comparable consistencies between the computed orbit solutions and satellite laser ranging observations at a level of 2 cm. In addition, the consistencies with single-satellite ambiguity fixed orbit solutions by the German Space Operations Center are at comparable levels for all the modes, with reduced-dynamic baseline consistency at a level of 1-3 mm for the pendulum Swarm-A/C formation and 3-5 mm for the high-dynamic Swarm-B/A and -B/C satellite pairs in different directions.

Gravity Field Recovery Using High-Precision, High–Low Inter-Satellite Links

Past temporal gravity field solutions from the Gravity Recovery and Climate Experiment (GRACE), as well as current solutions from GRACE Follow-On, suffer from temporal aliasing errors due to undersampling of the signal to be recovered (e.g., hydrology), which arise in terms of stripes caused by the north–south observation direction. In this paper, we investigate the potential of the proposed mass variation observing system by high–low inter-satellite links (MOBILE) mission. We quantify the impact of instrument errors of the main sensors (inter-satellite link and accelerometer) and high-frequency tidal and non-tidal gravity signals on achievable performance of the temporal gravity field retrieval. The multi-directional observation geometry of the MOBILE concept with a strong dominance of the radial component result in a close-to-isotropic error behavior, and the retrieved gravity field solutions show reduced temporal aliasing errors of at least 30% for non-tidal, as well as tidal, mass variation signals compared to a low–low satellite pair configuration. The quality of the MOBILE range observations enables the application of extended alternative processing methods leading to further reduction of temporal aliasing errors. The results demonstrate that such a mission can help to get an improved understanding of different components of the Earth system.

Hydrological signals in polar motion excitation – Evidence after fifteen years of the GRACE mission

We investigate the contribution of continental hydrology signals to polar motion excitation resulting from changes in mass distribution associated with terrestrial water and snow/ice based on a variety of datasets. The impact of the former is less well known than that of other geophysical fluids, namely atmospheric and oceanic angular momentum signals, especially for oscillations over shorter-than-annual periods. The geodetic excitation function of polar motion can be determined from observations, while the hydrological excitation function can be estimated either from global models of the land-based hydrosphere or from indirect observations of variations in the Earth’s gravity field. With more than 15 years of data, the pair of GRACE satellites has provided valid datasets for studying interannual polar motion excitation. Recent results are promising and indicate agreement between GRACE-derived interannual hydrological excitations and the corresponding geodetic observations. Our work focuses on the non-seasonal spectral range, and compares hydrological excitations based on recent GRACE solutions from different centres with signals from climate models, and as a residual from geodetic observations.

Atmospheric bending effects in GNSS tomography

Abstract. In Global Navigation Satellite System (GNSS) tomography, precise information about the tropospheric water vapor distribution is derived from integral measurements like ground-based GNSS slant wet delays (SWDs). Therefore, the functional relation between observations and unknowns, i.e., the signal paths through the atmosphere, have to be accurately known for each station–satellite pair involved. For GNSS signals observed above a 15∘ elevation angle, the signal path is well approximated by a straight line. However, since electromagnetic waves are prone to atmospheric bending effects, this assumption is not sufficient anymore for lower elevation angles. Thus, in the following, a mixed 2-D piecewise linear ray-tracing approach is introduced and possible error sources in the reconstruction of the bended signal paths are analyzed in more detail. Especially if low elevation observations are considered, unmodeled bending effects can introduce a systematic error of up to 10–20 ppm, on average 1–2 ppm, into the tomography solution. Thereby, not only the ray-tracing method but also the quality of the a priori field can have a significant impact on the reconstructed signal paths, if not reduced by iterative processing. In order to keep the processing time within acceptable limits, a bending model is applied for the upper part of the neutral atmosphere. It helps to reduce the number of processing steps by up to 85 % without significant degradation in accuracy. Therefore, the developed mixed ray-tracing approach allows not only for the correct treatment of low elevation observations but is also fast and applicable for near-real-time applications.

Chromosomal Karyotype Analysis of Four Varieties of Green Petioles Leaf Beet

Chromosome analysis of four varieties of green petioles leaf beet was performed by pressing plate’s method. The results revealed the chromosome numbers of all four varieties of green petioles leaf beet was 2n = 2x = 18. And their centromere type was mainly composed by metacentric chromosomes (m) and sub metacentric chromosomes (sm), while the number of m was more than sm. However, evident variation among varieties was presented on their karyotype formula, karyotype type, satellitic position, and arm ratio, ratio of chromosome length (L/S) and index of karyotypic asymmetry. Although they all had one pair of satellites, the location of the satellites was different. The karyotype type contained 1A from ‘Zhongnong’, ‘Huaxianzi’ and 2A from ‘Lifeng’, ‘Huaguan’. Statistically ratio of chromosome length for tested varieties ranged from 1.20 in ‘Lifeng’ to 1.52 in ‘Zhongnong’, and average arm ratio ranged from 1.42 in ‘Zhongnong’ to 1.62 in ‘Huaguan’. The index of karyotypic asymmetry of ‘Zhongnong’, ‘Huaxianzi’, ‘Lifeng’, ‘Huaguan’ were 58.50%, 59.21%, 58.79% and 61.32%, respectively. This study revealed the karyotypic characteristics of leaf beet from the cytogenetic aspect, and provided a reference for variety breeding of leaf beet.

Characteristics of BeiDou Navigation Satellite System (BDS) Code Observations for Different Receiver Types and Their Influence on Wide-Lane Ambiguity Resolution.

The Chinese BeiDou Navigation Satellite System (BDS) has been an important constitute of the Global Navigation Satellite System (GNSS), and the combination of GPS and BDS shows significant improvements when compared with single GPS system for real-time kinematic (RTK) positioning, and improves on availability and fixing rates, especially in the East Asian area. While network RTK might have different types of receivers, both for global and regional networks, different types of receiver may adopt different internal multipath mitigation methods and other techniques that result in different pseudorange characteristics, especially for a multipath. Then, the performance of wide-lane ambiguity resolution (WL AR) is affected. In this study, we first analyze and compare the characteristics of BDS dual-frequency observations for different types of receivers, including Trimble, Leica, Javad, and Septentrio, based on multipath (MP) observables, and then we assess their influence on double-differenced (DD) WL AR. The numerical results show that an obvious low-frequency component exists in MP observables of BDS geostationary earth-orbit satellites (GEOs) for Leica receivers, while its high-frequency measurement noise is very small. For geosynchronous orbit satellites (IGSOs) and medium earth-orbit satellites (MEOs), a slight fluctuation can also be observed that is similar to that of GPS satellites, except for the satellite-included code bias. In Trimble, Javad, and Septentrio receivers, the MP series are dominated by high-frequency measurement noise, both for GEOs and non-GEOs, except for satellite-included code bias. Furthermore, the characteristic of Leica receivers for BDS GEOs seriously affects WL AR and, even for a short baseline, it takes a long time for WL ambiguities to converge, or not converge for many GEO-related DD WL ambiguities, while Trimble, Javad, and Septentrio receivers perform well for short and medium baselines. Then, a time-difference method is proposed to mitigate the multipath of BDS GEOs for a Leica receiver. After applying the proposed method, WL ambiguity fixing rates of GEO-related satellite pairs are improved significantly and the convergence time is shortened from several hours to ten minutes.

Motion compensation of the transmitter and receiver in bistatic frequency‐modulated continuous‐wave synthetic aperture radar

Synthetic aperture radar (SAR) imaging with an independently moving transmitter and receiver introduces motion artefacts difficult to compensate for, especially between satellites in separate orbits that rendezvous over a target. A solution for polarisation-resolved bistatic frequency-modulated continuous-wave (FMCW) SAR with independently moving platforms is derived and demonstrated through simulation. This solution accounts for the polarisation of the source, susceptibility tensor of the target, and the velocity and acceleration of the transmitter and receiver. The accuracy of the solution is demonstrated by reconstructing simulated point ground targets with a pair of X-band SAR satellites of realistic parameters.

Radar Forest Height Estimation in Mountainous Terrain Using Tandem-X Coherence Data

In this paper, we consider the problem of radar estimation of forest canopy height in regions with dense forests and severe topography. We combine a reference digital elevation model with multiple satellite baselines from ascending and descending orbits to develop a merging algorithm relating single pass interferometric coherence to forest canopy height. We first describe the algorithm and processing steps used for height estimation and then apply the technique to a mountainous study site in British Columbia, Canada, using data from the Tandem-X satellite pair. We devise a new masking scheme to isolate potential problem areas in sloped terrain and apply the new merging algorithm by using multiple Tandem-X tracks to overcome the gaps left due to the masking procedure. The radar height products are validated by using a network of ground forest measurement sites and supporting lidar. The regression statistics show an r2 of 0.70 and rmse of 4.1 m between the radar and the field measured heights. By examining height errors, we implement a new test for the presence of canopy extinction, or subcanopy surface scattering, and demonstrate that in the dense and mountainous forests of British Columbia, there are significant canopy extinction effects in X-band imagery.

Volumetric assessment of satellite encounter rates

Presented here is a planning and characterization tool to estimate satellite encounter rates for a prospective orbit regime. The prescribed method can be used to estimate the satellite conjunction operational tempo (i.e., the pace of conjunction activity associated with the operation of a satellite) for a given orbit against a satellite catalog. The encounter volume is defined by an ellipsoid that is constant in size, shape, and orientation in the satellite's Radial-InTrack-CrossTrack frame. For a given pair of satellites, we evaluate volumetric proximity by defining a traveling ellipsoid about the second satellite's Keplerian orbit and assessing if/when the first satellite's orbit traverses it; if inside the volume it is considered proximate. To ensure that no encounter is missed, the ellipsoid is moved along the second satellite's circular or elliptical orbit in increments of true anomaly corresponding to in-track movement much smaller than the ellipsoid's minor axis. An incremental determination of encounter probability is made if/when the first satellite's orbit track contacts the ellipsoid. When this takes place, the orbit true anomalies at ellipsoid entry and exit are captured for both satellites and converted to their respective mean anomaly ranges. The likelihood that both satellites will simultaneously be inside that encounter region is then determined from these ranges. In addition to estimating probability, the number of expected encounters over a given time span can also be approximated. The method is valid for both coplanar and non-coplanar orbits. However, our assumption of uniformly-distributed, relative in-track positions is not applicable in all satellite pairings. This trending tool is also useful in identifying such regions as graveyard orbits that are least likely to produce encounters. It can also be used to estimate how often neighboring satellites will trigger volumetric warnings when considering a candidate orbit.

Coherence Difference Analysis of Sentinel-1 SAR Interferogram to Identify Earthquake-Induced Disasters in Urban Areas

This study proposes a workflow that enables the accurate identification of earthquake-induced damage zones by using coherence image pairs of the Sentinel-1 satellite before and after an earthquake event. The workflow uses interferometric synthetic aperture radar (InSAR) processing to account for coherence variations between coseismic and preseismic image pairs. The coherence difference between two image pairs is useful information to detect specific disasters in a regional-scale area after an earthquake event. To remove background effects such as the atmospheric effect and ordinal surface changes, this study employs the two-step threshold method to develop the coseismic coherence difference (CCD) map for our analyses. Thirty-four Sentinel-1 images between January 2015 and February 2016 were collected to process 30 preseismic image pairs and two coseismic image pairs for assessing multiple types of disasters in Tainan City of southwestern Taiwan, where severe damages were observed after the Meinong earthquake event. The coseismic unwrapping phases were further calculated to estimate the surface displacement in east-west and vertical directions. Results in the CCD map agree well with the observations from post-earthquake field surveys. The workflow can accurately identify earthquake-induced land subsidence and surface displacements, even for areas with insufficient geological data or for areas that had been excluded from the liquefaction potential map. In addition, the CCD details the distribution of building damages and structure failures, which might be useful information for emergency actions applied to regional-scale problems. The conversion of 2D surface displacement reveals the complex behavior of geological activities during the earthquake. In the foothill area of Tainan City, the opposite surface displacements in local areas might be influenced by the axis activities of the Kuanmiao syncline.

The Local Group Dwarf Galaxies. The Star Formation Histories derived using the Long Period Variable Stars

AbstractDwarf galaxies in the Local Group (LG) represent a distinct as well as diverse family of tracers of the earliest phases of galaxy assembly and the processing resulting from galactic harrassment. Their stellar populations can be resolved and used as probes of the evolution of their host galaxy. In this regard, we present the first reconstruction of the star formation history (SFH) of them using the most evolved AGB stars that are long period variable (LPV). LPV stars trace stellar populations as young as ∼ 30 Myr to as old as the oldest globular clusters. For the nearby, relatively massive and interacting gas-rich dwarf galaxies, the Magellanic Clouds, we found that the bulk of the stars formed ∼ 10 Gyr ago for the LMC, while the strongest episode of star formation in the SMC occurred a few Gyr later. A peak in star formation around 0.7 Gyr ago in both Clouds is likely linked to their recent interaction. The Andromeda satellite pair NGC147/185 show different histories; the main epoch of star formation for NGC 185 occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate (SFR). In the case of NGC 147, the SFR peaked only 6.9 Gyr ago, staying intense until ∼ 3 Gyr ago. Star formation in the isolated gas-rich dwarf galaxy IC 1613 has proceeded at a steady rate over the past 5 Gyr, without any particular dominant epoch. Due to lack of sufficient data, we have conducted an optical monitoring survey at the Isaac Newton Telescope (INT) of 55 dwarf galaxies in the LG to reconstruct the SFH of them uniformly. The observations are made over ten epochs, spaced approximately three months apart, as the luminosity of LPV stars varies on timescales of months to years. The system of galactic satellites of the large Andromeda spiral galaxy (M31) forms one of the key targets of our monitoring survey. We present the first results in the And I dwarf galaxy, where we discovered 116 LPVs among over 10,000 stars.