Number Of Ground Stations Research Articles

Evaluation of GPM IMERG Satellite Precipitation Products in Event-Based Flood Modeling over the Sunshui River Basin in Southwestern China

This study evaluates the applicability of hourly Global Precipitation Measurement Mission (GPM) Integrated Multi-satellitE Retrievals for GPM (IMERG) data for event-based flood modeling in the Sunshui River Basin, southwestern China, using the hydrologic modeling system (HEC-HMS) model. The accuracies of IMERG V6, IMERG V7, and the corrected IMERG V7 satellite precipitation products (SPPs) were assessed against ground rainfall observations. The performance of flood modeling based on the original and the corrected SPPs was then evaluated and compared. In addition, the ability of different numbers (one–eight) of ground stations to correct IMERG V7 data for flood modeling was investigated. The results indicate that IMERG V6 data generally underestimate the actual rainfall of the study area, while IMERG V7 and the corrected IMERG V7 data using the geographical discrepancy analysis (GDA) method overestimate rainfall. The corrected IMERG V7 data performed best in capturing the actual rainfall events, followed by IMERG V7 and IMERG V6 data, respectively. The IMERG V7-generated flood hydrographs exhibited the same trend as those of the measured data, yet the former generally overestimated the flood peak due to its overestimation of rainfall. The corrected IMERG V7 data led to superior event-based flood modeling performance compared to the other datasets. Furthermore, when the number of ground stations used to correct the IMERG V7 data in the study area was greater than or equal to four, the flood modeling performance was satisfactory. The results confirm the applicability of IMERG V7 data for fine time scales in event-based flood modeling and reveal that using the GDA method to correct SPPs can greatly enhance the accuracy of flood modeling. This study can act as a basis for flood research in data-scarce areas.

Monitoring Aerosol Optical Depth for Air Quality Through Himawari-8 in Urban Area West Java Province Indonesia

Air quality is a crucial parameter in human life. One air quality indicator can be observed through Aerosol Optical Depth (AOD). If these substances are pollutants such as particulate matter, aerosols, and ozone, it is confident that air quality will deteriorate, threatening human health and causing climate change. AOD monitoring can be used as a basis for policymakers and related parties to maintain the stability of air quality in the atmosphere. Many ground observation stations monitor air quality by obtaining data on PM2.5 and PM10 aerosol particles. However, the number of ground stations is limited, resulting in incomplete data. Fortunately, remote sensing satellites have the advantage of covering large areas and providing continuous observations, with the ability to gather information on large-scale aerosol and obtain spatiotemporal distribution. Therefore, this research aims to obtain AOD through Himawari-8 and analyze the spatiotemporal air quality in urban areas of West Java based on AOD. The research methodology used in this study is descriptive analysis with an empirical research approach. Assisted by remote sensing technology and Geographic Information Systems, this research generates AOD data extraction that can be obtained from the new generation satellite of Himawari-8. The distribution of AOD levels and spatiotemporal monitoring in urban areas of West Java is very dynamic depending on anthropogenic activity in a particular area and time. Keywords: Aerosol Optical Depth (AOD), Air Quality, Himawari-8

Simulation of the Use of Variance Component Estimation in Relative Weighting of Inter-Satellite Links and GNSS Measurements

Inter-satellite links (ISLs) can improve the performance of the Global Navigation Satellite System (GNSS) in terms of precise orbit determination, communication, and data-exchange capabilities. This research aimed to evaluate a simulation-based processing strategy involving the exploitation of ISLs in orbit determination of Galileo satellites, which are not equipped with operational ISLs. The performance of the estimation process is first tested based on relative weighting coefficients obtained with methods of variance component estimation (VCE) varying in the complexity of the calculations. Inclusion of biases in the ISL measurements allows evaluation of the processing strategy and assessment of the impact of three different sets of ground stations: 44 and 16 stations distributed globally and 16 located in Europe. The results indicate that using different VCE approaches might lower orbit errors by up to 20% with a negligible impact on clock estimation. Depending on the applied ISL connectivity scheme, ISL range bias can be estimated with RMS between 10% to 30% of initial bias values. The accuracy of bias estimation may be associated with weighting approach and the number of ground stations. The results of this study show how introducing VCE with various simulation parameters into the processing chain might increase the accuracy of the orbit estimation.

Investigation of Equatorial Medium Earth Orbits for Space Solar Power

Most existing space solar power concepts place one or more power stations in geosynchronous Earth orbit (GEO). However, due to the limited availability of GEO orbital slots, it may not be feasible to locate a power station in GEO. To overcome this limitation, this article presents a system analysis for a space solar power system that incorporates a constellation of power stations in a 20184 km altitude equatorial medium Earth orbit (MEO). The orbiting power stations are based on the Caltech Space Solar Power Project architecture. The constellation consists of multiple power stations in a shared equatorial MEO each transmitting to a nonequatorial receiving station. The analysis assumes a one-to-one correspondence between the number of power stations and the number of ground stations. Like a GEO-based system, this constellation architecture enables a MEO-based system to provide near continuous power (outside of eclipse) to each ground station. It is shown that a MEO constellation with three or more power stations provides comparable transmission efficiency to a GEO-based system. The levelized cost of electricity (LCOE) is then computed for MEO systems with three, four, and five power stations and compared to the LCOE for the GEO-based system. Ground station area is identified as a significant contributor to the LCOE for the MEO-based systems. The system analysis shows that a MEO constellation with as few as four power stations has an LCOE comparable to GEO, and hence, it is concluded that MEO is a viable alternative to GEO for space solar power.

Research on Evaluation Method of Time Transfer Performance Between Ground Stations in Two-Way Satellite Comparison Network

High-precision time synchronization between ground stations is essential for the precise measurement and determination of satellite orbits. Among the numerous time synchronization methods available, two-way satellite time and frequency transfer (TWSTFT) technology is widely used because of the approximate symmetry of its signal transmission path, such that most of the error terms can be offset to obtain subnanosecond time transmission accuracy. A C-band two-way satellite comparison network has been constructed at National Time Service Center (NTSC), Chinese Academy of Sciences for orbit determination and to compare the times between ground stations. The gradual increase in the number of ground stations has created a problem in need of urgent resolution: the facile and rapid evaluation of the time transfer performance between ground stations by a two-way satellite comparison network without the use of additional equipment. We solve this problem by applying the TWSTFT triangle closure method and the precise point positioning time transfer method to the NTSC two-way satellite comparison network and conduct joint observations between terrestrial stations to evaluate the network time transfer performance. We select the Xi'an, Kashi and Sanya stations of the NTSC two-way satellite comparison network to carry out verification tests. The results show that the NTSC two-way satellite comparison network can achieve subnanosecond time synchronization accuracy. The accuracy of time synchronization between stations can be checked conveniently and quickly without the use of additional equipment. This study presents the most recent developments for the NTSC two-way satellite comparison network and serves as a reference for evaluating the time transfer performance of similar networks.

Comparative Analysis of Terrestrial and Satellite Observations of Photospheric Magnetic Field in an Appendix to Simulation of Parameters of Coronal Holes and Solar Wind

Data from observations of the photospheric magnetic field at a number of ground stations (STOP, WSO, GONG) and by the SDO/HMI satellite for 2014–2019 are examined in terms of the accuracy of the model calculations of the speed of calm solar wind and the areas of coronal holes obtained on their basis. A high positive correlation was found (R > 0.5) between the simulation results and satellite observations (ACE, SDO/AIA193) for separate time periods. The best results in the modeling of the speed of the calm solar wind are given by the data from the ground stations WSO and STOP (Kislovodsk Mountain Astronomical Station, 2014). The temporal variability of the empirical coefficients of the model used in the forecasts was determined.

Research on the operational regional coverage of satellite and spacecraft tracking and controlling

In view of the problem of building ground stations for tracking and controlling of satellites and spacecraft, considering the fixed angle between the orbit of the satellite or spacecraft and the equatorial surface of the earth, and the difference of longitude between the two circles in succession of the satellite or spacecraft caused by the rotation of the earth, the operation area of the satellite or spacecraft was calculated by using the method of spherical projection of satellite orbit rotation, taking the earth as the reference system. The minimum number of ground stations needed for satellite tracking and controlling was calculated in three cases, by using the mathematical model of sphere ring area and honeycomb coverage. This model was validated by the launch and operation data of Shenzhou 7.

Estimation of Solar Potential for Thailand Using Adaptive Neurofuzzy Inference System Models

This research paper proposes a new method of global solar radiation prediction for Thailand using adaptive neurofuzzy inference system (ANFIS) models. Contrary to mathematical-based modeling approaches, the proposed models are able to estimate the monthly mean of daily global solar radiation at the ground level without using the earth's atmospheric layer model. The proposed technique alternately utilizes the 9-year long recorded spatiotemporal data of solar irradiance from meteorological ground stations in the modeling process. With a limited number of ground stations, it covered six regions of Thailand, ANFIS modeling; testing and restructuring have been performed repetitively; and finally, the best-fit models with minimum mean absolute percentage errors (MAPEs) corresponding to six regions of Thailand are obtained. Moreover, the ANFIS models have been tested comparatively to the measured data and the multilayer feed forward artificial neural network (ANN) models, which has a good agreement to real data for the proposed models, can be met with the average accuracy of 7.07% MAPE. By applying this model as a tool to estimate solar potential, the local government or the business sector can provide basic information, which is useful for solar energy system planning and project development.

Worldwide performance assessment of 75 global clear-sky irradiance models using Principal Component Analysis

This study evaluates the performance of 75 clear-sky global irradiance models against 75 ground stations worldwide covering five major Köppen-Geiger climate classifications and overall global performance. After quality control, clear-sky detection and data-availability criteria, there are 4.36 million 1-min valid global horizontal irradiance (GHI) data points for evaluation. This study represents the most encompassing evaluation of its kind in terms of number of models assessed, number of ground stations used, most consistent selection of input variables in terms of temporal and spatial resolution, also using the most rigorous and fair performance assessment criteria and suitable ranking system. A statistically rigorous Principal Component Analysis (PCA) ranking procedure is proposed to replace the conventional ordering method based on single and simple statistics. In particular, it is demonstrated that all 13 error metrics contribute to the variance of GHI estimation and thus must all be accounted for in the ranking procedure. The best performing models in each climate zone of equatorial, arid, temperate, cold and polar are REST2v9.1, REST2v9.1, MAC2, REST2v5 and CLS, respectively. Globally, the top three performing models are MAC2, REST2v5, and REST2v9.1. Many models appear to suffer from over-fitting empirical relationships to training data, resulting in inconsistent worldwide performance. Furthermore, six different formulations of the Linke Turbidity factor (TL) are evaluated through its application to five clear-sky models that require it as an input. The best globally performing TL formulations are either by Ineichen or Gueymard when combined with four of the five Linke-dependent clear-sky models, whilst the Grenier formulation performs best with the Ineichen & Perez model. Significant performance variation is observed depending on the TL formulation selected, hence, careful selection is required for an optimal application. The model codes are available in R [1].

First estimate of the value of the instrumental polarization of the RadioAstron space radio telescope using the results of an early scientific program for observing active galactic nuclei

To interpret radio interferometric observations sensitive to linear polarization it is necessary to take into account the effect of instrumental or “parasitic” polarization. In the case of ground-space very-long-baseline interferometry, this procedure involves a session of polarization-sensitive mapping with the sufficient number of ground stations and sufficiently small projections of ground-space baselines. In the paper, the problem of estimating the value of the instrumental polarization of the Spectr-R space radio telescope of the RadioAstron project using the results of the early scientific program for observing active galactic nuclei is addressed. We use a statistical approach to estimate the value of the instrumental polarization associated with the analysis of already obtained to present time intermediate results of the review of brightness temperatures. Estimates obtained for the frequency bands C and L (95% probability intervals [0.0646, 0.1267] and [0.0945, 0.1736] for 6 and 18 cm, respectively) suggest that the instrumental polarization of the radio telescope does not exceed the values typical for the ground VLBI stations.

Feasibility assessment of optical technologies for reliable high capacity feeder links

Space telecom scenarios like data relay satellite and broadband/broadcast service providers require reliable feeder links with high bandwidth/data rate for the communication between ground station and satellite. Free space optical communication (FSOC) is an attractive alternative to microwave links, improving performance by offering abundant bandwidth at small apertures of the optical terminals. At the same time Near-Earth communication by FSOC avoids interference with other services and is free of regulatory issues. The drawback however is the impairment by the laser propagation through the atmosphere at optical wavelengths. Also to be considered are questions of eye safety for ground personnel and aviation. In this paper we assess the user requirements for typical space telecom scenarios and compare these requirements with solutions using optical data links through the atmosphere. We suggest a site diversity scheme with a number of ground stations and a switching scheme using two optical terminals on-board the satellite. Considering the technology trade-offs between four different optical wavelengths we recommend the future use of 1.5µm laser technology and calculate a link budget for an atmospheric condition of light haze on the optical path. By comparing link budgets we show an outlook to the future potential use of 10µm laser technology.

RADARSAT-1 mission planning: Meeting customer needs over 5 years of evolving operations

The RADARSAT-1 Mission Management Office (MMO) has been planning and scheduling the SAR payload, ground reception, and processing facilities that supply the World's synthetic aperture radar (SAR) user community with timely RADARSAT-1 data since 1995. The goal of the MMO is to provide users with timely, high-quality data in a manner continuously being refined to meet changing needs. As the RADARSAT-1 client base and network of stations expanded, the demand for data increased, new applications were found, and new requirements were outlined. Over 5 years the number of ground stations grew from three reception facilities (two in Canada, and one in the United States) to 16 worldwide. To support new facilities, higher data demands, and new data applications, the MMO system required changes to support time-critical information transfer between facilities and improve data delivery timelines and image quality. This paper has two main purposes: describe the current RADARSAT-1 acquisition planning operations system, and summarize the operational improvements made over the last 5 years with the focus being how changes affected SAR users.

Study of Low-Cost Orbit Determination System for Tethered Satellites

The statistics of estimated errors when conducting the orbit determination of a tethered satellite system (TSS) by only using Doppler frequencies is shown. TSS consists of two satellites connected each other by a tether. The center of mass (CM) moves like a single satellite, so this system can be utilized in case of a single satellite as well as TSS. This is considered as a low-cost orbit determination system because it can be realized with simple instruments. This kind of low-cost ground station will charm institutions that are developing a micro satellite in a small budget. The result will provide a useful reference in constructing a ground station for the orbit determination by only Doppler frequencies. The TSS motion model is explained at first. The transformation from covariance matrix of TSS state into the easy-to-image coordinates is explained. Following simulations are examined by supposing 800-km altitude circle LEO with high inclination: Case of measurement from two satellites in 7-day span, Influence by different values of tether length or epoch swing amplitude, and Comparison with patterns of measurement or the number of ground stations. From the results, estimated standard deviations of TSS are quantified when using measurement data of two satellites. If either satellite’s data is only available, it is difficult to determine the tether length and the mass ratio. However, state of CM can be determined in accuracy of some extent.

Evidence for a global disturbance with monochromatic pulsations and energetic electron bunching

We present data from a number of ground stations and satellites that reveal an example of a previously unreported type of global event. The event is characterized by the occurrence of monochromatic pulsations of widely varying frequencies in different regions of local times and L shells. The pulsations appear to be modulated with a ∼45 min periodicity. Simultaneously, energetic particle fluxes observed at geosynchronous orbit (with energies resulting in a drift period of ∼45 min) appear to become phase bunched and the pulsations are observed to occur coincident with minima in the particle bunching. Corresponding data from GOES 5 show an increase in the compressional component of the magnetic field with this period. Data from South Pole Station show fluctuations in the east–west component of the magnetic field at half this period, along with a weak auroral signature and riometer absorptions. We conclude that the data show the existence of a global disturbance with a compressional magnetic field signature, and we suggest that this compression induces a radial electric field, based on Faraday's law. Phase bunching of energetic electrons (due to the induction electric field) and monochromatic pulsations are consequences of the global disturbance, although the mechanism responsible for exciting the pulsations is not clear.

Mesoscale circulation across the Alps: Preliminary simulation of transalp 1990 observations

This paper describes the simulation of the most important characteristics of the flow and temperature fields observed during the tracer release of the 1990 TRANSALP campaign. The goal of the campaign was to follow the tracer from Leventina Valley to the other side of the Alpine range to investigate whether a re-circulation process could occur due to the valley and mountain breezes. Model output is compared with observations of wind and temperature over an entire day (29 September 1990). Meteorological measurements were obtained at a number of ground stations. Vertical wind and temperature profiles were also available. Model initialization was performed using airborne meteorological measurements and profiles from the synoptic station of Payerne. RAMS results correctly describe the generation of the drainage flow during the night and the successive formation of the valley breeze due to the difference in temperature gradients in the various valleys of the domain. Simulated windfields are in general more accurate during the daytime than during the night hours, since vertical variability in stable conditions is more difficult to reproduce.

Time-varying reconstruction of the ionosphere. 2. Data source analysis

A time-varying computed ionospheric tomography (TVCIT) algorithm for reconstructing moving images of the ionosphere is proposed. Since the TVCIT algorithm provides more information in the reconstructions than is provided in reconstructions using conventional static computed ionospheric tomography, the TVCIT algorithm requires information from additional data sources for accurate reconstruction. There are several data sources that may be available for TVCIT reconstruction, including ionosonde data and total electron content data from additional satellites. This article evaluates the efficacy of various combinations of data sources for TVCIT reconstruction. In addition, a quantitative measure is used to compare the amount of information derived from several different combinations of data sources and to evaluate the effect of varying the number of ground stations. © 1998 John Wiley & Sons, Inc. Int J Imaging Syst Technol 9: 491–499, 1998

Rain Margin Improvement Using Resource Sharing in 12-GHz Satellite Downlinks

In this paper we consider the effectiveness of sharing a small pool of reserved time slots of a Time Division-Multiple-Access (TDMA) frame among a large number of ground stations to overcome rain fading. With this approach, a system dynamically assigns time slots from the reserved pool to ground stations experiencing fade depths above the built-in margin. Powerful error correcting codes can be introduced to occupy the extra time slots, providing 10 dB or more of extra fade margin. Because a large number of ground stations are competing for the limited reserved pool, blockage can occur if the number of simultaneous fades exceeds the maximum number that can be accommodated. Some factors that influence the effectiveness of resource sharing are the mutual fade statistics at the various sites, the traffic distribution within the network, the number of earth stations, the size of the reserved pool, and the rain outage objective. Since the mutual fade and traffic statistics are unavailable, we develop models that can be used to find a conservative bound on the required size of the reserved pool. The rain model accounts for diurnal, seasonal, and geographical correlation among attenuation events. Results for a maximum resource-sharing gain of 10 dB show that reserving six percent of the time slots ensures a realized fade gain in excess of 9 dB for a down-link outage objective of 0.005 percent if there are more than 50 ground stations in the network, each with two percent or less of the traffic.

The Stanford University VLF Wave Injection Experiment on the ISEE-A Spacecraft

A Stanford University VLF wave injection experiment will be carried out as part of the ISEE mission. This experiment consists essentially of three basic components; a broad-band VLF receiver on ISEE-A, a broad-band VLF transmitter located at Siple Station in the Antarctic, and a number of ground stations in the Antarctic and Canada. This experiment is an outgrowth of VLF wave injection experiments carried out over the past four years using the Stanford University transmitter at Siple Station, Antarctica. The purpose of this experiment is to study VLF-wave-particle interactions in the magnetosphere, with the goal of achieving a better understanding of this important portion of the earth's environment. In the present paper we sketch briefly the scientific background of the experiment and describe the functions of the ISEE-A instrument.

Probing the Earth’s gravity field by means of satellite-to-satellite tracking

The idea of tracking one spacecraft from another grew out of some tracking studies performed early in the Apollo programme (1962-3). The main practical advantage of such a technique is that ( a ) contact time with a low orbiting spacecraft can be increased considerably (approximately 50 min v . 5 min for a single ground station); ( b ) the number of ground stations can be reduced; ( c ) the dependency on stations on foreign soil can almost be eliminated; and ( d ) detailed studies of spacecraft motions due to small variations in the Earth’s gravity field (anomalies) may be detectable. This paper describes specifically two satellite-to-satellite tracking (s. s. t.) tests, namely ( a ) the ATS-6/Geos-3 and ( b ) the ATS-6/Apollo-Soyuz experiment and some of the results obtained. The main purpose of these two experiments was first to track via ATS-6 the Geos-3 as well as the Apollo-Soyuz and to use these tracking data to determine ( a ) both orbits, that is, ATS-6, Geos-3 and/or the Apollo-Soyuz orbits at the same time; ( b ) each of these orbits alone, and ( c ) test the ATS-6/Geos-3 and /or Apollo-Soyuz s. s. t. link to study local gravity anomalies; and, second, to test communications, command and data transmission from the ground via ATS-6 to these spacecraft and back again to the ground (Rosman, N. G.). Most of the interesting data obtained to date originate from the Apollo-Soyuz geodynamics experiment. Thus, it will be discussed in some detail. Gravity anomalies of say 3-5 mGal (3-5 × 10 -5 m s -2 ) or larger having wavelength of 500-1000 km on the Earth’s surface are important for studies of the upper layers of the earth. Such anomalies were actually ‘seen’ for the first time from space as signatures in the form of very small variation (order of ~ 1 to 2 cm/s) in the range rate between ATS-6, Geos-3 and Apollo-Soyuz. Since the measured range noise turned out to be only 0.03- 0.05 cm/s on the average, these signatures were detected with an excellent signal-to-noise ratio. Orbit determination examples using s. s. t. data from ATS-6 and Geos-3 are also discussed in detail together with errors associated with the orbits of Geos-3. Further, signature studies and gravity anomaly detections with s. s. t. data will be shown and discussed in detail.

Error Distributions of Best Estimate of Position from Multiple Time Difference Hyperbolic Networks

A mathematical model for the error distribution of the best estimate of position of a vehicle as determined from a set of simultaneous measurements of times of arrival of electromagnetic waves from an arbitrary number of ground stations is obtained. The major and minor axes of the error ellipse, as well as the angle that they make with the assumed axes, are determined. The results are in such form as to be easily adaptable for use with a computer.