Tracking Stations Research Articles

Editors' Note

We are living through a grim moment of global convergences, from emerging contagious diseases and the looming threat of catastrophic climate change to insufferable forms of social oppression and authoritarian regimes of surveillance. We invite our readers to contribute reflections, considered scholarly meditations, or short notes “from the field” to help illuminate the ongoing turmoil of world historical crises.Meanwhile, our inaugural issue of 2021 begins with the section “The African-Soviet Modern,” which brings together interdisciplinary scholarship that explores the multiplex connections between Africa and the Soviet Union. Organized by Elizabeth Banks, Robyn d'Avignon, and Asif Siddiqi, the section investigates the distinctive ethos of African-Soviet projects of modernization and modernity that diverged from Western capitalist projects. The essays explore these projects in their own terms rather than as a state socialism destined to fail or a sideshow to particular national or nationalist histories. Andrew Ivaska's study of the circulation of exiles from Mozambique's FRELIMO insurgency, Banks's history of the troubled relationship between Soviet and Mozambiquan women's groups, Siddiqi's examination of Soviet satellite tracking stations in Africa, and Steffi Marung's genealogy of Soviet paradigms of geography and development all point to the distinctive temporalities and the methodological challenges posed by foregrounding underexplored Soviet transregional connectivities.The essays by Owain Lawson and Natalie Koch bring together political geography with environmental histories to investigate the workings of local, regional views of the land—and its rivers and deserts—from Lebanese hydroelectric engineers to Saudi desert dairy farmers in the section titled “The Geopolitics of Nature.” Their focus on the relationship between modern technopolitics and the natural landscape draws attention to the transnational links and international vectors that moved through the concepts and constructions these actors engaged with, from international financiers, urban planners, and development ideologues to desert geographers and agriculturalists.Finally, we end with the section “Political Aesthetics,” which comprises an essay on the affective feminism of contemporary Iranian art by Foad Torshizi and an original interview with the Palestinian artist Rana Bishara. As Lila Abu-Lughod's interview with Bishara makes so clear, regarding art as an active form of reconstitution as much as representation—or as a form of activism in its own right—highlights the aesthetic as much as political possibilities and perceptual movements it activates. Torshizi draws attention to the contemporary artist GhazalehHedayat's haptic visualities as aesthetic movements that interrupt acts of representation. Meanwhile, Bishara brings to life a variety of everyday objects and scenes that animate their embeddedness in contemporary Palestinian life under Israeli occupation and control and yet simultaneously integrate the experiences of dislocation and suffering they aesthetically represent, such as the use of barbed wire (ubiquitous in the landscape of occupied Palestine) wrapped around a melting ice cube in Hunger Strike/Ticking Freedom, which appears on the cover of this issue. As Bishara recalls so evocatively when describing her series Blindfolded Histories, these bittersweet juxtapositions arethemselves political acts as much as aesthetic representations: “Each time a viewer steps into the space,” she states, “there is movement. An emotion is released in this space of frozen moments from the brutal realities of our history.” Indeed, this evocative reanimation of collective memory and loss reminds us all of the power and beauty of art as activism.

Performance Assessment of a Turn Around Ranging in Communication Satellite Orbit Determination

Satellite operators utilize a two-stations turn around ranging (TAR) system to reduce the ground station measurement system's complexity and cost while having the same or better orbit determination accuracy for communication satellites orbit determination recently. This study investigates two stations' performance, four-way ranging on communication satellite orbit determination, operational conformance, and cost. The observation data sets are collected using traditional single station tracking (SST) and the new method TAR. The computed results using the Monte Carlo method encourage the satellite operators to use a four-way ranging system to observe and measure required data sets. TAR performance is evaluated, taking SST as a reference. The six classical orbital elements (a, e, i, RAAN, AoP, and TA) are compared for large numbers of observation data. The SST and TAR results are very close to each other. The worst-case calculated Euclidian distance between SST and TAR is 1.893 km at the epoch below the 6 km success criteria. The TAR observation method is appropriate to collect data sets for precise orbit determination. This work result indicates that satellite operators should consider deploying TAR stations to collect two-station range data sets and compute the orbit for nominal north-south station-keeping maneuvers (NSSK) and east-west station-keeping (EWSK) maneuver operations. The TAR method is superior to SST in terms of accuracy, operational conformance, and costs.

Adaptive Array Antenna in Ground Station Control System for Massive LEO CubeSat Constellation Tracking

Since Low Earth Orbit has become a popular orbit to place satellites, CubeSat constellation tracking is challenging. The higher capacity and capability of a new Ground Station tracking system are in demand and this motivates researchers to come out with a solution to mitigate the traditional Ground Station degradation performance. This paper discusses the implementation of the Least Mean Square algorithm to the Adaptive Array Antenna for the Ground Station tracking system application. The simulation results show the main beam pattern’s capability to perform instantaneous tracking and steering towards the targeted CubeSat while suppressing the side lobes towards the other interfering CubeSats when massive CubeSats constellations take place. Planar and circular array antennas are analyzed by setting the optimum weighting factors to be adapted. Three scenarios of CubeSats constellations are simulated. The Adaptive Array Antenna’s performance for each scenario is analyzed based on the beam width characteristics and the Signal-to-Interference Ratio of the beam pattern. The number of CubeSats tracked by the Adaptive Array Antenna and the system’s capability to perform fast tracking as the number of antenna array elements is also discussed in this paper. Finally, solutions to mitigate the Least Mean Square algorithm’s limitation to perform fast tracking when massive interferer CubeSats are nearby are proposed.

A New Method for Determining an Optimal Diurnal Threshold of GNSS Precipitable Water Vapor for Precipitation Forecasting

Nowadays, precipitable water vapor (PWV) retrieved from ground-based Global Navigation Satellite Systems (GNSS) tracking stations has heralded a new era of GNSS meteorological applications, especially for severe weather prediction. Among the existing models that use PWV timeseries to predict heavy precipitation, the “threshold-based” models, which are based on a set of predefined thresholds for the predictors used in the model for predictions, are effective in heavy precipitation nowcasting. In previous studies, monthly thresholds have been widely accepted due to the monthly patterns of different predictors being fully considered. However, the primary weakness of this type of thresholds lies in their poor prediction results in the transitional periods between two consecutive months. Therefore, in this study, a new method for the determination of an optimal set of diurnal thresholds by adopting a 31-day sliding window was first proposed. Both the monthly and diurnal variation characteristics of the predictors were taken into consideration in the new method. Then, on the strength of the new method, an improved PWV-based model for heavy precipitation prediction was developed using the optimal set of diurnal thresholds determined based on the hourly PWV and precipitation records for the summer over the period 2010–2017 at the co-located HKSC–KP (King’s Park) stations in Hong Kong. The new model was evaluated by comparing its prediction results against the hourly precipitation records for the summer in 2018 and 2019. It is shown that 96.9% of heavy precipitation events were correctly predicted with a lead time of 4.86 h, and the false alarms resulting from the new model were reduced to 25.3%. These results suggest that the inclusion of the diurnal thresholds can significantly improve the prediction performance of the model.

Review of the Precise Orbit Determination for Chinese Lunar Exploration Projects

AbstractChina's lunar exploration program has developed and progressed for more than 13 years. Over this time, Precise Orbit Determination (POD) has guaranteed successful execution of the Chang’e missions, and is the basis for further scientific investigations using radio science data; for example, recovering the lunar gravity field to explore the inner structure and detect craters to study the evolutionary history of the moon. This study briefly reviews the Chang’e series mission orbit and radio tracking measurements. We reprocessed the tracking data and comprehensively summarized the evolution of the POD accuracy and the tracking system precision from the Chang’e 1 to Chang’e 4 missions using in‐house software. Our results show a progression of the tracking station, as the first Chang’e 1 mission only reached several millimeters per second level accuracy in the Doppler measurements and several meters level accuracy for the two‐way range measurements. Later on the accuracy of Chang’e 5T1 Doppler measurements reached about 0.35 mm/s and the accuracy of the Chang’e 3 two‐way range measurements below 0.7 m. Moreover, the newest Chang’e 4 relay satellite achieved POD residuals Root Mean Square for the two‐way range less than a meter and orbital accuracy at 200 m level, in contrast to meters level residual and the half‐kilometer level accuracy achieved during the Chang’e 1 mission. We can clearly see that the POD performance of the Chang’e spacecraft continuously improved. This research study is a reference for future Chinese Lunar exploration missions, as well as Chinese Mars and asteroid explorations.

Accuracy Analysis of GNSS Hourly Ultra-Rapid Orbit and Clock Products from SHAO AC of iGMAS

With the development of the global navigation satellite system(GNSS), the hourly ultra-rapid products of GNSS are attracting more attention due to their low latency and high accuracy. A new strategy and method was applied by the Shanghai Astronomical Observatory (SHAO) Analysis Center (AC) of the international GNSS Monitoring and Assessment Service (iGMAS) for generating 6-hourly and 1-hourly GNSS products, which mainly include the American Global Positioning System (GPS), the Russian Global’naya Navigatsionnaya Sputnikova Sistema (GLONASS), the European Union’s Galileo, and the Chinese BeiDou navigation satellite system (BDS). The 6-hourly and 1-hourly GNSS orbit and clock ultra-rapid products included a 24-h observation session which is determined by 24-h observation data from global tracking stations, and a 24-h prediction session which is predicted from the observation session. The accuracy of the 1-hourly orbit product improved about 1%, 31%, 13%, 11%, 23%, and 9% for the observation session and 18%, 43%, 45%, 34%, 53%, and 15% for the prediction session of GPS, GLONASS, Galileo, BDS Medium Earth Orbit (MEO), Inclined Geosynchronous Orbit (IGSO), and GEO orbit, when compared with reference products with high accuracy from the International GNSS service (IGS).The precision of the 1-hourly clock products can also be seen better than the 6-hourly clock products. The accuracy and precision of the 6-hourly and 1-hourly orbit and clock verify the availability and reliability of the hourly ultra-rapid products, which can be used for real-time or near-real-time applications, and show encouraging prospects.

Orbit determination of BDS-3 satellite based on regional ground tracking station and inter-satellite link observations

• Under regional GTS, ISL significantly improves BDS-3 orbit determination accuracy. • The initial weight ratio has little effect on the result when VCE algorithm is used. • VCE algorithm has contribute to fusion data processing of L-band and Ka-band. The BeiDou global navigation satellite system (BDS-3) has established the Ka-band inter-satellite link (ISL) to realize a two-way ranging function between satellites, which provides a new observation technology for the orbit determination of BDS-3 satellites. Therefore, this study presents a BDS satellite orbit determination model based on ground tracking station (GTS) observations and ISL ranging observations firstly to analyze the impact of the ISL ranging observations on the orbit determination of BDS-3 satellites. Subsequently, considering the data fusion processing, the variance component estimation (VCE) algorithm is applied to the parameter estimation process of the satellite orbit determination. Finally, using the measured data from China’s regional GTS observations and BDS-3 ISL ranging observations, the effects of ISL ranging observations on the orbit determination accuracy of BDS-3 satellites are analyzed. Moreover, the impact of the VCE algorithm on the fusion data processing is evaluated from the aspects of orbit determination accuracy, Ka-band hardware delay parameter stability, and ISL ranging observation residuals. The results show that for China’s regional GTSs, the addition of BDS-3 ISL ranging observations can significantly improve the orbit determination accuracy of BDS-3 satellites. The observed orbit determination accuracy of satellite radial component is improved from 48 cm to 4.1 cm. In addition, when the initial weight ratio between GTS observations and ISL ranging observations is not appropriate, the various indicators which include orbit determination accuracy, ISL hardware delay, and ISL observation residuals were observed to have improved after the adjustment of the VCE algorithm. These results validate the effectiveness of the VCE algorithm for the fusion data processing of the GTS observations and ISL ranging observations.

BeiDou-3 broadcast clock estimation by integration of observations of regional tracking stations and inter-satellite links

The BeiDou navigation satellite system (BDS) tracks medium earth orbit (MEO) satellites using only regional tracking stations in China. As a result, the broadcast clock accuracy of the MEO satellites decreases rapidly during the invisible arcs because of the lack of available observations. The inter-satellite link (ISL) technology of the third generation of BDS (BDS-3) can be used to extend the visible arcs of MEO satellites and to measure the relative inter-satellite clock in nearly real time. We propose a broadcast clock approach for BDS-3 by integrating observations from regional tracking stations and ISLs. The clock error between satellites is obtained through centralized estimation based on ISLs. The Ka-band hardware delay is calibrated by taking the double difference between ISL-centralized clock and the Multi-satellite Precise Orbit Determination clock. The deviation between the ISL-centralized clock and the BeiDou time is obtained using only one Two-way Satellite Time Comparison station or anchor station. To validate the algorithms, we analyze clock estimation and prediction accuracy, hardware delay stability, and time synchronization accuracy. The results show that the frequency stability of the BDS-3 onboard passive hydrogen maser (PHM) and rubidium atomic frequency standard (RAFS) is competitive to those of the GPS IIF RAFS and Galileo FOC PHM and better than those of GPS IIR RAFS. The root-mean-square error of the 2-h clock prediction is better than 0.25 ns, and the validation result relative to the post-processed precise clock product is better than 0.4 ns. The time synchronization accuracy of better than 1 ns can be obtained based on only one TSTC station or an anchor station, and the standard deviation of Ka-band hardware delay is about 0.12 ns. It is believed that the ISL and the proposed algorithms will bring a significant upgrade in the estimation of BDS-3 broadcast clock; the broadcast clock accuracy will be greatly improved, and reliance on the ground segment will also be reduced significantly.

Aloha Space Age: NASA and the Hawaiian Islands, 1957—1970

Aloha Space Age:NASA and the Hawaiian Islands, 1957—1970 David A. Smith (bio) I have been part of that generation so inspired by the space program. Nineteen sixty-one was the year of my birth, the year that Kennedy made his announcement. And one of my earliest memories is sitting on my grandfather's shoulders, waving a flag as astronauts arrived in Hawai'i. For me, the space program has always captured an essential part of what it means to be an American: reaching for new heights, stretching beyond what previously did not seem possible. And so as President, I believe that space exploration is ... an essential part of that quest. —US President Barack Obama, 2010 1 When scholars and the general public recall the early years of the National Aeronautics and Space Administration (NASA) space pro-gram, from Project Mercury through Project Apollo, the Cape and Kennedy Space Center in Florida and Manned Space Flight Center in Houston are the two earthbound locales that usually first come to mind. 2 Over the years, historians have also turned their attention to other sites outside of southeast Texas and Florida's space coast that played significant roles in space exploration. 3 One region that has [End Page 1] been almost universally overlooked by academic and popular authors alike, however, has been the State of Hawai'i, which became the fiftieth state just one year after NASA was established in November 1958. While the world's premier and perhaps most controversial astronomical site at Mauna Kea on the island of Hawai'i has been the subject of several studies (including one in this journal 4 ), the stories of the Hawaiian Islands' engagement with the movement of people to and from space and the Moon, including residents' responses to the Space Age, have not been told. From NASA's formative years through the early 1970s, the Hawaiian Islands played important and colorful roles in space exploration. On the island of Kaua'i, the Kōke'e tracking station was an integral part of the agency's tracking and communications network for the early human space flights that made up the Mercury, Gemini, and Apollo programs. O'ahu was the first port of call for astronauts returning from the Moon. The island of Hawai'i became the first choice for astronaut geology training and other lunar space-related activities. There were other significant, if less tangible, connections as well. From the aftermath of the Soviet Union's launch of Sputnik I through the dramatic return of the Apollo 13 astronauts in 1970, most, if not all, of Hawai'i's residents, local press, politicians, celebrities, schools, and cultural institutions consistently embraced space travel programs and related activities with a sense of enthusiasm and "great aloha" (especially in terms of greetings for returning astronauts) with a participation rate far out of proportion to Hawai'i's small population. A variety of schemes for space launch centers and facilities, related exhibition and educational programs, and spontaneous celebrations of NASA successes seemed to be everywhere in Hawai'i during the 1960s. Examining the Hawaiian Islands' engagement and welcome of these NASA initiatives fills an existing void in the literature and provides a window, from a mid-Pacific perspective, into understanding why the space program experienced the highest level of public approval of all major US federal initiatives during this era. 5 Rockets, Missiles, and Moonports Most scholars accept that the Space Age began when the Soviet Union launched the world's first man-made satellite, Sputnik, on October 4, 1957. [End Page 2] The United States quickly took notice and began focusing more attention on its own fledgling space initiatives. In the post-war period, the US Congress was alarmed by the perceived Soviet threat to America's security and to its now questionable lead in world technology. As historian Andreas Reichstein contends: "The reaction of the American people to the successful launch of Sputnik . . . has been compared to the one after Pearl Harbor." 6 Troubled O'ahu residents once again looked to the skies and, in November, reported spotting the companion rocket to the Soviet's "first artificial 'moon'" as it...

Assessment and Analysis of the Four-Satellite QZSS Precise Point Positioning and the Integrated Data Processing With GPS

The Quasi-Zenith Satellite System (QZSS), which serves Japan and its surrounding areas, is a regional navigation satellite system developed by Japan Aerospace Exploration Agency. The system is now in a four-satellite constellation with preliminary standalone navigation and positioning capabilities. In this paper, the performance of QZSS-only precise point positioning (PPP) in both static and kinematic modes is initially evaluated using the datasets spanning 11 days from 11 tracking stations in Asia-Pacific regions and the final precise orbit and clock products from the analysis centers GFZ and WUM. For completeness, the GPS/QZSS integrated data processing with GFZ and WUM final products as well as the L6E real-time orbit and clock corrections is also analyzed. The results indicate that the static positioning accuracy of QZSS-only PPP is approximately 4, 2 and 15 cm in the east, north and up directions, respectively, and the static convergence time of QZSS-only PPP with WUM and GFZ products can be 75.2, 44.3 and 45.6 min, and 153.5, 201.6 and 162.8 min in the three directions, respectively. As for the QZSS-only kinematic PPP solutions, the re-convergence repeatedly occurs due to limited available satellites. To achieve more reliable solutions, the inter-system bias parameter in GPS/QZSS PPP processing is recommended to be estimated as random walk process or white noise process. The improvement of post-processed GPS/QZSS PPP over GPS-only case is marginal on position accuracies, but can be several minutes on convergence time. Compared with post-processed GPS/QZSS PPP, the real-time one achieves worse position accuracies but comparable convergence time.

Improved Self-Calibration of a Multilateration System Based on Absolute Distance Measurement.

Multilateration tracking systems (MLTSs) are used in industrial three-dimensional (3D) coordinate measuring applications. For high-precision measurement, system parameters must be calibrated properly in advance. For an MLTS using absolute distance measurement (ADM), the conventional self-calibration method significantly reduces estimation efficiency because all system parameters are estimated simultaneously using a complicated residual function. This paper presents a novel self-calibration method that optimizes ADM to reduce the number of system parameters via highly precise and separate estimations of dead paths. Therefore, the residual function to estimate the tracking station locations can be simplified. By applying a suitable mathematical procedure and solving the initial guess problem without the aid of an external device, estimation accuracy of the system parameters is significantly improved. In three self-calibration experiments, with ADM repeatability of approximately 3.4 µm, the maximum deviation of the system parameters estimated by the proposed self-calibration method was 68.6 µm, while the maximum deviation estimated by the conventional self-calibration method was 711.9 µm. Validation of 3D coordinate measurements in a 1000 mm × 1000 mm × 1000 mm volume showed good agreement between the proposed ADM-based MLTS and a commercial laser tracker, where the maximum difference based on the standard deviation was 17.7 µm. Conversely, the maximum difference was 98.8 µm using the conventional self-calibration method. These results confirmed the efficiency and feasibility of the proposed self-calibration method.

Adaptation of the NeQuick2 model for GNSS wide-area ionospheric delay correction in China and the surrounding areas

As an empirical three-dimensional ionospheric electron density model, NeQuick2 can generally represent the temporal and spatial characterization of ionospheric Total Electron Content (TEC) at a global scale. However, mismodeling errors usually exist between NeQuick2 representation and actual state of short-term ionospheric variation in small scales. In this contribution, an adaptation method is proposed for the NeQuick2 model based on data ingestion with GNSS-derived ionospheric TEC data. The relationship between the background NeQuick2 Slant TEC (STEC) estimations and real STEC measurements can be properly modelled with a ‘scaling factor’ in a highly efficient manner. In order to verify the feasibility and usefulness of the algorithm, validation experiments are carried out with GNSS tracking stations in China and the surrounding areas under different levels of solar activity. The results show that the bias of NeQuick2 corrections after adaptation are mostly within 1.0 TECu, suggesting that the systematic bias of the original NeQuick2 model can be significantly reduced. In comparison with about 50%-70% STEC RMS errors corrected by the original NeQuick2 model, the NeQuick2 model after adaptation can generally correct about 80% − 90% STEC RMS errors, which is also better than the CODE final GIM product as it has smaller residual bias and uses fewer reference stations. Therefore, the proposed algorithm can be satisfactorily applied in wide area GNSS ionospheric corrections.

Potential of Multi constellation Global Navigation Satellite System in Indian Missile Test Range Applications

In this paper, the potentials of using Global Navigation Satellite System (GNSS) techniques in the complex calibration procedure of the tracking sensors for missile test range applications have been presented. The frequently used tracking sensors in test range applications are- electro-optical tracking stations (EOTS) and tracking radars. Over the years, the EOTS are used as the reference for bias estimation of the radars. With the introduction of GPS in test range applications, especially the DGPS, the reference for bias estimation got shifted to DGPS from the EOTS. However, the achievable position solution accuracy is limited to the order of a few meters for DGPS, EOTS, and Radars. With the evolution of Multi-constellation GNSS and carrier-phase based measurement techniques in satellite navigation, achievable position solution accuracies may be improved to sub-meter level. New navigation techniques like real time kinematic (RTK) and precise point positioning have the potentials for use in the calibration procedures of the missile test ranges to the accuracies of centimeter-level. Moreover, because of the availability of a large number of navigation signals over the Indian region, multi-constellation GNSS receivers can enhance signal availability, reliability, and accuracies during the calibration of missile test ranges. Currently available compact, low-cost GNSS modules also offer the possibilities of using these for cost-effective, networked RTK for dynamic calibration of test ranges reducing cost and resource requirements.

Tracking Area Boundary-aware Protocol for Pseudo Stochastic Mobility Prediction in LTE Networks

Accurate mobility prediction enables efficient and faster paging services in these networks. This in turn facilitates the attainment of higher bandwidths and execution of activities such as handovers at low latencies. The conventional mobility prediction models operate on unrealistic assumptions that make them unsuitable for cellular network mobile station tracking. For instance, the Feynman-Verlet, first order kinetic model and Random Waypoint assume that mobile phones move with constant velocity while Manhattan, Freeway, city area, street unit, obstacle mobility, and pathway mobility postulate that mobile station movement is restricted along certain paths. In addition, obstacle mobility model speculate that the mobile station signal is completely absorbed by an obstacle while random walk, random waypoint, Markovian random walk, random direction, shortest path model, normal walk, and smooth random assume that a mobile station can move in any direction. Moreover, the greatest challenge of the random direction model is the requirement that a border behavior model be specified for the reaction of mobile stations reaching the simulation area boundary. In this paper, a protocol that addresses the border behavior problem is developed. This protocol is shown to detect when the subscriber has moved out of the current tracking area, which is crucial during handovers.

Parallel resolution of large GNSS networks using carrier ranges

Owing to the increasing numbers of navigation satellites, available frequencies, and tracking stations, the computational efficiency of large GNSS networks has become a critical issue, especially when undifferenced (UD) pseudorange and carrier phase observations are used. By converting UD carrier phases into carrier ranges before implementing the integrated processing of a whole network, the UD calculation time can be greatly decreased. However, because a common multicore hardware platform is not fully utilized to optimize the algorithm, the processing of a whole network is still time consuming. In this paper, this problem is overcome by applying the multicore parallel computing technology, namely the Task Parallel Library (TPL). To improve the processing efficiency of large GNSS networks using UD carrier ranges and based on the task parallelism and data parallelism offered through TPL, the wide-lane (WL) and narrow-lane (NL) fractional cycle biases (FCBs), and carrier ranges are computed in parallel, and the integrated processing of a whole network is performed by establishing the normal equation in parallel and adopting parallel computing for Cholesky decomposition. The proposed strategy is validated using GPS, BDS and Galileo observation data recorded from January 6, 2019, to January 26, 2019 by the IGS network with a global distribution of approximately 500 stations. The average positioning accuracies in the east, north and vertical components are 2.54, 2.34 and 4.41 mm, respectively, and the average SDBS ambiguity fixing rate is 93.18%. The average time consumptions for step 1, 2 and 3 are reduced from 151.45, 248.80 and 44.35 min to 32.20, 52.71 and 14.25 min, respectively, with acceleration ratios of 4.70, 4.72 and 3.11 times, respectively, on a six-core platform.

Diurnal variation of precipitable water vapor over Central and South America

Annual and seasonal diurnal precipitable water vapor (PWV) variations over Central and South America are analyzed for the period 2007–2013. PWV values were obtained from Global Navigation Satellite Systems (GNSS) observations of sixty-nine GNSS tracking stations. Histograms by climate categories show that PWV values for temperate, polar and cold dry climate have a positive skewed distribution and for tropical climates (except for monsoon subtype) show a negative skewed distribution.The diurnal PWV and surface temperatures (T) anomaly datasets are analyzed by using principal components analysis (PCA). The first two modes represent more than 90% of the PWV variability. The first PCA mode of PWV variability shows a maximum amplitude value in the late afternoon few hours later than the respective values for surface temperature (T), therefore the temperature and the surface conditions (to yield evaporation) could be the main agents producing this variability; PWV variability in inland stations are mainly represented by this mode. The second mode of PWV variability shows a maximum amplitude at midnight, a possible explanation of this behavior is the effect of the sea/valley breeze. The coastal and valley stations are affected by this mode in most cases. Finally, the “undefined” stations, surrounded by several water bodies, are mainly affected by the second mode with negative eigenvectors. In the seasonal analysis, both the undefined and valley stations constitute the main cases that show a sea or valley breeze only during some seasons, while the rest of the year they present a behavior according to their temperature and the surface conditions. As a result, the PCA proves to be a useful numerical tool to represent the main sub-daily PWV variabilities.

GNSS with satcom networks to dramatically improve space situational awareness

With current technology prediction errors in Low Earth Orbit (LEO) are frequently greater than the estimated separation distance between objects undergoing conjunction. Current predictions are thus frequently false alarms, and real collisions have been missed resulting in expensive losses. The February 2009 collision between the active Iridium 33 and the inert Kosmos-2251 was predicted to have a miss distance of 584 metres, but this was less than the error, and so the prediction was ignored, and the actual collision and destruction of Iridium-33 came as a complete surprise. As well the inherent errors in existing ground-based measurements of spacecraft, a significant problem for satellite tracking and collision prediction is the lack of coverage of ground stations. Less than 20% of the Earth's surface area is suitable for siting ground stations [plus a limited number of isolated island sites]. For example, for BDS [a Chinese navsat system], inadequate ground stations and the poor distribution of the network have limited the accuracy of BDS orbit determination [1]. The increasing flux density of orbital objects is driving the community to improve orbital determination of all space objects, or as many as possible. Recently studies have determined that collision prediction accuracy can be greatly improved for spacecraft who carry on board GNSS transponders. We are developing a system using autonomous tracking devices (an application of our Pixie nanospacecraft) which are attached to a host satellite and communicate accurate location data autonomously of the main spacecraft. The data collected is sent to a central Earth station via an established satellite relay from the device. This intersatellite link is available continuously without regard to the position of the host satellite relative to the Earth's surface (e.g., whether it is over regions with no tracking stations). Consequently, positional and velocity data of the host satellite is available without gaps, allowing greatly improved knowledge of the orbital parameters and position of the host satellite versus existing methods. The attached device contains several sensors including a GPS or GNSS receiver, which allows determination of the host spacecraft position accurate to within 10 metres or better, compared with one kilometre (or worse) for conventional ground station methods. The improved knowledge of spacecraft position will allow greatly improved predictions of collisions between space objects, by orders of magnitude versus the existing technology, and yet is cheaper than the less accurate methods using ground stations. It is difficult to quantify the economic damage due to space object collisions since commercial operator might not publish all collision events, preferring to keep them confidential. Nevertheless, it is widely agreed that economic impact can be severe. In addition, damage to the International Space Station is a major concern, it is the largest “target”. Furthermore, it is becoming increasingly difficult to correctly identify the increasing number of satellites. There have been recent launches of multiple spacecraft on a single vehicle in which radio contact was not established with many of the objects, and hence it has become impossible to identify them. This is a concern for several reasons, including liability. Attaching Pixies will resolve this problem. Over time, the improvement in knowledge of space object position by installing the Pixie black boxes will reduce the number of collisions by improving positional knowledge accuracy and reducing the number of false alarms. Collision avoidance maneuvers can be planned and executed in a more controlled and reliable manner, reducing expenditure of propellant. Once Pixies have been successfully demonstrated then it is entirely possible that the international community could mandate the use of Pixie black boxes to be attached to all spacecraft, in a similar manner than Flight Data Recorders and Cockpit Voice Recorders are mandated on most passenger aircraft. The large quantity of new more accurate space object positional data generated will facilitate new methods for organizing large numbers of rapidly moving space objects. New algorithms will be developed to optimize multi-dimensional positional knowledge and situational awareness. Artificial intelligence would be appropriate to leverage the new data pool. Improved conjunction predictions will be facilitated, and probabilistic risk assessment methods should be applied to assist decision making, e.g. when to initiate collision avoidance maneuvers. Potentially this decision making could be automated.

Time delay bias between the second and third generation of BeiDou Navigation Satellite System and its effect on precise point positioning

To realize the deep integration of BeiDou-2 regional navigation satellite system (BDS-2) and BeiDou-3 global navigation satellite system (BDS-3), the time delay bias between BDS-2 and BDS-3 at the receiver side is analyzed for the joint BDS-2/BDS-3 precise point positioning (PPP). Twenty Multi-GNSS (Global Navigation Satellite System) Experiment tracking stations with different receiver types were selected to investigate the time-varying characteristics of time delay bias. We also make a comprehensive analysis about the influence of time delay bias on the ionosphere-free PPP and uncombined PPP. The results show that the daily mean values of time delay bias for consecutive days vary within 1 ns without apparent day-boundary discontinuities. In addition, the time delay bias for the receivers of the same brand can differ by several nanoseconds, especially for the TRIMBLE receivers. Without considering the time delay bias between BDS-2 and BDS-3, the pseudorange residuals, slant ionospheric delays, float ambiguities are subjected to change with the magnitude of the time delay bias, while the phase residuals and positioning accuracy are slightly affected. Moreover, the deviation of float ambiguity in ionosphere-free PPP caused by neglecting the time delay bias is larger than that of uncombined PPP.

Real-time monitoring of the dynamic variation of satellite orbital maneuvers based on BDS observations

The BeiDou Navigation Satellite System (BDS) frequently experiences orbital maneuvers. This study proposed a method for the real-time monitoring of the dynamic variation of satellite orbital maneuvers. Based on the epoch-differenced velocity estimation model, the epoch variation of the receiver clock error can be calculated for each station. The dynamic variation of the orbital maneuver can then be estimated real-time by the monitoring model, using observations from three or more BDS tracking stations. The feasibility and effectiveness of this method is validated by different datasets. The results show that the method can precisely detect orbital maneuvers, thereby extending the usable observations for about three hours. Thus, it is useful for improving the positioning, navigation, and timing service. The dynamic variations of orbital maneuvers can be monitored in real-time, which is useful for precise orbit control, and is helpful for keeping the satellite healthy and monitoring the navigation system operation.

SciFi, the new Tracker of the LHCb experiment

The LHCb Collaboration is currently in the final stage of constructing an upgraded detector that will allow the experiment to operate at higher luminosities and with a triggerless readout. The main tracking stations—which originally were subdivided in an Inner Tracker made from silicon strip sensors and an Outer Tracker built from straw-tubes—are being replaced by a single tracking detector, the Scintillating fibre Tracker (SciFi). The SciFi covers a total detector area of 340 m2 by using more than 10000 km of scintillating fibre with 250 μm diameter, enabling a spatial resolution of better than 100 μm for hits originating from charged particles. It is built from individual modules (0.5 m × 4.8 m) comprised of 8 scintillating fibre mats with a length of 2.4 m as the active detector material. The 13 cm wide fibre mats consist of 6 layers of densely packed blue emitting scintillating fibres. The scintillation light is detected with arrays of multi-channel silicon photomultipliers (SiPMs) cooled to −40̂C to minimize the expected high dark noise from neutron radiation. The readout of 524k channels occurs through custom-designed front-end electronics. Since it is the first time that this technology is being used as a large tracker and with a small granularity, many challenges have to be overcome. This document is intended to give an overview of the SciFi detector design, production, performance and the status of the detector assembly.