Availability Of Constellations Research Articles

An alternating renewal process to model constellation availability

Constellation availability is an important assessment criteria of a constellation usually modeled with the assumption of the Markov property. We show this assumption does not hold for the time to long-term outage distribution of GPS satellites. In this paper, we propose an alternate method to calculate slot availability by modelling the outage process as an alternating renewal process. This proposed method avoids the assumption of the Markov property. The method is then validated against the slot availability data from the GPS 24 constellation, where both long-term and short-term outages are shown to approximate independent alternate renewal processes. In addition, the similarities of various constellation state probabilities with the binomial distribution based on average slot availability are shown. Using the model, future constellation availability performance can be estimated directly from historical data and also provide accurate estimates of the expected number of satellite replacements.

Combining Swarm Langmuir probe observations, LEO-POD-based and ground-based GNSS receivers and ionosondes for prompt detection of ionospheric earthquake and tsunami signatures: case study of 2015 Chile-Illapel event

The study investigates ionospheric electric field responses to the earthquake (EQ) of magnitude 8.3, and the related seismic activity and tsunami triggered by the mainshock in Chile-Illapel region, at 22:54 UTC, in the evening of September 16, 2015. The work is a wider review of available ground and satellite data and techniques available to detect seismically induced traveling ionospheric disturbances (TID) and irregularities of smaller scale. The data used in the experiment includes several types of ground and satellite observations from low-Earth orbit (LEO) satellites. The number of techniques applied here is also extended and includes spectral analysis of LEO along-track data and composed analysis of ground GNSS data. The timeframe of the analyses is focused on September 16 and 17, 2015 but also extended to several adjacent days, where an enhanced seismic activity has been recorded. Several examples of seismically triggered TIDs are shown, as detected by combined observations from more than one source and applying different methods, including spectral analysis. These disturbances occur before the mainshock, just after, or in time following this large EQ, and can be found in close neighborhood of Chile-Illapel or far away from the epicenter. The objective of the work was to demonstrate an increasing number of available data and techniques, which can be limited when applied alone, but their combination can provide many advantages in the analysis of seismically disturbed ionosphere. The combination of LEO satellite data reaching all regions of the globe with local but dense ground-based GNSS data and ionospheric HF sounders looks promising, especially in view of the nearby availability of CubeSat constellations equipped with instruments for ionosphere sounding. An important conclusion coming from the study is a need for spectral analysis techniques in the processing of LEO along-track data and the requirement of the validation of LEO observations with separate LEO data or ground-based data. A general but key finding refers to the complementarities of different observations of the ionospheric electric field, which is critically important in the case of analyzing ionospheric irregularities in the extended and composed ionosphere, especially if not every sounding direction can successfully find it.

A Semi-2D Synthetic Storm Technique for Orbital Diversity on Earth-Space Propagation Paths

In this article, new assumptions are introduced into the well-known synthetic storm technique (SST) proposed by Drufuca and further developed by Matricciani, to address the orbital diversity in satellite links. The new approach receives as input information on collocated rain rate recordings and instantaneous estimates of the storm direction and speed; and assumes that the rain event is transversely constant to the storm movement. The achieved improvements are evaluated by analyzing two example measured rain attenuation events “seen” by two Ka -band beacon receivers looking at two different satellites spaced around in azimuth. A study is made of the cross correlation coefficient between rain attenuation series for these two links. Moreover, a methodology is sketched as to how multiple non-GEO satellite constellation availability (for global Internet access at Ka -band) could be evaluated in the frame of the SST.

A refined metric for multi-GNSS constellation availability assessment in polar regions

Human activity in the polar regions is increasing, and as a result of this, the positioning performance of global navigation satellite systems (GNSSs) in these regions is attracting more attention. Since the constellation design of GNSS systems (such as GPS, GLONASS, Galileo, and BDS) only provides superior coverage for human activity in the middle and low latitudes, the elevation angles of GNSS satellites are lower in the polar regions. In this study, the authors first analyzed the availability of GPS, GLONASS, Galileo, and BDS in the polar regions using the classic geometric dilution of precision (GDOP) metric. It was discovered that if only a stand-alone navigation system is employed, satellite visibility in the Arctic and Antarctic regions is excellent, but the GDOP is much higher than in the middle and low latitudes areas. Another interesting phenomenon is that a single navigation system other than GPS (i.e., GLONASS, Galileo, or BDS) will have some areas that cannot be located in the middle and low latitudes and will not appear in the polar regions. A simple solution of this problem is to combine multiple navigation systems to attain better GDOP. In reality, there are currently few achievable or practical solutions for the weight ratio of different satellites in the actual multi-system combined GDOP algorithm. Based on the signal-in-space analysis of different GNSS satellites, the authors propose a NEW WDOP metric for the spatial constellation configuration of multi-GNSSs, including detailed mathematical models and algorithms. The NEW WDOP metric was then used to assess the availability of multi-GNSSs in the polar regions with dual-system, three-system, and four-system combinations. This study thus draws some conclusions using the NEW WDOP model and measured data. In particular, when navigating and positioning in polar regions with a stand-alone GNSS, the mean of the NEW WDOP values is approximately 2.5, and there are many outlier values. Meanwhile, the mean of the NEW WDOP values with the dual GNSS combinations is <1.5, but the value of the NEW WDOP for some combinations in the polar regions is still extremely large and contains some outliers. However, the mean of the NEW WDOP with the three-system or four-system combinations in the polar regions is approximately 1, and the number of outliers is very small; in particular, the four-system combination has no outliers. The results of this study contribute some useful information for the GNSS application of multi-system combinations and satellite selection in the polar regions.

Sliding Time Master Digital Image Correlation Analyses of CubeSat Images for landslide Monitoring: The Rattlesnake Hills Landslide (USA)

Landslide monitoring is a global challenge that can take strong advantage from opportunities offered by Earth Observation (EO). The increasing availability of constellations of small satellites (e.g., CubeSats) is allowing the collection of satellite images at an incredible revisit time (daily) and good spatial resolution. Furthermore, this trend is expected to grow rapidly in the next few years. In order to explore the potential of using a long stack of images for improving the measurement of ground displacement, we developed a new procedure called STMDA (Slide Time Master Digital image correlation Analyses) that we applied to one year long stack of PlanetScope images for back analyzing the displacement pattern of the Rattlesnake Hills landslide occurred between the 2017 and 2018 in the Washington State (USA). Displacement maps and time-series of displacement of different portions of the landslide was derived, measuring velocity up to 0.5 m/week, i.e., very similar to velocities available in literature. Furthermore, STMDA showed also a good potential in denoising the time-series of displacement at the whole scale with respect to the application of standard DIC methods, thus providing displacement precision up to 0.01 pixels.

A modeling and analysis strategy of constellation availability using on-orbit and ground added launch backup and its application in the reliability design for a remote sensing satellite

To evaluate the system reliability and the constellation availability of remote sensing satellite, in this study, a novel single satellite availability modeling and analysis strategy using on-orbit...

Using external tropospheric corrections to improve GNSS positioning of hot-air balloon

High accurate global navigation satellite systems (GNSS) require to correct a signal delay caused by the troposphere. The delay can be estimated along with other unknowns or introduced from external models. We assess the impact of the recently developed augmentation tropospheric model on real-time kinematic precise point positioning (PPP). The model is based on numerical weather forecast and thus reflects the actual state of weather conditions. Using the G-Nut/Geb software, we processed GNSS and meteorological data collected during the experiment using a hot-air balloon flying up to an altitude of 2000 m. We studied the impacts of random walk noise setting of zenith total delay (ZTD) on estimated parameters and the mutual correlations, the use of external tropospheric corrections, the use of data from a single or dual GNSS constellation and the use of Kalman filter and backward smoothing processing methods. We observed a significant negative correlation of the estimated rover height and ZTD which depends on constraining ZTD estimates. Such correlation caused a degraded performance of both parameters when estimated simultaneously, in particular for a single GNSS constellation. The impact of ZTD constraining reached up to 50-cm differences in the rover height. Introducing external tropospheric corrections improved the PPP solution regarding: (1) shortened convergence, (2) better overall robustness, particularly, in case of degraded satellite geometry, (3) less adjusted parameters with lower correlations. The numerical weather model-driven PPP resulted in 9–12- and 5–6-cm uncertainties in the rover altitude using the Kalman filter and the backward smoothing, respectively. Compared to standard PPP, it indicates better performance by a factor of 1–2 depending on the availability of GNSS constellations, the troposphere constraining and the processing strategy.

Monitoring and evaluation algorithm of GNSS signal in space availability

In civil aviation and other navigation application fields, the availability of Global Navigation Satellite System (GNSS) in Signal in Space (SIS) is a key indicator to evaluate the performance. In this paper, the SIS availability of Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) are evaluated and analyzed. The model of satellite availability algorithm is constructed based on the Markov process, with its reliability investigated. Moreover, the evaluation algorithm of constellation availability is developed. Based on the evaluation models, the performance of GPS SIS and BDS SIS is evaluated by using the measured data, respectively. Combined with the availability standard of GPS Standard Positioning Service (SPS) performance standards and BDS Signal In Space Interface Control Document (Version 2.0), the proposed evaluation models of SIS availability are effective and the performance of GPS SIS and BDS SIS conform the availability of performance standards, respectively. Meanwhile, the results are instructive for the study of the availability performance monitoring and the evaluation of global BDS.

Monitoring dam structural health from space: Insights from novel InSAR techniques and multi-parametric modeling applied to the Pertusillo dam Basilicata, Italy

The availability of new constellations of synthetic aperture radar (SAR) sensors is leading to important advances in infrastructure monitoring. These constellations offer the advantage of reduced revisit times, providing low-latency data that enable analysis that can identify infrastructure instability and dynamic deformation processes.In this paper we use COSMO-SkyMed (CSK) and TerraSAR-X (TSX) data to monitor seasonal induced deformation at the Pertusillo dam (Basilicata, Italy) using multi-temporal SAR data analysis. We analyzed 198 images spanning 2010–2015 using a coherent and incoherent PS approach to merge COSMO-SkyMed adjacent tracks and TerraSAR-X acquisitions, respectively. We used hydrostatic-seasonal-temporal (HST) and hydrostatic-temperature-temporal (HTT) models to interpret the non-linear deformation at the dam wall using ground measurements together with SAR time-series analysis. Different look geometries allowed us to characterize the horizontal deformation field typically observed at dams. Within the limits of our models and the SAR acquisition sampling we found that most of the deformation at the Pertusillo dam can be explained by taking into account only thermal seasonal dilation and hydrostatic pressure. The different models show slightly different results when interpreting the aging term at the dam wall. The results highlight how short-revisit SAR satellites in combination with models widely used in the literature for interpreting pendulum and GPS data can be used for supporting structural health monitoring and provide valuable information to ground users directly involved in field measurements.

Reliability Analysis for the Navstar Based on State Probability of the Constellation

The definition for the constellation availability of the navigation system is given firstly, and then calculates the availability of a single satellite by using Markov processes, and establishes the model based on the state probability of the constellation, to achieve mapping model from component reliability of the single satellite to the constellation availability. Finally, we analysis the regional availability performance of the GNSS by the algorithm, which can fit and satisfy the requirements of contradiction between accuracy and rapidity for analysis of availability for constellation of GNSS.

Receiver autonomous integrity monitoring (RAIM) performances of combined GPS/BeiDou/QZSS in urban canyon

Multi-GNSS (global navigation satellite system) has become a trend to solve the urban canyon effect in order to meet the requirements of GNSS users, but the probability of mistaken satellites is also increasing with the increase in the number of visible satellites. The receiver autonomous integrity monitoring (RAIM) performance has therefore become very important. In this paper, the minimum RAIM availability (MRA) is proposed to evaluate the RAIM availability of combined GNSS constellations, and the minimal detectable effect (MDE) is applied to assess the RAIM fault detection capability. The significant improvements of RAIM performances in urban canyons when GPS/QZSS (quasi-zenith satellite system) are combined without and with BeiDou are quantified and analyzed by the simulations of the Asia-Pacific region snapshot and temporal variations in Beijing, Tokyo, Shanghai, and Hong Kong with different masking angles. The combining of BeiDou with GPS/QZSS has great realistic significance, especially in the Asia-Pacific region. The evaluation measure of RAIM performances can be used to predict the availability of GNSS constellations under different conditions, and it can be also used to assess the availability and integrity of combined constellations. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The Modeling and Simulation of Constellation Availability Based on Satellite Reliability

In aerospace, the design principles have transformed from the pursuit of performance to a balance among performance, costs, reliability and other aspects, and how to meet the system performance requirements under the premise of minimizing costs has become a serious problem. In this paper, the relationships between the constellation availability and the satellite reliability were modeled considering the replacement mode of the back-up satellite. A case study was presented to show the effects of the satellite reliability on constellation availability.