GNSS Constellations Research Articles

Dynamic positioning configuration and its first-order optimization

Traditional geodetic network optimization deals with static and discrete control points. The modern space geodetic network is, on the other hand, composed of moving control points in space (satellites) and on the Earth (ground stations). The network configuration composed of these facilities is essentially dynamic and continuous. Moreover, besides the position parameter which needs to be estimated, other geophysical information or signals can also be extracted from the continuous observations. The dynamic (continuous) configuration of the space network determines whether a particular frequency of signals can be identified by this system. In this paper, we employ the functional analysis and graph theory to study the dynamic configuration of space geodetic networks, and mainly focus on the optimal estimation of the position and clock-offset parameters. The principle of the D-optimization is introduced in the Hilbert space after the concept of the traditional discrete configuration is generalized from the finite space to the infinite space. It shows that the D-optimization developed in the discrete optimization is still valid in the dynamic configuration optimization, and this is attributed to the natural generalization of least squares from the Euclidean space to the Hilbert space. Then, we introduce the principle of D-optimality invariance under the combination operation and rotation operation, and propose some D-optimal simplex dynamic configurations: (1) (Semi) circular configuration in 2-dimensional space; (2) the D-optimal cone configuration and D-optimal helical configuration which is close to the GPS constellation in 3-dimensional space. The initial design of GPS constellation can be approximately treated as a combination of 24 D-optimal helixes by properly adjusting the ascending node of different satellites to realize a so-called Walker constellation. In the case of estimating the receiver clock-offset parameter, we show that the circular configuration, the symmetrical cone configuration and helical curve configuration are still D-optimal. It shows that the given total observation time determines the optimal frequency (repeatability) of moving known points and vice versa, and one way to improve the repeatability is to increase the rotational speed. Under the Newton’s law of motion, the frequency of satellite motion determines the orbital altitude. Furthermore, we study three kinds of complex dynamic configurations, one of which is the combination of D-optimal cone configurations and a so-called Walker constellation composed of D-optimal helical configuration, the other is the nested cone configuration composed of n cones, and the last is the nested helical configuration composed of n orbital planes. It shows that an effective way to achieve high coverage is to employ the configuration composed of a certain number of moving known points instead of the simplex configuration (such as D-optimal helical configuration), and one can use the D-optimal simplex solutions or D-optimal complex configurations in any combination to achieve powerful configurations with flexile coverage and flexile repeatability. Alternately, how to optimally generate and assess the discrete configurations sampled from the continuous one is discussed. The proposed configuration optimization framework has taken the well-known regular polygons (such as equilateral triangle and quadrangular) in two-dimensional space and regular polyhedrons (regular tetrahedron, cube, regular octahedron, regular icosahedron, or regular dodecahedron) into account. It shows that the conclusions made by the proposed technique are more general and no longer limited by different sampling schemes. By the conditional equation of D-optimal nested helical configuration, the relevance issues of GNSS constellation optimization are solved and some examples are performed by GPS constellation to verify the validation of the newly proposed optimization technique. The proposed technique is potentially helpful in maintenance and quadratic optimization of single GNSS of which the orbital inclination and the orbital altitude change under the precession, as well as in optimally nesting GNSSs to perform global homogeneous coverage of the Earth.

Assessing the GNSS scintillation climate over Brazil under increasing solar activity

We study ionospheric scintillation on GNSS signals at equatorial latitudes to draw a climatological picture of the low latitude ionosphere in the Brazilian sector during the ascending phase of the upcoming 2013 solar maximum. Such data have been acquired during the early stage of the CIGALA project (http://cigala.galileoic.org/), funded by the European Commission under the 7th Framework Program and the outcome of this work is part of the scientific achievements of the project itself. The considered network is based on the novel PolaRxS receivers, developed and deployed specifically to comply with the aims of the FP7 project. The PolaRxS is able to monitor ionospheric scintillation for all operational and upcoming GNSS constellations (GPS, GLONASS, Compass, GALILEO) and corresponding frequencies in the L-band. The ionosphere over the Brazilian territory, being close to the southern crest of the Equatorial Ionospheric Anomaly (EIA), is heavily affected by intense scintillation conditions. The sector under investigation is also very peculiar with respect to other low latitude regions, because of its proximity to the South Atlantic Magnetic Anomaly (SAMA). The application of the Ground Based Scintillation Climatology (GBSC) technique, for the first time simultaneously on GPS and GLONASS data and on both L1 and L2 frequencies, highlights the joint effect of the EIA and of the SAMA in producing the irregularities leading to scintillation.

Digital Surface Models for GNSS Mission Planning in Critical Environments

AbstractGlobal Navigation Satellite System (GNSS) surveys performed in critical environments (e.g., urban canyons, mountainous areas, or areas of dense vegetation) usually suffer from a lack of satellite coverage as a result of obstacles such as buildings and vegetation. GNSS mission-planning software provides an estimate of satellite visibility and dilution-of-precision (DOP) values along a planned trajectory to establish the best time frame over which to perform the survey. However, such an estimate is not reliable in a complex scenario because the surrounding environmental morphology is not considered. This paper introduces a new method to improve the prediction of GNSS satellite visibility. This method involves computing GNSS satellites position by means of the orbital parameters, as well as using three-dimensional digital surface models (DSMs) to develop a more reliable mission plan. The time evolution of key parameters describing the GNSS constellation is computed by means of a visibility georeferen...

The BeiDou Navigation Message

The article provides an overview of the BeiDou navigation message contents and highlights its specific communalities and differences with respect to other GNSS constellations. Making use of data collected by multi-GNSS monitoring stations of the MGEX and CONGO networks, the quality of BeiDou broadcast ephemerides is assessed through the analysis of satellite laser ranging measurements, comparison with post-processed orbit and clock products as well as positioning tests. Specific attention is given to signal-specific group delays and their proper consideration in the positioning.

Critical Elements for a Multi-Constellation Advanced RAIM

The GNSS environment is experiencing two major changes: the introduction of a new civil signal in the L5 band and the launch of new GNSS constellations. These improvements could enable worldwide vertical guidance using Advanced Receiver Autonomous Integrity Monitoring (ARAIM). Before ARAIM can be implemented, it will be necessary to develop a common understanding across service providers of the navigation requirements, the threat model, the airborne algorithm, and the role of ground monitoring. We outline a concept for the provision of integrity based on ARAIM. First, an interpretation of vertical guidance (LPV-200) requirements is proposed. Then, we describe a comprehensive threat model for GNSS which includes both nominal performance and faulted behavior. We will show how the threats could be mitigated through the use of ground monitoring and the Integrity Support Message in addition to the ARAIM airborne algorithm. Finally, examples of constellation configurations providing worldwide coverage of LPV-200 are presented. Copyright © 2013 Institute of Navigation.

Acquisition of Weak Signals in Multi-Constellation Frequency Domain Receivers

New positioning applications’ availability requirements demand receivers with higher sensitivities and ability to process multiple GNSS signals. Possible applications include acquiring one signal per GNSS constellation in the same frequency band and combining them for increased sensitivity or predicting acquisition of other signals. Frequency domain processing can be used for this purpose, since it benefits from parallel processing capabilities of Fast Fourier Transform (FFT), which can be efficiently implemented in software receivers. On the other hand, long coherent integration times are mainly limited due to large FFT size in receivers using frequency domain techniques. A new method is proposed to address the problems in frequency domain receivers without compromising the resources and execution time. A pre-correlation accumulation (PCA) is proposed to partition the received samples into one-code-period blocks, and to sum them together. As a result, the noise is averaged out and the correlation results will gain more power, provided that the relative phase between the data segments is compensated for. In addition to simplicity, the proposed PCA method enables the use of one-size FFT for all integration times. A post-correlation peak combination is also proposed to remove the need for double buffering. The proposed methods are implemented in a configurable Simulink model, developed for acquiring recorded GNSS signals. For weak signal scenarios, a Spirent GPS simulator is used as a source. Acquisition results for GPS L1 C/A and GLONASS L1OF are shown and the performance of the proposed technique is discussed. The proposed techniques target GNSS receivers using frequency domain processing aiming at accommodating all the GNSS signals, while minimizing resource usage. They also apply to weak signal acquisition in frequency domain to answer the availability demand of today’s GNSS positioning applications.

Using Outage History to Exclude High-Risk Satellites from GBAS Corrections

GNSS augmentation systems that provide integrity guarantees to users typically assume that all GNSS satellites have the same failure probability. The assumed failure probability is conservative such that variations among satellites in a given GNSS constellation are not expected to violate this assumption. A study of unscheduled GPS satellite outages from 1999 to 2011 shows that, as expected, older satellites are much more likely to fail than younger ones. In addition, satellites that have recently experienced unscheduled outages are more likely to suffer additional unscheduled outages. Combining these two factors suggests that it is possible for a subset of GPS satellites to violate the overall satellite failure probability assumption, although this has not yet been demonstrated. Potential rules for GPS satellite exclusion based upon satellite age and recent outages are investigated, and suggestions for including satellite geometry are explored. Copyright © 2012 Institute of Navigation.

Reliability of partial ambiguity fixing with multiple GNSS constellations

Reliable ambiguity resolution (AR) is essential to real-time kinematic (RTK) positioning and its applications, since incorrect ambiguity fixing can lead to largely biased positioning solutions. A partial ambiguity fixing technique is developed to improve the reliability of AR, involving partial ambiguity decorrelation (PAD) and partial ambiguity resolution (PAR). Decorrelation transformation could substantially amplify the biases in the phase measurements. The purpose of PAD is to find the optimum trade-off between decorrelation and worst-case bias amplification. The concept of PAR refers to the case where only a subset of the ambiguities can be fixed correctly to their integers in the integer least squares (ILS) estimation system at high success rates. As a result, RTK solutions can be derived from these integer-fixed phase measurements. This is meaningful provided that the number of reliably resolved phase measurements is sufficiently large for least-square estimation of RTK solutions as well. Considering the GPS constellation alone, partially fixed measurements are often insufficient for positioning. The AR reliability is usually characterised by the AR success rate. In this contribution, an AR validation decision matrix is firstly introduced to understand the impact of success rate. Moreover the AR risk probability is included into a more complete evaluation of the AR reliability. We use 16 ambiguity variance–covariance matrices with different levels of success rate to analyse the relation between success rate and AR risk probability. Next, the paper examines during the PAD process, how a bias in one measurement is propagated and amplified onto many others, leading to more than one wrong integer and to affect the success probability. Furthermore, the paper proposes a partial ambiguity fixing procedure with a predefined success rate criterion and ratio test in the ambiguity validation process. In this paper, the Galileo constellation data is tested with simulated observations. Numerical results from our experiment clearly demonstrate that only when the computed success rate is very high, the AR validation can provide decisions about the correctness of AR which are close to real world, with both low AR risk and false alarm probabilities. The results also indicate that the PAR procedure can automatically chose adequate number of ambiguities to fix at given high-success rate from the multiple constellations instead of fixing all the ambiguities. This is a benefit that multiple GNSS constellations can offer.

Acquisition algorithm for GNSS multi-constellation signals

Multi-constellation receivers will become popularity since the current satellite navigation systems are transforming from the solo GPS to multi GNSS constellations. The traditional fast acquisition algorithms in time domain and frequency domain for signal of special satellite system have their own shortages. So it cannot process multi-constella-tion signals which modulation and code length are different. A universal fast acquisition algorithm implemented in multi-constellation receiver will be introduced. It has been proved that the algorithm has practical value, and is compatible on acquiring different types of satellite signals, such as GPS, GLONASS, Galileo, and COMPASS, etc.

The perspective from Asia concerning the Impact of Compass/Beidou-2 on future GNSS

The next decade promises drastic improvements to global navigation satellite systems. The USA is modernizing GPS, Russia is refreshing GLONASS, Europe is moving ahead with its own Galileo system, and The People's Republic of China is expanding its Beidou-1 system from a regional navigation system to a full constellation global navigation satellite system known as Compass/BeiDou-2, which consists of thirty five satellites including geostationary satellites, MEO satellites, and geosynchronous satellites in the coming year. Extra satellites will improve performance for all applications, and especially where satellite signals can be obscured, such as in urban canyons, under tree canopies, or in open-cut mines. The benefits of the expected extra satellites and their signals are increased availability, accuracy, continuity, and reliability.The advent of hybrid GNSS constellation has made the study of compatibility and interoperability increasingly important. A number of performance analyses have been conducted on a global scale with respect to availability, reliability, accuracy, and integrity in different simulated scenarios (such as open sky and urban canyons) for each system individually as well as for all possible combinations.Since the Compass/BeiDou-2 has received increasing attention from GNSS communities, the main objective of this study is to investigate the performance of Compass/BeiDou-2 in comparison with GPS, GLONASS, and Galileo in the greater Asia region; and also to explore whether combining Compass/BeiDou-2 with another GNSS would yield performance improvements in the region.Performance analysis can be conducted using either signals or geometrical conditions. However, the scope of this study is limited to the impact of current and future GNSS based on geometrical conditions. Therefore, the satellite visibility and DOP values of each system or their possible combinations are used as the major indices for the performance evaluation with an emphasis on the addition of Compass. In addition, those indices are analyzed in terms of their spatial and temporal distributions with an emphasis on the greater Asia region. Moreover, the spatial performance analyses are conducted as both global and regional scales to provide more insightful comparisons to illustrate the importance of Compass for users in the greater Asia region.

Aegean Sea Level Network: Eastern Mediterranean Altimeter Calibration Network – eMACnet

ILRS Technical Workshop on SLR Tracking of GNSS Constellations (2009); Metsovo, Greece, Sept. 14-19, 2009

The performance comparison between GPs and BeiDou‐2/compass: A perspective from Asia

The next decade promises drastic improvements to global navigation satellite systems. The USA is modernizing GPS, Russia is refreshing GLONASS, Europe is moving ahead with its own Galileo system, and The People's Republic of China is expanding its BeiDou‐1 system from a regional navigation system to a full constellation global navigation satellite system known as BeiDou‐2/Compass, which consists of thirty five satellites including geostationary satellites, MEO satellites and geosynchronous satellites in the coming year. Extra satellites will make possible improved performance for all applications, and especially where satellite signals can be obscured, such as in urban canyons, under tree canopies or in open‐pit mines. The benefits of the expected extra satellites and their signals can be evaluated in terms of availability, accuracy, continuity, and reliability issues.The advent of a hybrid GNSS constellation has drawn a lot of attention to study compatibility and interoperability. A number of performance analyses have been conducted on a global scale with respect to availability, reliability, accuracy and integrity in different simulated scenarios (such as open sky and urban canyons) for each system individually as well as for combined systems with all the possible combinations. Since the BeiDou‐2/Compass has gained more attention from GNSS communities, the main objective of this paper is to study the performance of BeiDou‐2/Compass comparied to GPS in the greater Asia region; and also to explore whether the combination of BeiDou‐2/Compass with GPS would yield performance improvements in this region.The performance analysis can be analyzed by either the signal or the geometrical conditions. However, the scope of this study is limited to investigating the impact of current and future GNSS based on geometrical conditions. Therefore, the satellite visibility and DOP (Dilution of Precision) values of each system or possible combinations between them are used as the major indices for performance evaluation with the emphasis on the addition of Compass. In addition, those indices are further analyzed in terms of their spatial and temporal distributions with the emphasis on the greater Asia region. Moreover, the spatial performance analyses are conducted on both global and regional scales to provide more insightful comparisons to illustrate the importance of future Compass for users in the greater Asia region.

GPS-GLONASS-GALILEO: A Dynamical Comparison

The new millennium brought an asymptotic interest in space technology. Besides existing operational global positioning systems i.e. GPS and GLONASS, another two have already emerged: GALILEO and Compass/Beidou. Alternatives to GPS imposed the GNSS concept, which has crystallized in a short time. The true reasoning of this excessive need for positioning information is beyond the scope of this paper; instead, an analysis of orbital behaviour of GPS, GLONASS and GALILEO is conducted. GNSS orbits have similar characteristics permitting us to encompass them under the term Medium Earth Orbits (MEO). However, small differences between mean orbital elements of the three systems produce a significantly different reaction of their satellites' orbits to the natural perturbing factors. This study analyzes the three space segment characteristics, introduces the analytical method used to evaluate variations of orbital elements under the most significant perturbing influences and offers a broad comparative image of the dynamic behaviour of the three GNSS constellations.

Double-Difference Carrier-Phase Network Solution Using Nominal Gnss Constellations (Future Perception)

Double-Difference Carrier-Phase Network Solution Using Nominal Gnss Constellations (Future Perception)Global Navigation Satellite Systems (GNSS) have an endless number of applications in industry, science, military, transportation and recreation & sports. Two systems are currently in operation namely GPS (the USA Global Positioning System) and GLONASS (the Russian GLObal NAvigation Satellite System), and a third is planned, the European satellite navigation system GALILEO. The potential performance improvements achievable through combining these systems could be significant and expectations are high. The need is inevitable to explore the future of positioning accuracy using different nominal constellations. In this research paper, Bernese 5.0 software could be modified to simulate and process GNSS observations from three different constellations (GPS, Glonass and Galileo) using different combinations. This study presents results of double-difference carrier-phase solution for five stations-network using the three constellations and different combinations.

Galileo and the IGS: Taking advantage of multiple GNSS constellations

The paper reviews some objectives and preparatory activities of the International GNSS Service (IGS), in relation to the developing European Global Navigation Satellite System Galileo. Experience already acquired in the IGS in monitoring multiple global navigation satellite systems (GPS and GLONASS) is reviewed. Some significant features of the Galileo system are outlined, including constellation design, and some areas are identified in which Galileo will influence IGS operations in the future and in which the IGS and its active elements can continue to contribute to new GNSS developments. These include the IGS GNSS Working Group, the Galileo System Test Bed GSTB-V2 and the Galileo Geodetic Service Provider (GGSP) Prototype.

Code Single Point Positioning Using Nominal Gnss Constellations (Future Perception)

Code Single Point Positioning Using Nominal Gnss Constellations (Future Perception) Global Navigation Satellite Systems (GNSS) have an endless number of applications in industry, science, military, transportation and recreation & sports. Two systems are currently in operation namely GPS (the USA Global Positioning System) and GLONASS (the Russian GLObal NAvigation Satellite System), and a third is planned, the European satellite navigation system GALILEO. The potential performance improvements achievable through combining these systems could be significant and expectations are high. The need is inevitable to explore the future of positioning from different nominal constellations. In this research paper, Bernese 5.0 software could be modified to simulate and process GNSS observations from three different constellations (GPS, Glonass and Galileo) using different combinations. This study presents results of code single point positioning for five stations using the three constellations and different combinations.

Making Profitable GNSS RTK Network Infrastructure

The authors have acquired extensive experience in marketing and deploying GPS RTK Network infrastructures worldwide, including design, implementation, training and support. They have been regularly confronted by some crucial questions asked by potential GPS network operators on ways to retrieve investment and on how this infrastructure could be turned into a profitable business. In many ways, the GPS RTK Network infrastructure-based services are similar to the mobile communication business, and more particularly like a mobile information content provider business. The apparent difficulty in turning this high tech infrastructure into a profit-making venture is mainly due to the fact that the actual service/product providers are in a direct relationship with their users. Both are focused on technology, not on the business aspects. The benefits of GPS RTK Networks are numerous and cost savings are the most important of the adoption drivers. But for how long? New operator profiles are appearing and they would like to understand how to justify their investment. They are also looking to generate new services based on the GPS data streams. Today, with the advent of the second generation of GPS RTK Network software like the Leica GPS SpiderNET, things could change significantly. The network correction messages format will be soon standardized by the RTCM organization, but these represent only a fraction of the products that such software/infrastructure can generate. The widespread and easy access of high-speed Internet and various forms of wireless connection are now cutting significantly the fixed costs associated with running such infrastructures, and for accessing the GPS real-time data products in the field. New lower cost GPS RTK receivers and GPSintegrated Total Stations are increasing the number of users. Moreover, the ease of use of such devices by nonsurvey operators will also enlarge the customer community. The topic of this paper is the new business models, new coming business operators, and a totally fresh vision of what this technology can offer, so as to sustain the development of new products and services that most people are ignoring today. The new GNSS constellations like GALILEO will just amplify this trend that will capitalize on the accuracy, reliability and highspeed location addicts.

Low-cost relative navigation sensing: GNSS-like devices hosted on deployed tethers

Currently, the possibility to perform very precise GNSS-based relative navigation determination among two or more spacecraft in rendezvous, docking and/or formation flying scenarios is deemed as the baseline for several current and future missions. Due to the required centimetre-level accuracy, the pseudorange measurements accuracy is not sufficient, and carrier phase measurements shall be employed. Those algorithms making exclusive use of carrier phase measurements are usually called “kinematic” algorithms, and require for dedicated integer ambiguity resolution algorithms. Obviously, this relative navigation determination concept is only suitable when the spacecraft is in the range of the GNSS constellations coverage. In situations where the GNSS constellations do not provide coverage (from MEO/GEO orbits to non-Earth planetary orbits and passing through inter-planetary missions), systems based on radio frequency (RF) signal combining emitters and receivers on-board the same platform have been proposed and are currently being studied (i.e. DARWIN RF subsystem). Nevertheless, DARWIN scenario takes advantage of the multi-spacecraft scenario characteristic (four, at least, spacecraft hosting an emitter are required to provide three-dimensional navigation). Innovative approaches are required when considering three or just two spacecraft in formation flying. This paper illustrates the concept and supplies simulation-based accuracy results of a full GNSS-like-based relative navigation determination with RF emitters/receivers hosted on board two spacecraft flying in close formation or performing a RDV and docking manoeuvre, a target and a chaser respectively named. To fulfil concept requirements, at least three RF pseudolites shall be placed on-board or around the target spacecraft so to provide the required RF coverage meant for the chaser spacecraft. A key point to achieve the required system performance is related to the relative geometry between the emitting pseudolites and the receiving antennas. The ideal being the emitters uniformly distributed in the receiver antenna field of view. It is analysed how to achieve this spatial distribution, that cannot be accomplished by placing multiple emitters on-board the same vehicle structure (characteristic spacecraft dimension will be much smaller than the inter-spacecraft distance) by using a tether-based deployed pseudolites system. Tether deployment mechanisms, particularly gravity gradient assisted, are assessed in the paper, in order to achieve the best RF emitter pseudolites distribution for a maximum formation flying navigation determination accuracy.