H E positioning with satellitenavigation systems like GPS or Galileo is based on time of arrival measurements of the signals transmitted from the satellite at the receiver. The measured signal delays are converted to pseudo ranges by multiplication with the speed of light. The prefix ‘pseudo’ signifies the fact that these ranges do not correspond to the real geometrical distances to the satellites, because the receiver clock is not synchronized to the satellite system time and the measured delays include additional contributions by system and propagation errors which must be corrected for. For synchronization of the receiver clock to the system time measurement data from at least four satellites must be available. Then the receiver clock bias can be solved for as a fourth unknown together with the three unknown co-ordinates of the position x, y, z by triangulation. While system errors can normally not be corrected in real time, propagation errors can be reduced by models or advanced signal processing, where the remaining error depends on the type of measurements e.g. single frequency or dual frequency measurements, signal characteristics e.g. carrier frequency and bandwidth, and the quality of the models used.
Besides the ionosphere one of the most significant problems to achieve an accurate navigation solution in cities with GPS or GALILEO is the multipath reception. Various channel models do exist for ground to ground communications (e.g. COST 207 for the GSM system). But there is still a lack of knowledge for broadband satellite to earth channels . Therefore the German Aerospace Centre (DLR) performed a measurement campaign in 2002. In this campaign we used a Zeppelin to simulate a satellite transmitting a 100 MHz broadband signal towards earth. To ensure a realistic scenario the signal was transmitted between 1460 and 1560 MHz just nearby the GPS L1 band. This signal was received by a measurement van and was recorded using a regular time grid. The so gathered data was then passed through a super resolution algorithm to detect the single reflections. In a further step we tracked the detected reflections in time and gained a knowledge about the characteristics of any isolated reflection. This includes both Doppler shift and delay of the reflection. On top of this we gained knowledge about the direct path behaviour.
Besides the ionosphere one of the most significant problems to achieve an accurate navigation solution in cities with GPS or GALILEO is the multipath reception. Various channel models do exist for ground to ground communications (e.g. COST 207 for the GSM system). But there was a lack of knowledge for broadband satellite to earth channels . Therefore the German Aerospace Centre (DLR) performed a measurement campaign in 2002. In this campaign we used a Zeppelin to simulate a satellite transmitting a 100 MHz broadband signal towards earth. To ensure a realistic scenario the signal was transmitted between 1460 and 1560 MHz just nearby the GPS L1 band. This signal was received by a measurement van and was recorded using a regular time grid. The so gathered data was then passed through a super resolution algorithm to detect the single reflections. In a further step we tracked the detected reflections in time and gained knowledge about the characteristics of any isolated reflection. This includes both Doppler shift and delay of the reflection. On top of this we gained knowledge about the direct path behaviour.
performance of various signals had been made [1, 2]. Lack of accurate models for signal propagation in ur- ban multipath environments did result in diﬀerent hy- pothetical channel models . Their usage in simula- tions showed an extreme influence of the signal prop- agation on the navigation accuracy. This exposed the necessity of measurements for the satellite-to-land mobile channel. These measurements have been car- ried out in  and have brought an in-depth under- standing of signal propagation in urban, suburban, and rural environments. These measurements have been analyzed  and have resulted in a multipath channel model  which has been released into the public domain . The International Telecommuni- cation Union (ITU) has standardized this model [7,8]. The development of GALILEO  and the modern- ization of GPS  have been accompanied by a long discussion about new wideband signal options. These have resulted into the definition of the GALILEO sig- nal format , the invention of GPS M-code for mil- itary applications , and a second signal L2C on L5 for civil applications .
Since few years, the use of multi-element antenna arrays in global positioning navigation systems (GNSS) has been proposed, to provide robustness against in-band interference and multi-path propagation. This approach provides the opportunity of using sophisticated beamforming techniques to null-out the unwanted signals. However, the overall physical size is restricted by the inter-element separation of d = λ/2, i.e., about 10 cm, to minimize the detrimental mutual coupling effects.
As well-known from the literature, the angles-only navigation problem is not fully observable in the absence of maneuvers. Sufficient conditions to achieve observability are provided in Ref. 6 , 7 for the bearing- only target motion analysis. In the field of orbital rendezvous, they are presented in Ref. 8 . According to these contributions, a maneuver that produces a variation of the LOS natural trend makes the system observable. In this work it is shown that relative orbital elements can support a direct physical interpretation of the un-observability issues. Moreover results are generalized for the case in which the perturbations due to the equatorial bulge are taken into account. It is here shown that observability properties improve for certain families of relative orbits as the trends of the mean relative elements deviate from the one they have within the Kepler assumptions. The exploitation of relative orbital elements is of great benefit also when dealing with the assessment of the estimation accuracy achievable when maneuvers are performed. In the literature different methods to quantify the observability of the problems are suggested. They span from the definition of a range error metric, 9 to the exploitation of the covariance of the estimated state 10 or of the condition number of the observability matrix. 11 In our work simple metrics are proposed to quantify the effectiveness of the maneuvers performed. They are based on the immediate idea of what changes will take place in the LOS behavior due to the relative orbital elements variation determined by a maneuver. In this frame an optimization problem can be easily set up.
The rural channel model will allow the definition of tree distributions and user vehicle trajectory parameters as well as the position of the satellite as input parameters. Statistical processes will then output channel impulse responses which will correspond to those which were measured during the campaign taking the actual wave propagation effects into account. We expect first results of the modelling to be available later this year.
R. Prieto-Cerdeira received his Telecommunications Engineering degree in 2002 from the University of Vigo, Spain, and followed postgraduate studies on Space Science and Radioastronomy in Chalmers University of Technol- ogy, Gothenburg, Sweden. Since 2004, he has been with the European Space Agency (ESA/ESTEC) in the Wave Interaction and Propagation Section where he is respon- sible of the activities related to radiowave propagation in the ionosphere and local environment for Global Naviga- tion Satellite Systems (GNSS) such as Galileo and EGNOS projects, satellite mobile communications, and remote sens- ing and planetary radio science projects.
In this paper, we have introduced a novel two-fold marginal- ized Bayesian filter for multipath mitigation in satellitenavigation receivers. Our approach allows us to exploit the constrained channel dynamics within a typical satellite-to- user propagation scenario in an urban environment. We have proposed an e ﬃcient implementation of the filter by applying the concept of marginalization, where we proposed to estimate impinging multipath replicas in a typical track- before-detect approach. Our approach is able to adapt to the channel dynamics and favors implicitly the most likely channel configuration for a given sequence of channel observations. This has been shown to be of particular benefit in case the LOS path is shadowed or blocked, since unlike other approaches, the presented filter does not synchronize on powerful replicas during such periods. We have shown that our approach requires a significantly reduced number of particles compared to previous work, which is achieved as a result of the implicit use of phase information. Our results for a real urban environment show that our approach is practically viable and confirm its benefits. They also provide insights on how many simultaneous multipath replicas a future Bayesian navigation receiver should consider. Our findings reveal that the LOS tracking performance of our Bayesian filter tends to saturate rapidly when increasing of the number of simultaneously detectable multipath replicas.
hundreds of stations distributed over Europe and a period of nearly two decades were presented. The comparison of GNSS and NWM ZTDs and horizontal delay gradients demonstrated a good agreement with standard deviations of 10 mm and 0.4 mm respectively. [ 12 ] provided more details for the same stations and time period. In fact, for several stations they find clear artificial signals in horizontal delay gradients which are caused by problems to receive low-elevation observations. They show that the standard deviations between GNSS and NWM ZTDs and horizontal delay gradients have a clear seasonal dependency, that is, the standard deviation is larger in summer than in winter. This can be explained by the fact that NWMs have difficulties to predict the high water vapour variability in summer. In addition, it appears that the standard deviation for the ZTD does not change over the years whereas the standard deviation for the horizontal delay gradient decreased over the years. This is an indication that the quality of GNSS horizontal delay gradients improved over the years. The GNSS ZTDs are of high quality since their introduction in meteorology in the early 90’s. In summary, recent studies begin to indicate that a single station does not only provide high-quality ZTD estimates but high-quality horizontal delay gradient estimates as well.
Received: 13 March 2019; Accepted: 23 April 2019; Published: 25 April 2019 Abstract: Conventional relative kinematic positioning is difficult to be applied in the polar region of Earth since there is a very sparse distribution of reference stations, while precise point positioning (PPP), using data of a stand-alone receiver, is recognized as a promising tool for obtaining reliable and accurate trajectories of moving platforms. However, PPP and its integer ambiguity fixing performance could be much degraded by satellite orbits and clocks of poor quality, such as those of the geostationary Earth orbit (GEO) satellites of the BeiDou navigationsatellite system (BDS), because temporal variation of orbit errors cannot be fully absorbed by ambiguities. To overcome such problems, a network-based processing, referred to as precise orbit positioning (POP), in which the satellite clock offsets are estimated with fixed precise orbits, is implemented in this study. The POP approach is validated in comparison with PPP in terms of integer ambiguity fixing and trajectory accuracy. In a simulation test, multi-GNSS (global navigationsatellite system) observations over 14 days from 136 globally distributed MGEX (the multi-GNSS Experiment) receivers are used and four of them on the coast of Antarctica are processed in kinematic mode as moving stations. The results show that POP can improve the ambiguity fixing of all system combinations and significant improvement is found in the solution with BDS, since its large orbit errors are reduced in an integrated adjustment with satellite clock offsets. The four-system GPS+GLONASS+Galileo+BDS (GREC) fixed solution enables the highest 3D position accuracy of about 3.0 cm compared to 4.3 cm of the GPS-only solution. Through a real flight experiment over Antarctica, it is also confirmed that POP ambiguity fixing performs better and thus can considerably speed up (re-)convergence and reduce most of the fluctuations in PPP solutions, since the continuous tracking time is short compared to that in other regions.
In Table 2 we additionally show simulation results for vari- ous other selection scenarios as well as the all-in-view case as reference. Selecting 10 instead of 12 satellite with the pseudo HPL heuristic still ensures 100% availability but already leads to significantly larger average HPLs. Going even further to 8 satellites increases the 99.5% HPL average almost to the alert limit, no longer proving full availabil- ity. This indicates a limit of 8-10 satellites when using this selection in terms of a multi-constellation scenario. Looking at the results for the geometric selection reveals problems even in case of 12 satellites. Even though the availability is approximately 100%, the average 99.5% HPL is already 30 m and therefore almost twice as high compared to the heuristic. When using this method at least more than 12 satellites need to be selected to ensure a stable performance without availability issues due to occasional unfavorable geometries.
Within the constellation all data are transported via optical links. The divergence of those links is so small that the area illuminated by the signal has a radius of around 700 m at the inter-satellite link distance! As a consequence, these optical links are essentially impossible to jam or spoof. The L-band receive antenna of the LEO is upward looking at an altitude of 1209 km, which makes jamming and spoofing difficult from the ground. The robustness of the receiver can be further increased by spatial processing. Attacks that we could think of are either space-based or of a military nature. This compares favorably with current GNSS systems.
We introduced probability propagation nets (PPNs) on the basis of PN representation of propositional Horn clauses. This makes it possible to represent deduction (and abduction) pro- cesses as reproduction of the empty marking . Touchstones for our approach are the representation of probabilistic Horn abduction and the propagation of λ- and π-messages in BNs. In our opinion, it is valuable to introduce specific PN con- cepts into the field of propagations. In particular t-invariants as an elementary means to structure PNs turned out to be quite fruitful. The minimal t-invariants, on the one hand when reproducing the empty marking describe exactly the flows of λ- and π-messages, thus structuring the PNs (and so the BNs) in a very natural way. On the other hand, they reveal the true complexity behind the simply structured BNs.
Keywords: Routenplanung, A*, rasterbasierter Ansatz, Bahnplanung
Die Navigation in Straßennetzen ist ein gut erforschtes Gebiet. Basierend auf Dijkstras Algorithmus  oder A*  ist es möglich, Pfade mit minimalen Kosten effizient zu berechnen. Hierzu erforderlich ist die Modellierung der befahrbaren Um- welt als Graph mit Knoten (Kreuzungen) und Kanten (Straßenabschnitten zwischen Kreuzungen). Kantengewichte repräsentieren die zu minimierenden Kosten, z.B. die Durchfahrungszeit oder Distanz.
Für die Implantologie in der Zahnheilkunde wurde ein Planungs- und Navigation entwickelt, daß sich durch mehrere Komponenten über den Stand der Technik hinaus geht: Das System hat eine rein passive Trackertechnologie auf Glaskugelbasis, verfügt über einen Miniaturbildschirm zur Navigation und kann vollständig ohne Tastatur bedient werden. Das CE zertifizierte System wird von der Straumann AG vertrieben.
Eine Integration mit den Arbeiten der Aktivitätserkennung, die derzeit ohnehin in der Regel mit Beschleunigungssensoren vorgenommen wird, kann eine neue Art ortsbezogener Dienste ermöglichen: Nämlich solche Dienste, die neben dem Aufenthaltsort auch die derzeitige Aktivität des Nutzers verwenden, um die passenden Informationen zum passenden Zeitpunkt proaktiv anzubieten. Grundlegend für höherwertige ortsbezogene Dienste in Gebäuden ist natür- lich ein algorithmisches Verständnis des Gebäudes selbst. Nur dann, wenn alle wichtigen Aspekte des Gebäudes von kontextabhängigen Diensten verwendet werden können, wird sich die Vision des „Ubiquitous Computing“, nämlich, dass die Unterstützung durch Informationstechnologie unsichtbar wird, um- setzen lassen. Als mögliche Grundlage für solche Integrationen wurden im Kapitel 4 sehr einfache Umgebungsmodelle konzipiert, die aber dennoch in der Lage sind, die Anforderungen einer Indoor-Navigation zu erfüllen, und die darüber hinaus leicht erweiterbar sind. Wenn sich besonders einfache Um- gebungsmodelle durchsetzen, besteht die Hoﬀnung, dass viele Privatpersonen und Gebäudeeigentümer geeignetes Kartenmaterial zusammenstellen. Für den Außenbereich ist mit der OpenStreetMap ja genau solch ein Vorgehen bereits extrem erfolgreich.
A 10 served as the reference method in Step 2 using the same approach as in Step 1, the spatial differences between A–E and A 10 were calculated each 0.02 s from start to finish for the elevation masks 10°, 30° and 40°. GNSS measurements of the unit on the athletes were started (warm start) on average 69 s (minimum 53 s, maximum 92 s) before skiers reached the speed of 2 m/s, while the measurement on the base station unit was started earlier. Between the start of the GNSS measurement and the time point when skiers left the start gate, skiers stood in an upright position in an open area with no obstacles other than the topography affecting satellite signal reception. The same statistics as in Step 1 were used to characterize the spatial differences between A–E and A 10 .
In this section we briefly highlight some of the possibil- ities to overcome the limitations of the SAR system under investigation due to adverse meteorological conditions. The ideal strategy to identify rain induced signatures would be a multi-frequency SAR system, because attenuation through rain is wavelength dependent. Since current SAR systems are not equipped with such a multi frequency sensor set, other ways have to be taken into consideration. Wherever available, ground based weather radar would assist in the process to iden- tify precipitation induced distortions in SAR images. However, world-wide coverage of ground based weather radars is limited to industrialized regions such as the USA or Europe. Espe- cially over the oceanic regions no ground based weather radar data are available and so it is for the polar regions of the Earth. One simple measure to avoid or mitigate precipitation effects are multi-temporal acquisitions over the same scene, since the statistical probability for rain is rather low, as already shown before. As the look angle determines the path length of the propagation path through the precipitation media, steep look angles would reduce the attenuation due to rain, and conversely shallow look angles cause increased values of attenuation. The question to what extent scan-on-receive techniques are capable to improve, respectively enhance precipitation effects was addressed by , and deserves further consideration. A mitigation of the backscattering can be obtained. However, the attenuation will still be present for transmit and receive patterns as is diagrammatically shown in Fig. 14.
During adult life of mammals skeletal muscles show a remarkable ability to regenerate. Muscle regeneration includes necrosis of the damaged tissue, inflammation, activation of myogenic stem cells and, as a result of this activation, formation of new myofibers and reconstitution of a functional contractile apparatus (Fig. 6). This highly synchronized process, which requires satellite cell activation, proliferation, migration and terminal differentiation, is activated and controlled by a complex network of signalling pathways and requires collaboration of different cell types (Hansen-Smith and Carlson, 1979; Hansen-Smith et al., 1980). Quiescent satellite cells become active and start to repair muscles under a wide variety of conditions (injury, overwork, denervation, exercise, stretch). Regeneration is regulated by a complex signalling network and requires cell-cell and cell- matrix interactions. Muscle injuries have been shown to cause the release of biologically active molecules into extracellular space. Extracts of crashed muscle contain mitogens for satellite cells (Bischoff, 1986). Peptide signalling molecules that stimulate, inhibit or regulate cellular functions are called growth factors. The action of growth factors can be autocrine (secreted by and acting on the same cell), paracrine (acting on other cells in the local environment), juxtacrine (membrane bound and requiring cell-cell contact) or endocrine (acting on distant target cells).