bound for the covariance matrix of the estimates. In the case ofGNSS receivers, the FIM associated to the one-step ML solution (i.e., DPE) has been calculated in , but no insights on the performance associated to the delay/Doppler two- step approach were provided. Even if DPE has been shown to be asymptotically efficient, it suffers from a huge computational burden which prevents its use in real-time applications, then being theoretically appealing but with minor prac- tical interest. On the other hand, the two-step approach is suboptimal because it relaxes the links existing among all delays and Dopplers and simply consid- ers them as independent measurements. However, it has been recently shown to be asymptotically efficient when using an appropriated weighting . Such optimal weights are obtained by resorting to the EXtended Invariance Principle (EXIP) , which states that using a re-parametrization of the problem can lead to a simpler solution while preserving the asymptotic performances. More precisely, the intermediate estimates obtained from a simpler first step can be refined to asymptotically achieve the performanceof the initial model using an appropriate WLS minimization. Obviously, this optimal solution must exploit not only pseudoranges to each satellite in view (i.e., delays) but also Doppler measurements.
For RTK positioning, the new CRB and the proposed analysis provided even more interesting results. In fact, it was shown that the SNR threshold region is driven by the time-delay precision and not the phase one. Using fast codes, we may have up to 10 dB of gain in the threshold, which in turn implies the validity of such RTK solutions in a wider range of applications. Also, notice that this threshold can be used to determine for which operation regions it is worth to exploit phase measurements, because above the threshold the RTK fixed solution rapidly converges to the float (i.e. real) one. These results hold whatever the signal carrier frequency. To summarise, if a newGNSS signal was to be designed for precise positioning, the recommendation would be to use a carrier frequency as high as possible and a signal modulation with the largest signal bandwidth, the former driving the asymptotic RTK performance, and the latter the threshold region.
The GNSS signals broadcast by the different satellite constellations are typically built as a multilayer structure: (i) a low rate navigation message, encoded as binary phase-shift keying (BPSK) bits; (ii) a fast rate ranging code, so-called pseudo-random noise (PRN) code, with good autocorrelation and cross-correlation properties in order to allow individual satellite signals’ processing (i.e., quasi orthogonality); (iii) a subcarrier that modulates the PRN code and shapes the autocorrelation function (ACF), i.e., no subcarrier is employed for the legacy GPS L1-C/A signal or binary offset carrier (BOC)-type subcarriers in modernized GPS and some Galileo signals; (iv) a carrier that is used to allocate the complete signal into the corresponding frequency. Notice that the signal may have data bits or not, depending on whether it belongs to a data component or a pilot component. In the sequel, and without loss of generality, we do not account for navigation data bits within the observation time. In general, the signal at the receiver antenna is the superposition of a set of signals at different frequency bands, plus environmental effects such as multipath and/or interferences. The latter two effects are out of the scope of this contribution, and because of the quasi orthogonal PRN code design, we can focus on one of these signals to define the model to be exploited. Therefore, we consider the transmission of a band-limited GNSS signal c ( t ) (bandwidth B), which contains both the PRN and subcarrier, over a carrier frequency f c (λ c = c/ f c ), from a transmitter (satellite) T to a receiver
In this research there are two cases: military aircraft, TORNADO is chosen； Large civil aircraft, AIRBUS 320. Both of them are drawn in CATIA to get the 3D models.
3D model can be used to generate the antenna masking due to the obscuration of wing and fuselage and tails. They are also useful in calculating the satellite visibility during different flight phase combined with the GNSS constellation simulator. 3D models will be used in the simulation of multipath when calculating the wing reflection and fuselage reflection via geometrical optics method.
The evaluation of the filter output points out that the short- and medium-term stability of rubidium and cesium clock offsets with respect to the Composite Clock (implicit mean produced by the Kalman filter) is very close to the stability of these clocks in a free-running mode. On the other hand, the instability of the AHMs-CC offsets is considerably higher compared to a free-running AHM due to instabilities of the CC itself. Furthermore, the instability of the cesium-clocks-CC offset is disturbed in the long term. Hence, the Composite Clock with the used q-parameter configuration is not able to benefit from the long- term stability of the cesium clocks.
The robustness of GPS Composite Clock using 3-state clock models is investigated by simulating a “light” GNSS clock ensemble of 10 satellite Rubidium clocks and two ground stations each equipped with 3 Cesium-clocks and one Active Hydrogen Maser (AHM). Figure 2 shows the Allan Deviation of clock simulations of every clock type.
The L-band signals transmitted by the MEO satellites are received and analyzed by the LEO satellites. Since the LEO satellites are globally synchronized with the MEO satellites, the measurements of time differences in the coordinate system K are ranges. Remember that the time component of the coordinate system is TCG or a similar choice. Additionally, all LEO satellites perform (two-way) range measurements to at least two MEO satellites and all MEO satellites perform similar measurements with respect to their neighbors. This creates a mesh of L-band and optical measurements, which are used for orbit determinations. Since the plasma density above 1209 km is extremely low, these measurements are not much affected by the atmosphere, Additionally, L-band pseudoranges are measured by one single ground station to maintain the alignment with earth rotation. Clearly, several ground stations may be used for robustness but are not needed to achieve the performance described below. The orbit determination capabilities were analyzed by G. Michalak et. al. . They performed both simulations without any impairment beyond measurement noise, as well as simulations with a number of mismodeling assumptions. The latter included: phase center offsets, differing models in the simulation and in the estimation for solar pressure, air drag, gravity field, earth tides and ocean loading. They used a simple force model to absorb the mismodeling and estimated the associated parameters in 30 minutes intervals. The estimation was performed by least squares - which will need to be extended to a Kalman filter approach in the future. Nevertheless the quality of the estimates are indicative. The measurement noise was assumed to be 50 cm for L-band code measurements, 3 mm for L-band phase measurements and 1 mm for optical range measurements. The worst case errors in the estimation of the orbit for the MEO are 5.0 cm rms and 0.6 cm in the important radial direction with float ambiguities. The LEO orbital errors are at 1.4 cm rms. The code noise and mismodeling assumptions are very conservative. Fixing the ambiguities further improves the results. The use of least squares estimation needs to be revisited but is not expected to change the picture significantly.
The emerging of safety-critical applications subject to accurate GNSS positioning and timing, such as autonomous cars or assisted landing, has urged the GNSS community into identifying jamming as one of the major menaces [ 17 – 19 ]. Thus, lately there have been numerous studies related to detecting and counteracting the impact of jamming attacks. In [ 20 ], the performanceof a broad range of consumer grade GPS receivers under the interference of a low cost PPD is analyzed. The test was carried out for a static scenario within a confined space, where the performanceof the receiver in terms of positioning accuracy and solution availability was analyzed for both the jamming-free and jamming scenarios of different intensities. Surprisingly, the tested receivers coped properly with the interference during the light jamming attack (with a jamming-to-signal ratio of 15 dB) with only a marginal influence on the quality of the position solution. However, during the severe jamming attack (jamming-to-noise-ratio of 25 dB), the availability of the position solution was reduced by more than 75%, while the positioning accuracy heavily degraded. It appears that jamming merely introduces additional noise in the measured pseudoranges, while the positioning degradation occurs under powerful interference due to the loss ofsatellite signal track and a consequent poor satellite geometry. A series of works addressing the impact of jamming attacks on the maritime navigation has been recently presented. For example, a trial was conducted on the East coast of United Kingdom using a professional L1 band jammer [ 21 ]. This study evaluated the jamming impact on the safety of maritime navigation and the quality of on-shore services such as vessel traffic management. The lack of GNSSs triggers numerous alarms and failures of interfaces (like the ECDIS) on the bridge of the vessel, causing discomfort to a vessel crew that additionally needs to face the challenge of quickly reverting to traditional means of navigation. This study nicely underlines the necessity for a backup for GNSS-based positioning on board a vessel.
Received: 9 October 2019; Accepted: 4 November 2019; Published: 5 November 2019 Abstract: Real-time multi-GNSS precise point positioning (PPP) requires the support of high-rate satellite clock corrections. Due to the large number of ambiguity parameters, it is difficult to update clocks at high frequency in real-time for a large reference network. With the increasing number of satellites of multi-GNSS constellations and the number of stations, real-time high-rate clock estimation becomes a big challenge. In this contribution, we propose a decentralized clock estimation (DECE) strategy, in which both undifferenced (UD) and epoch-differenced (ED) mode are implemented but run separately in different computers, and their output clocks are combined in another process to generate a unique product. While redundant UD and/or ED processing lines can be run in offsite computers to improve the robustness, processing lines for different networks can also be included to improve the clock quality. The new strategy is realized based on the Position and Navigation Data Analyst (PANDA) software package and is experimentally validated with about 110 real-time stations for clock estimation by comparison of the estimated clocks and the PPP performance applying estimated clocks. The results of the real-time PPP experiment using 12 global stations show that with the greatly improved computational efficiency, 3.14 cm in horizontal and 5.51 cm in vertical can be achieved using the estimated DECE clock.
In this paper, two prototypes of EVNet raw data processors have been prototyped to investigate the behavior of amplitude and phase measurements based on modified Javad high rate receivers (50 Hz). It has been shown that both processors are suitable to be used as basic processors in the context ofGNSS signal assessment in real time. In this context the assessment primary comprises ionospheric state assessment, raw data assessment, receiver validation and local augmentation. A general conclusion consists in the requirement to apply comparable measurement equipment (receivers) to avoid misinterpretations of processing results. As a new feature the phase processor comes with the quality to implement the cycle slip detection and correction without any need for range measurements. Furthermore, it could be shown that high-rate processing improves the separation potential between occurred cycles slips and occurred phase fluctuations and increases therefore the provision of continuous phase segments.
In this paper simulation results of DLR’s ultra-wideband land mobile satellite (LMS) multipath channel model are presented. The goal is the evaluation ofGNSSperformance in the rail environment. For this safety of life (SOL) application an urban railway was identified as the most critical scenario. The channel model is based on a measurement campaign of the land mobile multipath channel performed in 2002. From this data a model was derived that is synthesising the measured channel impulse response. It allows the realistic simulation of the multipath channel by approximating every single reflection. This model includes time variant reflectors approaching and receding in dependency of the azimuth and elevation of the satellite. All the signal processing had been realised independently of the transmitted signal. Therefore the usability for both, navigation systems GPS as well as GALILEO is given.
The Global Navigation Satellite System (GNSS) signals are always available, globally, and the signal structures are well known, except for those dedicated to military use. They also have some distinctive characteristics, including the use of L-band frequencies, which are particularly suited for remote sensing purposes. The idea of using GNSS signals for remote sensing - the atmosphere, oceans or Earth surface - was first proposed more than two decades ago. Since then, GNSS remote sensing has been intensively investigated in terms of proof of concept studies, signal processing methodologies, theory and algorithm development, and various satellite-borne, airborne and ground-based experiments. It has been demonstrated that GNSS remote sensing can be used as an alternative passive remote sensing technology. Space agencies such as NASA, NOAA, EUMETSAT and ESA have already funded, or will fund in the future, a number of projects/missions which focus on a variety ofGNSS remote sensing applications. It is envisaged that GNSS remote sensing can be either exploited to perform remote sensing tasks on an independent basis or combined with other techniques to address more complex applications. This paper provides an overview of the state of the art of this relatively new and, in some respects, underutilised remote sensing technique. Also addressed are relevant challenging issues associated with GNSS remote sensing services and the performance enhancement ofGNSS remote sensing to accurately and reliably retrieve a range of geophysical parameters. Review
pilots and controllers on ground), this additional delay can get critical for meeting the operational requirements, e.g. the maximum message expiration time (see  for further details). Random Access (RA) schemes are therefore being considered an interesting candidate for this low-duty cycle traffic , avoiding the signalling overhead of DAMA and providing short transmission latencies, since avoiding the signalling RTT. RA schemes have a long history and were a topic for research in all different facets. Starting from the initial publication of ALOHA by Abramson  a vast amount of evolutions of RA protocols can be found in the literature. In the following only the ones which are most relevant for this work shall be reviewed for pointing out the novelties and difference of the new CRA scheme proposed in this work with respect to them. The original proposal of ALOHA was done for sharing a broadcast channel by multiple users in an asynchronous manner, i.e. without the need to have the users synchronized to any time reference or organizing the channel access among them. In ALOHA, users transmit their packets at random times and any collisions among two or more transmissions results in a loss of the collided packets. As is well known, the ALOHA scheme reaches a maximum achievable throughput of T = e −2 pkt slot ≈ 0.18 slot pkt due to this. The second well known evolution followed by Roberts in . In this work the main innovation w.r.t. ALOHA was the segmentation of time into slots and restrictions that packet transmissions may only happen at the beginning of a slot and that a packet transmission lasts exactly one slot. This way the throughput of the so called Slotted ALOHA (SA) protocol increased to
As a result, the Galileo-GPS combination can provide worldwide performance suitable for LPV-200 compliant ap- proaches. Unless the actual fault probability measured for satellites of future modernized constellations will be unusually high (i.e. of over 200 faults/year in each constellation with a 6-hr fault latency), the VPL requirement is met even when one satellite is taken offline for maintenance. Surprisingly, the newly discussed EMT requirement poses a more stringent limitation on availability for all scenarios. However, the spe- cific EMT requirements published in the ICAO Annex 10  are still currently subject to interpretation. The authors of this study recommend that the wording of the EMT requirement be further clarified, with hindsight to the significant impact these requirements can have on satellite navigation availability for civil aviation.
To obtain a precise new range, the aircraft’s horizontal position has to be known in order to take the correct terrain elevation from the database. This is a critical step since misleading position information automatically translates into a false terrain elevation information and thus directly into an error in the new additional range. To avoid this, a horizontal position with integrity information is needed for the look-up of the terrain elevation. If the position, together with the horizontal protection level (HPL) is available, the area where the plane is located can be considered in the DTED. Depending on the resolution of the map several data points might lie within this region. Together with the beam width characteristics of the radar altimeter signals and the aircraft’s attitude a certain area on the ground is defined which could be seen by the aircraft. With increasing altitude this area gets significantly large, nevertheless this method leads to promising results for the final approach for two reasons: (1) the altitude above ground is rather small, thus the visible area for the radar altimeter is limited and (2) the terrain under the final approach path of an airport certified for CAT-II/III operations is reasonably flat. Taking all data points in the area and their first neighbors on the outside of the area to avoid uncertainty due to interpolation methods between data points, thus combines all error sources discussed in this paragraph and results in an uncertainty of the terrain elevation which can be directly derived from the database.
ANASTASIA (Airborne New and Advanced Satellite techniques and Technologies in A System Integrated Approach) was an integrated project funded by the European Community’s Sixth Framework Programme (DG research); see www.anastasia-fp6.org. The core of ANASTASIA research was to provide on-board Communication, Navigation and Surveillance (CNS) solutions to cope with the expected increase in air traffic by 2020. A receiver mock-up has been designed under the Thales expertise for three Galileo bands (L1, E5a, E5b), which is compliant to the MOPS current standards . A DME measurement campaign was carried out and the receiver was tested up to its limits regarding interferences, multi-paths and low level signals.
The European Space Agency (ESA) PolarGap airborne gravimetry campaign was carried out from December 7, 2015 to January 19, 2016 to cover the gravity gap over central Antarctica. A Twin- Otter aircraft was used with three antennas named AIR2, 0158 and SPAN installed on the aircraft to collect the GNSS data. Here, we choose the data collected on day 19 December 2015 for analysis, which covers about 10.5 h from 10:30 to 21:00 (UTC). Most of the flight tracks were done from the South Pole along the meridian to the edge of the region and then back. A dedicated reference station SP2X was installed at the South Pole, i.e., in the middle of the region. Such a track pattern is beneficial for traditional relative kinematic positioning. However, the observation data of SP2X on day December 19 is not available, and we use the data from another reference station called FD83 which was installed in a tented field camp (Figure 7). As the observed satellites on these kinematic receivers are at low elevation angles (less than 60°), a cut-off angle of 7° is applied to fully use all satellites. Since AIR2 observes GR data, 0158 and SPAN observe GPS data, only GPS data is processed for the three antennas in DD, PPP and POP mode for validation. For DD processing only one reference
The Composite Clock algorithm  is applied to process the time offset measurements between the control stations and the satelliteclocks and to estimate the satellite and control station clocks with respect to its implicitly defined mean. Due to the fact that N clocks with three states involve N-1 measurements, the formal covariance of the solution parameters will grow without bound. This obscures intuitive understanding of the solutions and also can result in problems due to computer limitations in handling large numbers. However, it can be shown  that no measured quantity is affected if the Kalman Filter covariance estimates C(t) are reduced as follows: