The orbit of a satellite around the earth is constantly disturbed by various factors, such as variations in the gravitational field and the solar wind pressure. The drift of the satellite position can compromise the mission, and even lead to a crash or a fall in the atmosphere. The station-keeping operations therefore consist in performing an accurate measurement of the satellite trajectory and then in using its thrusters to correct the drift. The conventional solution is to measure the position with the help of a ground based radar. This solution is expensive and does not allow to have the satellite position permanently : the trajectory corrections are therefore infrequent. A positioning and autonomous navigation system using constellations of navigation satellites, called Global Navigation Satellite System (GNSS), allows a significant reduction in design and operational maintenance costs. Several studies have been conducted in this direction and the first navigation systems based on GPS receivers, are emerging. A receiver capable of processing multiple navigation systems, such as GPS and Galileo, would provide a better service availability. Indeed, Galileo is designed to be compatible with GPS, both in terms of signals and navigation data. Continuous knowledge of the position would then allow a closed loop control of the station keeping. Initially, we defined what the specifications of a multi-mission space receiver are. Indeed, the constraints on such a receiver are different from those for a receiver located on the surface of the Earth. The analysis of these constraints, and the performance required of a positioning system, is necessary to determine the specifications of the future receiver. There are few studies on the subject. Some of them are classified ; others have, in our view, an analytical bias that distorts the determination of specifications. So we modeled the system : GNSS and receivers satellite orbits, radio frequency link. Some parameters of this link are not given in the specification or manufacturers documents. Moreover, the available theoretical data are not always relevant for realistic modeling. So we had to assess those parameters using the available data. The model was then used to simulate various scenarios representing future missions. After defining analysis criteria, specifications were determined from the simulation results. Calculating a position of a satellite navigation system involves three main phases. For each phase, there are several possible algorithms, with different performance characteristics, the circuit size or the computation load. The development of new applications based on navigation also drives the development of new adapted algorithms. We present the principle for determining a position, as well as GPS and Galileo navigation signals. From the signal structure, we explain the phases of the demodulation and localization. Through the use of GPS and Galileo constellations, standard algorithms achieve the performance required for space applications. However, these algorithms need to be adapted, thus some parts were specifically designed. In order to validate the choice of algorithms and parameters, we have simulated the various operating phases of the receiver using real GPS signals. Finally, impact and prospects are discussed in the conclusion.

The orbit of a satellite around the earth is constantly disturbed by various factors, such as variations in the gravitational field and the solar wind pressure. The drift of the satellite position can compromise the mission, and even lead to a crash or a fall in the atmosphere. The station-keeping operations therefore consist in performing an accurate measurement of the satellite trajectory and then in using its thrusters to correct the drift. The conventional solution is to measure the position with the help of a ground based radar. This solution is expensive and does not allow to have the satellite position permanently : the trajectory corrections are therefore infrequent. A positioning and autonomous navigation system using constellations of navigation satellites, called Global Navigation Satellite System (GNSS), allows a significant reduction in design and operational maintenance costs. Several studies have been conducted in this direction and the first navigation systems based on GPS receivers, are emerging. A receiver capable of processing multiple navigation systems, such as GPS and Galileo, would provide a better service availability. Indeed, Galileo is designed to be compatible with GPS, both in terms of signals and navigation data. Continuous knowledge of the position would then allow a closed loop control of the station keeping. Initially, we defined what the specifications of a multi-mission space receiver are. Indeed, the constraints on such a receiver are different from those for a receiver located on the surface of the Earth. The analysis of these constraints, and the performance required of a positioning system, is necessary to determine the specifications of the future receiver. There are few studies on the subject. Some of them are classified ; others have, in our view, an analytical bias that distorts the determination of specifications. So we modeled the system : GNSS and receivers satellite orbits, radio frequency link. Some parameters of this link are not given in the specification or manufacturers documents. Moreover, the available theoretical data are not always relevant for realistic modeling. So we had to assess those parameters using the available data. The model was then used to simulate various scenarios representing future missions. After defining analysis criteria, specifications were determined from the simulation results. Calculating a position of a satellite navigation system involves three main phases. For each phase, there are several possible algorithms, with different performance characteristics, the circuit size or the computation load. The development of new applications based on navigation also drives the development of new adapted algorithms. We present the principle for determining a position, as well as GPS and Galileo navigation signals. From the signal structure, we explain the phases of the demodulation and localization. Through the use of GPS and Galileo constellations, standard algorithms achieve the performance required for space applications. However, these algorithms need to be adapted, thus some parts were specifically designed. In order to validate the choice of algorithms and parameters, we have simulated the various operating phases of the receiver using real GPS signals. Finally, impact and prospects are discussed in the conclusion.

Undoubtedly, the idea of global network is the founding concept of the next generation of communication systems. In the future, a user should therefore be able to maintain an excellent quality of communication regardless of where he is or moves without noticing the underlying technology changes. As a niche market of communications, be part of this dynamic is crucial for satellite communications. However, in the recent years, many studies have shown that the integration of the satellite segment in a global context was complicated by the different characteristics of considered technologies. In fact, some of the most important protocols such as TCP that warranty the quality of a communication are strongly suffering from the RTT duration and so are inadequate for the satellite link. Since the 2000s, satellite community therefore deployed specific solutions in the form of TCP-PEP. They offer very good performance, but marginalize the satellite link from the others by breaking the essential concept of end-to-end communication, and then make its integration among other technologies difficult. Also, if a solution that enables a better integrability and efficiency is not found, the satellite may be excluded from this ambitious project, despite its numerous strengths. We first conducted an extensive set of studies regarding the behavior of TCP latest flavors initiated by the main Operating Systems. They suggest that relevant end-to-end solutions, designed to fit terrestrial networks, can eventually offer similar performance in a satellite environment than the TCP-PEP solution. However, these optimisations only improve long-lived connections. The poor performance of short-lived connections, that are a majority in the Internet, continues therefore to justify the use of TCP-PEP. Consequently, we focused on improving end-to-end transport protocols for short-lived connections and proposed a mechanism called Initial Spreading that allows significant performance improvements regardless of the context. Its simple concept aims to overcome the RTT dependence that strongly damages the short-lived flows by emitting a large amount of data segments just after the connection establishment. We pay close attention to the consequences of releasing a large group of segments (burst) in a congested network. So while solutions such as RFC 6928, proposed by Google, see their performance sharply deteriorated in such an environment, our mechanism ensures very good performance using an accurateand regulated spreading of the first sent segments. Many simulations in NS2 first allowed us to validate the usefulness and scope of our mechanism. A mathematical model of short-lived TCP connections then allowed us to corroborate these results and to understand in an accurate way the consequences of the transmission of bursts of segments on the average performance of a communication. Finally, we implemented the Initial Spreading in the Linux kernel in order to test its behavior and efficiency in terrestrial and satellite networks and show the merits of our proposal in a real environment. All these evaluations allowed us to refine our mechanism to significantly improve the performance of short-lived TCP connections regardless of the context in question and the state of the network. We finally submitted our proposal to the IETF in the form of an “Internet Draft”.

Satellite navigation signals demodulation performance is historically tested and compared in the Additive White Gaussian Noise propagation channel model which well simulates the signal reception in open areas. Nowadays, the majority of new applications targets dynamic users in urban environments; therefore the GNSS signals demodulation performance has become mandatory to be provided in urban environments. The GPS L1C signal demodulation performance in urban environments is thus provided in this paper. To do that, a new methodology adapted to provide and assess GNSS signals demodulation performance in urban channels has been developed. It counteracts the classic method limitations which are the fluctuating received C/N0 in urban environments and the fact that each received message is taken into account in the error rate computation whereas in GNSS it is not necessary. The new methodology thus proposes to provide the demodulation performance for ‘favorable’ reception conditions together with statistical information about the occurrence of these favorable reception conditions. To be able to apply this new methodology and to provide the GPS L1C signal demodulation performance in urban environments, a simulator SiGMeP (Simulator for GNSS Message Performance) has been developed. Two urban propagation channel models can be tested: the narrowband Perez-Fontan/Prieto model and the wideband DLR model. Moreover, the impact of the received signal phase estimation residual errors has been taken into account (ideal estimation is compared with PLL tracking).

With a vast majority of Internet connections shorter than 10 segments, designing a new fast start-up TCP mechanism is a major concern. While enlarging the Initial Window (IW) up to 10 segments is the fastest solution to deal with a short-lived connection in uncongested networks, numerous researchers are concerned about the impact of the large initial burst on congested networks.

New mobile technology generations succeed in achieving high goodput, which results in diverse applications profiles exploiting various resource providers (Wifi, 4G, 5G, . . . ). Badly set parameters on one of the network component may severely impact on the transmission delay and reduce the quality of experience. The cross layer impact should be investigated on to assess the origin of latency. To run cross-layer (from physical layer to application layers) simulations, two approaches are possible: (1) use physical layer models that may not be exhaustive enough to drive consistent analysis or (2) use real physical traces. Driving realistic measurements by using real physical (MAC/PHY) traces inside network simulations is a complex task. We propose to cope with this problem by introducing Cross Layer InFormation Tool (CLIFT), that translates real physical events from a given trace in order to be used inside a network simulator such as ns-2. Our proposal enables to accurately perform analysis of the impact of link layer reliability schemes (obtained by the use of real physical traces) on transport layer performance and on the latency. Such approach enables a better understanding of the interactions between the layers. The main objective of CLIFT is to let us study the protocols introduced at each layer of the OSI model and study their interaction. We detail the internal mechanisms and the benefits of this software with a running example on 4G satellite communications scenarios.

Leon Chua introduced memristors in 1971 [1] as an ideal two-terminal circuit element in complement to the already known three basic circuit elements: resistor, inductor and capacitor (RLC). Memristors are defined by a non-linear memristance that relates the flux (or integral of voltage across the device) to the charge (or integral of current in the device). Because of this definition the memristor will generate passive intermodulation products and their power will depend on the memory of the past current that is contained in the device.

In the context of satellite communications, random access methods can significantly increase throughput and reduce latency over the network. The recent random access methods are based on multi-user multiple access transmission at the same time and frequency followed by iterative interference cancellation and decoding at the receiver. Generally, it is assumed that perfect knowledge of the interference is available at the receiver. In practice, the interference term has to be accurately estimated to avoid performance degradation. Several estimation techniques have been proposed lately in the case of superimposed signals. In this paper, we present an overview on existing channel estimation methods and we propose an improved channel estimation technique that combines estimation using an autocorrelation based method and the Expectation-Maximization algorithm, and uses pilot symbol assisted modulation to further improve the performance and achieve optimal interference cancellation.

Delay/Doppler radar altimetry is a new technology that has been receiving an increasing interest, especially since the launch of Cryosat-2 in 2010 , the first altimeter using this technique. The Delay/Doppler technique aims at reducing the measurement noise and increasing the along-track resolution in comparison with conventional pulse limited altimetry. A new semi-analytical model with five parameters has been recently introduced for this new technology. However, two of these parameters are highly correlated resulting in bad estimation performance when estimating all parameters. This paper proposes a new strategy improving estimation performance for delay/Doppler altimetry. The proposed strategy exploits all the information contained in the delay/Doppler domain. A comparison with other classical algorithms (using the temporal samples only) allows to appreciate the gain in estimation performance obtained when using both temporal and Doppler data.