The automatic identification system (AIS) is a system allowing ships and coast stations to exchange some information by VHF radio. This information includes the identifier, status, location, direction and speed of the emitter. The aim of this thesis is to allow the reception of AIS messages by low Earth orbit satellites without modifying the existing ship equipments. With this system, it becomes possible to know the position of all ships over the Earth. As a consequence, several new services become available, such as global traffic monitoring or determining boat location (for ship-owners). Satellite reception of AIS signals is subjected to a higher noise level when compared to ground level reception. This noise makes classical demodulation and decoding methods unusable. A first contribution of this thesis is to develop new demodulators using error correction methods. These demodulators take advantage of the presence of a cyclic redundancy check (CRC) block in the messages as well as known information about the structure of messages and data. Generalizations of the proposed receiver have also been studied in order to take into account the phase noise of the received signals and the possible collision of messages sent simultaneously by several vessels. The last part of this thesis is devoted to the study of localization methods for ships that do not transmit their location in AIS messages. This localization takes advantage of information contained in the received messages such as the propagation delay and the carrier frequency shift due to the Doppler effect, and a ship movement model.

This thesis focuses on jointly improving the spectral efficiency and the power efficiency of satellite transmission schemes. The emergence of new services and the increasing number of actors in this field involve higher transmission rates with increasingly limited resources. Recent progress in the embedded technologies and in digital communications offered to consider transmission schemes with higher spectral and power efficiency. Nevertheless, the major current challenge consists in making efficient use of resources. The study developed in this thesis explores the possibilities of jointly improving the spectral and power efficiency by offering a combination of the Cyclic- Code-Shift Keying modulation (CCSK), which power efficiency increases with the degree of modulation, with a multiplexing technique such as Code-Division Multiplexing (CDM) to offset the deterioration on the spectral efficiency due to the spread spectrum induced by CCSK. Two approaches based on the use of Gold sequences of length N are defined : • a multi-stream approach with an optimal receiver implemented through sphere decoding. The complexity due to the receiver optimality leads to limited spectral efficiencies but the study of performance, confirmed by simulations, shows an increase in power efficiency with spectral efficiency. • a single-stream approach justified by the appearance of redundancy in the patterns following the sequences multiplexing. The single-stream approach offers spectral efficiencies equivalent to the adopted schemes in the DVB-S2 standard, with improved power efficiency from a certain level of signal to noise ratio compared to those schemes. Subsequently, the study focuses on the implementation of several modulation symbols on the subcarriers of an OFDM modulator and the benefits and advantages of such an approach. It concludes with the contribution of channel coding based on nonbinary block codes such as Reed-Solomon and LDPC codes. The proposed waveform offers operating points with high spectral efficiency and high power efficiency with attractive perspectives. In the current context, its application is limited by its amplitude fluctuations but is possible in a multicarrier transmission context, as expected in the years to come.

This thesis focuses on jointly improving the spectral efficiency and the power efficiency of satellite transmission schemes. The emergence of new services and the increasing number of actors in this field involve higher transmission rates with increasingly limited resources. Recent progress in the embedded technologies and in digital communications offered to consider transmission schemes with higher spectral and power efficiency. Nevertheless, the major current challenge consists in making efficient use of resources. The study developed in this thesis explores the possibilities of jointly improving the spectral and power efficiency by offering a combination of the Cyclic- Code-Shift Keying modulation (CCSK), which power efficiency increases with the degree of modulation, with a multiplexing technique such as Code-Division Multiplexing (CDM) to offset the deterioration on the spectral efficiency due to the spread spectrum induced by CCSK. Two approaches based on the use of Gold sequences of length N are defined : • a multi-stream approach with an optimal receiver implemented through sphere decoding. The complexity due to the receiver optimality leads to limited spectral efficiencies but the study of performance, confirmed by simulations, shows an increase in power efficiency with spectral efficiency. • a single-stream approach justified by the appearance of redundancy in the patterns following the sequences multiplexing. The single-stream approach offers spectral efficiencies equivalent to the adopted schemes in the DVB-S2 standard, with improved power efficiency from a certain level of signal to noise ratio compared to those schemes. Subsequently, the study focuses on the implementation of several modulation symbols on the subcarriers of an OFDM modulator and the benefits and advantages of such an approach. It concludes with the contribution of channel coding based on nonbinary block codes such as Reed-Solomon and LDPC codes. The proposed waveform offers operating points with high spectral efficiency and high power efficiency with attractive perspectives. In the current context, its application is limited by its amplitude fluctuations but is possible in a multicarrier transmission context, as expected in the years to come.

This paper deals with the problem of detecting a collision target in ground clutter, using a long integration time. A single reception channel being available, classical space time adaptive processing (STAP) cannot be used. After range processing, ground clutter can be modeled as a known interference subspace in the Doppler domain depending on its radial and orthoradial speeds. We exploit this a priori knowledge to perform an adpative detection of a collision target supposed to lie in a known and different subspace. A GLRT detector is first derived for known clutter covariance matrix. Then, the unknown covariance matrix is adaptively estimated from the projection of the data onto the modeled clutter subspace, and is plugged in the GLRT to form a suboptimal detector. The proposed scheme can be viewed as a synthetic STAP, for which the space domain is replaced by a clutter orthoradial information and longer integration time.

Waveforms transmitted by the elements of a MIMO radar system may differ in power and bandwidth. This raises the question of optimal resource allocation among the radar elements. Specifically, we are asking, given constrained resources, what is the optimal bandwidth and optimal joint power and bandwidth allocation for best target localization performance. Using the Cramér-Rao Lower Bound (CRLB) as the figure of merit for localization accuracy, the resource allocation optimization problem turns out to be non-convex. We apply a Difference of Convex functions programming approach to develop quasi-optimal algorithms for solving the resource allocation problems. A lower bound is also developed to help assess the quality of our solutions. Numerical examples demonstrate that bandwidth allocation has considerably more impact on performance than power allocation, and that the best performance is obtained with joint power and bandwidth allocation.

This paper proposes an improvement of the random multiple access scheme for satellite communication named Multislot coded ALOHA (MuSCA). MuSCA is a generalization of Contention Resolution Diversity Slotted ALOHA (CRDSA). In this scheme, each user transmits several parts of a single codeword of an error correcting code instead of sending replicas. At the receiver level, the decoder collects all these parts and includes them in the decoding process even if they are interfered. In this paper, we show that a high throughput can be obtained by selecting variable code rates and user degrees according to a probability distribution. With an optimal irregular degree distribution, our system achieves a normalized throughput up to 1:43, resulting in a significant gain compared to CRDSA and MuSCA. The spectral efficiency and the implementation issues of the scheme are also analyzed.

This paper addresses the problem of error correction of AIS messages by using the a priori knowledge of some information in the messages. Indeed, the AIS recommendation sets a unique value or a range of values for certain fields in the messages. Moreover, the physics can limit the range of fields, such as the speed of the vessel or its position (given the position of the receiver). The repetition of the messages gives also some information. Indeed, the evolution of the ship position is limited between messages and the ship ID is known. The constrained demodulation algorithm presented in this article is an evolution of the constrained Viterbi algorithm (C-VA). It is based on a modified Viterbi algorithm that allows the constraints to be considered in order to correct transmission errors by using some new registers in the state variables. The constraints can be either a single value or a range of values for the message fields. Simulation results illustrate the algorithm performance in terms of bit error rate and packet error rate. The performance of the proposed algorithm is 2 dB better than that obtained with the receiver without constraints.

The concept of delay/Doppler radar altimeter has been under study since the mid 90’s, aiming at reducing the measurement noise and increasing the along-track resolution in comparison with the conventional pulse limited altimeters. This paper introduces an analytical model of the mean backscattered power waveform acquired by a radar altimeter operating in SAR mode, as well as an associated least squares estimation algorithm. As for conventional altimetry, the mean power can be expressed as the convolution of three terms: the flat sea surface response, the sea wave height probability density function and the time/frequency impulse response of the radar. An important contribution of our work has been to derive an analytical formula for the flat sea surface response associated with Doppler altimetry. The double convolution defining the mean power can then be computed numerically. The resulting single-look model depends on three parameters: the epoch, the sea surface wave height and the amplitude. A multi-look model is obtained by summing all the reflected power from the along track beam surface of interest after applying appropriate delay compensation. The second contribution of our work concerns the estimation of the parameters associated with the single and multi-look analytical Doppler models. A least squares approach is investigated by means of the Levenberg-Marquardt algorithm (which does not need computing the exact model derivatives). Simulations conducted on synthetic altimetric waveforms allow the performance of the proposed estimation algorithm to be appreciated. The proposed analytical model (and the associated estimation algorithm) are finally compared with other models developed by the Centre National d'Etudes Spatiales (CNES) and the company Collecte Localisation Satellites (CLS) both located in Toulouse, France. The analysis of a huge number of Cryosat waveforms shows an improvement in parameter estimation when compared to the conventional LRM mode altimeter. These results are very promising.

Since the rise of technologies using GNSS positioning systems, development of carrier phase tracking receiver for precise point positioning in hostile environments is becoming one of the most important challenges for future satellite navigation applications. Because phase locked loops (PLL) that track carrier phase suffer from cycle slips phenomenon, noise robustness of the formers has to be reinforced if one wants to use precise positioning techniques in the widest range of challenging environments. The purpose of this article is to propose a new PLL design using a phase unwrapping algorithm that effectively corrects cycle slips due to phase noise in low CN0. Unlike phase unwrapping algorithms using a threshold approach for cycle slips detection, the algorithm implemented in our PLL structure is based on a system of prediction and pre-compensation of the phase dynamic stress. In order to reduce the cycle slips and enforce noise robustness of phase tracking, this algorithm is adapted to tracking loops with the aim to propose two innovative PLL structures. A comparative study is performed to show the effectiveness of the two proposed structures in case of noisy environment. 47