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

Propagation of monochromatic electro-magnetic waves in free space results in a widening of the spectral line. On the contrary, propagation preserves monochromaticity in the case of acous-tic waves. In this case, the propagation can be modelled by a linear invariant filter leading to attenuations and phase changes. Due to the Beer-Lambert law, the associated transfer function is an exponential of power functions with frequency-dependent parameters. In recent papers, we have proved that the acoustic propagation time can be modelled as a random variable following a stable probability distribution. In this paper, we show that the same model can be applied to the propagation in coaxial cables.

Positioning and navigation by GNSS in urban context are always challenging tasks, because of signal propagation problems such as shadowing effects and multipath. When not enough GNSS signals are received in line-of-sight (LOS), classical approaches mitigating multipath effects become insufficient because there is not enough reliable information available. Consequently, positioning errors can be about tens of meters, especially in urban canyons. In this paper, we introduce a NSS positioning approach that uses constructively non-line-of-sight (NLOS) signals in order to have enough information to compute the user’s position. In this work, we use the SE-NAV software to predict the geometric paths of NLOS signals using a high realistic 3D model of the environment. More precisely, we propose a new version of the extended Kalman filter augmented by the information provided by SE-NAV, referred to as 3D AEKF, for GNSS navigation in NLOS context. In the proposed approach, the measurement model traditionally based on the trilateration equations is constructed from the received paths estimated by SENAV. The Jacobian of the measurement model is calculated through knowledge of the objects on which the reflections have occured. To use even less reliable measurements, we propose a robust version of the 3D AEKF. Simulations conducted in realistic scenarios allow the performance of the proposed method to be evaluated.

Increased sensitivity and reduced fast time to first fix (TTFF) are key performance indicators for GNSS receivers, which depend on the surrounding environment, receiver design and available aiding information. To reduce the effect of attenuations, dynamics and navigation data bits transition, the GNSS acquisition engines employ both coherent and post-coherent signal integration strategies, namely non-coherent and differentially coherent. Understanding the advantages and drawbacks of each post-coherent integration strategy is fundamental in the process of optimization of the acquisition scheme, as well as the effect of high-dynamics in both coherent and post-coherent operations. In this paper, we study three important issues of GNSS acquisition. First, we propose a closed form expression to quantify the effect of a linearly changing Doppler frequency on the coherent integration output. Second, we derive a formula capable of characterizing the sensitivity gain of a differential integration detector. Third, we compare the effect of dynamics on both non-coherent and differential integration. The main objective of this work is to combine these three contributions for overall optimization of the acquisition scheme. More precisely, we mitigate the effect of large Doppler errors without compromising receiver’s sensitivity and ideally without additional computational cost.

We present a solution to evaluate the performance of transport protocols as a function of link layer reliability schemes (i.e. ARQ, FEC and Hybrid ARQ) applied to satellite physical layer traces. As modelling such traces is complex and may require approximations, the use of real traces will minimise the potential for erroneous performance evaluations resulting from imperfect models. Our Trace Manager Tool (TMT) produces the corresponding link layer output, which is then used within the ns-2 network simulator via the additionally developed ns-2 interface module. We first present the analytical models for the link layer with bursty erasure packets and for the link layer reliability mechanisms with bursty erasures. Then, we present details of the TMT tool and our validation methodology, demonstrating that the selected performance metrics (recovery delay and throughput efficiency) exhibit a good match between the theoretical results and those obtained with TMT. Finally, we present results showing the impact of different link layer reliability mechanisms on the performance of TCP Cubic transport layer protocol.

We propose a novel DTN routing algorithm, called DQN, specifically designed for quasi-deterministic networks with an application to satellite constellations. We demonstrate that our proposal efficiently forwards the information over a satellite network derived from the Orbcomm topology while keeping a low replication overhead. We compare our algorithm against other well-known DTN routing schemes and show that we obtain the lowest replication ratio without the knowledge of the topology and with a delivery ratio of the same order of magnitude than a reference theoretical optimal routing.

This paper addresses the problem of error correction in a multi-user trellis coded system in the presence of bit stuffing. In particular, one considers the situation in which automatic identification system (AIS) signals are received by a satellite. The proposed receiver uses a cyclic redundancy check (CRC) for error correction. A Viterbi algorithm based on a so-called extended trellis is developed. This trellis is defined by extended states composed of a trellis-code state and a CRC state. Moreover, special conditional transitions are defined in order to take into account the possible presence of bit stuffing. The proposed algorithm was first developed in a single-user context. It is generalized in this paper to a multi-user scenario by designing an interference mitigation method. This method allows one to derive a demodulation algorithm whose complexity is almost identical to that obtained in the single-user context. Some performance results are presented in the context of AIS and compared with results provided by existing techniques.