This thesis deals with the improvement of Spread Spectrum digital receiver robustness characteristics especially those of GPS. A manner to reach this objective is to improve the post-correlation robustness of the GPS digital channels. The technique, called here TASAP, uses this principle of robustness by providing a velocity aiding from the Navigation Filter (PVT) itself to each GPS channel. As soon as the C/No is too weak to allow the normal use of the receiver tracking mode, the PLL and DLL loops switch to the AVIA mode (Internal and Autonomous Velocity Aiding). The NCOs of that carrier tracking loop (PLL) and Digital Delay Lock Loop (DDLL) are then pushed by this velocity aiding (internal to the receiver). This memorandum brings the theoretical demonstrations that reducing the noise bandwidth allows the receiver to reduce the acquisition and tracking thresholds of GPS signals between 5 to 15 dB depending of the situation.

A large panorama of potential original techniques to reduce the acquisition and tracking thresholds is detailed. The theoretical elements are exposed to allow the better understanding of the advantages and the efficiencies of AVIA Loops and to demonstrate the actual C/No thresholds.
The classical models of phase-locked loop are described and with this straightforward model, the code loop is studied more in details in the case of an external velocity aiding resulting from an Inertial Navigation System (INS). Finally, a simple model of the PLL with an autonomous velocity aiding coming from the GPS Navigator itself is carried out.. The model takes into account all the perturbations which corrupt the functionality of the PLL (SA, pseudorange noise, velocity error, etc).

The modelisation of an entire communication system is designed in software with the conception of a generic baseband GPS receiver simulator called MATNAV. The loop filter, the PLL and DLL loop discriminators, the impulse and step response analysis, the loop bandwidths and several simulations can also be found in this work. Our simulations show an improvement of 5 to 15dB on the C/No threshold by using an internal and autonomous velocity aiding from the GPS Navigation Filter. Moreover, the integration of 7 velocity aiding modes to MATNAV simplify the task to the user for measurement comparisons.

Robustness - Threshold - Acquisition - Tracking - GPS - Navigation - Numerical GPS Receiver - Velocity Aiding - INS - Inertial Aiding - MATNAV - TASAP - AVIA Loop - Digital Signal Processing - ARD Capsule - DIOGENE.