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Session C2: Advances in High Accuracy Positioning

Enabling Metasignal Processing Technology for GNSS Signals for Highly Accurate and Resilient PNT Solution in Harsh Environments
A. Emmanuele, L. Danelli, A. Ferrario, M. Libertone, L. Scaciga, Thales Alenia Space Italia
Alternate Number 3

Abstract: New global navigation satellite system (GNSSs) such as GALILEO second generation and modernized GPS will be reality, exhibiting higher performance with respect to older GPS and Galileo first generation [1] [2] and other systems like GLONASS, BEIDOU.
In all these systems, several improvements have been achieved taking full advantage of the gains obtained in the last years on concepts and technologies, such as new materials and components for spacecraft or advanced digital signal processing, just to cite a few.
New signals and navigation schemes have been specifically designed in modernized GPS [3], [4] and will be introduced by GALILEO second generation [5], [6] to make the GNSS functional for any daily-life request and a valid solution concerning safety-critical and non-safety-critical applications.
The ultimate goal of satellite positioning systems is enhanced user position and timing accuracy, that is directly related to the accuracy of time-delay estimation (TDE) of ranging signals.
As a matter of fact, timing recovery represents the most critical function in every radio-location systems, including those based on satellite positioning. Current GNSSs are in fact based on the capability of a receiver to estimate the propagation times of a set of spread spectrum (SS) signals broadcast by multiple satellites placed at known locations. When at least four propagation times are available, the receiver can unambiguously obtain its own spatial coordinates and the time reference. In this scenario, positioning accuracy depends on the accuracy in TDE between transmitted codes and local replicas and it is apparent that the more accurate the TDE is, the more precise the user position will be.
Current activities at system level aim at enhancing the overall navigation performance by providing better navigation signals to those available today, i.e., by optimizing signal modulation schemes, content and structure of the navigation messages.
The aim of this work is to ensure enhancements in TDE accuracy and resilience in harsh environmental conditions, still adopting current legacy park of signals available, based on special digital signal processing to be applied at professional or non-professional receivers.
A set of innovative techniques are here proposed, recalling the concept of “metasignal” processing [7], [8], to combine two (or more) signal components transmitted from the same GNSS satellite, typically ranging on different carriers of the GNSS spectrum and finally remapped into the same digital base-band domain in such a way to optimize the TDE accuracy [9][10].
Two different approaches are here proposed for metasignal processing: coherent and semi-coherent schemes, depending on how the relative differences of carrier phase and code phase of the two or more components to be combined are treated in time.
In a complementary way, to enable the metasignal techniques, an advancement on the concept is considered, concerning the capability of the receiver to demodulate in parallel the same radio-frequency (RF) signal component on different intermediate frequencies (IF), such that at digital domain a set of spectrum replicas are made available for further processing.
Advantages and disadvantages of this procedure are considered, as function of the attained relative frequency offset between the replicas and w.r.t. the reference carrier frequency of the signal in use.
As benchmark, the E5AltBOC signal-in-space is here considered as ready-to-use test bench for the set of metasignal processing techniques, taking benefit of the wideband GALILEO signal which is already constituted by two signal components, namely E5a and E5b, separated into two frequency offsets w.r.t. E5 and also separated in code domain, carrying different PRN codes.
Real HW in the loop results will be presented, obtained developing the digital signal processing algorithms on Thales Alenia Space Italia (TASI) receiver.
The views presented in the paper represent solely the opinion of the authors.
Keywords: Metasignal processing, signals combination, Time Delay Estimation (TDE), high accuracy, harsh environments, Galileo, GPS, GNSS Receivers, Position Navigation Timing (PNT), resilient PNT.

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