Implementation issues and performance characterization of FDAF for a GNSS receiver

Abstract : The future use of the Galileo E5 and GPS L5 bands by the civil aviation community raises new issues, notably concerning pulsed interference. These bands suffer concomitants radio frequency emissions from DME (Distance Measuring Equipment), TACAN (TACtical Air Navigation), JTIDS (Joint Tactical Information Distribution System) and MIDS (Multifunctional Information Distribution System) systems. These interferences, if ignored, significantly disturb GNSS receivers functioning and prevent them from meeting civil aviation requirements. In order to be used onboard civil aircrafts, the GPS L5 and Galileo E5 signals have to be processed so that they allow specified minimum performances. In the following, the study will focus on the E5a/L5 band, as this band is more impacted by pulsed interference than the E5b one [Bastide, 2004]. Also, it can be shown that the impact of JTIDS/MIDS signals on GPS L5 and Galileo E5a signal processing is very low with respect to DME/TACAN signals? one. Consequently they will be omitted herein for simplification purposes. The hot spot is defined as the place where the influence of DME/TACAN signals is the largest on the victim GNSS receivers. Studying the behaviour of a GNSS receiver in this particular interference environment is necessary to guarantee the receiver reaches the minimum performance specified by civil aviation authorities. The usual Figure Of Merit (FOM) used to look at the impact of interference is the degradation of the postcorrelation C/N0. Indeed, it constitutes a good indicator of acquisition, tracking and data demodulation performance, which are critical operations for the receiver. A GNSS receiver that would not use an Interference Mitigation Technique (IMT) would experience severe post-correlation C/N0 degradations [Bastide, 2004]; often leading to loss of locks, in the hot spot environment. Thus, two IMTs implemented in the RF front end were proposed. The first one is called the Temporal Blanker (TB). Its digital implementation is described in [Grabowsky, 2002], and its benefits in terms of postcorrelation C/N0 largely studied in [Bastide, 2004]. It shows that GNSS receivers using the technique would comply with ICAO requirements: using TB, no loss of lock is experienced and the C/N0 ratio stays above the minimum requirements. On the other hand, FDAF is proposed as an algorithm that would guarantee higher C/N0 levels in presence of interference. This has already been studied through simulation results in [Raimondi, 2008], neglecting real receivers front-end effects. In addition, the postcorrelation C/N0 degradation was the only observed FOM, whereas some other indicators are necessary. The paper first describes the threat of introducing new GNSS signals in an already occupied band. Then, the impact of pulsed interference, in terms of post-correlation C/N0 and AGC gain disturbance will be presented. Then the FDAF algorithm is depicted, along with its capacity to remove interference. Finally, FDAF impact on postcorrelation C/N0, but also AGC convergence, crosscorrelation functions and signal phase continuity are analysed.
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ENC-GNSS 2008, Conférence Européenne de la Navigation, Apr 2008, Toulouse, France. 2008
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Mathieu Raimondi, Olivier Julien, Christophe Macabiau. Implementation issues and performance characterization of FDAF for a GNSS receiver. ENC-GNSS 2008, Conférence Européenne de la Navigation, Apr 2008, Toulouse, France. 2008. 〈hal-01022135〉

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