Compliance of Single Frequency Ionospheric Delay Estimation and Cycle Slip Detection with Civil Aviation Requirements

Christophe Ouzeau, Christophe Macabiau, Anne-Christine Escher, and Benoit Roturier

Abstract: Ionosphere is a dispersive medium that can strongly affect GPS and GALILEO signals. Ionospheric delay affecting the GPS and GALILEO single frequency pseudorange measurements is the largest source of ranging error. In addition, this perturbation is difficult to model and thus difficult to predict. Nominal dual frequency measurements provide a good estimation of ionospheric delay. In addition, the combination of GPS and GALILEO navigation signals at the receiver level is expected to provide important improvements for civil aviation. It could, potentially with augmentations, provide better accuracy and availability of ionospheric correction measurements. Indeed, GPS users will be able to combine GPS L1 and L5 frequencies, and future GALILEO signals will bring their contribution as some of them will be transmitted at the same frequencies as the GPS signals. However, if affected by radio frequency interference, a receiver can lose one or more frequencies leading to the use of only one frequency to estimate ionospheric code delay. Therefore, it is felt by the authors as an important task to investigate the performance of techniques trying to sustain multi-frequency performance when a multi-constellation receiver installed in an aircraft loses dual frequency capability, during critical phases of flight. After a loss of several frequencies leading to a single frequency degraded mode, a receiver can use code and carrier phase pseudoranges made on only one carrier frequency to estimate the ionospheric delay. To achieve this estimation, the receiver can use the difference between code and carrier phase measurements. Indeed, this quantity can be modelled as twice the ionospheric delay plus noise, multipath, and the carrier phase ambiguity. The ionospheric delay can then be extracted from this, provided the ambiguity is properly removed. This can be achieved after convergence of a Kalman Filter for example, but then cycle slips need to be monitored. The probability of a cycle slip to occur is low but not negligible for civil aviation purposes. Several causes of cycle slips may be identified. For instance multipath, dynamics, signal blockage and ionospheric scintillation may be sources of this type of rupture in carrier phase measurements. Cycle slips may have random magnitudes. Those ones have to be detected and corrected with a performance compliant with civil aviation requirements for integrity, continuity, accuracy and availability. This problem of cycle slip detection is a priority before analyzing the accuracy of the single frequency iono corrected pseudorange. We propose to follow the methodology exposed below to assess the performance of potential algorithms of detection (and estimation) of cycle slips. First, the cycle slip detection and correction ability will be defined by the smallest cycle slip detectable with a required probability of missed detection. This smallest detectable cycle slip implies a bias on position error depending on geometry. Therefore, availability of protection against cycle slips compatible with APV 1 and APV 2 for instance, depends on geometry and must be computed at every second. The main goal of this paper is to know exactly the impact of the capability of cycle slip detection algorithms on the availability of reliable single frequency iono corrected pseudoranges.
Published in: Proceedings of the 2007 National Technical Meeting of The Institute of Navigation
January 22 - 24, 2007
The Catamaran Resort Hotel
San Diego, CA
Pages: 1296 - 1305
Cite this article: Ouzeau, Christophe, Macabiau, Christophe, Escher, Anne-Christine, Roturier, Benoit, "Compliance of Single Frequency Ionospheric Delay Estimation and Cycle Slip Detection with Civil Aviation Requirements," Proceedings of the 2007 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2007, pp. 1296-1305.
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