A GNSS Multipath Error Model for Airport Surface Operations

The Global Navigation Satellite System (GNSS) is a worldwide position and time determination system currently used in civil aviation to support En-route to Precision Approach (PA) operations. Extending the use of GNSS to aircraft guidance during surface operations remains a challenge. Indeed, during taxi and parking operations, GNSS pseudo range measurements suffer from higher multipath errors than whilst in flight because of signal reflections from the aircraft structure, and from additional sources of multipath, such as the ground and obstacles surrounding the airborne antenna [2]. This can result in horizontal positioning errors that reach several meters [3]. The multipath ranging error model currently standardized for En-route to PA operations [8] is not designed to protect users from the effects of multipath from the ground and obstacles in airport environments. Hence, it is necessary to develop a multipath ranging error model adapted to airport environments in order to protect the user from the effects of multipath during surface operations (taxi and parking) and possibly correct these effects. Several steps are needed to set up such a model. The first step, which is the main objective of this paper, is to provide an analysis of the error on the raw code and smoothed code GPS L1 C/A pseudo range measurements due to multipath from: - the aircraft structure and the ground, - the aircraft structure, the ground, and a single obstacle surrounding the airborne antenna, in both static and dynamic configurations. Firstly, the analytical models of the raw and smoothed errors in the time domain assuming that the aircraft and the satellite are both static are provided. The derived models are parameterized by a time-independent, space-dependent coefficient. The physical meaning of this coefficient is provided, and its variation in the space domain is investigated. This coefficient is statistically modeled and over-bounded by a non-zero Gaussian distribution in the case where the pseudo range measurement is affected by multipath from the ground, the aircraft structure and a single obstacle. The influence of the type of obstacle and of the satellite elevation angle on the mean and the standard deviation of the Gaussian distribution are analyzed. A mathematical model of the variation of the standard deviation as a function of the elevation angle for a given obstacle is proposed. Secondly, the definition of "dynamic regime" is discussed. The models of the raw code and smoothed code multipath ranging errors affecting an aircraft that performs a straight line trajectory with a constant speed in this airport are provided. In the case where the pseudo range measurement is affected by multipath from the ground, the aircraft structure and a single obstacle, these errors are over bounded by a stationary non-zero mean Gaussian distribution. The influence of the obstacle, the elevation angle and the aircraft dynamic on the Gaussian distribution mean and standard deviation are analyzed.