Joint Positioning and Time Synchronization for APNT

M. Suess, B. Belabbas, M. Meurer

Abstract:	Global navigation satellite systems (GNSS) have been selected to be primary means of navigation in aeronautics. However, several ATM programs (i.e. NextGen, SESAR) have identified the need for an Alternate Positioning Navigation and Time service (APNT), which can act as backup in cases of major outage or disturbance of GNSS. DLR proposed the usage of the LDACS1 (L-band Digital Aeronautical Communication System 1) system, which was originally being planned for communication purposes only, also for an alternate navigation [1]. This system consists of ground stations and an airborne subsystem. The ranging performance is right now under evaluation within a demonstration campaign and already provides very promising results in terms of e.g. ranging accuracy. This current statement motivates us to investigate advanced and alternate positioning and timing techniques that take into account the technical properties of a ground based APNT. The state of the art of multi-lateration concept assumes that the ground station network is sufficiently and accurately synchronized or can be corrected to a common system time by applying time parameters to mitigate the time synchronization error. An aircraft user processes the pseudo range measurements to calculate its three-dimensional position (and its derivatives) and time relative to the system time. Data of additional sensors is included if available. In such a situation, the classical PVT-principle is applied to solve for unknowns of the aircraft position considering the availability of ground station precise coordinates and time offset parameters. However, the apparent simple approach includes additional challenging aspects like geometry of the resulting ranges, signal design, multipath and time synchronization concepts which have an important impact on the architecture of the overall APNT. Accurate synchronization of the ground stations is a major requirement of the aforementioned PVT-principle. However, these requirements can heavily being relaxed if the determination of the PVT is suitably adapted. The major objective of the paper is to present and study a novel sophisticated PVT-method that allows to determined position and timing solutions in cases of only roughly or even non-synchronized ground stations and, therefore, also provides PVT in fall back situations where the synchronization of the ground station network is disturbed or not available. In that situation, the time offsets of the ground stations are to be traded as additional states within the PVT-equations. To compensate for that underdetermined system of equations, additional observations are required to provide a unique existence of the solution. Therefore, at each epoch, a sequence of range measurements (batch) is processed to estimate the joint aircraft positions and time offsets along with the ground station time offsets. By definition of the model, this solution is parameterized with the number of ground stations (identical with the number of pseudo ranges), the number of epochs within the epochs and the time interval between epochs that ensure statistical independency of observations. It is stated that joining measurements in batches allows determining the position solution without the need of a synchronized ground station network; however, it can require that the growths of the involved clock offsets are within a certain range. The impact of the batch length on the PVT-solution is assessed by simulation of different stations and aircraft clock scenarios. Exemplary, the approach can be used to process horizontal and vertical solution of an aircraft in the situation of having four range measurements. Typical ground station clock scenarios such as equipped with low cost Rubidium clocks or high/standard performance Cesium clocks are simulated and evaluated. A trade-off is performed to characterize a feasible ground station clock configuration for the presented approach of joint positioning and time synchronization. The geometry of the simulation shall cover different flight heights and ground station distributions approximating APNT demonstration campaigns. To complement the introduced technique an integrity monitoring concept will be investigated taking into account the specificities of the joint processing algorithm, which means the envisagement of an over-bound error model and the parameters of the problem. A first part of the integrity monitoring concept consists of over-bounding the nominal errors and the empirical calculation of protection bounds for a given integrity risk (protection level concept generalized to a joint positioning and time synchronization algorithm). The second part of the integrity monitoring concept consists of the detection capabilities of anomalous behavior of different components of the APNT processing chain: clock jumps (time and frequency) in a given station, stochastic variations of the time and frequency offset within one batch of observables. The paper will start with an introduction of the APNT service and an overview of existing APNT systems and demonstrations. Within the second section, the mathematical description and details of the novel joint positioning and time synchronization algorithm are given. The following section defines the APNT nominal and operational scenarios to be simulated and evaluate its performances. It is finally concluded with a trade-off of the batch length onto the estimation and integrity performance. Additionally, robustness and operational aspects are quantified and addressed. [1] B. Belabbas, M. Felux, M. Meurer, N. Schneckenburger, M. Schnell “LDACS1 for an Alternate Positioning Navigation and Time Service.” Proceedings GNSS Signals, Toulouse, 2011.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3508 - 3515
Cite this article:	Suess, M., Belabbas, B., Meurer, M., "Joint Positioning and Time Synchronization for APNT," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3508-3515.
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