Chip Scale Atomic Clocks are a recently developed technology, that used together with a GNSS receiver helps to improve the performance of GNSS navigation solutions in some particular conditions [1]. Current GNSS receivers include a Temperature Compensated Cristal Oscillator (TCXO) clock which is characterized by a short term stability (τ = 1 second) of 2·10 -9 seconds that leads to an error of 0.6 meters in pseudorange measurements. While a Chip Scale Atomic Clock (CSAC) [1] has a stability (τ = 1 second) of 2.5·10 -10 seconds that implies a range error of 0.075 meters. The use of TCXO forces the inclusion of a time parameter in the navigation algorithms, which reduces the positioning performance in poor satellite constellation conditions, reflected in the Dilution of Precision (DoP) values [2]. The aim of the study presented on this paper is to characterize the impact of the Cesium Atomic clocks technology, specifically a CSAC clock, in high-grade GNSS receivers by evaluating the position scattering and the holdover.

Chip Scale Atomic Clocks (CSAC) are recently-developed electronic instruments that, when used together with a Global Navigation Satellite Systems (GNSS) receiver, help improve the performance of GNSS navigation solutions in certain conditions (i.e., low satellite visibility). Current GNSS receivers include a Temperature Compensated Cristal Oscillator (TCXO) clock characterized by a short-term stability (τ = 1 s) of 10 −9 s that leads to an error of 0.3 m in pseudorange measurements. The CSAC can achieve a short-term stability of 2.5 × 10 −12 s, which implies a range error of 0.075 m, making for an 87.5% improvement over TCXO. Replacing the internal TCXO clock of GNSS receivers with a higher frequency stability clock such as a CSAC oscillator improves the navigation solution in terms of low satellite visibility positioning accuracy, solution availability, signal recovery (holdover), multipath and jamming mitigation and spoofing attack detection. However, CSAC suffers from internal systematic instabilities and errors that should be minimized if optimal performance is desired. Hence, for operating CSAC at its best, the deterministic errors from the CSAC need to be properly modelled. Currently, this modelling is done by determining and predicting the clock frequency stability (i.e., clock bias and bias rate) within the positioning estimation process. The research presented in this paper aims to go a step further, analysing the correlation between temperature and clock stability noise and the impact of its proper modelling in the holdover recovery time and in the positioning performance. Moreover, it shows the potential of fine clock coasting modelling. With the proposed model, an improvement in vertical positioning precision of around 50% with only three satellites can be achieved. Moreover, an increase in the navigation solution availability is also observed, a reduction of holdover recovery time from dozens of seconds to only a few can be achieved.

Current GNSS (Global Navigation Satellite System) receivers include an internal quartz oscillator, such as TCXO (Temperature Compensated Crystal Oscillator) or similar, limited by its frequency stability and a poor accuracy, being one of the main sources of uncertainty in the navigation solution (also multipath and ionosphere effects are an important error sources.) Replacing the internal TCXO clock of GNSS receivers by a higher frequency stability clock such a CSAC (Chip Scale Atomic Clock) can improve the navigation solution in terms of availability, positioning accuracy, tracking recovery, multipath and jamming mitigation and spoofing attacks detection. For achieving these benefits, the deterministic errors from the CSAC need to be modelled, by determining and predicting the clock frequency stability in the positioning estimation process. The procedure of calculating a position without the need of estimating continually the clock error parameter is also known as clock coasting. The presented research shows the potential of the clock coasting method in order to be able to obtain position with only three satellites, improve the vertical positioning accuracy and increase the navigation solution availability.

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ABSTRACT:This paper presents the concept of an architecture for a wheeled robot system that helps researchers in the field of geomatics to speed up their daily research on kinematic geodesy, indoor navigation and indoor positioning fields. The presented ideas corresponds to an extensible and modular hardware and software system aimed at the development of new low-cost mapping algorithms as well as at the evaluation of the performance of sensors. The concept, already implemented in the CTTC's system ARAS (Autonomous Rover for Automatic Surveying) is generic and extensible. This means that it is possible to incorporate new navigation algorithms or sensors at no maintenance cost. Only the effort related to the development tasks required to either create such algorithms needs to be taken into account. As a consequence, change poses a much small problem for research activities in this specific area. This system includes several standalone sensors that may be combined in different ways to accomplish several goals; that is, this system may be used to perform a variety of tasks, as, for instance evaluates positioning algorithms performance or mapping algorithms performance.