CSAC Characterization and Its Impact on GNSS Clock Augmentation Performance

Abstract

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.

Operational Experience of GNSS Receivers With Chip Scale Atomic Clocks for Baseline Measurements

“…where ( , ) t t Dρ − τ are the increments of geometric distances between the receiver and the satellite; 1 r A is the rate of clock of GNSS receiver, which generates a random process such as "white noise" ( ) t ω (Enric, Calero, & Parés, 2017;Krawinkel & Schön, 2015); DΣ is the total increment of errors due to atmospheric delays, multipath effect, and measurement noise.…”

“…where c is the speed of light, c ≈ 3×10 8 m/s. The relative clock instability for quartz frequency oscillators with temperature compensation used in GNSS geodesic receivers with clock instability of 10 −9 (Enric et al, 2017;Krawinkel & Schön, 2015 ) that at surveillance sam- Figure 1. GSPG network and O. P. Suchkov Standard Spatial Basis (SSB) of the SSUGT ple rate Dt = 15 sec will lead to an error in the increment of the phase pseudoranges F δD = 4.5 m (linear measure).…”

“…GSPG network and O. P. Suchkov Standard Spatial Basis (SSB) of the SSUGT ple rate Dt = 15 sec will lead to an error in the increment of the phase pseudoranges F δD = 4.5 m (linear measure). For CSACs with relative clock instability of 10 −12 (Enric et al, 2017;Krawinkel & Schön, 2015), the corresponding error is reduced by three orders of magnitude, to a value equal to 4.5 mm, which is a prerequisite for increasing the accuracy of baseline measurements.…”

“…Other positive effects of CSACs in GNSS receivers include improvement of the navigation solution, increasing the availability of the solution, improving signal capture, reducing multipath effect, eliminating interference, and spoofing detection (Kitching, 2007;Shkel, 2011;Chan, Joerger, Khanafseh, Pervan, & Jakubov, 2014;Antonovich, Kosarev, & Lipatnikov, 2014;Beard & Senior, 2017;Enric et al, 2017;Krawinkel & Schön, 2015).…”

Low-Cost GNSS Software Receiver Performance Assessment

“…The estimated receiver clock offsets for static Precise Point Positioning (PPP) tests were around 32, 33, and 18 nanoseconds from GPST, GLONASST, and BDT, respectively. In [14], Fernandéz et al presented an analysis of the correlation between temperature and clock stability noise and the impact of its proper modeling on the holdover recovery time and on the positioning performance. They found that fine clock coasting modeling leads to an improvement in vertical positioning precision of around 50% with only three satellites; an increase in the navigation solution availability was also observed; a reduction of holdover recovery time from dozens of seconds to only a few could be achieved.…”