Search

GPS precise point positioning (PPP) technique has been widely used in time and frequency transfer. The performance of PPP can be improved if the phase ambiguities are fixed to the correct integer number. In this study, GPS PPP integer ambiguity resolution (PPP AR) with CNES/CLS products is applied in time and frequency transfer. To eliminate the day boundary discontinuities and achieve continuous time and frequency transfer with GPS PPP AR, a method of modified satellites carrier phase continuity is proposed when multi-day GPS PPP AR solutions are calculated. In addition, to guarantee the robustness of time transfer results, a three-segment factor is used to adjust the variance of integer ambiguities constraints. Test results in comparison of PPP AR with an optical time link show that standard deviation is deduced about 40% and the frequency stability is improved about 50% for 3.5d averaging than BIPM PPP method. Besides, GPS PPP AR shows a better short-term frequency stability than BIPM IPPP while they show similar STD value and long-term frequency stability. Moreover, test results with zero-baseline, short-baseline and long-baseline time transfer links show that the average frequency stability improvement rates are about 40% compared with PPP float solutions, and about 30% compared with IGS final products when the averaging time is 21.3 h. [ABSTRACT FROM AUTHOR]

Based on the polarization difference between the multipath and the line-of-sight (LOS) signal, a method for multipath detection using a single antenna is proposed. The antenna has two channels to receive two orthogonal linear polarized components of the multipath and LOS signal, respectively. A hypothetical model of the antenna is employed such that the antenna patterns of the channels are assumed identical regarding amplitude and phase and are independent of azimuth. The antenna gain in the direction below the local horizon is assumed to be larger than in the direction toward LOS signals. Parallel cross-cancellation is used to remove the LOS signal from the received signals based on the magnitude and phase difference between the two orthogonal components. Then the residual signals are processed by a conventional digital processor of global navigation satellite system. The multipath can be detected by parallel cross-cancellation in the receiver in real time. The proposed method makes use of the polarization and spatial information of the multipath and LOS signal, and can detect short-delay multipath. [ABSTRACT FROM AUTHOR]

A technique for finding a priori position with coarse-time assistance and partial measurements is proposed to improve time-free positioning algorithm. The positioning method can be described as follows. First, three satellites are selected by considering the balance of the geometric properties and search space in estimating the rough position with the altitude hypothesis. Additionally, the relationship between the maximum pseudorange difference and inter-satellite distance is analyzed and can be used to compress the search space. Second, the most reasonable position is chosen using the residual minimum root-sum-of-squares criterion. Finally, full pseudoranges are constructed and the user position can be estimated employing time-free positioning. Experimental results showed that, for sub-millisecond measurements, high success rates of 99.93, 99.75 and 99.58 % were achieved for time offsets of 1, 3 and 5 s, respectively, using the data file 'mars3650.14d.z' of the International GNSS Service, while the average number of search candidates was about 672. In addition, the success rate can still reach 100 % even if the time offset is hundreds of seconds with sub-20-ms measurements. [ABSTRACT FROM AUTHOR]

The successful launch of two new-generation satellites of the BeiDou Navigation Satellite System (BDS), named BeiDou M1-S and BeiDou M2-S, marks another step in expanding BeiDou into a navigation system with global coverage. An initial characterization and analysis of the navigation signals transmitted by BeiDou M2-S is presented. For data collection, a 7.3-m high-gain antenna of the National University of Defense Technology (NUDT) ground station in Changsha (China) has been used to record the signal spectrum and the modulated chip sequences. Based on a prototype receiver, code and phase observations of BeiDou M2-S have also been collected for further analysis. The signal quality and performance of this new-generation BDS satellite are described and analyzed; a backward compatibility with older-generation satellite signals is shown. [ABSTRACT FROM AUTHOR]

Due to the limitation of Geostationary Earth Orbit (GEO) satellite resources, only very few TWSTFT links with parallel counterpart operating via different GEO satellites between two ground stations are available. There are 5 GEO satellites in BeiDou Navigation System, which enables an improved TWSTFT with Multi-GEO. Two algorithms are put forward to take the advantage of Multi-GEO links to improve the performance of time and frequency transfer respectively. The dynamic weight average algorithm based on a newly proposed dynamic time variance aiming at improving the performance of time transfer is shown to be able to reduce the dynamic time deviation of the two-way measurement result to 1/ $\sqrt N $ dynamic time deviation of the single link using N-GEO links, if each link has similar level of link noises. The algorithm of cancelling out the link noise Allan variances with 2-GEO links is expected to obtain the Allan variance of clock difference between two ground stations. The experiment results using 2-GEO links in BeiDou Navigation System show that the dynamic weight average algorithm can improve the dynamic time deviation of the two-way measurement to around 0.6 of that of the single link in an average concept. And the estimated Allan deviation of the clock difference using the algorithm for cancelling the noise Allan variances of both links can reach 7×10( τ) for the averaging time 1-10 s which is within the same order of magnitude as the estimated Allan deviation based on carrier Doppler measurement for the averaging time 10 s-10 s, despite the noisy links with the Allan deviation follows 3 × 10( τ). [ABSTRACT FROM AUTHOR]

We consider linear second order singularly perturbed two-point boundary value problems with interior turning points. Piecewise linear Galerkin finite element methods are constructed on various piecewise equidistant meshes designed for such problems. These methods are proved to be convergent, uniformly in the singular perturbation parameter, in a weighted energy norm and the usual L norm. Supporting numerical results are presented. [ABSTRACT FROM AUTHOR]

This paper is concerned with a high order convergent discretization for the semilinear reaction-diffusion problem: $-\varepsilon^2 u'' + b(x,u) = 0$ , for $x\in(0,1)$ , subject to $u(0)=u(1)=0$ , where $\varepsilon\in(0,1]$ . We assume that $b_u(x, u)>b_0^2>0$ on $[0, 1] \times {\Bbb R}^1$ , which guarantees uniqueness of a solution to the problem. Asymptotic properties of this solution are discussed. We consider a polynomial-based three-point difference scheme on a simple piecewise equidistant mesh of Shishkin type. Existence and local uniqueness of a solution to the scheme are analysed. We prove that the scheme is almost fourth order accurate in the discrete maximum norm, uniformly in the perturbation parameter $\varepsilon$ . We present numerical results in support of this result. [ABSTRACT FROM AUTHOR]