Clock Offsets Research Articles

Spoofing GPS Receiver Clock Offset of Phasor Measurement Units

We demonstrate the feasibility of a spoofing attack on the GPS receiver of a phasor measurement unit (PMU). We formulate the attack as an optimization problem where the objective is to maximize the difference between the PMU's receiver clock offset (with respect to the GPS time measured by the onboard satellite clocks) before and after the attack. Since the PMU uses this clock offset to compute a synchronized time stamp for its measurements, an error in the receiver clock offset introduces a proportional phase error in the voltage or current phase measurements provided by the PMU. For the most general case, the decision variables in the optimization problem are the satellites' ephemerides, pseudoranges, and the receiver's Earth-Centered Earth-Fixed (ECEF) coordinates. The constraints are cast such that the decision variables and the satellite positions computed from the solution of the optimization problem are close to their pre-attack values, so as to avoid possible detection schemes that check for large jumps in the values of these variables. We show that the spoofing attack is feasible for any number of visible satellites. As an illustration of the impact of such spoofing attacks, we present simulation results in which the attack induces errors in a real-time voltage stability monitoring algorithm that relies on the phase information from measurements provided by PMUs.

Optimizing Wideband Cyclostationary Spectrum Sensing Under Receiver Impairments

In the context of Cognitive Radios (CRs), cyclostationary detection of primary users (PUs) is regarded as a common method for spectrum sensing. Cyclostationary detectors rely on the knowledge of the signal's symbol rate, carrier frequency, and modulation class in order to detect the present cyclic features. Cyclic frequency and sampling clock offsets are the two receiver impairments considered in this work. Cyclic frequency offsets, which occur due to oscillator frequency offsets, Doppler shifts, or imperfect knowledge of the cyclic frequencies, result in computing the test statistic at an offset from the true cyclic frequency. In this paper, we analyze the effect of cyclic frequency offsets on conventional cyclostationary detection, and propose a new multi-frame test statistic that reduces the degradation due to cyclic frequency offsets. Due to the multi-frame processing of the proposed statistic, non-coherent integration might occur across frames. Through an optimization framework developed in this work that can be performed offline, we determine the best frame length that maximizes the average detection performance of the proposed cyclostationary detection method given the statistical distributions of the receiver impairments. As a result of the optimization, the proposed detectors is shown to achieve the performance gains over conventional detectors given the constrained sensing time. We derive the proposed detector's theoretical average detection performance, and compare it to the performance of the conventional cyclostationary detector. Our analysis shows that gains in average detection performance using the proposed method can be achieved when the effect of sampling clock offset is less severe than that of the cyclic frequency offset. The analysis given in this paper can be used as a design guideline for practical implementation of cyclostationary spectrum sensors.

DVB-T Passive Radar Signal Processing

This paper provides a detailed overview of the Digital Video Broadcasting Terrestrial (DVB-T) signal structure and the implications for passive radar systems that use these signals as illuminators of opportunity. In particular, we analyze the ambiguity function and explain its delay and Doppler properties in terms of the underlying structure of the DVB-T signal. Of particular concern for radar range-Doppler processing are ambiguities consistent in range and Doppler with targets of interest. In this paper we adopt a mismatched filtering approach for range-Doppler processing. We also recognize that while the structure of the DVB-T signal introduces ambiguities, the structure can also be exploited to better estimate the transmitted signal and channel, as well as any mismatch between transmitter and receiver (e.g., clock offsets). This study presents a scheme for pre-processing both the reference and surveillance signals obtained by the passive radar to mitigate the effects of the ambiguities and the clutter in range-Doppler processing. The effectiveness of our proposed scheme in enhancing target detection is demonstrated using real-world data from an (Australian) 8k-mode DVB-T system. A 29 dB reduction in residual ambiguity levels over existing techniques is observed, and a 36 dB reduction over standard matched filtering; with only a 1 dB reduction in the zero-delay, zero-Doppler peak.

Robust Sampling Clock Recovery Algorithm for Wideband Networking Waveform of SDR

A novel technique for sampling clock recovery in a wideband networking waveform of a software defined radio is proposed. Sampling clock recovery is very important in wideband networking radio operation as it directly affects the Medium Access adaptive time slot switching rate. The proposed Sampling clock recovery algorithm consists of three stages. In the first stage, Sampling Clock Offset (SCO) is estimated at chip level. In the second stage, the SCO estimates are post-filtered to improve the tracking performance. We present a new post-filtering method namely Steady-State State-Space Recursive Least Squares with Adaptive Memory (S4RLSWAM). For the third stage of SCO compensation, a feedforward Lagrange interpolation based algorithm is proposed. Real-time hardware results have been presented to demonstrate the effectiveness of the proposed algorithms and architecture for systems requiring high data throughput. It is shown that both the proposed algorithms achieve better performance as compared to existing algorithms.

Estimation and Confidence Intervals for Clock Offset in Networks with Bivariate Exponential Delays

There is a large and increasing literature on statistical modeling-based estimation of the offset between two clocks. Recent work has focused on the construction of confidence intervals for offset. However, in most of this work it has been assumed that the network delays that occur during the synchronization process are independent. The network delays are often modeled as independent exponential random variables. Thus, we introduce the use of a bivariate exponential distribution to capture the anticipated correlation between the network delays and derive a maximum likelihood estimator and a confidence interval procedure for the offset parameter. We then illustrate how use of the independent model for network delays can lead to improper inference about the offset parameter.

Real-time clock offset prediction with an improved model

The GPS orbit precision of the IGS ultra-rapid predicted (IGU-P) products has been remarkably improved since 2007. However, the satellite clock offsets of the IGU-P products have not shown sufficient high-quality prediction to achieve sub-decimeter precision in real-time precise point positioning (RTPPP), being at the level of 1---3 ns (30---90 cm) RMS in recent years. An improved prediction model for satellite clocks is proposed in order to enhance the precision of predicted clock offsets. First, the proposed prediction model adds a few cyclic terms to absorb the periodic effects, and a time adaptive function is used to adjust the weight of the observation in the prediction model. Second, initial deviations of the predictions are reduced by using a recomputed constant term. The simulation results have shown that the proposed prediction model can give a better performance than the IGU-P clock products and can achieve precision better than 0.55 ns (16.5 cm) in real-time predictions. In addition, the RTPPP method was chosen to test the efficiency of the new model for real-time static and kinematic positioning. The numerical examples using the data set of 140 IGS stations show that the static RTPPP precision based on the proposed clock model has been improved about 22.8 and 41.5 % in the east and height components compared to the IGU-P clock products, while the precisions in the north components are the equal. The kinematic example using three IGS stations shows that the kinematic RTPPP precision based on the proposed clock model has improved about 30, 72 and 44 % in the east, north and height components.

클락 오프셋 추정 방식을 이용한 TWR WPAN 측위 시스템

Compared to OWR (One-Way Ranging) method that requires precise network time synchronization, TWR (Two-Way Ranging) method has advantages in building an indoor WPAN (Wireless Personal Area Network) location system with lower cost. However, clock offsets of nodes in WPAN system should be eliminated or compensated to improve location accuracy of the TWR method. Because conventional clock offset elimination methods requires multiple TWR transactions to reduce clock offset, they produce network traffic burden instead. This paper presents a clock offset estimation method that can reduce clock offset error with a single TWR transaction. After relative clock offsets of sensor nodes are estimated, clock offsets of mobile tags are estimated using a single TWR communication. Simulation results show that location accuracy of the proposed method is almost similar to the conventional clock offset elimination method, while its network traffic is about a half of the conventional method.

Improved GRACE kinematic orbit determination using GPS receiver clock modeling

Kinematic positions of Low Earth Orbiters based on GPS tracking are frequently used as pseudo-observations for single satellite gravity field determination. Unfortunately, the accuracy of the satellite trajectory is partly limited because the receiver synchronization error has to be estimated along with the kinematic coordinates at every observation epoch. We review the requirements for GPS receiver clock modeling in Precise Point Positioning (PPP) and analyze its impact on kinematic orbit determination for the two satellites of the Gravity Recovery and Climate Experiment (GRACE) mission using both simulated and real data. We demonstrate that a piecewise linear parameterization can be used to model the ultra-stable oscillators that drive the GPS receivers on board of the GRACE satellites. Using such a continuous clock model allows position estimation even if the number of usable GPS satellites drops to three and improves the robustness of the solution with respect to outliers. Furthermore, simulations indicate a potential accuracy improvement of the satellite trajectory of at least 40 % in the radial direction and up to 7 % in the along-track and cross-track directions when a 60-s piecewise linear clock model is estimated instead of epoch-wise independent receiver clock offsets. For PPP with real GRACE data, the accuracy evaluation is hampered by the lack of a reference orbit of significantly higher accuracy. However, comparisons with a smooth reduced-dynamic orbit indicate a significant reduction of the high-frequency noise in the radial component of the kinematic orbit.

Combined covariance reductions for Kalman filter composite clocks

The impact of combining two covariance matrix reduction methods on the resulting Kalman filter timescale estimates and covariances is addressed. The Brown and Greenhall reduction operations commute, and the combination of both reductions gives the same Kalman filter estimates as the Greenhall reduction alone. The benefit of combining both methods is that the Greenhall reduced covariances are further reduced in the positive semi-definite sense. A numerical example demonstrates the combined reduction by calculating the trace of the original and reduced covariance matrices. It is pointed out that the corrected clock offsets can follow the system time offset with different precisions, but the mutual disagreements of the corrected time offsets (called synchronization error) can be identical.

Autonomous Prediction of GPS and GLONASS Satellite Orbits

A method to predict satellite orbits in a stand-alone consumer-grade GNSS navigation device is presented. The motivation for this work was to reduce the startup time of a navigation device without the use of network assistance. The presented orbit prediction method works for both GPS and GLONASS, achieving median accuracies of 58 and 23 m in satellite position, respectively, for prediction up to four days ahead. A simple method for prediction of the satellite's clock offsets is also discussed. A basic analysis indicates that the method gives a line of sight range error of 15 m, for both GPS and GLONASS, with most of the error due to the clock offset prediction. Copyright © 2012 Institute of Navigation.

TerraSAR-X precise orbit determination with real-time GPS ephemerides

For active and future Earth observation missions, the availability of near real-time precise orbit information is becoming more and more important. The latency and quality of precise orbit determination results is mainly driven by the availability of precise GPS ephemerides and clocks. In order to have high-quality GPS ephemerides and clocks available at real-time, the German Space Operations Center (GSOC) has developed the real-time clock estimation system RETICLE. The system receives data streams with GNSS observations from the global tracking network of the International GNSS Service (IGS) in real-time. Using the known station position, RETICLE estimates precise GPS satellite clock offsets and drifts based on the most recent available ultra rapid predicted orbits provided by the IGS. The clock offset estimates have an accuracy of better than 0.3ns and are globally valid. The latency of the estimated clocks is approximately 7s after the observation epoch.Another limiting factor is the frequency of satellite downlinks and the latency of the data transfer from the ground station to the operations center. Therefore a near real-time scenario using GPS observation data from the TerraSAR-X mission is examined in which the satellite has about one ground station contact per orbit or respectively one contact in 90min. This test campaign shows that precise orbits can be obtained in near real-time. With the use of estimated clocks an orbit accuracy of better than 10cm (3D-RMS) can be obtained. The evaluation of satellite laser ranging (SLR) observations shows residuals of 2.1cm (RMS) for orbits using RECTICLE and residuals of 4.2cm (RMS) for orbits using the IGS ultra rapid ephemerides and clocks products. Hence the use of estimated clocks improves the orbit determination accuracy significantly (∼factor 2) compared to using predicted clocks.

Synchronized OFDM System Using Inter-Symbol Pilots

OFDM is a multicarrier modulation technique which has many advantages like spectral efficiency, robust to multipath fading effect etc. But it is very sensitive to Synchronization Error which increases the error rate and degrades the system performance. The cause for this error is Carrier Frequency Offset (CFO) and Sampling Clock Offset (SCO) which reduces the orthogonality and leads to Inter-Symbol Interference and Inter-Carrier Interference. For increasing the performance of the OFDM system Inter-Symbol Pilot aided synchronization method is introduced in this paper, this method is used to estimate both these offset. These estimated offsets are compensated using interpolation and Inverse Carrier Frequency Techniques.

Sample Clock Offset Detection and Correction in the LTE Downlink

The narrow subcarrier spacing and wide bandwidth arrangement in the LTE downlink produce a vulnerability to sample clock mismatch between the transmitting and receiving data converters. Without high precision sampling clock frequencies, a high level of inter-carrier interference (ICI) is introduced, yielding undesirable performance. In this article, a method to jointly estimate and correct sampling frequency mismatch is proposed. The proposed method uses information already known to the receiver, operates strictly in the time domain and does not require the aid of pilot symbols or other frequency domain information. The method allows clocks with lower precision to be used with minimal performance degradation. Results are presented using MATLAB simulation as well as an FPGA hardware implementation.

An Evaluation of the Beidou Time System (BDT)

The evaluation of Beidou system time is one of the important tasks in constructing and developing the Beidou satellite navigation system. The evaluation methods based on two-way satellite time and frequency transfer method (TWSTFT) and GPS common-view method are proposed in this paper. Tests and performance evaluation of the Beidou time system (BDT) were conducted. Results show that BDT is stable and reliable. The BDT frequency deviation, Allan deviations and clock offsets, relative to UTC (NTSC), are 10e-14, 10e-15 and less than 30 ns respectively.

Robust Time-Based Localization for Asynchronous Networks

Time-based localization approaches attract a lot of interest due to their high accuracy and potentially low cost for wireless sensor networks (WSNs). However, time-based localization is tightly coupled with clock synchronization. Thus, the reliability of timestamps in time-based localization becomes an important yet challenging task to deal with. In this paper, we propose robust time-based localization strategies to locate a target node with the help of anchors (nodes with known positions) in asynchronous networks. Two kinds of asynchronous networks are considered: one only with clock offsets, labeled quasi-synchronous networks, whereas the other with not only clock offsets but also clock skews, labeled fully asynchronous networks. A novel ranging protocol is developed for both networks, namely asymmetric trip ranging (ATR), to reduce the communication load and explore the broadcast property of WSNs. Regardless of the reliability of the timestamp report from the target node, closed-form least-squares (LS) estimators are derived to accurately estimate the target node position. As a result, we counter the uncertainties caused by the target node by ignoring the timestamps from this node. Furthermore, in order to simplify the estimator in fully asynchronous networks, localization and synchronization are decoupled. A simple yet efficient method is proposed to first Calibrate the Clock Skews of the anchors, and then Estimate the Node Position (CCS-ENP). Finally, Cramer-Rao bounds (CRBs) and simulation results corroborate the efficiency of our localization schemes.

BeiDou Navigation Satellite System and its time scales

The development and current status of BeiDou Navigation Satellite System are briefly introduced. The definition and realization of the system time scales are described in detail. The BeiDou system time (BDT) is an internal and continuous time scale without leap seconds. It is maintained by the time and frequency system of the master station. The frequency accuracy of BDT is superior to 2 × 10−14 and its stability is better than 6 × 10−15/30 days. The satellite synchronization is realized by a two-way time transfer between the uplink stations and the satellite. The measurement uncertainty of satellite clock offsets is less than 2 ns. The BeiDou System has three modes of time services: radio determination satellite service (RDSS) one-way, RDSS two-way and radio navigation satellite service (RNSS) one-way. The uncertainty of the one-way time service is designed to be less than 50 ns, and that of the two-way time service is less than 10 ns. Finally, some coordinate tactics of UTC from the viewpoint of global navigation satellite systems (GNSS) are discussed. It would be helpful to stop the leap second, from our viewpoint, but to keep the UTC name, the continuity and the coordinate function unchanged.

Black Burst Synchronization (BBS) – A protocol for deterministic tick and time synchronization in wireless networks

In this paper, we present Black Burst Synchronization ( BBS), a modular protocol for multi-hop tick and time synchronization in wireless ad hoc networks, located at MAC level. For the successful operation of BBS, it is crucial that collisions of synchronization messages that are sent (almost) simultaneously by two or more nodes are non-destructive. This is achieved by collision-protected bit encodings with black bursts, periods of transmission energy of defined length on the medium, starting at determined points in time. Under reasonable assumptions, BBS provides low and bounded tick and clock offsets, guarantees a very small and constant convergence delay, has low and bounded complexity regarding computation, storage, time, and structure, and is robust against topology changes at runtime. This makes it a candidate for user level applications such as data fusion and networked control systems, and especially for system level tasks such as duty cycling and multi-hop medium slotting. To validate its predicted behavior, we have implemented and deployed BBS on MICAz motes.

SU-E-I-176: Impact of Residual Activity and Clock Synchronization on Quantitative Treatment Response Assessment

Purpose: The accuracy of quantitative PET is influenced by many factors whose uncertainties need to be characterized for its reliable use in treatment response assessment. Among the factors are residual activity and lack of synchronization between PET‐scanner and dose‐calibrator clocks which are frequently overlooked. Therefore, we characterized their effects on PET‐ based treatment response assessment. Methods: Fifteen patients receiving targeted molecular therapy underwent whole body [18F]FLT PET/CT scans at multiple time points. Response was calculated as the change in SUV between subsequent scans. Residual‐activity left in syringe after injection effects were assessed by comparing response measures when SUV values were corrected for residual activity (range: 0.07mCi‐0.61mCi) to when they were not. Uncertainty in response was also compared between scans where the synchronization of the PET‐scanner and dose‐calibrator clocks were randomly offset (range: 1min‐60mins) to when they were synchronized. The effect of both factors on PERCIST classification of treatment response was assessed. Results: Residual‐activity and lack of clock synchronization had significant effects on treatment response (p=0.05). 3% change in clock offsets between subsequent scans resulted in 2.5% absolute difference in response. The mean difference in responses due to desynchronized clocks was 18% (maximum=35%). 3% change in residual activity resulted in 0.1% absolute difference in response. Not accounting for residual‐activity caused mean difference of 1% (maximum=6%) in the response. Changes in clock synchronization and not accounting for residual activity resulted in reclassification of the PERCIST response status of over 30% of the patients. Conclusions: Lack of synchronization between PET‐scanner and dose‐ calibrator clocks and not accounting for residual activity caused a difference in response. Administration procedures that minimize residual activity (e.g. flushing syringe with saline) have to be implemented. Also, all clocks used with dose‐calibrator have to be synchronized with PET‐scanner clock periodically. Otherwise patients can be wrongly classified as partial, stable, or progressive disease.

Fundamental Limits on Synchronizing Clocks Over Networks

We characterize what is feasible concerning clock synchronization in wireline or wireless networks. We consider a net work of n nodes, equipped with affine clocks relative to a designated clock that exchange packets subject to link delays. Determining all unknown parameters, i.e., skews and offsets of all the clocks as well as the delays of all the communication links, is impossible. All nodal skews, as well as all round-trip delays between every pair of nodes, can be determined correctly. Also, every transmitting node can predict precisely the time indicated by the receiver's clock at which it receives the packet. However, the vector of unknown link delays and clock offsets can only be determined up to an (n - 1)-dimensional subspace, with each degree of freedom corresponding to the offset of one of the (n - 1) clocks. Invoking causality, that packets cannot be received before they are transmitted, the uncertainty set can be reduced to a polyhedron. We also investigate structured models for link delays as the sum of a transmitter-dependent delay, a receiver-dependent delay, and a known propagation delay, and identify conditions which permit a unique solution, and conditions under which the number of the residual degrees of freedom is independent of the network size. For receiver-receiver synchronization, where only receipt times are available, but no time-stamping is done by the sender, all nodal skews can still be determined, but delay differences between neighboring communication links with a common sender can only be characterized up to an affine transformation of the (n - 1) un known offsets. Moreover, causality does not help reduce the uncertainty set.

Optical OFDM Synchronization With Symbol Timing Offset and Sampling Clock Offset Compensation in Real-Time IMDD Systems

End-to-end real-time Optical Orthogonal Frequency Division Multiplexing (OOFDM) signal synchronization that is capable of simultaneously compensating for both Symbol Timing Offset (STO) and Sampling Clock Offset (SCO) is experimentally demonstrated, for the first time, over 64-quadrature amplitude modulated (QAM)-encoded, 11.25-Gb/s, 25-km, Intensity-Modulation and Direct-Detection (IMDD) single mode fiber (SMF) systems employing directly modulated distributed feedback lasers. In comparison with manual synchronization, almost-perfect compensations of arbitrary STOs are achieved. The OOFDM synchronization technique can also compensate for the SCO effect with an accuracy of <; 1 ppm for initial SCOs as large as 4000 ppm. The technique has a number of salient advantages including low complexity, fast tracking speed, high accuracy, and suitability for high-speed optical transmission systems.