1PPS Signal Research Articles

ARCHITECTURAL AND CIRCUIT MODEL OF NAVIGATION DEVICE FOR ULTRA-LIGHT LAUNCH VEHICLE

The paper considered the architectural and circuitry model of a navigation device for an ultra-lightweight launch vehicle designed to improve the accuracy and reliability of positioning and orientation in flight conditions. The study utilized modern approaches to the integration of global navigation satellite system (GNSS) and inertial navigation system (INS) to achieve high accuracy and stability of the system. The key components of the device including UM980 module, LSM6DSV32X inertial measurement module, STM32 F7 series microcontroller, E220-400T30S radio module, LMR50410XDBVR converter based power supply and RS422 interface were described in detail. The UM980 module, including SAW and LNA filter blocks, demonstrated the ability to efficiently process incoming RF signals. The NebulasIV main function block combined GNSS BB, GNSS RF, interfaces and power management unit (PMU) modules, supporting multiple communication interfaces such as UART, SPI and I²C, as well as RTK and PPS signals. The study analyzed the integration and accuracy aspects of the module in navigation systems. The signal processing scheme of the inertial navigation system module was also presented, including digital filters, free-fall and activity/inactivity detection functions, and interfaces for data communication. User bias functions and filter modes are considered to ensure the accuracy and stability of the navigation device. The work lays the groundwork for further improvements in navigation technology, especially in the context of increasing availability and accuracy for small scale space launches.

Bi-directional PRN laser ranging and clock synchronization for TianQin mission

Laser frequency coupling noise due to unbalanced arm lengths is the dominant noise source for space-based gravitational wave detection. Time-delay interferometry is a proposed technique to eliminate the coupling noise by constructing a virtual equal-arm-length interferometer, where the real-time information of both absolute inter-satellite distance and clock synchronization is required. Inter-satellite laser ranging based on pseudo-random noise (PRN) sequence modulation has become the main technical means for extraction of the inter-satellite distance and clock difference information. This paper shows a bi-directional laser ranging scheme based on PRN code modulation and experimentally demonstrates its function by using an optical fiber of 10 km long. The measured results of the transmission time duration and the clock difference are compared with independent methods of pulse laser ranging and 1PPS signal measurement, respectively, both of which coincide well. It is also evaluated that the laser ranging accuracy is 1.2 m, and the clock synchronization accuracy reaches 6.7 ns, applicable to TianQin with the preliminarily determined requirement of 3 m and 10 ns. This laser ranging scheme can be applied to satellite navigation and space-based gravitational wave detection.

FPGA-Based Autonomous GPS-Disciplined Oscillatorsfor Wireless Sensor Network Nodes

Numerous devices in distributed wireless sensor arrays require a high-accuracy timing reference. Although the GPS-disciplined oscillators have been developed for decades, the hardware design still has performance limitations. In this context, we present the hardware implementation for a GPS-disciplined oscillator with an automatic adaptive drift correction algorithm, which is implemented in a low-cost, high-speed field-programmable gate array (FPGA) device. The system design and the hardware implementation are presented to demonstrate the advantages of the proposed oscillator. To verify this oscillator in real-time applications, we tested the device in multiple environments and compared it to state-of-the-art designs. The experimental results showed that our proposed device has a low cost and high performance. This device can achieve less than 80 ns and 356 ns in 1PPS signal drift in the indoor environment test and the outdoor environment test, respectively, after 24 h of working without a GPS signal.

A Method of Low-Cost Pure-Digital GPS Disciplined Clock for Improving Frequency Accuracy and Steering Frequency Through TAF-DPS Frequency Synthesizer

One of the key characteristics of modern devices is time-awareness, which demands frequency source of good accuracy, precision, and stability. Unfortunately, all high-quality sources are expensive and bulky. An alternative is to add a companion clocking circuit to the primary source for performing frequency compensation in real time, under the guidance of a good time or frequency reference. The companion circuit aims at counteracting the primary source's systematic error and environment-induced variations. In this work, a frequency synthesizer having the features of small frequency granularity and fast frequency switching is adopted as such companion circuit. It functions as a tuning tool for periodically adjusting device's output frequency under the supervision of 1 PPS signal locked to global positioning system (GPS). This clock disciplining method can also be used for steering output frequency away from the specified value and toward a user-defined target. The main focus of this method is on improving frequency accuracy. The key advantage of the proposed method is low-cost and high-accessibility. It provides a new perspective, or devises a new instrument, for dealing with the sophisticated task of real-time frequency compensation. Our primary goal is to significantly simplify the engineering effort of this task without sacrificing the quality of clock signal noticeably. An field-programmable gate array (FPGA)-based system is created to demonstrate its capability of frequency compensation and steering. Further, 20 such systems are used to study the precision of a multi-node organization, as an example of using this instrument as a syntonization tool. The handy implementation on FPGA illustrates the advantage of DIY (do it yourself), making this method conveniently applicable for a variety of user scenarios.

Fiber-optic joint time and frequency transfer with the same wavelength

Optical fiber links have proven their powerful ability to transfer the ultrastable clock signals over a national and continental metrological optical network. In this Letter, we propose and demonstrate a new scheme that time and frequency signals are modulated on the same laser wavelength with phase and intensity modulation by an independent electro-optic modulator, respectively. Different wavelengths are used in two opposite directions to prevent Rayleigh scattering. The noise changes caused by the fluctuations of the transfer links are actively canceled at the local site by optical delay lines. Radio frequency with 1 GHz and time signal with one pulse per second (1 PPS) are transmitted over 110 km fiber spools. The experimental results demonstrate that frequency instabilities of 1.7 E − 14 at 1 s and 5.9 E − 17 at 10 4 s and time interval transfer of 1 PPS signal with sub-ps stability after 1000 s are obtained. This novel scheme offers advantages with respect to reduce the channel in the fiber network, and can keep the time and frequency signal free from interference with each other.

Integrated time–frequency synchronisation method for cooperative bistatic radar

Bistatic radar system has some advantages over conventional monostatic radar system. However, time and frequency synchronisation is a crucial issue for bistatic radar. This study proposes an integrated time–frequency synchronisation method for cooperative bistatic radar based on GPS disciplined frequency standard. The method uses a high-precision GPS-disciplined frequency source as the frequency reference of the transmitter and receiver, and the frequency synchronisation is realised by full-coherent frequency synthesis. Using GPS time information and PPS signal to generate the PRT trigger signal, and using the local oscillator frequency source to maintain the PRT trigger signal and triggering again within a fixed time, time synchronisation is realised. Through the experiments, the effectiveness of the time-frequency synchronisation method is verified, and it can well meet the imaging requirements of bistatic radar.

Time Synchronization for Wireless Sensors Using Low-Cost GPS Module and Arduino

Time synchronisation for wireless sensors is important for a proper interpretation of measurements, particularly for acceleration measurements to estimate mode-shapes correctly. This paper presents a new time synchronisation method working independently on each node without exchanging time-sync packets among nodes. This stand-along operation can make field measurement campaigns very time-efficient without a need of constructing and validating the wireless sensor network. The proposed method firstly time-stamps measurements using the accurate time-source from a GPS module on each node, and secondly re-samples the time-stamped data to get time-synchronised data. The time-stamping method proposed in the study utilises Pulse-Per-Second (PPS) signals and NMEA (National Marine Electronics Association) sentences generated by a low-cost GPS module, and the internal timer/counter unit of Arduino. Error analysis on the proposed time-stamping method was carried out and derived an analytical expression for the maximum variance of time-stamping error of the proposed method. Four experiments have been carried out to observe 1) the long-term operational stability of the GPS module, 2) the accuracy of the PPS signals, 3) the accuracy of the proposed time-stamping method, and 4) the validity of the proposed time-synchronisation method for output-only modal analysis on a laboratory floor structure. The GPS module was found to operate or to resume operating stably for the entire test period of seven days even with the limited field of view to the sky. The relative time errors of two PPS signals from four GPS modules were found to be within +/-400 nsec. The time-stamping error measured by two identical time-stamping Arduinos for common trigger signals was found to have a standard deviation of 40.8 nsec, which agreed well with the maximum value of 42.0 nsec predicted by the error analysis. From the output-only modal analysis, the estimated modal parameters were found to agree well with that from the wired acceleration sensors. The phase angle of the cross spectral density of the two wireless accelerations showed that there was no apparent time-synchronisation error observable. These observations indicated a successful operation of the proposed time-synchronisation method.

IEEE 1588 time synchronisation performance for IEC 61850 transmission substations

Accurate timestamping with 1 µs precision is required by power system monitoring and control equipment, including: Phase Measurement Units (PMUs) for real-time measurement and control; travelling wave detectors for fault location, protection and control; and sample value measurement techniques associated with the IEC 61850 process bus. The conventional way of achieving this is to transmit 1-PPS or IRIG-B timing signals using dedicated cables but such an approach is expensive and difficult to update as secondary control systems evolve. IEEE 1588 precision time protocol (PTP) allows the transmission of time information with sub-microsecond accuracy in an Ethernet network. However, a comprehensive study is required to enhance the confidence of the power industry in using PTP in present and future IEC 61850 based substations. This paper delivers a design and experimental validation of the real-time time synchronisation characteristics achievable with IEEE 1588. Several factors which can potentially affect the synchronisation accuracy in delivering a time reference from the master clocks to the end devices during both steady-state and transient changes were investigated. The objective is to increase the understanding of the real-time performance of a complete IEEE 1588 timing system applied in transmission substations, and identify the limitations of commercially-available devices.

Research on Rubidium Atomic Clock Disciplined Method Based on LabWindows/CVI

With the increasing demand for stable, accurate and reliable frequency standard sources in the field of measurement and control, rubidium atomic clocks with strong environmental adaptability and low price are widely used but are limited by long-term frequency drift. In this paper, 1PPS signal as the reference signal provided by the National Time Service Center, and a rubidium atomic clock frequency automatic calibration system was designed and implemented. Experimental results show that, the clock difference is less than 8ns between the rubidium atomic clock and the master clock of the NTSC, the Modified Allan Deviation is 1.83E-12@100s, and 2.70E-13@10000s. The experiment proves that the self-calibrating frequency system gives rubidium atomic clock a good accuracy and long-term stability.

GNSS Time Synchronization in Vehicular Ad-Hoc Networks: Benefits and Feasibility

Time synchronization is critical for the operation of distributed systems in networked environments. It is also demanded in vehicular ad-hoc networks (VANETs), which, as a special type of wireless networks, are becoming increasingly important for emerging cooperative intelligent transport systems. Global navigation satellite system (GNSS) is a proven technology to provide precise timing information in many distributed systems. It is well recognized to be the primary means for vehicle positioning and velocity determination in VANETs. However, GNSS-based time synchronization is not well understood for its role in the coordination of various tasks in VANETs. To address this issue, this paper examines the requirements, potential benefits, and feasibility of GNSS time synchronization in VANETs. The availability of GNSS time synchronization is characterized by almost 100% in our experiments in high-rise urban streets, where the availability of GNSS positioning solutions is only 80%. Experiments are also conducted to test the accuracy of time synchronization with 1-PPS signals output from consumer grade GNSS receivers. They have shown 30-ns synchronization accuracy between two receivers of different models. All these experimental results demonstrate the feasibility of GNSS time synchronization for stringent VANET applications.

A Modem with a Fiber-Optic Communication Line for Dissemination of Frequency and Time Reference Signals

The features of a multichannel modem that has been developed for transmission of frequency and time reference signals on a fiber-optic communication line with phase instability compensation are examined. Its capabilities for transmitting signals of reference frequencies of active hydrogen masers (such as the Ch1-1003M), we well as a 1 PPS signal with an error of synchronization no greater than 250 psec over 100 km without the use of optical amplifiers are assessed. The results of experimental verification of the metrological characteristics of the modem are given.

Fiber-Optic UTC(k) Timescale Distribution With Automated Link Delay Cancelation.

In this paper, we describe the idea and practical realization of an automated calibrator of fiber-optic UTC(k) distribution system, along with a high-resolution shifter of 1 PPS signal, which allows us to cancel the propagation delay and, thus, to produce the 1 PPS at the remote system output with practically zero offset. The solution was experimentally verified with 10 different optical paths, up to 300 km long. The rms offset of the output timescale in our experiments was 7.1 ps, and maximum absolute value did not exceed 15 ps.

Method and system for disciplining a local reference oscillator by GPS 1PPS signal

Method and system for disciplining a local reference oscillator by GPS 1PPS signal

Low cost GNSS receivers in time synchronization systems

The GNSS (GNSS — Global Navigation Satellite Systems) receivers can be utilized to obtain accurate time markers. The preliminary results of the cheap GNSS receivers’ tests are presented in the paper. The one receiver’s price (including antenna) does not exceed 30 $. The studies on the use of receivers in the time synchronization systems were executed. Three identical models of receiver modules were used. The 1PPS (1PPS — 1 Pulse Per Second) signals available on the receiver’s output were used. The 1PPS’s main time characteristics were described. Delay times between different receivers 1PPS signals were measured. Measurements were taken using 1 GHz oscilloscope and precise time/frequency counter T3200U. Keywords: time synchronization, 1PPS, GNSS, GPS time

Application of VCSELs in next-generation telescope array networks such as the Square Kilometre array

The distribution of precise and accurate optical clock signals across fibre links has a significant impact in various fields. Many applications require the transfer of stable and precise timing signals from a single central location to multiple remote users via a fibre network. One such application is the big data instrument, the Square Kilometre Array (SKA). An increase in demand for more bandwidth by broadband providers has prompted the expansion and development of attractive and intelligent, modern communication networks. Vertical cavity surface-emitting lasers (VCSELs) are an attractive solution and a potential candidate for realizing low cost and high bandwidth data transmission. In this paper a novel and unique, all-optical technique for measuring the one-way transmission time delay of the propagating signal along an optical fibre is presented. This has been successfully achieved by optically injecting a pulse-per-second (PPS) signal into the secondary mode of a 1550 nm VCSEL located at the client end. A round-trip latency time of 113.2 μs was experimentally measured over a 22 km G.652 single mode fibre (SMF). A novel VCSEL capacity upgrade technique is further demonstrate. This was experimentally achieved by simultaneously modulating a single 1310 nm VCSEL with a 10 Gbps 27 − 1 pseudorandom binary sequence (PRBS) and a polarization based PPS clock signal. An error free transmission over 11 km of G.652 SMF was reported, with a measured transmission penalty of 0.52 dB when the VCSEL was simultaneously modulated with the 27 − 1 PRBS and the polarization based PPS signal.

Time and Frequency Synchronization on the Virac Radio Telescope RT-32

Abstract One of the main research directions of Ventspils International Radio Astronomy Centre (VIRAC) is radio astronomy and astrophysics. The instrumental base for the centre comprised two fully steerable parabolic antennas, RT-16 and RT-32 (i.e. with the mirror diameter of 16 m and 32 m). After long reconstruction, radio telescope RT-32 is currently equipped with the receiving and data acquisition systems that allow observing in a wide frequency range from 327 MHz to 9 GHz. New Antenna Control Unit (ACU) allows stable, fast and precise pointing of antenna. Time and frequency distribution service provide 5, 10 and 100 MHz reference frequency, 1PPS signals and precise time stamps by NTP protocol and in the IRIG-B format by coaxial cable. For the radio astronomical observations, main requirement of spatially Very Long Base Line Interferometric (VLBI) observations for the observatory is precise synchronization of the received and sampled data and linking to the exact time stamps. During October 2015, radio telescope RT-32 performance was tested in several successful VLBI experiments. The obtained results confirm the efficiency of the chosen methods of synchronization and the ability to reproduce them on similar antennas.

Effective PPS Signal Generation with Predictive Synchronous Loop for GPS

Effective PPS Signal Generation with Predictive Synchronous Loop for GPS

Partial discharge location in power cables using a double ended method based on time triggering with GPS

Partial discharge (PD) diagnostics is the most widely used tool to assess the insulation condition of insulated power cables which facilitates informed maintenance planning leading to extended service life of ageing assets. Time domain reflectometry (TDR) using a single ended or double ended approach is the most widely used method for locating PD sources. The success of the single ended method is dependent upon cable network design. However, by monitoring PDs at both ends of the cable, i.e. double-ended PD monitoring, higher accuracy of PD location can be achieved with a higher success rate. The double ended method is not widely used due to its complex system design, time synchronization and communication requirement between measurement units. This paper proposes a double ended PD location system which triggers on the predefined one pulse per second (1PPS) signal obtained from a global positioning systems (GPS) using novel time based triggering logic (TBTL) implemented in field programmable gate arrays (FPGA). This system ignores non-useable (not synchronized) data caused by flywheel 1PPS from GPS receiver due to any short-term loss of satellite signals which eventually reduces the PD location accuracy. Furthermore TBTL also ignores spurious triggering pulses radiated from noise sources within the substation. With the use of a communication link between two ends of the cable provided via mobile broadband together with TBTL, eliminated the acquisition of non-useable(not synchronized) data. Based on laboratory tests and on-site measurements PD location accuracy of less than ± 10 m can be achieved. The system design, laboratory tests and on-site measurements are discussed.

Precision timing in ocean sensor systems

This paper discusses the use of the IEEE 1588 standard in ocean observatories. The performance result of prototype implementations of this standard in an Ethernet Marine Sensor Network (MSN) is presented. The performance tests emulate an underwater-cabled observatory with a Master Clock synchronized with GPS, located in an on-shore station, and with underwater instruments requiring high-precision PPS (pulse s−1) signals for synchronization purposes. These signals will be provided to the underwater station by an IEEE 1588 GPS Emulator connected to the observatory's Local Area Network (LAN). The experimental setup emulates the underwater-cabled observatory OBSEA where this technology will be installed due to synchronization requirements of marine instruments such as ocean bottom seismometers.

PHASE NOISE SUPPRESSION IN GPS-DISCIPLINED FREQUENCY SYNCHRONIZATION SYSTEMS

Global positioning system (GPS)-disciplined quartz oscillator can provide a precise time and frequency standard. However, possible phase errors and biases caused by oscillator phase noise must be suppressed or compensated for some applications, e.g., distributed radar systems, in which a rigorous phase or frequency synchronization precision is required. Moreover, the quartz oscillator is necessary to provide a holdover of time or frequency when the GPS signals are lost. This letter presents a signal processing and hardware design strategy for suppressing phase noise in GPS-disciplined frequency synchronization systems. Firstly, the quartz oscillator frequency is disciplined by the GPS pulse-per-second (PPS) signals. Next, a recursive algorithm is applied to suppress the possible jitters when the GPS PPS signals are lost. Finally, a direct digital synthesizer (DDS)-based multiple frequency tuning synthesizer is presented to generate the whole system frequencies. Although the use of GPS signal to discipline oscillator for frequency transfer is not a new concept, the originality of this letter lies in the proposed phase noise suppression methods, along with the hardware design strategies.