GPS frequency transfer using carrier phase measurements

Combining GPS and TWSTFT considerably increases the robustness of the accurate time transfer result due to the complete independence between these two spatial techniques. In this paper we present the last improvements of a time and frequency transfer method based on a least-square analysis of GPS code and carrier phase measurements in common view, constrained by TWSTFT data. The combined method developed here provides a time transfer solution which benefits from the high short-term stability and high resolution of the GPS data and the high accuracy of the TWSTFT data. It allows to solve the large day boundary discontinuities that exist in some clock solutions obtained with geodetic time transfer, but some discontinuities appear occasionally in the CV+TW solution, due to the noise and diurnal perturbation in the TWSTFT data.

In this contribution, a novel opto-acoustic indoor navigation system for linear distance measurement is presented. The novelty consists in the simultaneous use of envelope time-of-flight (ETF) and carrier phase (CPH) measurements. In particular, we establish a two-channel link consisting of an ultrasound and infrared channel, the latter using a carrier frequency equal to the ultrasound frequency. In order to render the system extendible for multiple transmitters and receivers, code-division-multiple-access using Gold codes is employed, as all ultrasound and infrared signals share the same carrier frequency. This is also beneficial for multipath scattering suppression. The time difference between the carrier phase of the received ultrasound and infrared signals offers a high resolution but ambiguous distance signal. On the other hand, correlation of the received signals yields a coarse but unique distance signal. Experimental results show that a measurement resolution in the submillimeter range is feasible.

GAGAN (GPS - Aided Geo Augmented Navigation) is an Indian SBAS (Satellite Based Augmentation Systems). In contrast to the European EGNOS, this system already supports the navigation function and it transmits signals both in L1 and L5 frequency channels. We used the GAGAN signals for an experimental common-view time transfer between IPE Prague and PTB Braunschweig which is a distance of 370 km and we also tested the time transfer properties using a single clock common-view. The L1 and ionosphere-free code measurements have markedly lower accuracy compared to a GPS common-view because of rather narrow bandwidth of the SBAS signal in L1 frequency channel. L5 code measurement provides much better precision. It results from the single clock common-view that the observed fluctuations can be described as white noise with standard deviations of 6 ns, 1.3 ns and 14 ns for L1, L5 and ionosphere-free combination. The results obtained from the carrier phase measurements are promising. The single clock common-view precision was approximately 30 ps RMS even for the ionosphere-free combination. Interesting results followed from the comparison at the long distance where relatively fast fluctuations induced by ionosphere are obvious in the single frequency measurements. We believe that using GAGAN and other SBASs for permanent carrier phase frequency transfer could be an ideal solution for continuous comparisons of precise frequency sources.

This paper proposed heave estimation using time-differenced GPS carrier phase measurement and compensated barometer measurement with applying error model. The carrier phase measurement makes a precise of the altitude more than code measurement because carrier phase based pseudo range has mm error. But integer ambiguity resolution problem must be solved when carrier phase measurement is used. Time-Differenced(TD) carrier phase measurements can be used to find a relative altitude using single GPS without an integer ambiguity resolution since a time-invariant. Integer ambiguity is removed by TD operation. Barometer error model estimated using Gauss-Morkov process. Output is result of kalman filter using barometer and carrier phase measurement. Performance of implemented system was evaluated through the experiments at various scenarios including static test in rural and urban areas.

Carrier phase measurements are much more precise than pseudorange measurements and can be used to achieve very accurate positioning solutions. However, carrier phase measurements require resolution of integer ambiguities before precise positioning can be achieved. The GPS receiver can keep track of the integer number of cycles as long as the receiver maintains lock to the satellite signal. However, in reality, the GPS signal could be interrupted momentary by some disturbing factors leading to a discontinuity of an integer number of cycles in the measured carrier phase. This interruption in the counting of cycles in the carrier phase measurements is known as a cycle slip. When a cycle slip occurs, the Doppler counter would restart causing a jump in the instantaneous accumulated phase by an integer number of cycles. Thus, the integer counter is reinitialized meaning that ambiguities are unknown again. In this event, either the ambiguities need to be resolved again or cycle slips need to be corrected to resume the precise positioning/navigation process. These cycle slips can, to some extent, be detected and fixed to avoid delay and computation complexity attributed to integer ambiguity resolution. Researchers have been addressing the problem of cycle slip detection and correction for the last two decades. This paper provides a detailed survey for available techniques to tackle the problem showing their pros and cons.

A new TWSTFT modem was developed for both time and frequency transfer. The code and carrier phase measurements become available by it. We evaluated the performance in a common-clock and non common-clock measurements. The results encourage us to widely spread the use of the new modem.

The model of BDS/GPS carrier phase time and frequency transfer is given in this paper, and the transformation method of time and coordinate reference are researched. Adopting precise satellite orbits and clocks provided by GFZ, data from MGEX are processed to analysis the performance of BDS/GPS carrier phase time and frequency transfer. The results show that the precision of BDS/GPS carrier phase time and frequency transfer is not better than the single system obviously when the visible satellites are more than 7. BDS convergence time is significantly longer than GPS and BDS/GPS, due to fewer MEO satellites. The influence for single system time and frequency transfer is more obvious than BDS/GPS under complex environment. When the elevation mask angle is over 30°, BDS have better performance than GPS due to BDS’s special constellation in asian-pacific region, and GPS time transfer results are nonsequence. When the elevation mask angle is over 40°, BDS time transfer is also nonsequence at sometime, and GPS cannot be used for time transfer, but BDS/GPS is still get the high precision results; the convergence speed and stability are better than the single system.

In this study, the carrier acceleration-ramp-step test was applied to GPS carrier phase measurements, and the results were compared and analyzed. In the carrier acceleration-ramp-step test, the acceleration, ramp, and measurements are estimated using 10 consecutive carrier phase measurements for satellites observed at the same time based on the least square method. As for the characteristic of this test, if failure occurs in the measurement, the value jumps significantly compared to the previous result; but it judges that failure has occurred in all the satellites although failure has occurred in one satellite. Therefore, in this study, a method that eliminates a satellite with failure was suggested, and thresholds of the carrier acceleration, ramp, and step were suggested. The evaluation of the failure detection performance of carrier phase measurement using the suggested thresholds showed that failure could be detected when the carrier phase measurement changed abruptly by more than about 0.1 cycles.

In this paper we show that the judicious use of all the information available from C/A code can provide results better than the reported accuracy of P code with much lower cost. This performance however, is limited to low-dynamic, low-jamming environments which are typical of civilian applications. The elegance of P-code is primarily manifested in high dynamic vehicles (jets and missiles) and in jamming environments. Results are provided in stationary and low-dynamic cases based on code-phase, carrier-phase, doppler, and integrated doppler measurements from TRIMBLE 4000 receivers. In particular, it is shown that the relative or differential accuracy of better than 10 centimeters can be achieved in low-dynamic applications. Position fixes are obtained every second. after a short time of initial calibration and tracking. The improved accuracy is based on aiding the code phase measurements with integrated doppler. Each one second position fix. which is provided in both polar and cartisian coordinates is based on a complete and independent set of measured data, with no hypothetical modeling, interpolation, or extrapolation. INDEX TERMS: GLOBAL POSITIONING SYSTEM, GPS DOPPLER PROCESSING. TRIMBLE 4000A LOCATOR. INTRODUCTION Even a brief survey of the growing body of GPS literature, leaves little doubt that GPS will indeed replace all existing positioning and timing systems in the near future. The attractiveness of GPS is not only due to its technical superiority, but also to economic considerations. It is interesting to observe that, even in this early stage of the GPS program, production of low cost GPS receivers is a reality. Today C/A code GPS receivers can be manufactured, in large volumes, at a lower cost than LORAN receivers [1]. Of the two classes of GPS signals, C/A and P code, only C/A code will be available for civilian applications [2]. This restriction might lead one to conclude that the precision positioning and timing capabilities of GPS will not be available to civilian users. This is a false conclusion. Judicious use of all the information available from C/A code can provide results equivalent to P code, with much lower cost. In this paper we prove this claim by providing results based on code phase, doppler, integrated doppler and carrier phase measurements from TRIMBLE C/A code receivers. It is our belief that even if P code was available for civilian applications it adds very little, if any, to the precision that is available from C/A code. The elegance of P code is mostly manifested in military applications such as in high dynamic vehicles (jets and missiles) and in jamming environments. In this paper we distinguish between four levels of accuracy that can be obtained from the C/A code system:LEVEL A: Accuracy of about 10 meters,LEVEL B: Accuracy of about 1 meter,LEVEL C: Accuracy of about 10 centimeters,LEVEL D: Accuracy of about 1 centimeter, Each one of these accuracy levels can be achieved with a different type of C/A code information processing, and with a different operational requirement.

Meteor burst communication equipment has been successfully utilized in Russia for two-way time comparisons between standards of several time-keeping services. For its operation, the equipment depends on carrier phase measurements at several frequencies. Time transfer with accuracy better than 1 ns for a distance up to 2000 km is possible via this self-contained and low-cost equipment. To test the equipment and to estimate the time transfer performance, a series of time transfer experiments has been made on several meteor links in 1991-1993. >

Carrier phase-based ionospheric observables using PPP models

Current cooperative positioning with global navigation satellite system (GNSS) for connected vehicle application mainly uses pseudorange measurements. However, the positioning accuracy offered cannot meet the requirements for lane-level positioning, collision avoidance and future automatic driving, which needs real-time positioning accuracy of better than 0.5 m. Furthermore, there is an apparent lack of research into the integrity issue for these new applications under emerging driverless vehicle applications. In order to overcome those problems, a new extended Kalman filter (EKF) and a multi-failure diagnosis algorithm are developed to process both GNSS pseudorange and carrier phase measurements. We first introduce a new closed-loop EKF with partial ambiguity resolution as feedback to address the low accuracy issue. Then a multi-failure diagnosis algorithm is proposed to improve integrity and reliability. The core of this new algorithm includes using Carrier phase-based Receiver Autonomous Integrity Monitoring method for failure detection, and the double extended w test detectors to identify failure. A cooperative positioning experiment was carried out to validate the proposed method. The results show that the proposed closed-loop EKF can provide highly accurate positioning, and the multi-failure diagnosis method is effective in detecting and identifying failures for both code and carrier phase measurements.

The precise point positioning (PPP)-based real-time-kinematic (RTK) method attracts increasing attention from both academia and industry because of its potential for high accuracy positioning with a shorter convergence time compared to the traditional PPP. Besides high accuracy, integrity monitoring (IM) is indispensable for safety–critical real-time land vehicle and aviation applications. As the traditional advanced receiver autonomous integrity monitoring (ARAIM) method is designed for (smoothed) pseudorange-based positioning, the complexity of multi-frequency multi-constellation PPP-RTK using carrier phase measurements has not been given sufficient consideration. This study proposes an IM scheme for multi-frequency multi-constellation uncombined PPP-RTK applying the ARAIM theory, with a new comprehensive threat model to accommodate not only pseudorange measurements, but also carrier phase measurements, and other fault events arising from the network corrections that support PPP-RTK. Characteristics of different types of faults are analyzed with the aid of numerical experiments. In addition, the impact of ambiguity-fixed solutions on PPP-RTK integrity performance is investigated. The authors have also conducted case studies, including static and real-kinematic positioning experiments. Experiments have demonstrated that fast convergence in accuracy and the position error bounds, or protection levels, with a given integrity risk, in horizontal position components of PPP-RTK could be achieved. For the open sky environments on a highway, the protection levels estimated by PPP-RTK solutions have the potential to meet the alert limit requirement for road transportation using ambiguity-fixed PPP-RTK positioning under the assumption that the risks of wrong ambiguity fixing are very small and can be ignored.

Recently developed Navigation Indian Constellation (NavIC) signal (L5-1176.45 MHz and S-2492.028 MHz) pseudo range and carrier phase measurements are to be used for position, navigation and time computation in different engineering, science and other applications over the Indian sub-continent and its surroundings. To ensure the quality services analysis of signals that are being received is essential. Since, NavIC signal uses complex BPSK on L5 and BoC(5,2) on S-band different analysis methods are to be employed for evaluating the quality. In this research work, NavIC-L5 signal quality is analyzed using zero baseline double difference pseudo range and carrier phase measurements with real time data collected through Septentrio PolaRX5 receivers. Results show that the NavIC signal quality is slightly better (less residual) for GSO satellites. However, the residual is minimum for IRNSS 1C when IRNSS 1B is considered as a reference. Moreover, NavIC GSO signals are less consistent than other 3 GEO satellites signals.

Global Navigation Satellite System raw measurements from Android smart devices make accurate positioning possible with advanced techniques, e.g., precise point positioning (PPP). To achieve the sub-meter-level positioning accuracy with low-cost smart devices, the PPP algorithm developed for geodetic receivers is adapted and an approach named Smart-PPP is proposed in this contribution. In Smart-PPP, the uncombined PPP model is applied for the unified processing of single- and dual-frequency measurements from tracked satellites. The receiver clock terms are parameterized independently for the code and carrier phase measurements of each tracking signal for handling the inconsistency between the code and carrier phases measured by smart devices. The ionospheric pseudo-observations are adopted to provide absolute constraints on the estimation of slant ionospheric delays and to strengthen the uncombined PPP model. A modified stochastic model is employed to weight code and carrier phase measurements by considering the high correlation between the measurement errors and the signal strengths for smart devices. Additionally, an application software based on the Android platform is developed for realizing Smart-PPP in smart devices. The positioning performance of Smart-PPP is validated in both static and kinematic cases. Results show that the positioning errors of Smart-PPP solutions can converge to below 1.0 m within a few minutes in static mode and the converged solutions can achieve an accuracy of about 0.2 m of root mean square (RMS) both for the east, north and up components. For the kinematic test, the RMS values of Smart-PPP positioning errors are 0.65, 0.54 and 1.09 m in the east, north and up components, respectively. Static and kinematic tests both show that the Smart-PPP solutions outperform the internal results provided by the experimental smart devices.