A New TWSTFT Modem with Code and Carrier Phases

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.

GPS time transfer is usually carried out with C/A code measurements using the common-view technique. An uncertainty of a few nanoseconds can be obtained for an integration time of about 1000 seconds when ionospheric measurements and precise orbits are used. This limits the capability of frequency comparison to the level of 3 parts in 10/sup 14/ for an integration time of one day. Recently new frequency standards have been developed that claim an accuracy at or below 1/spl times/10/sup -14/. For existing time transfer techniques it is a challenge to compare their frequency over a sufficiently short interval. Previous studies have already shown the potential of carrier-phase and P-code measurements from geodetic GPS receivers. This paper draws up tentative uncertainty budgets for clock frequency comparisons using phase and code measurements from commercially available dual-frequency and multi-channel GPS receivers. Although the level of noise involved in carrier phase measurements is two or three orders of magnitude lower than that of code measurements in the short term, other error sources such as tropospheric delay of GPS signals, multipath propagation and the phase ambiguity problem lead to a limitation in the accuracy of frequency transfer at the 10/sup -15/ level over an averaging time of one day. Carrier phase measurements, however, provide an improvement over the performance expected from code measurements for all averaging times smaller than one day.

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.

The BeiDou global satellite navigation system (BDS) is an important tool for precise remote time and frequency transfer. The current day-boundary discontinuity in the data process of the BDS-3 carrier phase (CP) time transfer limits the exploitation of the full potential of the BeiDou system, particularly over extended periods (e.g. >1 day). This study focused on the daily discontinuities in BDS-3 ground time and frequency transfer induced by the current satellite orbit and clock products from the individual Multi-GNSS experiment (MGEX) analysis center. An interpolation approach that bridges the daily jump of satellite products is proposed to improve the continuity of BDS-3 time and frequency transfer when using the CP technique. Only third-order polynomial is required to smooth and bridge the impact of the daily jump for medium earth orbit (MEO) and inclined geosynchronous satellite orbit (IGSO) satellite products. Eight MGEX network stations equipped with various types of receivers and antennas with dual-frequency BDS-3 signals were used to establish four time transfer links (i.e. BRUX – PTBB, AMC4–HOB2, LCK4–USUD, and ONS1–LCK3) and evaluate their effectiveness. These results reveal that the current BDS-3 satellite product discontinuity from individual analysis center easily caused the drift of phase residuals near the day-boundary epoch, with mean amplitudes of 0.081 m and 0.434 m for MEO and IGSO satellite products, respectively. The proposed approach can contribute to effectively eliminating this drift in time and frequency transfer. Furthermore, it achieved a noticeable reduction in the percentage of negative day-boundary discontinuities in BDS-3 ground time and frequency transfer results. The average values of the jump were 0.143 ns, 0.367 ns, 0.138 ns, and 0.131 ns for the BRUX – PTBB, AMC4–HOB2, LCK4–USUD, and ONS1–LCK3 time links, respectively, representing improvements of 11.4%, 40.5%, 4.5%, and 12.0%, respectively, compared with the traditional approach.

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.

Apart from communications, the Fifth Generation (5G) technology is also motivated by positioning requirements down to the sub-meter level across industry verticals. In this work, we utilize Universal Software Radio Peripheral to transmit and receive the 5G New Radio signals. The obtained samples are then processed by the proposed 5G code and carrier phase software defined receiver. In this process, the dedicated 5G Positioning Reference Signal (PRS) is adopted to obtain time of arrival estimates using more accurate carrier phase measure-ments instead of code phase measurements. The continuously transmitted 5G PRSs over time enable accurate carrier phase tracking and avoid ambiguity problems during the tracking stage in line-of-sight environments. Furthermore, a roadside unit is set up to observe the same signals and cancel out the measurement error introduced by the clock offset between two transmitters. Finally, some field experiments are carried out, and the experimental results indicate that using 5G double-difference carrier phase measurements from only four transmitters, a position root mean square error of 0.790m is achievable.

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.

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.

The expected positioning accuracies of civil users utilizing modernized GPS and Galileo are derived using the error analysis in this paper. Since, in general, the performance of DLL, PLL and FLL is proportional to chip lengths and wavelengths, the positioning accuracies from various measurements of modernized GPS and Galileo are derived as function of chip length and wavelength. These results are compared with that from GPS Ll measurement. In absolute positioning, compared to GPS C/A code only case, more than 17 times performance improvement is expected when all civil code signals of modernized GPS and Galileo (L1, L2, L5, E1, E5A and E5B) are used. In relative positioning, compared to GPS L1 carrier phase only case, more than 2 times performance improvement is expected when all civil signals of modernized GPS and Calileo are used. Furthermore, the relationship between GDOP and RGDOP in single frequency case is expanded to general case where multiple frequencies and both code and carrier phase measurements are used.

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.

The carrier phase (CP) technique based on the BeiDou Global Satellite Navigation System (BDS-3) has proven to be a crucial spatial tool for remote time and frequency transfer. The current CP technique models the receiver clock offset as a white noise stochastic process and easily absorbs some unmodeled errors, thus compromising the time and frequency transfer performance. To further improve the performance of time and frequency transfer, a new BDS-3 receiver clock estimation algorithm based on the epoch difference (ED) model is presented, and the mathematical principles and applied modes are discussed. The algorithm makes full use of both observations of the current epoch and practical variations of the receiver clock offset, further improving the performance of time and frequency transfer. Five Multi-Global Navigation Satellite System Experiment network stations equipped with various types of receivers and antennas with dual-frequency BDS-3 signals were used to establish four time transfer links (i.e., AMC4–PTBB, BRUX–PTBB, OP71–PTBB, and WTZS–PTBB) to evaluate their effectiveness. The ED model improves all the four time links in terms of noise level, with improvements of 17.0%, 18.3%, 20.3%, and 5.9%, respectively, when compared with the results from a non-ED model. The ED model outputs were better than the raw solutions in terms of frequency stability at all time links, particularly for average time intervals (tau) < 1000 s. The mean improvement was 8.1% for AMC4–PTBB, 16.1% for BRUX–PTBB, 10.0% for OP71–PTBB, and 18.6% for WTZS–PTBB when the average time (tau) was less than 1000 s.

Engineering Test Satellite VIII (ETS‐VIII) is planning to be launched in 2005 for fundamental studies on satellite positioning technologies and is the one first to have installed a highly precise cesium clock in Japan. To measure the clock offset between the satellite and the TCE earth station, Time Comparison Equipment (TCE) has been developed by NICT. The TCE will be used to measure both code and carrier phases and this system will improve two‐way time and frequency transfer. This equipment makes it possible to compare two clocks with high precision. Moreover, TCE and TCE earth stations have abilities to remove the delay caused by terrestrial ionosphere and that between the receivers and transmitters.We estimated measurement precision in code and carrier phases. Furthermore, from the examinations using TCE‐EM (Engineering Model), we confirmed that TCE is able to measure the clock phases with a subnanosecond precision by code phase observation and a precision of a few picoseconds by carrier phase observation. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(3): 61–71, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20385

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.

In this study a temperature controlled environment is used in order to quantify the thermal influence on all major parts of state of the art geodetic GPS receiving equipment. Temperature variations, effective as time delay variations, were identified as a dominating error source that degrades the capabilities of carrier phase GPS based time and frequency transfer considerably. For purely code-based measurements with uncertainties in the ns range is temperature rarely an issue. In contrast carrier phase observations offer potentially a two orders of magnitude better accuracy and are therefore suitable for exploiting the characteristics of maser quality clocks. However, the stability of the environment around the receiver equipment defines the achievable accuracy. Four distinct parts of the receiver chain were subject to systematic measurements of the temperature-delay dependency: antenna preamplifier, antenna and clock cables, power distribution devices and geodetic receivers. A temperature controllable climate chamber was deployed with the respective component to follow a long time-constant temperature stepping. Signal through devices were mainly tested in a vector-voltmeter approach. Zero base line GPS processing was used to test receivers. With individual component temperature dependence being far above the expected accuracy of carrier phase based time and frequency transfer it underlines the necessity to include temperature as an important parameter into time/frequency solutions.

Digital Beam Forming and nulling are viable means of GPS array signal processing that enable multipath mitigation and provide protection against interference. As reported in the literature [1], nulling causes errors in the carrier phase and code phase measurements. The output of multi element antenna arrays (Controlled Reception Pattern Antennas – CRPAs) are subject to significant variations in their phase patterns that arise from the application of time varying adaptive array weights. This phase error can be compensated in the carrier phase tracking domain in the baseband GPS receiver. Classical approaches implement a constrained beam steering mechanism to control the phase response of the array in the look direction. An alternative approach using unconstrained adaptive array processing with compensation for the subsequent phase distortion of the GPS signal is demonstrated using a software defined GPS radio.