Receiver Clock Research Articles

Precise real-time navigation of LEO satellites using a single-frequency GPS receiver and ultra-rapid ephemerides

Precise (sub-meter level) real-time navigation using a space-capable single-frequency global positioning system (GPS) receiver and ultra-rapid (real-time) ephemerides from the international global navigation satellite systems service is proposed for low-Earth-orbiting (LEO) satellites. The C/A code and L1 carrier phase measurements are combined and single-differenced to eliminate first-order ionospheric effects and receiver clock offsets. A random-walk process is employed to model the phase ambiguities in order to absorb the time-varying and satellite-specific higher-order measurement errors as well as the GPS clock correction errors. A sequential Kalman filter which incorporates the known orbital dynamic model is developed to estimate orbital states and phase ambiguities without matrix inversion. Real flight data from the single-frequency GPS receiver onboard China's SJ-9A small satellite are processed to evaluate the orbit determination accuracy. Statistics from internal orbit assessments indicate that three-dimensional accuracies better than 0.50 m and 0.55 mm/s are achieved for position and velocity, respectively.

A 12-bit 200MS/s pipeline ADC with 91 mW power and 66 dB SNDR

A 12-bit 200MS/s pipeline analog-to-digital converter (ADC) with sample-and-hold amplifier (SHA) is proposed in this paper. An innovative timing control technology is applied to the SHA in order to eliminate its nonlinear kickback, while the SHA eliminates bandwidth and timing mismatches between the input networks of the flash (sub-ADC) and of the multiplying DAC (MDAC) in the first stage, to improve the analog input bandwidth and to meet the requirements of a high-IF sampling ADC. A novel full complementary op-amp is utilized to reduce the overall power consumption of the ADC, which makes the power consumption of the SHA and of the first stage less than 20mW. A new fully differential reference buffer and a clock receiver with duty-cycle stabilizer (DCS) are applied in the prototype ADC. The jitter of the clock receiver is less than 150 fs to avoid the SNR degradation. The ADC is implemented in 0.18µm CMOS process and consumes 91mW from a 1.8V supply. The ADC achieves an SNDR of 66.10dB and an SFDR of 84.45dB with 10MHz input signal, while maintaining an SNDR >65dB and SFDR >80dB up in the entire Nyquist band. Its input signal frequency is up to 500MHz.

APPLICATION OF PPP-KINEMATIC METHOD AND AIRCRAFT POSITIONING SOFTWARE FOR CALCULATION OF PLANE’S POSITION

Results of application of GPS tech-nique in the air transport are presented in the paper. Aircraft’s coordinates were obtained using GPS code observations for PPP-Kinematic method in the APS software. Cessna 172 aircraft was applied in a flight test at Dęblin military airport. The paper de-scribes the PPP-Kinematic method and con-figuration of input parameters for this meth-od. In addition, the values of such parameters as: accuracy of receiver clock bias, accuracy of aircraft’s coordinates, DOP coefficients, accuracy of HPL and VPL terms, results of statistical test Chi-square are presented in the paper.

Formal Uncertainty and Dispersion of Single and Double Difference Models for GNSS-Based Attitude Determination.

With multi-antenna synchronized global navigation satellite system (GNSS) receivers, the single difference (SD) between two antennas is able to eliminate both satellite and receiver clock error, thus it becomes necessary to reconsider the equivalency problem between the SD and double difference (DD) models. In this paper, we quantitatively compared the formal uncertainties and dispersions between multiple SD models and the DD model, and also carried out static and kinematic short baseline experiments. The theoretical and experimental results show that under a non-common clock scheme the SD and DD model are equivalent. Under a common clock scheme, if we estimate stochastic uncalibrated phase delay (UPD) parameters every epoch, this SD model is still equivalent to the DD model, but if we estimate only one UPD parameter for all epochs or take it as a known constant, the SD (here called SD2) and DD models are no longer equivalent. For the vertical component of baseline solutions, the formal uncertainties of the SD2 model are two times smaller than those of the DD model, and the dispersions of the SD2 model are even more than twice smaller than those of the DD model. In addition, to obtain baseline solutions, the SD2 model requires a minimum of three satellites, while the DD model requires a minimum of four satellites, which makes the SD2 more advantageous in attitude determination under sheltered environments.

Advanced receiver autonomous integrity monitoring using triple frequency data with a focus on treatment of biases

Most current Advanced Receiver Autonomous Integrity Monitoring (ARAIM) methods are designed to use dual-frequency ionosphere-free observations. These methods assume that receiver bias is absorbed in the common receiver clock offset and bound satellite biases by nominal values. However, most multi-constellation Global Navigation Satellite Systems (GNSS) can offer triple frequency data that can be used for civilian applications in the future, which can improve observation redundancy, solution precision and detection of faults. In this contribution, we explore the use of this type of observations from GPS, Galileo and BeiDou in ARAIM. Nevertheless, the use of triple frequency data introduces receiver differential biases that have to be taken into consideration. To demonstrate the significance of these additional biases we first present a method to quantify them at stations of known coordinates and using available products from the International GNSS service (IGS). To deal with the additional receiver biases, we use a between-satellite single difference (BSSD) observation model that eliminates their effect. A pilot test was performed to evaluate ARAIM availability for Localizer Performance with Vertical guidance down to 200feet (LPV-200) when using the triple-frequency observations. Real data were collected for one month at stations of known coordinates located in regions of different satellite coverage characteristics. The BSSD triple-frequency model was evaluated to give early indication about its feasibility, where the implementation phase still requires further comprehensive studies. The vertical position error was always found to be bounded by the protection level proven initial validity of the proposed integrity model.

UART Receiver Synchronization: Investigating the Maximum Tolerable Clock Frequency Deviation

Objectives: To analyse the maximum tolerable clock frequency deviation in UART receiver for accurate data reception and to derive mathematical expressions for the same. Methods/Analysis: General design techniques and synchronisation problems in UART receivers have been investigated and a mathematical analysis of the designs is done. Various parameters taken into consideration are baud rate, parity/no parity, oversampling ratio and phase difference between transmitter and receiver clocks. Findings: The maximum tolerable frequency deviation, Δ max improves with increasing oversampling ratio at the UART Receiver side. It is also found that the Δ max is independent of the baud rate of operation. Improvements: Improved expressions may be derived by analyzing the effects of clock jitter, rise and fall times of the signals.

Autonomous Formation Keeping of Geostationary Satellites with Regional Navigation Satellites and Dynamics

In this paper, the reduced dynamic autonomous formation control of geostationary-Earth-orbit satellites using Indian Regional Navigation Satellite System observables is presented. Generally, the navigation signal is used to determine the position, velocity, and time kinematically. The kinematic formulation, however, is geometric and vulnerable to data outage and cycle slip. Weak geometry of navigation satellites causes high dilution of precision. In this situation, dynamics can provide the estimates with little degradation in accuracy. This requires combining Indian Regional Navigation Satellite System observables with orbital dynamics of the geostationary-Earth-orbit satellites in an extended Kalman filter setup, resulting in improved position and velocity estimates. Moreover, solar radiation pressure coefficient, ephemeris error, and any other unaccounted minute forces are estimated through an empirical acceleration model, along with receiver clock bias and drift, in a reduced dynamics formulation. Furthermore, it is demonstrated that these position and velocity estimates permit autonomous station and formation keeping of geostationary-Earth-orbit satellites. The classical technique using the sun and moon and solar radiation perturbing accelerations to advantage is used to control eccentricity and inclination vectors of each satellite in the formation. The formation-keeping control maneuvers are performed autonomously, whenever an individual satellite’s eccentricity or inclination limit is exceeded.

NavIC Time Transfer With Satellite Common View: An Analysis on Choosing Different Reference Times

The present work analyses the use of common view time transfer in the Indian Regional Navigation Satellite System (IRNSS) with two different IRIMS (IRNSS Range and Integrity Monitoring Stations) as the reference station. IRNSS is an independent navigation system designed by India with an operational name as NavIC (Navigation with Indian Constellation). Common view time comparison technique has been implemented across the IRIMS stations of IRNSS to transfer the reference time precisely to all the IRIMS stations. Analyses were done by comparing the common view receiver clock offsets obtained with respect to IRIMS Bangalore time and the ones obtained with respect to IRIMS Lucknow time. Observations obtained reveal the consistency of the method and therefore it can be utilized for NavIC. Further analyses show the stability with both cases establishing the credibility of the results.

Optimum stochastic modeling for GNSS tropospheric delay estimation in real-time

In GNSS data processing, the station height, receiver clock and tropospheric delay (ZTD) are highly correlated to each other. Although the zenith hydrostatic delay of the troposphere can be provided with sufficient accuracy, zenith wet delay (ZWD) has to be estimated, which is usually done in a random walk process. Since ZWD temporal variation depends on the water vapor content in the atmosphere, it seems to be reasonable that ZWD constraints in GNSS processing should be geographically and/or time dependent. We propose to take benefit from numerical weather prediction models to define optimum random walk process noise. In the first approach, we used archived VMF1-G data to calculate a grid of yearly and monthly means of the difference of ZWD between two consecutive epochs divided by the root square of the time lapsed, which can be considered as a random walk process noise. Alternatively, we used the Global Forecast System model from National Centres for Environmental Prediction to calculate random walk process noise dynamically in real-time. We performed two representative experimental campaigns with 20 globally distributed International GNSS Service (IGS) stations and compared real-time ZTD estimates with the official ZTD product from the IGS. With both our approaches, we obtained an improvement of up to 10% in accuracy of the ZTD estimates compared to any uniformly fixed random walk process noise applied for all stations.

Simplified ultra-tightly coupled BDS/INS integrated navigation system

Due to advantages such as low cost, small size, low weight, low power consumption, powerful capability of anti-jamming, and high dynamics, ultra-tightly coupled GNSS/MIMU integrated navigation systems are extensively used in projectile and precision-guided munitions. To overcome the nonlinearity of baseband signal processing and the complexity of the navigation filter, in this paper, we design a novel architecture for the ultra-tightly coupled (UTC) Beidou navigation satellite system (BDS)/INS integration based on the BDS B3 signal, including the combination of code phase and carrier phase dynamics baseband prefilter model, and the navigation filter navigation error propagation and measurement models. To eliminate the clock state components, i.e., the receiver clock offset and clock drift, the pseudorange, delta pseudorange, and delta pseudorange rate residuals obtained from baseband signal prefilters are differenced between two satellites. Based on the BDS/INS simulator, a test system for the UTC BDS/INS integrated navigation system is developed and UTC performance is evaluated. Some preliminary conclusions are formed about the effectiveness of the UTC BDS/INS integration approach.

Ranging Performance for Spoofer Localization using Receiver Clock Offset

Ranging Performance for Spoofer Localization using Receiver Clock Offset

A closed-form method for single-point positioning with six satellites in dual-GNSS constellations

With the impact of the Global Navigation Satellite System (GNSS), dual-GNSS constellations are playing an increasingly significant role in positioning, navigation and timing (PNT) applications. Aiming at improving from the existing method, i.e., linearization, of solving the single-point positioning problem under a dual-GNSS, this paper develops a closed-form method for solving PNT problems in the case of six satellites. This method reduces the positioning problem to a simple mathematical problem of finding solutions to a quadratic equation, thereby needing only one receiver clock bias (RCB) as variable. By solving the RCB, the positioning information in three dimensions is obtained by utilizing a linear equation. Compared with the existing method, the closed-form method requires no initial position or iterations. This method thus provides a direct solution to single-point positioning. Further, how to check the uniqueness and the validity of the solutions is also derived. Experimental results verify the validity, applicability and efficiency of the proposed method.

Modeling and Assessment of GPS/BDS Combined Precise Point Positioning.

Precise Point Positioning (PPP) technique enables stand-alone receivers to obtain cm-level positioning accuracy. Observations from multi-GNSS systems can augment users with improved positioning accuracy, reliability and availability. In this paper, we present and evaluate the GPS/BDS combined PPP models, including the traditional model and a simplified model, where the inter-system bias (ISB) is treated in different way. To evaluate the performance of combined GPS/BDS PPP, kinematic and static PPP positions are compared to the IGS daily estimates, where 1 month GPS/BDS data of 11 IGS Multi-GNSS Experiment (MGEX) stations are used. The results indicate apparent improvement of GPS/BDS combined PPP solutions in both static and kinematic cases, where much smaller standard deviations are presented in the magnitude distribution of coordinates RMS statistics. Comparisons between the traditional and simplified combined PPP models show no difference in coordinate estimations, and the inter system biases between the GPS/BDS system are assimilated into receiver clock, ambiguities and pseudo-range residuals accordingly.

A FPGA Software based GPS Receiver Implementation with Signal blocker through Simulink

Background/Objectives: Field-Programmable Gate Array (FPGA) software based Global Positioning System (GPS) receiver has been designed and developed using the C language interface environment. GPS Receivers are used for tracking a signal and calculates the current position of the signal in real time. GPS Receiver is a real time application used for tracking purpose. Methods/Analysis: A GPS Receiver works on specially coded GPS satellite signals. The signal is processed in a GPS receiver for computing position, velocity and time. A GPS uses minimum four satellite signals for computing positions in three dimensions using the real time receiver clock. Findings: In this paper, it is planned to build a FPGA-based software GPS receiver using a high level Matlab design Simulink tool. This GPS Receiver is used to design such components that require huge computation like baseband signal processing correlator, C/A code generator, Discriminator Code Loop (DCL) are designed by the Xilinx FPGA block and implemented in Matlab/Simulink. Novelty: The “GPS Receiver Processing Captured Satellite data” model of GPS system without jammer, the signal level display shows the value 0.1779; but in case of a jammer, the signal level is reduced to 0.1447 which is 81% of the initial signal level. So, jammer reduces the signal strength and makes the signal weaker.

An approach to improve the GPS positioning performance under urban environment conditions

Abstract Satellite deficiency degrades the performance of GPS single point positioning (SPP) in city urban areas. If the number of observed satellites is smaller than that of the unknown independent parameters in the data processing, extended Kalman filter (EKF) methods will usually be employed to transit the variable values epoch by epoch. In this case, the prediction model in EKF is very important for the positioning results. This paper focuses on the receiver clock offset models in the prediction equation. The investigation shows that, the clock offset model with clock shift is able to improve the accuracy of the clock offset prediction for internal crystal clocks within a short time span, compared with the clock offset model without clock shift. Besides, in order to further improve the performance of positioning in the case of satellite deficiency, an adaptive EKF is developed to enhance the clock prediction. The experiments show that this approach can improve the positioning accuracy in the scenarios that only fewer satellites are available.

Precise Point Positioning Using Triple GNSS Constellations in Various Modes.

This paper introduces a new dual-frequency precise point positioning (PPP) model, which combines the observations from three different global navigation satellite system (GNSS) constellations, namely GPS, Galileo, and BeiDou. Combining measurements from different GNSS systems introduces additional biases, including inter-system bias and hardware delays, which require rigorous modelling. Our model is based on the un-differenced and between-satellite single-difference (BSSD) linear combinations. BSSD linear combination cancels out some receiver-related biases, including receiver clock error and non-zero initial phase bias of the receiver oscillator. Forming the BSSD linear combination requires a reference satellite, which can be selected from any of the GPS, Galileo, and BeiDou systems. In this paper three BSSD scenarios are tested; each considers a reference satellite from a different GNSS constellation. Natural Resources Canada’s GPSPace PPP software is modified to enable a combined GPS, Galileo, and BeiDou PPP solution and to handle the newly introduced biases. A total of four data sets collected at four different IGS stations are processed to verify the developed PPP model. Precise satellite orbit and clock products from the International GNSS Service Multi-GNSS Experiment (IGS-MGEX) network are used to correct the GPS, Galileo, and BeiDou measurements in the post-processing PPP mode. A real-time PPP solution is also obtained, which is referred to as RT-PPP in the sequel, through the use of the IGS real-time service (RTS) for satellite orbit and clock corrections. However, only GPS and Galileo observations are used for the RT-PPP solution, as the RTS-IGS satellite products are not presently available for BeiDou system. All post-processed and real-time PPP solutions are compared with the traditional un-differenced GPS-only counterparts. It is shown that combining the GPS, Galileo, and BeiDou observations in the post-processing mode improves the PPP convergence time by 25% compared with the GPS-only counterpart, regardless of the linear combination used. The use of BSSD linear combination improves the precision of the estimated positioning parameters by about 25% in comparison with the GPS-only PPP solution. Additionally, the solution convergence time is reduced to 10 minutes for the BSSD model, which represents about 50% reduction, in comparison with the GPS-only PPP solution. The GNSS RT-PPP solution, on the other hand, shows a similar convergence time and precision to the GPS-only counterpart.

A new method to improve the performance of multi-GNSS pseudorange positioning in signal-degraded environment

There are two existing multi-GNSS positioning methods, (1) separate receiver clock parameter is set for each constellation, (2) estimating intersystem biases (ISBs) in advance to obtain position solutions with only four unknowns. The former is the regular method and its unknowns include three receiver-coordinate parameters and several receiver clock parameters (depend on the number of participant constellations), so that it may disable when few satellites belonging to different GNSS are in view. The latter is one workable way to obtain position solution with only four visible satellites. In addition to the disabled regular method, the positioning results by the ISB-corrected method are often unsatisfactory for navigation users in signal-degraded environment. Both the deviation of ISB-solutions and the remaining measurement errors of the need corrected observations are factors to degrade location precision. Apart from these, fewer visible satellites usually cause a low robustness of the positioning model, which cause the negative influence of various errors is amplified. Based on the variation of various measurement errors, we propose a new ISSB-corrected method with observations corrected by corresponding satellite-dependent parameters. The new parameter contains the difference of time scales, hardware delays and uncorrected measurement errors between the corresponding satellite and reference, in other words, it can synthetically consider measurement errors and the ISB. By the ISSB-corrected method, we not only achieve positioning solutions with four satellites, but also significantly reduce the accuracy loss. Many experiments are conducted to present the superiority of the ISSB-corrected method. In open-area, the accuracies of regular and ISB-corrected methods are nearly equal. Apart from a similar accuracy in horizontal, the accuracy is improved by approximate 10% in up direction with respect to the two existing methods. Given the high redundancy of model in open-area, the new method may not improve the accuracy remarkably. However, it can make great contributions in signal-degraded environments. In order to compare the performance of ISB- and ISSB-corrected methods in environment with limited visible satellites, we simulate several scenarios (different satellites participant or various receivers) with only four participant satellites in the positioning solution. By the ISSB-corrected method, the 3D RMS of positioning results with four satellites is about 15m, while it is usually worse than 25m for the ISB-corrected method. In an urban vehicular test, the horizontal positioning error of the ISSB-corrected method is less than 20m; and the ISB-corrected method may reach up to 70m.

Dilution of Precision Relationship between Time Difference of Arrival and Time of Arrival Techniques with No Receiver Clock Bias

Dilution of Precision (DOP), as a measure of positioning accuracy, is an essential factor. Therefore, the DOP relationship between systems is very important. In this paper, the DOP relationship between TDOA and TOA in systems lacking clock bias is derived analytically and verified experimentally. Also, using those of earlier studies, the DOP relationship in each of defined systems is derived analytically.

Nonlinear filtering for autonomous navigation of spacecraft in highly elliptical orbit

In support of Canada's proposed Polar Communication and Weather mission, this study examined the accuracy to which GPS-based autonomous navigation might be realized for spacecraft in a Molniya orbit. A navigation algorithm based on the Extended Kalman Filter was demonstrated to achieve a three-dimensional root-mean-square accuracy of 58.9m over a Molniya orbit with 500km and 40,000km perigee and apogee altitudes, respectively. Despite the inclusion of biased and non-white error models in the generated GPS pseudorange measurements – a first for navigation studies in this orbital regime – algorithms based on the Unscented Kalman Filter and the Cubature Kalman Filter were not found to improve this result; their benefits were eclipsed due to the accurate pseudorange measurements which were available during periods of highly nonlinear dynamics.This study revealed receiver clock bias error to be a significant source of navigation solution error. For reasons of geometry, the navigation algorithm is not able to differentiate between this error and a radial position error. A novel dual-mode dynamic clock model was proposed and implemented as a means to minimize receiver clock bias error over the entire orbital regime.

Multi-antenna synchronized global navigation satellite system receiver and its advantages in high-precision positioning applications

The multi-antenna synchronized global navigation satellite system receiver is a high precision, low cost, and widely used emerging receiver. Using this type of receiver, the satellite and receiver clock errors can be eliminated simultaneously by forming between antenna single-differences, which is equivalent to the conventional double-difference model. However, current multi-antenna synchronized global navigation satellite system receiver products have not fully realized their potential to achieve better accuracy, efficiency, and broader applications. This paper introduces the conceptual design and derivable products of multi-antenna synchronized global navigation satellite system receivers involving the aspects of attitude determination, multipath effect mitigation, phase center variation correction, and ground-based carrier phase windup calibration. Through case studies, the advantages of multi-antenna synchronized global navigation satellite system receivers in high-precision positioning applications are demonstrated.