Methods of calibration for GNSS one-way timing link and real-time GNSS system time monitoring

Traceable method for differential code bias estimation in PPP one-way timing

Metrological traceability in dynamic scenarios timing

The metrological traceability of time and frequency has been established and demonstrated in static stations, such as, the metrological traceability to UTC of UTC(k) in the frame established by CCTF key comparison: CCTF-K001.UTC. In dynamic scenarios, most mobile stations use the GNSS one-way time transfer to achieve time synchronization. However, the time reference of the output of the receiver is not clearly identified, which means the time and frequency values obtained by the user will not be metrologically traceable (This is attributable to the time broadcast by GNSS, which includes both the GNSS system time (GNSST) and the prediction of UTC broadcast by the GNSS system.). Mobile stations such as sailing ships, self-driving cars, and high-speed railway trains have put forward a demand for high-precision time synchronization. To ensure the reliability and legitimacy of the time and frequency values obtained by the mobile stations, a study on metrological traceability in dynamic scenarios timing should be conducted. The construction of the traceable link from the mobile station to the ‘reference’ is an important part of this study (The ‘reference’ can be any station that achieves traceability to UTC). A time transfer link is designed, which is constructed by two calibrated GNSS time transfer receivers located separately at the mobile station and the ‘reference’. The real-time position of the mobile station is calculated to generate the result of the 30-second difference between the mobile station and GNSST. The 30-second calculation of the difference between the ‘reference’ and GNSST is performed at the ‘reference’ as well, which then is transmitted to the mobile station via the network every 30 seconds. Through the differentiating procedure, the real-time time transfer result between the mobile station and the ‘reference’ is obtained. In this way, it can be ensured that the time and frequency values obtained by the mobile station are derived from a clear and definite reference. We construct a platform for the establishment of traceability of time in dynamic scenarios and conduct the experiments on this platform. A trolley, a car traveling on a city road, and a railway train are the mobile stations, and our lab which keeps an atomic time scale TS(BJTU) is employed as the ‘reference’. The real-time time transfer was successfully achieved between the mobile stations and the ‘reference’. The time transfer results calculated based on the post-processing Precise Point Positioning (PPP) technology are used for the evaluation of link noise level. In three dynamic time transfer scenarios, the noise level distributes between 2 ns and 7 ns. Further analysis of the uncertainty of time transfer in dynamic scenarios will be conducted to accomplish this study.

Development of Low Power GNSS Time-based System with Fault Tolerance and DMA Driven Data

Among all possible time keeping schemes in modern timekeeping Embedded systems requiring real-time synchronization, GNSS (Global Navigation Satellite System) is one of the widely used ones. GNSS Receivers use signals transmitted by satellites orbiting the Earth. GNSS based system, during integration and testing, the most widespread issue observed in the system is either absence of satellite acquisition or failure to get connection with the satellites. At such situations RTC (Real Time Clock) becomes a crucial component. Traditional polling-based methods demand a lot of power and CPU cycle, which are not feasible for low power embedded systems. In this paper, a system employing USART (Universal Synchronous Asynchronous Receiver and Transmitter) DMA (Direct Memory Access) with circular buffer and FreeRTOS to implement asynchronous reception of GNSS data in an optimized manner with minimal processor usage has been proposed. The system possesses all the attributes of a low power design, such as sleep mode operation, interrupt-driven task switching, and RTC-driven wake-up from sleep mode, and has been proven to be an efficient, expandable, and robust solution for embedded low power time keeping applications such as IOT devices, autonomously working devices, industrial instrumentation, and real time navigation devices.

Robust positioning using hybridization of GNSS with other measurements of opportunity

(English) Achieving robust positioning across ground and UAV platforms remains challenging under multipath, partial satellite visibility, and rapidly changing measurement quality, especially in urban and embedded scenarios. At the same time, modern smartphones and embedded receivers increasingly provide multi-constellation, dual-frequency observations, carrier-phase measurements, and IMU streams that can be exploited for aided positioning. The present thesis addresses these conditions through a robust GNSS/IMU integration framework, evaluated across heterogeneous sensor grades, from navigation-grade platforms to consumer smartphones. The framework is designed not only for offline post-processing, but also for real-time and embedded operation, following a deterministic execution structure suitable for on-board use. The principal conclusions are: First, a unified processing framework has been developed for GNSS/IMU integration using raw, undifferenced, uncombined GNSS observables (code, carrier phase, Doppler) and inertial measurements. Second, a Square-Root Information Filter (SRIF) architecture has been adopted as the estimator core, enabling a numerically robust implementation and a shared software structure across GNSS-only processing, loosely coupled (LC) fusion, and tightly coupled (TC) fusion modes. Third, Allan deviation analysis has been used systematically to identify inertial noise parameters and to configure the process-noise model of the navigation filter across different IMU classes, improving consistency of tuning when detailed manufacturer specifications are incomplete. Fourth, experimental validation on multiple independent datasets, spanning different GNSS conditions and equipment grades, shows that LC fusion provides the most consistent practical gains, especially in continuity and robustness during short GNSS degradations, and often improves typical solution behaviour when inertial data quality is adequate. Fifth, in dense urban conditions with strong multipath and masking, positioning performance remains fundamentally constrained by GNSS measurement quality and correction level; inertial aiding mitigates short-term disruptions but does not eliminate the GNSS-side error ceiling. Finally, the thesis demonstrates a transferable and numerically stable GNSS/IMU integration framework, a reproducible Allan-based methodology for configuring heterogeneous IMUs in the filter, and a realistic path toward robust positioning under practical field conditions. (Català) Assolir un posicionament robust en plataformes terrestres i UAV continua sent un repte en presencia de multipath, visibilitat satel·lital parcial i canvis rapids en la qualitat de les observacions, especialment en escenaris urbans i embeguts. Al mateix temps, els telefons intel·ligents i els receptors embeguts moderns proporcionen cada cop mes observacions multiconstel·lacio i de doble frequencia, mesures de fase portadora i fluxos IMU aprofitables per al posicionament assistit. La present tesi aborda aquestes condicions mitjancant un marc robust d'integracio GNSS/IMU, avaluat en sensors de diferents graus, des de plataformes de grau de navegacio fins a telefons intel·ligents de consum. El marc esta concebut no nomes per al post-processament fora de linia, sino tambe per a operacio en temps real i en sistemes encastats, seguint una estructura d'execucio determinista adequada per a aquest tipus de desplegament. Les conclusions de la recerca son: Primer, s'ha desenvolupat un marc unificat de processament per a la integracio GNSS/IMU utilitzant observables GNSS crus, no diferenciats i no combinats (codi, fase portadora i Doppler) i mesures inercials. Segon, s'ha adoptat una arquitectura de Filtre d'Informacio en Arrel Quadrada (SRIF) com a nucli de l'estimador, la qual cosa permet una implementacio numericament robusta i una estructura de programari compartida per al processament GNSS-only, la fusio en acoblament lax (LC) i la fusio en acoblament estret (TC). Tercer, l'analisi de desviacio d'Allan s'ha utilitzat de manera sistematica per identificar parametres de soroll inercial i per configurar el model de soroll de proces del filtre de navegacio en diferents classes d'IMU, millorant la coherencia de l'ajust quan les especificacions del fabricant son incompletes. Quart, la validacio experimental en multiples conjunts de dades independents, que abasten diferents condicions GNSS i graus d'equipament, mostra que la fusio LC aporta les millores practiques mes consistents, especialment en continuitat i robustesa davant degradacions breus de GNSS, i sovint millora el comportament tipic de la solucio quan la qualitat de les dades inercials es adequada. Cinque, en entorns urbans densos, amb multipath intens i emmascarament, el rendiment del posicionament continua limitat de manera fonamental per la qualitat de la mesura GNSS i el nivell de correccions; l'ajuda inercial mitiga interrupcions de curt termini, pero no elimina el sostre d'error del costat GNSS. Finalment, en conjunt, la tesi demostra un marc d'integracio GNSS/IMU transferible i numericament estable, una metodologia reproduible basada en Allan per configurar IMUs heterogenis al filtre, i una via realista cap a un posicionament robust en condicions de camp. (Español) Lograr un posicionamiento robusto en plataformas terrestres y UAV sigue siendo un reto en presencia de multitrayectoria, visibilidad satelital parcial y cambios rapidos en la calidad de las observaciones, especialmente en escenarios urbanos y embebidos. Al mismo tiempo, los telefonos inteligentes y receptores embebidos modernos proporcionan cada vez mas observaciones multiconstelacion y de doble frecuencia, medidas de fase portadora y flujos IMU aprovechables para posicionamiento asistido. La presente tesis aborda estas condiciones mediante un marco robusto de integracion GNSS/IMU, evaluado en sensores de distintos grados, desde plataformas de grado navegacion hasta telefonos inteligentes de consumo. El marco esta concebido no solo para post-procesado fuera de linea, sino tambien para operacion en tiempo real y en sistemas embebidos, siguiendo una estructura de ejecucion determinista adecuada para uso a bordo. Las principales conclusiones son: Primero, se ha desarrollado un marco unificado de procesamiento para la integracion GNSS/IMU utilizando observables GNSS brutos, no diferenciados y no combinados (codigo, fase portadora y Doppler) y medidas inerciales. Segundo, se ha adoptado una arquitectura de Filtro de Informacion en Raiz Cuadrada (SRIF) como nucleo del estimador, lo que permite una implementacion numericamente robusta y una estructura software compartida para el procesamiento GNSS-only, la fusion en acoplamiento laxo (LC) y la fusion en acoplamiento estrecho (TC). Tercero, el analisis de desviacion de Allan se ha utilizado de forma sistematica para identificar parametros de ruido inercial y para configurar el modelo de ruido de proceso del filtro de navegacion en distintas clases de IMU, mejorando la coherencia del ajuste cuando las especificaciones del fabricante son incompletas. Cuarto, la validacion experimental en multiples conjuntos de datos independientes, que abarcan distintas condiciones GNSS y grados de equipamiento, muestra que la fusion LC aporta las mejoras practicas mas consistentes, especialmente en continuidad y robustez ante degradaciones breves de GNSS, y a menudo mejora el comportamiento tipico de la solucion cuando la calidad de los datos inerciales es adecuada. Quinto, en entornos urbanos densos, con multipath intenso y enmascaramiento, el rendimiento del posicionamiento sigue limitado de forma fundamental por la calidad de la medida GNSS y el nivel de correcciones; la ayuda inercial mitiga interrupciones de corto plazo, pero no elimina el techo de error del lado GNSS. Por ultimo, en conjunto, la tesis demuestra un marco de integracion GNSS/IMU transferible y numericamente estable, una metodologia reproducible basada en Allan para configurar IMUs heterogeneos en el filtro, y una via realista hacia un posicionamiento robusto en condiciones de campo.

Real-time estimation of Earth Rotation Parameters from GPS based on filter method

Performance evaluation of GPS and BeiDou hybrid positioning using a low cost GNSS receiver in Nigeria

This study evaluates the performance of Global Positioning System (GPS)-only, BeiDou-only, and hybridized GPS+BeiDou Global Navigation Satellite System (GNSS) constellations in Nigeria, with emphasis on positional stability and the benefits of multi-constellation integration. A dual-frequency u-blox ZED-F9P GNSS receiver was deployed at a static reference station at the Department of Pure and Applied Physics, Ladoke Akintola University of Technology, Ogbomoso, Nigeria. Continuous 24-hour GNSS observation data were collected at 4 Hz over a 14-day period in May 2025. The raw data were converted to Receiver Independent Exchange (RINEX) Format using the Real-Time Kinematic Library (RTKLIB) suite and processed with RTKPOST to obtain positioning solutions. The data were analyzed in terms of carrier-to-noise density ratio (C/N₀), number of satellites visible (NSV), satellite elevation angle, positional stability, and Single Point Positioning (SPP) performance. The results showed that GPS provided more stable satellite visibility, with visible satellites ranging from 6 to 11 per epoch and a daily average NSV of 8.22 ± 0.05, while BeiDou ranged from 4 to 8 visible satellites per epoch, with a daily average NSV of 6.05 ± 0.11. GPS maintained 31 visible Pseudo-Random Noise (PRN) satellites throughout the observation period, whereas BeiDou varied between 28 and 31 PRNs. The C/N₀ analysis showed that GPS L1 provided the strongest signal quality, with values ranging from 36.26 dB-Hz to 39.05 dB-Hz and a mean value of 38.10 ± 0.55 dB-Hz. In terms of positional stability, GPS recorded lower standard deviations in latitude, longitude, and altitude, ranging from 1.15 m to 1.75 m, 1.38 m to 1.94 m, and 4.16 m to 5.69 m, respectively. BeiDou showed higher variability, with corresponding values ranging from 2.07 m to 3.39 m, 2.07 m to 4.12 m, and 6.36 m to 9.85 m. However, BeiDou provided a complementary elevation-angle advantage, with a mean elevation angle of 29.06 ± 2.08°, compared with 25.71 ± 2.39° for GPS. The hybridized GPS+BeiDou solution delivered the best positioning performance, with average Two-Distance Root Mean Square (2DRMS), Circular Error Probable (CEP), Spherical Error Probable (SEP), and Mean Radial Spherical Error (MRSE) values of 3.93 m, 1.64 m, 3.66 m, and 4.83 m, respectively. These values represent an improvement of approximately 14% over GPS-only and about 46–49% over BeiDou-only, depending on the positioning performance metric considered. The findings demonstrate that GPS+BeiDou hybridization improves GNSS positioning performance, reliability, and solution stability over the study area.