R. Piriz

GNSS interoperability: offset between reference time scales and timing biases

Within the GIOVE Mission (GIOVE-M), two experimental satellites called GIOVE-A and GIOVE-B have been launched by the European Space Agency. This paper analyses the different issues involved in GPS/GIOVE interoperability for positioning and timing, including GGTO (the GPS to Galileo time offset) and timing biases, and presents practical experience and results related to EGGTO, the GIOVE-M experimental version of GGTO broadcast within the GIOVE navigation messages.

Use of two traveling GPS receivers for a relative calibration campaign among European laboratories

We report about a GPS receiver relative calibration campaign, which took place between five European National Metrology Institutes or Designated Institutes: LNE-SYRTE in Observatoire de Paris (Paris, France), where the reference receiver of the campaign was located, ROA (San Fernando, Spain), SP (Borås, Sweden), PTB (Braunschweig, Germany) and INRIM (Torino, Italy). We used as traveling equipment two main units, both connected to a single antenna, and we kept track of the offset between both traveling units in all the visited sites. An external validation of the resulting hardware delays is provided against the time scale differences derived from the UTC - UTC(k) data published by BIPM in its monthly Circular T. Thanks to a very good stability of the traveling equipment, we obtained expanded uncertainty estimates within 2.0 ns (k = 2) for the hardware delays.

Network time and frequency transfer with GNSS receivers located in time laboratories

In this paper we investigate a possible network solution, similar to the IGS analysis center solutions, that can be easily managed by a network of timing institutes to solve for all the clock differences (in addition to other quantities) in a unique system to understand the feasibility and the advantages of this approach in time and frequency transfer. The investigation is based on a suite of global navigation satellite system (GNSS) software products that allows the users to perform a wide range of calculations and analyses related to GNSS, from the evaluation of performances at the user level to the computation of precise GNSS orbits and clocks, including the calculation of precise receiver coordinates. The time and frequency transfer capabilities of the network solution (named ODTS) are evaluated and compared with PPP solutions as well as to other time transfer results.

The Time Validation Facility (TVF): An all-new key element of the Galileo operational phase

In the Galileo FOC phase (Full Operational Capability), GMV is the prime contractor for the Time and Geodetic Validation Facility (TGVF), a contract of the European Space Agency (ESA). Within the TGVF, the Time Validation Facility (TVF) is the subsystem in charge of steering Galileo System Time (GST) to UTC, among other duties. The new TVF is operated at GMV headquarters near Madrid, Spain. TVF operations rely on the contribution of five European timing laboratories, located at INRiM, OP, PTB, ROA, and SP. This paper provides a general description of the TVF element and its related activities for the FOC phase, and presents the main results and findings of the TVF operation until now.

Relative Calibration of Galileo receivers within the Time Validation Facility (TVF)

GMV is the prime contractor for the Time and Geodetic Validation Facility (TGVF) in the Galileo FOC phase (Full Operational Capability), a contract of the European Space Agency (ESA). Within the TGVF, the Time Validation Facility (TVF) is the subsystem in charge of steering Galileo System Time (GST) to UTC, among other duties. The TVF is operated at GMV headquarters near Madrid, Spain. Calibrated Galileo receivers are needed in the frame of TVF activities to, among other tasks, assess the UTC-GST offset broadcast in the Galileo navigation message. Absolute receiver calibration is a complex activity involving the availability of a signal simulator and of a calibrated reference antenna. An alternative data-based method to evaluate the Galileo receiver delay in E5 signals relative to GPS is also possible. The key feature of such method is the cancellation of the GPS and Galileo ionospheric delays when combining pseudoranges from two satellites with a close position in the sky. A software tool called gecal has been developed by GMV within the Galileo TVF, implementing such method and processing RINEX 3 observation files. Satellite positions are read from a SP3 orbit file. The tool allows the rapid E5 calibration of a new or existing Galileo receiver. This paper describes the tool and the calibration of three Galileo receivers used in the TVF. Galileo Signal-In-Space validation results obtained using such receivers are also presented.

A cost-efficient regional synchronization system

GNSS timing is currently used worldwide in critical real-time systems that require precise synchronization or time-stamping at geographically dispersed sites, such as wireless telecom stations, electrical power grids, and financial services. For applications where multiple equipment needs to be deployed, it is desirable to use low-cost, single-frequency receivers. Increasing demands in cost-saving and accuracy will make the availability of precise single-frequency solutions more and more interesting. In the space operations domain, an application example of GNSS-based synchronization is passive ranging for geostationary satellites. This paper analyzes the application of low-cost GNSS receivers to multi-site synchronization for geostationary satellite orbit determination based of Time Difference of Arrival (TDOA) measurements. The receiver timing signals are measured at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. By comparing the receiver output timepulse with the reference UTC(PTB) timescale clock using a Time Interval Counter (TIC), one can fully characterize the behavior of the GNSS-derived time solution, and also calibrate the average receiver chain delay. It is expected that the largest contribution to the GNSS timepulse error will be the ionosphere.

Performance of the NeQuick G iono model for single-frequency GNSS timing applications

GNSS timing is currently used worldwide in critical real-time systems that require precise synchronisation or time-stamping at geographically dispersed sites, such as wireless telephone stations, electrical power grids and financial services. For applications where multiple timing equipment needs to be deployed, it is desirable to use low-cost, single-frequency receivers. Together with calibration issues, the ionospheric delay is the major limiting factor for accurate timing in single-frequency setups. Increasing demands in cost-saving and accuracy will make the availability of precise single-frequency solutions more and more interesting. In this paper we analyse single-frequency timing accuracy using the Galileo NeQuick G model, as compared with the standard GPS Klobuchar model, and taking the dual-frequency iono-free solution as reference. The analysis is done by postprocessing RINEX files from two calibrated GNSS receivers, one located at mid-latitude and the other one close to the equator. Unlike Klobuchar, NeQuick G is based on a rather complex mathematical algorithm. For offline applications in postprocessing this is not an issue, but for a real-time implementation it might be necessary to evaluate the model not too frequently, in order to improve execution speed. NeQuick G efficiency is analysed from this point of view also.

Smartphone application for the near-real time synchronization and monitoring of clocks through a network of GNSS receivers

The promising potentialities for time and frequency transfer of a network solution of geodetic GNSS receivers located in National Timing Laboratories aiming at comparing their time scales at the maximum level of precision is presented.

Near-real time synchronization through a network of GNSS receivers located in timing laboratories

Today, an increasing number of users need high-quality GLASS products, such as precise satellite orbit and clock estimations and predictions, accurate receiver coordinates or tropospheric delays, for their applications (e.g., precise point positioning, GLASS augmentation services, weather services, etc). In recent years, many national timing laboratories have collocated geodetic GLASS receivers together with their traditional GPS/GLOLAASS AV/CV and TWSTFT equipments. Many of geodetic GLASS receivers hosted in national timing laboratories, operate continuously within the International GLASS Service (IGS), and their data are regularly processed by IGS Analysis Centers. Whereas participating stations must agree to adhere to certain strict standards and conventions which ensure the quality of the IGS LAetwork, a number of products and tools have been developed in order to allow time and frequency transfer without taking part to the IGS. Among these, magicGLASS, a web application (http://magicgnss.gmv.com) for high-precision GLASS data processing developed by GMV in Madrid, allows the users to perform a wide range of calculations and analyses related to GLASS, from the evaluation of performances at user level, to the computation of precise GLASS orbits and clocks, including the calculation of precise receiver coordinates. The algorithms that process station data to generate products in magicGLASS, are called ODTS, which stands for Orbit Determination & Time Synchronization, and PPP. ODTS is a network solution requiring a set of stations distributed worldwide. PPP is a single-station solution (although several stations can be processed together for convenience). The advantages of a network solution compared to PPP are that the estimates of each station can benefit from the measures of all stations being in principle more robust and precise. Starting from promising results achieved in recent works, in this paper we want to investigate a possible network solution, similar to the IGS analysis center solutions, that can be easily managed by a network of timing centers to solve in a unique system all the clock differences (besides other quantities), to understand the feasibility and the advantages of this approach in time and frequency transfer. In particular, thanks to the automation of the ODTS process, the possibility to use magicGLASS as a tool for the near-real time synchronization of atomic clocks and time scales worldwide distributed with a latency of at least 30 minutes, is evaluated. Furthermore, the near-real time synchronization capabilities of the ODTS network solution are compared to PPP solutions provided by magicGLASS and other tools, as well as to IGS products and estimates generated by the traditional time and frequency transfer techniques.