GPS Application to Time Transfer and Dissemination

International audience

We present a simple model of the signals transmitted by global positioning system (GPS) satellites and of the reception of these signals by a GPS receiver, neglecting atmospheric effects. The model idealizes the receiver and transmitter as electric dipole antennas. We derive the open-circuit voltage induced in the antenna of the receiver as a function of time, position, velocity, and orientation of the respective antenna. As an application of this model, we show that by using a single dipole antenna, a user of the GPS can compute his position, time, and two angles of attitude, by tracking the phases of the open-circuit antenna voltages for six GPS satellites.

Lockheed Martin Mission Systems is developing a high fidelity simulator for the Air Force's Global Positioning System (GPS) satellite command and control operations. To accomplish this goal, each spacecraft subsystem was broken down into a series of algorithms that describes the subsystem dynamic behavior. The measurable effects produced by these algorithms were compared to actual GPS satellite data for algorithm validation. Several subsystems that posed particular difficulty include orbit determination, thermal control, and reaction control. The primary algorithms used to simulate these subsystems are included herein as well as a discussion pertaining to issues surrounding the development of these algorithms. Finally, some of the required anomalies are discussed along with their relation to the derived algorithms.

<p>Precise measurement of time and frequency has been instrumental in the development of modern geosciences. It has enabled us to quantify many observations, including plate motion, the variations of Earth rotation, and modern-day sea level rise.</p><p>Over the past decade, European National Metrology Institutes (NMIs), together with National Research and Education Networks (NRENs) and partners from universities and research institutes have pioneered the dissemination of ultra-stable optical frequency and timing signals via optical fibers. Initially started as proof-of-concept experiments, this technology has matured to aim for a paradigm change: making precise time and frequency signals available to the wider scientific community and thereby enabling new research avenues.</p><p>The CLOck NETwork Services Design Study (CLONETS-DS) is a research and innovation action intended to facilitate the vision of a sustainable, pan-European optical fiber network for precise time and frequency reference dissemination.</p><p>Here, we will present the envisioned technology, its performance parameters, and discuss potential applications, requirements and limitations for geophysical applications, for example in geodesy (chronometric levelling, gravity field observation), seismology, and very-long-baseline interferometry (VLBI).</p>

Both National Institute of Standards and Technology (NIST) and United States Naval Observatory (USNO) now operate a commercial 12-channel common-view receiver. The receiver tracks the GPS (Global Positioning System) satellites and the GLONASS (Global Navigation Satellite System) satellites. In this paper, we evaluate the receiver's performance using the common-clock, common-view calibration data. We also study the performance of multichannel GPS/GLONASS common-view between NIST and USNO. The common-clock, common-views using multichannel GLONASS C/A (Coarse/Acquisition) code and single-channel GLONASS P-code (precise code) contained very little transfer noise. A diurnal effect was noticed in the multi-channel GPS C/A code common-clock, common-view calibration. For the common-view comparison between NIST and USNO, the increased number of daily common-view tracks and the more precise measurements with GLONASS P-code reduced the transfer noise in short term. The multi-channel GPS/GLONASS common-view was influenced by some periodic systematic errors in long term.

: Software GPS receiver development has been undertaken. We are particularly interested in improving the GPS signal-to-noise/interference ratio using beam forming techniques. The phase relationship among the antenna array elements requires careful calibration. In this study, we will report a phase calibration technique for a 2 by 2 GPS antenna array using both simulated and real GPS signals. This technique is based on the GPS signal-processing algorithm developed for the software GPS receiver. A four-channel digital data collecting system was used in the experiment. For a simulated GPS signal, the experiment was conducted in an anechoic chamber in which a GPS simulation system was facilitated. For real GPS signals, we conducted the experiment on a rooftop to receive the signal from GPS satellites. The calibration verified the coherent nature of the signals among the elements. The results also allowed the source's direction to be determined.

Precise frequency linking method based on phase group synchronization

The Differential Global Positioning System (DGPS) is a marine navigation system operating at frequencies of 283.5–325 kHz, which is now the primary method for locating vessels in coastal shipping, as well as hydrography and mapping systems worldwide. Its positioning accuracy is determined by the following: the pseudorange error to Global Positioning System (GPS) satellites, the age of pseudorange corrections, and the value of the Horizontal Dilution of Precision (HDOP), which, in terms of accuracy, is crucial in positioning using GPS satellites. In 2020, the International Hydrographic Organization (IHO) introduced a new (the highest) order of hydrographic surveys, i.e., the Exclusive Order, which requires a positioning system to provide an accuracy of 1 m (p = 0.95). The aim of this article is to provide an answer to the question as to whether the maritime DGPS system, whose positioning accuracy is constantly increasing with that of the GPS system, fulfils the requirements for the hydrographic surveys of harbours. To this end, an extensive experimental study on the maritime DGPS system, involving a total of nearly 3.5 million fixes, was conducted. Statistical analyses showed that when ensuring the HDOP values range from 0.8 to 1.4, the DGPS system can be used in hydrographic surveys of harbours.

Position determination using satellite navigation system has grown significantly. It provides geospatial with global coverage called GNSS (Global Navigation System Satellite). GNSS satellites consists of GLONASS, GPS (Global positioning system) and Galileo.GPS is the most commonly used system and it is known to its capability to determine 3D position on the surface of the earth. In order to determine the position, a GPS receiver must be able to receive signals from at least four GPS satellites. However, the determination of position in condensed areas such as tunnels, area surrounded by high rise buildings, highly forested and in other closely-knit areas is not achieved because satellite signals cannot reach the receiver in the above-mentioned areas and also others where the signals are reflected before being received by a GPS receiver. In this paper, we present the algorithm to fuse GPS and the inertial measurement unit (IMU) to enable positioning in the above-mentioned Condensed Areas. The standard deviations of the two measurements show that GPS-IMU is better than GPS alone, the standard deviation when satellite outages occurred is - 4.57475 for GPS-IMU measurements and 0.218675 for GPS observations. We presented the results in graphics and it shows that GPS measurements are easily disturbed by external influence such as multipath but GPS-IMU graphic is continuous and robust. The advantages and disadvantages of GPS and INS are complementary and make them work together to enable the accurate measurements in the areas mentioned above. Integration of INS and GNSS can be classified into three types, loosely coupled Kalman filter, tightly coupled Kalman filters and ultratight coupled Kalman filter. In this research we used loosely coupled Kalman filter and tightly coupled Kalman filters to combine GPS and INS in one system.

Summary form only given. The USAF has embarked on an initiative known as Future Flight Safety System (FFSS) that will significantly reduce Test Range Operations and Maintenance (O&M) costs by closing facilities and decommissioning ground assets. In support of the FFSS, a GPS Metric Tracking (MT) system based on the Global Positioning System (GPS) satellite constellation has been developed that will allow Ranges to divest some of their radar assets. Successful flight testing and qualification of these systems has been conducted at Vandenberg Air Force Base (VAFB) on the Western Test Range and Cape Canaveral Air Force Station (CCAFS) on the Eastern Test Range. The GPS MT system has provided precise LV position, velocity and timing information that can replace ground radar tracking resource functionality. The GPS MT system provides an independent position/velocity S-Band telemetry downlink to support the current man-in-the-loop ground-based commanded destruct of an anomalous flight. The system utilizes a 50 channel digital receiver capable of navigating in high dynamic environments and high altitudes fed by antennas mounted diametrically opposed on the airframe skin. To enhance cost effectiveness, the GPS MT System design implemented existing commercial parts and common environmental and interface requirements for Launch Vehicles. The GPS MT System design is complete, successfully qualified, and has demonstrated in flight that the system performs as designed and simulated. Development of the next generation of GPS MT systems is ongoing and qualification testing is being conducted.

This study investigated the effects of solar flare activity using a baseline during solar minimum from July 2019 through December 2019 to determine a threshold and then assessing the impacts during the solar maximum period from January 2022 through May 2022 on Global Positioning System (GPS) timing sources. GPS satellite data received at the National Institute of Standards and Technology (NIST) was collected and overlaid with the solar activity archive data from SpaceWeatherLive.com to determine whether any correlation existed to showcase effects of solar flare activity on GPS satellite timing sources. Focus of the resulting data was on M-class and X-class flares due to the severity based on prior research papers on the topic. The resulting data analysis of the datasets indicates that each flare class not only impacted the GPS satellite offset value in nanoseconds, but that after each M-class and X-class flare, it was determined that GPS offset value in nanoseconds remains high for several days after the event. Given that the sun is entering solar maximum, more research is needed to better understand the impacts of higher-class solar events on GPS satellite technology (i.e., timing, navigation, etc.) to ensure systems are better protected before events occur especially upon a direct impact from a solar flare.

Ionospheric imaging at mid-latitudes using both GPS and ionosondes

The Global Positioning System (GPS) provides satellite-based navigation signals, which are employed in many fields, including agriculture, transportation, aviation, and military/personal navigation. In an effort to minimize interference among GPS satellites and to enable GPS receivers to discern satellite identity, each satellite is assigned a specific pseudorandom noise (PRN) sequence that is used to modulate the phase of the corresponding signal. The codes that modulate the current GPS landscape are constructed in such a way that cross-correlation among codes is kept to a bounded minimum, which should significantly limit harmful signal interference. In this study, the efficacy of the current PRN-based modulation system is called into question as GPS signal amplitude and carrier phase data over the past decade show frequent interference between satellite signals.

Since the beginning of the International Global Navigation Satellite System (GLONASS) Experiment, IGEX, in October 1998, the Center for Orbit Determination in Europe (CODE) has acted as an analysis center providing precise GLONASS orbits on a regular basis. In CODE's IGEX routine analysis the Global Positioning System (GPS) orbits and Earth rotation parameters are introduced as known quantities into the GLONASS processing. A new approach is studied, where data from the IGEX network are combined with GPS observations from the International GPS Service (IGS) network and all parameters (GPS and GLONASS orbits, Earth rotation parameters, and site coordinates) are estimated in one processing step. The influence of different solar radiation pressure parameterizations on the GLONASS orbits is studied using different parameter subsets of the extended CODE orbit model. Parameterization with three constant terms in the three orthogonal directions, D, Y, and X (D = direction satellite–Sun, Y = direction of the satellite's solar panel axis), and two periodic terms in the X-direction, proves to be adequate for GLONASS satellites. As a result of the processing it is found that the solar radiation pressure effect for the GLONASS satellites is significantly different in the Y-direction from that for the GPS satellites, and an extensive analysis is carried out to investigate the effect in detail. SLR observations from the ILRS network are used as an independent check on the quality of the GLONASS orbital solutions. Both processing aspects, combining the two networks and changing the orbit parameterization, significantly improve the quality of the determined GLONASS orbits compared to the orbits stemming from CODE's IGEX routine processing.

The author describes a procedure for instantaneous GPS (Global Positioning System) attitude determination, i.e., a solution for the GPS integrated carrier Doppler wavelength ambiguities using only measurements at a single epoch. Most previous techniques for solving the phase ambiguity problem have required some form of time history processing relying on GPS satellite and/or user motion to provide enough geometry change to eliminate false solutions. The algorithm described assumes three nonlinear antennas and integrated carrier Doppler measurements from four or more satellites. Double-difference processing provides at least three independent observables for the two antenna separation vectors to compute the three attitude Euler angles. The results obtained also suggest that arbitrarily large separations can be resolved since unambiguous carrier-smoothed pseudorange measurements could provide a coarse measurement with an accuracy falling within the resolution range of the algorithm presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>