Space Weather and the Global Positioning System
2008; American Geophysical Union; Volume: 6; Issue: 6 Linguagem: Inglês
10.1029/2008sw000400
ISSN1542-7390
Autores Tópico(s)Advanced Frequency and Time Standards
ResumoThe ability to monitor space weather in near–real time is required as our society becomes increasingly dependent on technological systems such as the Global Positioning System (GPS). Certain critical applications such as railway control, highway traffic management, emergency response, commercial aviation, and marine navigation require high-precision positioning. As a consequence, these applications require real-time knowledge of space weather effects. In recent years, GPS itself has become recognized as one of the premier remote sensing tools to monitor space weather events. For this reason, Space Weather has opened a special section called “Space Weather Effects on GPS. “ Papers in this section describe the use of GPS as a monitor of space weather events and discuss how GPS is used to observe ionospheric irregularities and total electron content gradients. Other papers address the implications that these space weather features may have on GPS and on Global Navigation Satellite System (GNSS) operations in general. Space weather impacts on GPS include the introduction of range errors and the loss of signal reception, both of which can have severe effects on marine and aviation navigation, surveying, and other critical real-time applications. To recognize the many important contributions that were made during the pioneering years of the 1980s and early 1990s and to provide some insight as to what can be expected in the years to come, we introduce the GPS special section by reviewing the 25-year history of GPS as an ionospheric monitoring system. This review will help give readers the context they need to understand ideas presented in this special section. GPS was developed by the U.S. Department of Defense with the primary goal of being an all-weather space-based navigation system. GPS system design is heavily dependent on the accuracy of atomic clocks on board the satellites, and the design of this satellite system during the early 1970s took advantage of advances in clock technology. However, the GPS signals must transit the ionosphere to communicate with ground receivers. This transit introduces signal propagation errors because the ionosphere affects the propagation speed and direction of all radio signals (including GPS). In addition, electron density irregularities in the ionosphere can introduce amplitude and phase fluctuations, a process known as scintillation. The ionosphere is a dispersive medium, meaning that these changes in speed and direction are a function of frequency. They are also proportional to the varying electron density along the line of sight between the receiver and the satellite. Thus, the cumulative effect at the receiver is proportional to the total electron content, or TEC, which is equal to the total number of electrons in a column with a cross-sectional area of 1 square meter along the line of sight between the satellite and the receiver. For GPS, the change in propagation speed introduces both a range delay, the equivalent of measuring a slightly longer distance to the satellite than is actually the case, and a phase advance in its observables. To obtain very accurate positions from GPS, this ionospheric delay/advance must be removed. To this day, the ionosphere remains the largest error source in the GPS navigation solution. The GPS system was designed to operate at two frequencies, L1 (1575.42 megahertz) and L2 (1227.60 megahertz). While the majority of handheld GPS receivers operate only on the L1 frequency, higher-quality GPS receivers, such as those used by the geodetic community, operate on both frequencies and are designed to take advantage of the frequency dependence of the ionospheric terms. If the GPS range and phase can be measured at both frequencies, the ionospheric range delay and phase advance can be computed. For the ionospheric scientist, this also means that the TEC can be measured almost exactly. Ionospheric scientists were quick to recognize that the GPS dual-frequency measurements could be used to measure the ionospheric TEC at multiple locations [MacDoran, 1979]. As soon as the first experimental GPS satellites were launched (between 1978 and 1985), scientists began using GPS signals to monitor the ionosphere [Clynch et al., 1983; Royden et al., 1984; Klobuchar, 1985]. There were 10 experimental satellites (the GPS Block I satellites), and they formed the GPS Demonstration System, which was designed to test and validate the use of a satellite-based system for real-time global navigation and timing. The early GPS measurements, collected primarily using stand-alone receivers, were compared with ionospheric measurements taken with other instruments, such as Faraday rotation sensors or incoherent scatter radar platforms [Klobuchar et al., 1986; Bishop et al., 1987; Lanyi and Roth, 1988; Coster and Gaposchkin, 1989; Coco et al., 1990; Coster et al., 1990; Coco, 1991; Klobuchar, 1991]. GPS was also quickly recognized as a useful instrument for the study of propagation parameters and scintillation [Clynch et al., 1989; Bishop et al., 1990, 1991; Wanninger, 1993; van Dierendonck et al., 1993; Doherty et al., 1994; Coker et al., 1995, 1996; Aarons et al, 1996; Pi et al., 1997; Beach and Kintner, 1999]. Starting in 1989, the first fully operational GPS satellites were launched. These satellites were launched into a 55° inclination orbit, lower than the 63° inclination of the Block I orbits. Furthermore, the operational satellites had the full suite of signal capabilities, including selective availability (SA) and antispoofing (AS), both of which deliberately degraded the precision of the GPS signals for nonmilitary users. SA involved destabilizing the satellite oscillators and altering broadcast orbit and clock parameters. Typical range accuracies for civilians with GPS SA turned on were around 100 meters. SA was permanently turned off in May 2000. Of more importance to ionospheric scientists was the implementation of AS, which was turned on full time in early 1994. Essentially, AS involves an encryption of the GPS code on the L2 frequency, and only users with a decryption key are allowed full access to the code. However, cross-correlation techniques were developed that allowed receiver manufacturers to reproduce a degraded GPS L2 signal with significantly less gain than GPS L1. Ionospheric scientists have based most of their GPS TEC measurements on this degraded GPS L2 signal. With GPS modernization, a new civilian signal that is not encrypted is being broadcast on L2. This new signal, called L2C, is available on the new GPS Block IIR-M satellites, which are in the process of being launched (see Figure 1). As part of the modernization program, GPS satellites will also be broadcasting another civilian signal at a third frequency, L5 (1176.45 megahertz). The first GPS satellite with this capability will be launched in the near future. By the mid-1980s, two primary issues were recognized with the use of GPS data to measure ionospheric TEC. Both fall under the category of unknown signal contributions and occur due to either multipath or system hardware differential delays. Multipath delays are propagation phenomena that result in radio signals reaching the receiving antenna by two or more paths. Multipath delays can result in GPS calculating its position erroneously, and can make the determination of the absolute TEC value extremely difficult. This issue is considerably worse at low elevation angles. GPS multipath issues were described by Bishop et al. [1985, 1994] and remain a concern, although they may be mitigated by the use of higher elevation cutoffs and the use of choke-ring antennas. The second significant issue is the additional differential delay between the two GPS frequencies introduced by the receiver and satellite hardware. These differential delays, commonly referred to as the receiver and satellite biases, can be significant and if not removed correctly, can corrupt the GPS TEC measurements. Initially, the community was uncertain how to estimate either the receiver or the satellite biases. Work at the NASA Jet Propulsion Laboratory [e.g., Lanyi and Roth, 1988] led to initial algorithms for bias estimation. Other methods were developed using single receivers [Coco et al., 1991; Gaposchkin and Coster, 1993] and were followed by more powerful techniques based on the global network of receivers [Wilson and Mannucci, 1993; Sardón et al., 1994]. The estimation of satellite and receiver biases remains an area of significant and active investigation within the ionospheric community. New and enhanced techniques have been recently developed that estimate receiver differential biases for all available GPS stations (typically around 1000 sites) on a daily basis [Komjathy et al., 2005; Rideout and Coster, 2006]. The research community needs more efficient and improved estimation algorithms to properly perform process and quality checks on the large amount of GPS data currently available on a daily basis. The International GPS Service (IGS, now the International GNSS Service) network of GPS receivers played a central role in developing the use of GPS for monitoring the ionosphere. The concept for the IGS was initiated in 1989 by a group of scientists affiliated with the International Union of Geodesy and Geophysics. These scientists recognized that GPS offered tremendous potential for use in Earth science research provided that all errors, including those introduced by signal propagation, are minimized. Scientific investigations require the highest precision data possible. To understand what is meant by the term “highest-precision data,” it is simplest to consider how the GPS position estimate is obtained. The GPS estimate of a user's position depends on knowledge of (1) the position of the GPS satellites, (2) the time as measured by the satellite and receiver clocks, and (3) the estimated ranges from different GPS satellites to the GPS receiver. To obtain the highest-precision GPS measurements, all of the above must also be known with the highest precision possible. The GPS community achieves this by estimating the precision ephemerides for the GPS satellites (a precision ephemeris is essentially a table of values that provides an extremely accurate position of a GPS satellite as a function of time) and additional clock information (such as the clock drift rates for the atomic clocks on board the GPS satellites and the receiver's clock offset). A global network of receivers was (and is) needed to best estimate these parameters. The requirement of a global network can be understood in context of the GPS satellite orbits. To improve upon the GPS orbits, data need to be collected at multiple points along the entire orbit. Observations collected in a single hemisphere or country alone cannot provide this. With support from numerous scientific organizations, the IGS was founded in 1992 with a network of about 20 geodetic receivers worldwide. At its tenth anniversary, IGS consisted of more than 200 actively contributing organizations in more than 80 countries and a global network of more than 350 stations. IGS was launched around the same time that the World Wide Web was created. The World Wide Web was invented in the late 1980s [Berners-Lee and Fischetti, 1999]. By the end of 1992, there were more than 50 Web servers in the world, many located at universities and other research centers. For the IGS, the World Wide Web enabled the easy transfer of data files and products. More important is the fact that the World Wide Web helped GPS researchers organize their standardized products and services, such as the Receiver Independent Exchange Format (RINEX) files [Gurtner et al., 1989]. The importance of the IGS service to developing the use of GPS for ionospheric monitoring purposes and for studying the structure of worldwide space weather events cannot be overstated. The IGS service provides high-quality data in a standard format that are freely available and easily accessible through the World Wide Web to all scientists at http://igscb.jpl.nasa.gov/. With the establishment of the IGS network, ionospheric scientists began investigating the use of producing maps of TEC based on the global network of receivers [Wilson et al., 1992; Mannucci et al., 1993; Wilson et al., 1995; Komjathy and Langley, 1996; Komjathy, 1997; Mannucci et al., 1998]. The ionospheric working group of IGS was established in 1998 [Feltens and Schaer, 1998]. Currently, four Ionospheric Associate Analysis Centers (IAACs) contribute with their rapid and final vertical TEC (VTEC) maps to the IGS products. The four IAACs are the NASA/California Institute of Technology/Jet Propulsion Laboratory, the Center of Orbit Determination in Europe (CODE), the European Space Agency (ESA), and the Technical University of Catalonia (UPC). IAACs compute the global distribution of TEC independently using different models. As an IGS final product, the four independently derived VTEC maps are combined for the IGS community. This global TEC information is used for purposes such as calibration of single-frequency GPS receivers and altimeters, and investigations of the global temporal and spatial behavior of ionospheric TEC. Data from about 1000 GPS receivers are currently available on a daily basis to monitor the temporal and spatial variability of the global ionosphere. These receivers include networks such as the Continuously Operating Reference Stations (CORS) in addition to the IGS network of receivers. Algorithms have been developed to process all of these data sets in a time-efficient manner, enabling daily monitoring of the quiet and storm-time ionosphere that affects satellite-based radio navigation systems such as GPS, the Russian Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), and Galileo, a new European satellite navigation system that is currently under development. Real-time ionospheric monitoring systems based on GPS data were first developed in the early 1990s [Coster et al., 1992; Coco et al., 1993]. In 1994, work began on a larger networked real-time system called the Wide Area Augmentation System (WAAS). WAAS was developed jointly by the U.S. Department of Transportation (DOT) and the Federal Aviation Administration (FAA) with the goal of becoming the future primary means of civil air navigation. WAAS is designed to augment GPS—without WAAS, clock drift, satellite orbit errors, and ionospheric effects (including disturbances) create undesirable error and uncertainty in the GPS signal. WAAS is needed to meet the very strict civil aviation requirements for integrity, accuracy, availability, and continuity. WAAS is based on a network of ground-based GPS reference stations in the North American continent that continuously measure ionospheric slant delays (the line of sight range delays between the GPS receiver and the satellite). All of these data are transferred to master stations, where the integrity of the system is assessed and the measurements are combined to compute ionospheric vertical delays (the delay due to the ionosphere looking directly overhead) for a virtual set of ionospheric grid points (IGPs) that is spaced over much of North America. By using a bilinear interpolation algorithm, WAAS avionics use the IGPs to estimate the adjustments needed to correct for receiver-to-satellite ionospheric delays. Although WAAS was the first GPS-based augmentation aviation system in operation, similar systems are currently in development in, for example, Europe, Japan, and India. Collectively, these systems are called Satellite Based Augmentation Systems (SBAS). Prior to 2000, the density of GPS receivers in the IGS network was relatively sparse, forcing scientists to integrate GPS TEC data into ionospheric models to produce continuous TEC maps. In 2001, the Massachusetts Institute of Technology Haystack Observatory was the first group to make use of all available GPS data to produce strictly data-driven plots of the TEC using no underlying models to smooth out gradients. Because of this lack of smoothing, the narrow plumes of storm-enhanced density (SED) [Foster, 1993] that form over the United States during geomagnetic storms could clearly be observed in the GPS TEC maps, first reported by Coster et al. [2001]. In a ground-breaking paper, Foster et al. [2002] linked the narrow plumes seen in GPS TEC observations as measured from the ground with the plasmaspheric plumes as seen from NASA's Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite in space at 8 Earth radii. This last observation clearly demonstrated that a dense network of ground-based receivers could play a significant role in measuring the coupling between the ionosphere and the magnetosphere. GPS TEC maps produced by this method have now been widely disseminated throughout the atmospheric research community and have become one of the standard means to study the effects of geomagnetic storms. Ground-based GPS receivers allow for good data coverage over land but not over the oceans. As proposed by Hajj et al. [1994], putting GPS receivers in space is one way of addressing this lack of coverage. The idea of using GPS receivers in space to sense properties of the atmosphere grew out of earlier work in the remote sensing of planetary atmospheres—in the 1970s, the atmospheres of Mars, Venus, and Jupiter were probed using the technique of radio occultation. Planetary occultation is when a smaller astronomical body passes behind a larger astronomical body, wholly obscuring its view. Similarly, radio occultation can be thought of as when the line of sight to a satellite is obscured by a planet or, in the case of GPS, the Earth. For GPS, a satellite in low-Earth orbit (LEO) tracks the signal from a GPS satellite. The LEO satellite typically orbits between 500 and 800 kilometers above Earth's surface, and GPS satellites are in nearly semisynchronous orbits (they travel around the Earth approximately twice a day) at an altitude of approximately 20,200 kilometers. From the perspective of the LEO satellite, GPS satellites rise and set several times a day. As the occultation occurs, the signal that is measured from the GPS satellite is refracted, or “bent,” by differing amounts as it propagates through different layers of the atmosphere. By measuring the amount of refraction as the GPS satellite is rising or setting, scientists are able to reconstruct properties of the different layers of the atmosphere (e.g., relative humidity and temperature profiles in the troposphere, and electron density in the ionosphere). The Global Positioning System/Meteorology (GPS/MET) experiment was designed as a proof-of-concept mission to demonstrate that GPS signals occulted by the Earth's atmosphere could be used to measure properties of our atmosphere and ionosphere [Ware et al., 1996]. The GPS/MET mission lasted from April 1995 to March 1997 and was highly successful. Numerous ionospheric studies were reported based on GPS/MET data [e.g., Hajj and Romans, 1998; Rius et al., 1998; Sokolovskiy et al., 2002]. The success of GPS/MET spawned a number of other radio occultation missions to provide ionospheric measurements over oceanic regions, including the Argentine Satelite de Aplicanciones Cientificas-C (SAC-C), the U.S.-funded Ionospheric Occultation Experiment (IOX), and Germany's Challenging Minisatellite Payload (CHAMP) [Jakowski et al., 2002]. The joint U.S./Taiwan Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC; http://cosmic-io.cosmic.ucar.edu/cdaac/index.html), a new constellation of six satellites, nominally provides up to 3000 ionospheric occultations per day (Figure 2). COSMIC measurements have been provided officially for public use since 28 July 2006. Researchers have begun integrating COSMIC-derived TEC measurements with ground-based GPS TEC data and assimilating these data into models (such as JPL's Global Assimilative Ionospheric Model (GAIM), originally sponsored by the U.S. Department of Defense) so that three-dimensional global electron density structures and ionospheric drivers can be estimated. The large amount of space-based measurements provided by the current COSMIC mission have made this possible [e.g., Komjathy et al., 2007]. 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Edwards (1995), Subdaily Northern Hemisphere ionospheric maps using an extensive network of GPS receivers, Radio Sci., 30, 639–648. Anthea Coster works on measuring and understanding the geophysics of adverse space weather at the Massachusetts Institute of Technology's Haystack Observatory in Westford, Mass. Attila Komjathy is a space weather scientist at the NASA/California Institute of Technology/Jet Propulsion Laboratory in Pasadena, Calif.
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