Global Positioning System Augmentation Research Articles

Integrity Concept for Maritime Autonomous Surface Ships' Position Sensors.

The primary means for electronic position fixing currently in use in majority of contemporary merchant ships are shipborne GPS (Global Positioning System) receivers or DGPS (Differential GPS) and IALA (International Association of Lighthouse Authorities) radio beacon receivers. More advanced GNSS (Global Navigation Satellite System) receivers able to process signals from GPS, Russian GLONASS, Chinese Beidou, European Galileo, Indian IRNSS, Japan QZSS, and satellite-based augmentation systems (SBAS), are still relatively rare in maritime domain. However, it is expected that such combined or multi-system receivers will soon become more common in maritime transport and integrated with gyro, inertial, radar, laser, and optical sensors, and they will become indispensable onboard maritime autonomous surface ships (MASS). To be prepared for a malfunction of any position sensors, their state-of-the-art integrity monitoring should be developed and standardized, taking into account the specificity of MASS and e-navigation safety. The issues of existing requirements, performance standards, and future concepts of integrity monitoring for maritime position sensors are discussed and presented in this paper.

Adaptive multisensor data fusion technique for train localisation and detection of accidental train parting

This study proposes a bilinear recursive least square based adaptive multisensor data fusion technique for the precise localisation of railway vehicles and detection of an accidental train parting to be used with the train collision avoidance system (TCAS) in Indian railways. The accurate localisation of railway vehicles during the absence of global positioning system (GPS) is a challenging task for the TCAS. One of the reliable solutions for this task may be the augmentation of GPS with the onboard multisensor system. A bilinear recursive least square adaptive filter is used here to estimate and compensate the position error of the onboard multisensor system. The impact of slack in coupling is considered for the analysis of parting detection. The performance of the proposed technique is compared with the observation error based approach, bounded offset based approach and pseudo-measurement state constraining technique. The simulation results indicate that the proposed technique is superior in terms of positional accuracy and for the detection of an accidental train parting with a minimum parting distance.

Comparing performance bounds for chi-square monitors with parameter uncertainty

This paper compares methods for evaluating the performance of chi-square monitors while conservatively accounting for parameter uncertainty. Chi-square monitors, like the signal deformation monitors used in global positioning system augmentation, detect failures that threaten safety-critical navigation. A chi-square monitor creates a quadratic test statistic from a random vector (nominally zero mean and Gaussian distributed). Gaussian model parameters, which may be poorly characterized for a real system, strongly influence chi-square monitor performance. Through a combination of theory and simulation, it is established that tight yet conservative modeling of parameter uncertainty is possible with a generalized chi-square bound for false-alarm risk and with an ellipsoid bound for missed-detection risk.

Medium-scale traveling ionospheric disturbances in the Korean region on 10 November 2004: Potential impact on GPS-based navigation systems

Extreme medium-scale traveling ionospheric disturbances (MSTIDs) occurred at midlatitudes in East Asia during a geomagnetically active time on 10 November 2004. Using the Global Positioning System (GPS) observation data from Korean GPS reference stations, the characteristics of the MSTIDs on 10 November 2004 and their potential impact on GPS-based navigation systems in the Korean region are analyzed. The MSTIDs were first observed in the northeast part of South Korea at about 10:00 UT and propagated southwestward with successive wavefronts which extended from northwest to southeast. The peak-to-peak amplitudes of vertical total electron content (TEC) disturbances decreased from about 29 to 10 total electron content unit (1 TECU = 1016 el m−2), and the wavelengths lengthened from about 360 to 580 km from 12:53 to 14:38 UT. The propagation velocity of MSTID wavefronts was estimated using three nearby reference stations showing that velocity gradually decreased from about 254 m/s at 11:46 UT to 76 m/s at 21:26 UT. The ionospheric irregularities in small-scale regions accompanied by the MSTIDs were spatially and temporally varied from about 10:00 to 22:00 UT in response to the movement and intensity change of the MSTIDs. This event also generated anomalously large ionospheric spatial gradients which could cause unacceptable residual pseudorange errors for users of GPS augmentation systems. Frequent loss of the GPS signals, which occurred due to the intense ionospheric irregularities, could also degrade the continuity and availability of GPS-based navigation systems.

Cooperative Positioning for Vehicular Networks: Facts and Future

Intelligent transportation systems (ITSs) are increasingly being considered to mitigate the impacts of road transportation, including road injuries, energy waste, and environmental pollution. Vehicular positioning is a fundamental part of many ITS applications. Although global navigation satellite systems (GNSSs), e.g., Global Positioning System (GPS), are applicable for navigation and fleet management, the accuracy and availability of GNSSs do not meet the requirements for some applications, including collision avoidance or lane-level positioning. Cooperative positioning (CP) based on vehicular communications is an approach to tackle these shortcomings. The applicability of vehicular CP techniques proposed in the literature is questionable due to viability issues, including internode distance estimation, which is an important part of many CP techniques. Conventional CP systems such as differential GPS (DGPS) and other augmentation systems are also effectively incapable of addressing the given ITS applications. In this paper, modern and conventional CP systems are discussed, and the viability of radio ranging/range rating and constraints of vehicular communications as main pieces of modern CP systems are investigated. The general performance boundaries for modern CP systems are explained, as is the gap existing between the positioning accuracy required for crucial ITS applications and what modern CP can provide. This is followed by introduction of a novel trend for vehicular CP research, which is a potential reliable solution using a modified concept of real-time kinematic (RTK) GPSs for vehicular environments.

The Development of GPS Navigation Systems in Civil Aircraft

The industry of civil aviation is developing in a fast manner to occupy the increasing needs for a fast, safe and comfortable transportation. To achieve these needs, some effective programs, plans, and systems designs are required. The current reliable aircraft navigation and landing systems have relatively a low level of accuracy especially in aircraft landing stage. The best landing category could be achieved requires a runway visual range up to 50 meters and automatic landing system is not in operation yet all over the world. Aircraft navigates using Radar, Radio and Satellite navigation systems. The Federal Aviation Administration (FAA) plans to replace legacy navigation systems with satellite based navigation technology. Currently, the Global Positioning System (GPS) and its augmentations is the main satellite navigation system used all over the world for air navigation applications. The development of aircraft navigation has been performed with the enhancement of GPS augmentation systems. In this paper, aircraft navigation systems and techniques will be presented in this paper to evaluate the integrity and reliability of each system, and to make a comparison among these systems according to accuracy, integrity and availability.

Delayed PIC for Postcorrelation Mitigation of Continuous Wave and Multiple Access Interference in GPS Receivers

Several techniques are available to mitigate the problem of multiple access interference (MAI) caused by the limited dynamic range of the 1023-chip global positioning system (GPS) coarse acquisition Gold codes. MAI causes difficulties in several high-sensitivity applications, including indoor GPS and the use of pseudolites for GPS augmentation. This paper provides details on a variation on the existing techniques of successive interference cancellation (SIC) and parallel interference cancellation (PIC), but with the advantage of being more readily applied. This postcorrelation method is suitable for use in both hardware and software GPS receivers and is able to use commercially available GPS front-end chips employing 2-bit quantization of the received radio signal. The method can also be used to cancel some forms of continuous wave (CW) interference. Simulation results providing statistics on the effectiveness of the technique are provided, as well as results obtained when the technique was implemented in hardware on a GPS receiver based on a field programmable gate array correlator. The proposed method is also compared to some other MAI-mitigating techniques.

Analysis of Iridium-Augmented GPS for Floating Carrier Phase Positioning

Carrier-phase ranging measurements from Global Positioning System (GPS) and low-Earth-orbiting Iridium telecommunication satellites are integrated in a precision navigation system named iGPS. The basic goal of the system is to enhance GPS positioning and timing performance, especially under jamming. In addition, large satellite geometry variations generated by fast-moving Iridium spacecraft enable rapid estimation of floating cycle ambiguities. Augmentation of GPS with Iridium satellites also guarantees signal redundancy, which enables Receiver Autonomous Integrity Monitoring (RAIM). In this work, parametric models are developed for iGPS measurement error sources and for wide-area corrections from an assumed network of ground reference stations. A fixed-interval positioning and cycle ambiguity estimation algorithm is derived and a residual-based carrier-phase RAIM detection method is investigated for integrity against single-satellite step and ramp-type faults of all magnitudes and start-times. Predicted overall performance is quantified for various ground, space, and user segment configurations.

Unaugmented GPS-Based Flight Inspection System

A unaugmented Global Positioning System (GPS)-based flight inspection system (FIS) that performs airborne inspection of the instrument landing system (ILS) is introduced. This novel system relies on a TV positioning system (TVPS) that measures the horizontal position over the runway threshold and a radar altimeter (RA) to determine a reference point that is able to remove the GPS biases. Because of the near-real-time nature of flight inspection, it is possible to reconstruct an accurate approach trajectory even though the reference point occurs after the completion of the trajectory. The precise relative positioning (PRP) algorithm is applied to achieve the desired results. It includes a first-order fit to the ionospheric error, based on code and carrier measurements, that enables a significant reduction in this error source. This system has an autonomous integrity feature, called FIS-RAIM (receiver autonomous integrity monitoring), that is equipped in this system to protect against GPS satellite failures that may cause significant positioning errors. The FIS-RAIM is specifically designed for a flight inspection problem, and much of its detail is also described. Experimental flight tests strongly suggest that the system architecture and algorithm could meet FIS accuracy requirements for CAT III ILS with better performance than current flight inspection systems in terms of cost and efficiency. The system is not dependent on any GPS augmentation system; therefore the unaugmented GPS-based FIS provides low cost and high efficiency with worldwide availability.

Effects of Pseudolite Positioning on DOP in LAAS

In this paper, effects on DOP (Dilution of Precision) due to augmentation of Global Positioning System (GPS) with pseudolites are investigated. For this purpose, a typical Local Area Augmentation System (LAAS) scenario is consi-dered by placing pseudolites in various positions. It is found that only properly located pseudolites can improve the DOP. DOP values with two pseudolites located on either side of the run way are found to be the best. Geometric DOP (max) was found to be nearly 4 due to only GPS and came down to approximately 2 due to augmentation with two pseudolites. Implementation aspects of Bayes and Kalman filters while estimating DOP values are also examined.

Ambiguity resolution in precise point positioning with hourly data

Precise point positioning (PPP) has become a powerful tool for the scientific analysis of Global Positioning System (GPS) measurements. Until recently, ambiguity resolution at a single station in PPP has been considered difficult, due to the receiver- and satellite-dependent uncalibrated hardware delays (UHD). However, recent studies show that if these UHD can be determined accurately in advance within a network of stations, then ambiguity resolution at a single station becomes possible. In this study, the method proposed by Ge et al. J Geod 82(7):389–399, 2007 is adopted with a refinement in which only one single-difference narrow-lane UHD between a pair of satellites is determined within each full pass over a regional network. This study uses the EUREF (European Reference Frame) Permanent Network (EPN) to determine the UHD from Day 245 to 251 in 2007. Then 12 International GNSS Service stations inside the EPN and 15 outside the EPN are used to conduct ambiguity resolution in hourly PPP. It is found that the mean positioning accuracy in all hourly solutions for the stations inside the EPN is improved from (3.8, 1.5, 2.8) centimeters to (0.5, 0.5, 1.4) centimeters for the East, North and Up components, respectively. For the stations outside the EPN, some of which are over 2,000 km away from the nearest EPN stations, the mean positioning accuracy in the East, North and Up directions still achieves (0.6, 0.6, 2.0) centimeters, respectively, when the EPN-based UHD are applied to these stations. These results demonstrate that ambiguity resolution at a single station can significantly improve the positioning accuracy in hourly PPP. Particularly, UHD can be even applied to a station which is up to thousands of kilometers from the UHD-determination network, potentially showing a great advantage over current network-based GPS augmentation systems. Therefore, it is feasible and beneficial for the operators of GPS regional networks and providers of PPP-based online services to provide these UHD estimates as an additional product.

Altitudinal variation of midlatitude localized TEC enhancement from ground‐ and space‐based measurements

We present terrestrial and space‐based dual‐frequency observations of a region of enhanced total electron content (TEC) over the southeastern United States at local nighttime during the geomagnetic storm of 29–31 October 2003. The apparently localized, large‐amplitude, and nearly Earth‐fixed midlatitude ionosphere disturbance contained about 10 m higher delay at Global Positioning System (GPS) L1 frequency than the nighttime background ionosphere TEC. Using the dual‐frequency altimeter on board the Jason satellite, we show evidence that nearly all of the electron content was below its orbital altitude of 1300 km at 0000 local time on 31 October 2003. Dual frequency GPS measurements from the receiver on board the SAC‐C satellite indicate that some portion of the electron content existed above the 700 km orbit altitude of SAC‐C. We develop a horizontally piecewise constant regional model of the enhancement. We compare the model prediction of TEC with the SAC‐C satellite GPS data to constrain the altitude of this enhanced TEC region. Our model indicates that the peak density of the anomalous region is at slightly higher altitude and greater in amplitude than that of the background. The TEC enhancement provides a concrete case study of an extreme scenario that both space‐based and ground‐based GPS augmentation systems must take into account in order to offer high‐accuracy, high‐integrity corrections to GPS for safety‐of‐life applications.

Bounding higher‐order ionosphere errors for the dual‐frequency GPS user

Civil signals at L2 and L5 frequencies herald a new phase of Global Positioning System (GPS) performance. Dual‐frequency users typically assume a first‐order approximation of the ionosphere index of refraction, combining the GPS observables to eliminate most of the ranging delay, on the order of meters, introduced into the pseudoranges. This paper estimates the higher‐order group and phase errors that occur from assuming the ordinary first‐order dual‐frequency ionosphere model using data from the Federal Aviation Administration's Wide Area Augmentation System (WAAS) network on a solar maximum quiet day and an extremely stormy day postsolar maximum. We find that during active periods, when ionospheric storms may introduce slant range delays at L1 as high as 100 m, the higher‐order group errors in the L1–L2 or L1–L5 dual‐frequency combination can be tens of centimeters. The group and phase errors are no longer equal and opposite, so these errors accumulate in carrier smoothing of the dual‐frequency code observable. We show the errors in the carrier‐smoothed code are due to higher‐order group errors and, to a lesser extent, to higher‐order phase rate errors. For many applications, this residual error is sufficiently small as to be neglected. However, such errors can impact geodetic applications as well as the error budgets of GPS Augmentation Systems providing Category III precision approach.

A new multi-position calibration method for MEMS inertial navigation systems

The Global Positioning System (GPS) is a worldwide navigation system that requires a clear line of sight to the orbiting satellites. For land vehicle navigation, a clear line of sight cannot be maintained all the time as the vehicle can travel through tunnels, under bridges, forest canopies or within urban canyons. In such situations, the augmentation of GPS with other systems is necessary for continuous navigation. Inertial sensors can determine the motion of a body with respect to an inertial frame of reference. Traditionally, inertial systems are bulky, expensive and controlled by government regulations. Micro-electro mechanical systems (MEMS) inertial sensors are compact, small, inexpensive and most importantly, not controlled by governmental agencies due to their large error characteristics. Consequently, these sensors are the perfect candidate for integrated civilian navigation applications with GPS. However, these sensors need to be calibrated to remove the major part of the deterministic sensor errors before they can be used to accurately and reliably bridge GPS signal gaps. A new multi-position calibration method was designed for MEMS of high to medium quality. The method does not require special aligned mounting and has been adapted to compensate for the primary sensor errors, including the important scale factor and non-orthogonality errors of the gyroscopes. A turntable was used to provide a strong rotation rate signal as reference for the estimation of these errors. Two different quality MEMS IMUs were tested in the study. The calibration results were first compared directly to those from traditional calibration methods, e.g. six-position and rate test. Then the calibrated parameters were applied in three datasets of GPS/INS field tests to evaluate their accuracy indirectly by comparing the position drifts during short-term GPS signal outages.

Performance evaluation of GPS augmentation using quasi-zenith satellite system

The current global positioning system (GPS) provides limited availability and capability for a country like Japan where mountainous terrain and urban canyons do not allow a clear skyline to the horizon. At present, the Japanese Quasi-Zenith Satellite System (QZSS) is under investigation through a cooperative effort between the government and the private sector. QZSS is considered a multi-function satellite system, as it is able to provide communication, broadcasting, and positioning services for mobile users in a specified region with a high elevation angle. The additional GPS compatible signals from QZSS can remarkably improve the availability, accuracy, and capability of GPS positioning. This work focuses on the performance of GPS augmentation using the proposed QZSS. The QZSS satellite constellation and signal structure are briefly reviewed. Positioning with pseudo-range and carrier phase are discussed. The performance of GPS augmentation using QZSS in the Asian-Pacific and Australian area is studied using software simulations. The results are presented using the number of visible satellites as a measure of availability, GDOP as a measure of accuracy, and ambiguity success rate as a measure of capability of carrier-phase-based positioning with spatial and temporal variations. The results show that the QZSS will improve not only the availability and accuracy of GPS positioning, but will also enhance the capability of the GPS carrier-phase-based positioning in Japan and neighboring regions.

Local area augmentation of GPS for the precision approach of aircraft

The Federal Aviation Administration (FAA) is investigating the feasibility of using the Global Positioning System (GPS), together with local augmentation as the primary navigation system for the Category II and III precision approach of aircraft. This augmented system would be known as a local area augmentation system (LAAS) because it would place subsystems on the ground at or near an airport to greatly enhance the performance of airborne GPS receivers approaching that airport. The LAAS would complement space-based augmentations of GPS, such as the wide area augmentation system. When complete, the integrated system will meet stringent requirements on accuracy, integrity, continuity, and availability. Indeed, the LAAS will improve the airborne accuracy from approximately 100 m for stand-alone GPS to better than 1 m. This high accuracy is required so that LAAS can guarantee the integrity and continuity of the aircraft guidance during the approach phase of flight. Integrity requires the LAAS to detect any situations which threaten the safety of the landing, and notify the aircraft within 2 s of such a threat. Continuity is a competing consideration-it requires that the total number of alarms, true plus false, sent to the aircraft is very small. High continuity guarantees that the total number of aborted approaches is manageably small. Finally, availability is the fraction of the time during which LAAS is operational-providing position fixes with the specified accuracy, integrity and continuity. This paper describes the LAAS which has been proposed by the FAA. It discusses the fundamental operation of the LAAS.