Satellite Based Augmentation System Research Articles

Analysis of Ionospheric Range Delay Corrections for Navigation in South American Low-Latitude Regions

Ionospheric conditions for South American low- and mid-latitude scenarios are simulated. The performance of an ionospheric correction algorithm on positioning is analysed for this region. This correction is of similar nature to the Satellite Based Augmentation System (SBAS) type algorithm. The mismodelling produced by each ionospheric simulated approximation can be separately quantified: 1) the single layer shell representation of the ionosphere and 2) the simple geometric mapping function. The effects of both components on positioning are evaluated and discussed for periods with different levels of ionospheric activity: winter, summer, and austral spring equinox. The results show that the mapping function is the most important contributor to the ionospheric error. Its effect on the height component is the most important. Besides, on north and east components, the principal error contributor is the Vertical Total Electron Content (VTEC) mismodelling. The application was also tested on real data during a spring equinox of a mid-low solar activity year (2005) and the results are similar and coherent with those obtained using simulated data.

Quality Control in SBAS: Protection Levels and Reliability Levels

This contribution extends the common documented approach of integrity through Protection Levels in Satellite-Based Augmentation System (SBAS) positioning for aeronautics, to reliability on the basis of statistical hypothesis testing, and as such provides a safeguard against model misspecifications as anomalies and outliers in the measurements. It is shown that when integrity is monitored through Protection Levels and reliability added through Reliability Levels, the availability of the SBAS position solution is more than 99% for APV-I precision approach. The availability for CAT-I is currently just a few percent. When the Galileo constellation is added, and current performance is copied ahead, the percentage for CAT-I increases to beyond 95%.

Flight test results and analysis of SBAS-like algorithm from the implementation of the Asia-Pacific GNSS test bed

AbstractThe Asia-Pacific GNSS Test Bed is a regional collaborative programme that brings together aviation practitioners in the field of satellite navigation within the Asia-Pacific to study the performance of global navigation satellite systems (GNSS) and to develop a regional plan that will lead to a successful implementation of GNSS in the region. The program is a work program under the Asia-Pacific Economic Cooperation GNSS Implementation Team (APEC GIT), a working group under the APEC. The Test Bed has been in operation since August 2006. The system architecture reflects hybrid architecture between a satellite-based augmentation system (SBAS) and a ground-based augmentation system (GBAS). Test reference stations (TRSs), consisted of a dual-frequency GPS receiver, a communication interface, and a data archival hardware, have been installed in the participating economies. GPS data collected at the TRSs will be transmitted to the test master station (TMS) in Bangkok, Thailand. SBAS messages will be generated and the messages will be broadcasted through a test VHF station (TVS). A test user platform (TUP) will receive GPS signal and the SBAS messages broadcast from TVS and then calculate and archive the TUP’s positions.This paper discusses performance analysis and flight test results conducted in Thailand in September 2007. During flight trials, noting that the Test Bed broadcasts SBAS messages through a VHF data link (VDL) similar to that of GBAS, the impacts of VHF broadcast, such as transmitting power, terrain effects, and impacts of different aircraft attitudes and positions are analysed. Preliminary analysis result indicates that, while the GBAS VDL coverage is very good through out the 30nm airport terminal area, during approach, and even during taxi, considerations should be given to levels of transmitting power to eliminate possible intermittent loss of VHF messages received by the aircraft receiver. This intermittent loss of VHF messages results in very strong fluctuation of horizontal protection levels (HPL) and vertical protection levels (VPL) calculated by the TUP. To yield with possible problem, windowing techniques are proposed and analysed.

Modelling of low-latitude ionosphere using modified planar fit method for GAGAN

Several countries have been developing satellite-based augmentation systems (SBAS) for improving positional accuracy of global positioning system (GPS). India is also developing one such system popularly known as GPS aided geo augmented navigation (GAGAN) system. Modelling of ionospheric effects is one of the major challenges in developing precise and reliable GAGAN. The high values of total electron content (TEC), the large diurnal and seasonal variability and intense irregularities present in the low-latitude ionosphere, lead to unacceptable positional errors in GAGAN service region. Todd Walter et al. of Stanford University, USA have made significant contribution in the area of SBAS ionospheric grid modelling by developing the popularly known planar fit model. It is reported by Walter et al. (2000) that a constant decorrelation value of 35 cm was proposed over the wide area augmentation system (WAAS) service region. To provide accurate estimation of ionospheric delays at user ionospheric pierce points (IPP) during a storm or intense irregularities, an irregularity detector and a decorrelation adaptor are incorporated in the modelling. As planar fit model is not adequate to model intense irregularities of Indian ionosphere, a modified planar fit model is applied for GAGAN data on similar lines as was done for WAAS and the results are encouraging.

Position error bound calculation for GNSS using measurement residuals

In safety-of-life applications of satellite navigation, the protection level (PL) equation translates what is known about the pseudo-range errors into a reliable limit on the positioning error. The current PL equations for satellite-based augmentation systems (SBAS) rely on Gaussian statistics. This approach is very practical: the calculations are simple and the receiver computation load is small. However, when the true distributions are far from Gaussian, such a characterization forces an inflation of the PLs that degrades performance. This happens in particular with errors with heavy tail distributions or for which there is not enough data to evaluate the distribution density up to small quantiles. We present a way of computing the optimal protection level when the pseudo-range errors are characterized by a mixture of Gaussian modes. First, we show that this error characterization adds a new flexibility and helps account for heavy tails without losing the benefit of tight core distributions. Then, we state the positioning problem using a Bayesian approach. Finally, we apply this method to PL calculations for the wide area augmentation system (WAAS) using real data from WAAS receivers. The results are very promising: vertical PLs are reduced by 50% without degrading integrity.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

Preliminary Experiments of GPS/INS Based Integrity Monitoring Using MSAS Differential Correction Data

Japan Aerospace Exploration Agency (JAXA) has developed several GPS/INS systems called GAIA (GPS Aided Inertial navigation Avionics) for over ten years and succeeded in automatic landing of unmanned experimental vehicle in differential mode. In order to use GAIA for civil aviation, JAXA has been developing a fault detection and exclusion (FDE) software for integrity monitoring based on filter bank method. On the other hand, a Japanese satellite based augmentation system, MSAS (MTSAT Satellite-based Augmentation System) is planned to be operational in 2007, and the test signal is transmitted. A flight experiment to collect MSAS data was conducted and GPS/MSAS data were used to compute horizontal protection level (HPL). The GPS/INS filter bank method showed a reduced HPL, compare to MSAS-based HPL.

New Algorithm and Results of WAAS Ionospheric Delay Correction

电离层介质的色散性是影响电磁波信号进行卫星导航定位精度的重要因素之一.配合北斗二代分系绩研制任务,提出了一种新的电离层折射校正算法,并利用2000年7月1日到3日的双频GPS观测数据对6个用户站进行试算,进一步将试算所得均方根误差和电离层网格算法得到的误差进行比较.结果表明,对于中纬区域的用户站,估算的TEC误差约为0.5 m左右;而低纬用户误差相对增大,为1 m左右.文中给出的算法与电离层网格模型所提供的精度相差不多,在未来中国自主的卫星增强系统中采用新方法进行电离层进行修正是可行的及有效的.

Development of the EGNOS Pseudolite System

In order to access the Satellite Based Augmentation System (SBAS) service, the end user needs to have a direct line of sight to at least one of the Geostationary Earth Orbit (GEO) satellites transmitting the augmentation messages. This requirement is critical for users in environments such as city canyons, valleys and fjords since high buildings and mountains in the vicinity of the end user can easily block the lines of sight to the GEO satellites. The situation becomes worse at high latitudes because of the low elevation angles to the GEO satellites. Even a very low obstacle can block the lines of sight to the GEO satellites. This limitation reduces SBAS Signal in Space (SIS) availability significantly at high latitude especially for land applications. This paper presents a solution of transmitting the European Geostationary Navigation Overlay Service (EGNOS) SIS using a pseudolite. The EGNOS pseudolite functions in a similar way as a GEO satellite. It will provide not only a terrestrial-based solution for transmitting the EGNOS SIS, but also a ranging measurement for the navigation solution. The EGNOS pseudolite system mainly consists of a Master Control Station, an EGNOS Data Server, EGNOS pseudolites and the user terminal. A preliminary test on a surveyed site has been carried out to verify the functionalities of the system. The data set collected from the test has been processed with two scenarios: one with four GPS satellites, while the other with three GPS satellites plus an EGNOS pseudolite. Both data processing scenarios have similar satellite geometries. The test result shows that the positioning accuracies are similar for both scenarios.

Global navigation satellite system (GNSS): a utility for sustainable development in Ghana

The rapid spread of modern information and communication technologies (ICT) is dependent on the unprecedented increase in information relating to our natural, economic and social environment, and for Ghana and Africa to be in tune with the current trends in ICT, spatial information and methods to acquire them should be our priority. GNSS is therefore the obvious choice. This technology has an enormous potential to contribute to the management of environment, natural disasters, provide food security, emergency response, improve the efficiency in surveying and mapping. Land, water and air navigation will undergo a dramatic improvement with the application of GNSS. This is just to mention a few of the expected benefits. This paper focuses on the benefits to be derived by Ghana as a developing country and Africa in general from the introduction of a functional Multipurpose Global Navigation Satellite System. It specifically highlights the needed basic infrastructure for setting up a Continuously Operating reference System and the need for acquiring the necessary infrastructure to utilize the European Satellite-Based Augmentation System (SBAS), EGNOS. Areas of probable application and benefits have been outlined and the necessary considerations for a successful implementation and its sustenance have been suggested. It looks at how this space technology can support our sustainable development as developing nations.Journal of Science and Technology (Ghana) Vol. 27 (2) 2007: pp. 130-138

SBAS Algorithm Performance in the Implementation of the ASIAPACIFIC GNSS Test Bed

This paper discusses the preliminary performance analysis result of the Asia-Pacific Global Navigation Satellite Systems (GNSS) Test Bed. Currently, seven Asia-Pacific economies are participating in the Test Bed project, namely Australia, Chinese Taipei, Indonesia, Malaysia, the Philippines, Thailand and Vietnam. The Test Bed was commissioned in May 2006. The discussion topics in this paper include Test Bed system architecture and preliminary analysis of the system performance. As presented in this paper, while current Satellite-Based Augmentation System (SBAS) algorithms can improve the accuracy performance of GPS positioning results, it cannot fulfill the integrity performance required by civil aviation community. This paper analyzes the limitation of the algorithm and proposes future research topics related to the limitation.

衛星型GNSS補強システムにおける対流圏遅延補正方式

A strategy to realize the tropospheric delay correction in satellite based augmentation systems is presented. The tropospheric delay is one of the major error sources of the GNSS signals. It is attempted to generate the correction information from the numerical weather prediction data to realize the tropospheric delay correction in real-time for mobile positioning. To introduce the correction information into the satellite based augmentation system, the amount of the correction data must be minimized without degrading the correction accuracy. In this paper, through several statistical evaluations using the long-term observation data, the appropriate contents and the data amount of the correction message are investigated. It is shown that the correction message should consist of the information of the tropospheric zenith delay and its average gradient in height. The amount of the correction data is also successfully minimized, and users in the region of Japan can receive the sufficient correction information by receiving only 3 messages. The results of the analyses clearly show the feasibility of the presented strategy.

Improving the ionospheric delay correction of satellite-based augmentation systems in equatorial regions

In 2005, China designed and constructed independent satellite systems ‘Beidou II’, which indicated the start of a new area in Chinese space-based positioning, navigation and timing systems. It is necessary and important to investigate the performance of ionospheric delay correction in the region of China. In this paper, a new algorithm of satellite-based augmentation systems (SBAS) ionospheric delay correction is proposed and tested using observational data at 18 GPS stations. The data are used during periods from January 4 to 23, 2000. The computing results show that precision is high for user stations in the higher mid-latitudes with the average root mean square (RMS) of around 0.4 m. The precision is relatively lower for the lower latitude, which is more prominent for the equatorial region from latitude 20°N to 25°N and from longitude 100°E to 120°E. Quite a few prediction errors surpass 2 m, and the worst case reaches a maximum of 3 m.

Geostationary L-band signal scintillation observations near the crest of equatorial anomaly in the Indian zone

Microwave L-band signal at 1.5 GHz from geostationary satellite INMARSAT (64°E) has been recorded on a continuous basis since 1990 at Calcutta (22.58°N, 88.38°E geographic; 32°N magnetic dip). The location of Calcutta, virtually under the northern crest of the equatorial anomaly in the Indian longitude sector, provides an excellent platform for studying equatorial scintillations. This paper presents a long-term study of scintillations at 1.5 GHz observed from Calcutta extending from minimum to high sunspot number (1996–2000). The variations of scintillations with local time, season, solar and magnetic activity are presented. L-band scintillations have been found to be essentially a pre-midnight phenomenon during the equinoctial months of high sunspot number years. Both the occurrence and intensity increase dramatically with solar activity level. During the solar maximum phase of 1998–2000, scintillations in excess of 20 dB occurred for 20% of total time in the pre-midnight hours of March 2000. Considering individual hours, a maximum percentage occurrence of 37 was recorded in the interval 19–20 LT. Although the percentage occurrence appears to be low, scintillations with Scintillation Index (SI)>20 dB were recorded on 23 out of 31 nights during the same month, indicating that scintillations do not occur continuously over a long interval of time, but in small patches with clear gaps in between. The distribution of patch duration during years 1998–2000 has a maximum in the time interval of 1–3 min. From the cumulative distributions of patch duration with SI⩾10 and 20 dB, the 99th percentile values of patch duration have been found to be 49.3 and 39.6 min, respectively. This information may be used to obtain an estimate of longitudinal separation of geostationary satellites to be used in Satellite Based Augmentation System (SBAS). During the equinoctial months of August–October and February–April, occurrence of scintillations is less during magnetically active (Kp=3+–9) conditions than during quiet (Kp=0−3). In the local summer months of May–July, the probability of occurrence of scintillations is more under magnetically active condition than in quiet condition. Equatorial scintillation is essentially controlled by the pre-reversal enhancement of the eastward electric field near sunset, which in turn has season and solar activity dependences. During magnetic activity, the generation of equatorial irregularities resulting in scintillations may be ascribed to the prompt penetration of magnetospheric electric field and disturbance dynamo effects.

Scattering height estimation using scintillating Wide Area Augmentation System/Satellite Based Augmentation System and GPS satellite signals

An experiment to measure equatorial amplitude scintillations on the geostationary Wide Area Augmentation System (WAAS) Satellite Based Augmentation System (SBAS) signal was conducted in Cachoeira Paulista (22.70°S, 45.01°W geographic coordinates; −17.74°N, 21.74°E geomagnetic coordinates), Brazil from December 2003 through February 2004. The purpose of this paper is to estimate the scattering height of the irregularities using the WAAS signal scintillations as compared to nearby Global Positioning System (GPS) signal scintillations. Estimating the scattering height is important because the calculated zonal drift velocity of the irregularities using the measured scintillation pattern velocity on the ground is height dependent. Accurate height estimation is also required if one wishes to develop phase screen scintillation models. The difference in the pattern velocities is due to the different signal puncture point velocities with respect to the ionospheric drift. Two east‐west receivers are used to measure the scintillation pattern drift velocity and to compare the results of the geostationary WAAS satellite signal to that of a GPS satellite signal, which has a nonzero ionospheric signal puncture point velocity. By varying the assumed scattering height for the measurements from the nearby GPS signal, the zonal velocity measurements from the GPS scintillations can be matched to those of the WAAS scintillations, and a scattering height estimate can be made. When the puncture points have minimal separation, the inferred ionospheric irregularity zonal velocities should be equal. On the two nights for which data are available, scattering height estimates of 669 ± 209 km for the first night and 388 ± 139 km for the second night were obtained. On the second night, the reported mean hmF2 as calculated using a collocated Digisonde was 385 ± 17 km over the same period as the GPS/WAAS scattering height estimate. The geometry of this experiment was not optimal, but analysis demonstrates that future experiments could be substantially more accurate.

위성항법시스템 적용을 위한 전리층 지연값 기울기 연구

Ionospheric storms, caused by the interaction between Solar and geomagnetic activities, may degrade the differential GNSS(Global Navigation Satellite Systems) performance significantly, and the importance of the ionospheric storm research is growing for the GBAS(Ground-Based Augmentation System) and SBAS(Satellite-Based Augmentation System) development. In order to support Korean GNSS augmentation system development, a software tool for analyzing the regional ionosphere is being developed and its preliminary results are discussed. After brief description of the ionosphere and ionospheric storm, the research topics on the GBAS applications are discussed. The need for ionospheric spatial gradient analysis is described and some results on the ionospheric spatial gradient during recent storm periods are discussed.

Flying EGNOS Approaches in the Swiss Alps

The European Geostationary Navigation Overlay Service (EGNOS) system is being developed in Europe to provide Global Positioning System (GPS) and GLONASS regional augmentation services to aviation, maritime and land users. The EGNOS system, as any other Wide Area Augmentation System (WAAS), relies on the broadcast of differential correction and integrity information in the pseudo-range domain, which are then used to provide a solution in the position domain. EGNOS is a major element of the European Satellite Navigation Program, which is jointly being implemented by the Commission of the European Union, the European Space Agency (ESA) and Eurocontrol (the European Organisation for the Safety of Air Navigation). It is also the first European step to the GALILEO system.As part of the EGNOS validation activities, flight trials have been organised by ESA and the EGNOS Industrial Consortium at various locations in Europe during Spring 2005. To demonstrate the system capability in a challenging mountainous environment, tests have been conducted at Lugano airport in the Swiss Alps. Due to the difficult topography of the airport and its surroundings, the use of conventional ground based navigation aids present some limitations. For the trials, a new Satellite Based Augmentation System (SBAS) procedure has been designed to take advantage of the system flexibility. In particular, a reduction of the approach glide path angle has been achieved, potentially allowing more aircraft types to fly the approach than today. This article presents the operational benefits that could be obtained with the new test procedure. The very impressive EGNOS performance is also described in details, showing that it can support Approach Procedure with Vertical guidance (APV) operations even in a very challenging environment.

Estimation of Satellite-Based Augmentation System Grid Size at Low Latitudes in the Indian Zone

The Total Electron Content (TEC) measured from a station situated near the northern crest of the equatorial anomaly is compared with that obtained from models such as the Parameterized Ionospheric Model (PIM 1.6) and International Reference Ionosphere (IRI-95) for well over one solar cycle (1977–1990). The limitations of conversion from vertical to slant TEC and vice versa as required for GPS ionospheric corrections in the equatorial region are discussed. It is found that the correspondence among the vertical TEC at the ionospheric pierce point, geometrically (sec χ) converted slant TEC, and slant TEC along a GPS signal propagation path becomes poor for elevation angles of less than 80 deg. Based on this finding, an optimum grid size for reliable operation of the Satellite-Based Augmentation System (SBAS) in the Indian subcontinent (GAGAN, or GPS and Geo Augmented Navigation) is estimated. The suggested grid size is much smaller than the standard 5 × 5 deg.

The ionospheric impact of the October 2003 storm event on Wide Area Augmentation System

The United States Federal Aviation Administration’s (FAA) Wide-Area Augmentation System (WAAS) for civil aircraft navigation is focused primarily on the Conterminous United States (CONUS). Other Satellite-Based Augmentation Systems (SBAS) include the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-transport Satellite-based Augmentation System (MSAS). Navigation using WAAS requires accurate calibration of ionospheric delays. To provide delay corrections for single frequency global positioning system (GPS) users, the wide-area differential GPS systems depend upon accurate determination of ionospheric total electron content (TEC) along radio links. Dual-frequency transmissions from GPS satellites have been used for many years to measure and map ionospheric TEC on regional and global scales. The October 2003 solar-terrestrial events are significant not only for their dramatic scale, but also for their unique phasing of solar irradiance and geomagnetic events. During 28 October, the solar X-ray and EUV irradiances were exceptionally high while the geomagnetic activity was relatively normal. Conversely, 29–31 October was geomagnetically active while solar irradiances were relatively low. These events had the most severe impact in recent history on the CONUS region and therefore had a significant effect on the WAAS performance. To help better understand the event and its impact on WAAS, we examine in detail the WAAS reference site (WRS) data consisting of triple redundant dual-frequency GPS receivers at 25 different locations within the US. To provide ground-truth, we take advantage of the three co-located GPS receivers at each WAAS reference site. To generate ground-truth and calibrate GPS receiver and transmitter inter-frequency biases, we process the GPS data using the Global Ionospheric Mapping (GIM) software developed at the Jet Propulsion Laboratory. This software allows us to compute calibrated high resolution observations of TEC. We found ionospheric range delays up to 35 m for the day-time CONUS during quiet conditions and up to 100 m during storm time conditions. For a quiet day, we obtained WAAS planar fit slant residuals less than 2 m (0.4 m root mean square (RMS)) and less than 25 m (3.4 m RMS) for the storm day. We also investigated ionospheric gradients, averaged over distances of a few hundred kilometers. The gradients were no larger than 0.5 m over 100 km for a quiet day. For the storm day, we found gradients at the 4 m level over 100 km. Similar level gradients are typically observed in the low-latitude region for quiet or storm conditions.

Evaluating the accuracy of ionospheric range delay corrections for navigation at low latitude

Abstract La Plata Ionospheric Model (LPIM) corrections in ionospheric range delay can be studied by analysing their effect on geodetic positioning. To evaluate these corrections, a scheme similar to that used in Satellite-Based Augmentation System (SBAS) is employed in this work. So, in this context an appropriate model for computing the ionospheric range delay and its error is required. By means of numerical simulation, the accuracy of LPIM for computing the ionospheric range delay correction at low latitude is evaluated. The slant range delays for the observing network are simulated using the NeQuick ionospheric model. The simulated ionospheric range delay correction applied to geodetic positioning is analysed at different levels of ionospheric activity as well as to solstices and equinoxes. The large errors of the ionospheric correction on geodetic positioning are obtained in high solar activity during the equinox. This work shows that the accuracy achieved using simulated data at low latitude is similar to the accuracy reported using real data at mid latitude.