Space Based Augmentation System Research Articles

Comparison of standard TEC models with a Neural Network based TEC model using multistation GPS TEC around the northern crest of Equatorial Ionization Anomaly in the Indian longitude sector during the low and moderate solar activity levels of the 24th solar cycle

The highest Total Electron Content (TEC) values in the world normally occur at Equatorial Ionization Anomaly (EIA) region resulting in largest ionospheric range delay values observed for any potential Space Based Augmentation System (SBAS). Reliable forecasting of TEC is crucial for satellite based navigation systems. The day to day variability of the location of the anomaly peak and its intensity is very large. This imposes severe limitations on the applicability of commonly used ionospheric models to the low latitude regions. It is necessary to generate a mathematical ionospheric forecasting and mapping model for TEC based on physical ionospheric influencing parameters. A model, IRPE-TEC, has been developed based on real time low latitude total electron content data using GPS measurements from a number of stations situated around the northern crest of the EIA during 2007 through 2011 to predict the vertical TEC values during the low and moderate solar activity levels of the 24th solar cycle. This model is compared with standard ionospheric models like International Reference Ionosphere (IRI) and Parameterized Ionospheric Model (PIM) to establish its applicability in the equatorial region for accurate predictions.

Performance Evaluation of EGNOS in Challenging Environments

Space-based augmentation systems (SBAS), such as EGNOS, are largely used to complement GPS for accurate and reliable positioning, which is required by rapidly growing location-based services (LBS). However, it is challenging to use EGNOS in the environments including urban areas and marginal area of the monitoring networks, where many LBS are delivered. Through the experiments in the challenging observation conditions, this study first evaluates the performance of EGNOS in these environments. Challenges consist in two aspects: EGNOS signals may be interrupted by blockages; EGNOS messages are not produced at all for marginal geographical areas due to the lack of raw satellite measurements. In order to use EGNOS for enhanced positioning performance in these environments, this paper then discusses several potential solutions. It is concluded that the two autonomous approaches, i.e. using aged corrections and mixing corrected and uncorrected satellites, can improve the positioning accuracy with a stand-alone receiver, and a full EGNOS positioning performance can be achieved in urban areas via a terrestrial access to EGNOS data, for example, the Internet connection with a smartphone. This paper discusses the effectiveness and usability of these approaches.

On constellation design of multi-GNSS radio occultation mission

The FORMOSAT-3/COSMIC mission which is a joint Taiwan-US mission for meteorological, climatological, ionospheric, and geodetic studies has successfully provided a significant amount of GPS radio occultation (RO) data for better modeling of climate model, forecasting of weather, and monitoring of ionosphere. Owing to the success of the FORMOSAT-3 program, the follow-on FORMOSAT-7 mission is being planned. In the perceivable future, sources for RO mission can be originated from multiple global navigation satellite systems (GNSSs) including GPS, Galileo, GLONASS, and Beidou as well as several regional navigation satellite systems (RNSSs) and space-based augmentation systems (SBASs). To facilitate the operational capability for weather forecast and space weather monitoring, the FORMOSAT-7 constellation must be designed to ensure a sufficient and uniform distribution in space and time for GNSS/RNSS/SBAS RO events. In addition, it is desired that the RO events in certain region can be maximized to account for severe weather. As the distribution of RO events due to GNSS, RNSS, and SBAS are distinct and the requirements appear to be multi-objective, the constellation design becomes very complicated. The paper applies genetic algorithm (GA) to design constellation parameters. It is shown that the adopted approach can be used to account for non-uniform distribution of RO events due to RNSSs and SBASs. The GA-based design approach is demonstrated to result in constellations that are superior in both global distribution and regional emphasis of RO events to those of the existing baseline design.

An algorithm for estimation and separation of ephemeris and clock errors in SBAS

The estimation and separation of ephemeris and clock errors is an integral part of a SBAS (Space Based Augmentation System). Generally, the global solution is based on the full state approach for satellite errors (ephemeris and clock) and station errors, using a large least square estimator; or the other way is to sequentially estimate the ephemeris and clock through a Kalman filter, using a complex model of the satellite dynamics. In this paper, the estimation and separation of ephemeris and clock errors is addressed through a unique approach of combining both the methods. The algorithm employs measurements, which are pre-processed for various errors and known biases. A single difference technique is used to separately estimate the ephemeris and clock components. The ephemeris Kalman filter uses a priori information of ephemeris errors along with measurements through a minimum variance estimator to provide ephemeris error estimate. A similar approach is adopted in the clock error estimation process, to provide clock and clock rate estimates. The algorithm results are presented using simulated data for known errors in ephemeris/clock and subsequent retrieval. This algorithm estimates these errors as corrections to the broadcast Global Positioning System (GPS) navigation data, required by a SBAS user for accuracy improvement.

MATLAB Algorithm Availability Simulation Tool

This paper describes a set of MATLAB functions currently being developed for Space Based Augmentation System (SBAS) availability analysis. This toolset includes simulation algorithms that are constantly being developed and updated by various working groups. This set of functions is intended for use as a fast, accurate, and highly customizable experimental test bed for algorithm development. A user-friendly interface has also been developed for the tool. It is open source and can be downloaded from the Stanford GPS Research Laboratory web site (http://waas.stanford.edu/~wwu/maast/maast.html). Therefore, it provides a common ground for different working groups to compare their results. This paper demonstrates the utility of this toolset by analyzing the SBAS service volume models for the Conterminous United States. The paper describes the functionality provided within the software, and shows an example set of contour plots which are the means for investigating how different algorithms and parameters impact availability.

Multi‐constellation GNSS/RNSS from the perspective of high accuracy users in Australia

Global Navigation Satellite Systems (GNSSs) involve satellites, ground stations and user equipment, and are now used to support many activities within modern societies. The U.S. Global Positioning System (GPS) is the best known, and only currently fully operational GNSS. Russia also operates its own (not yet fully deployed) GNSS called GLONASS. Fuelling growth in applications during the next decade will be next generation GNSSs that are currently being developed and deployed. Next generation GNSSs will include the U.S.’s modernized GPS and planned GPS‐III, Russia's revitalised GLONASS, Europe's GALILEO system, and China's planned COMPASS system. Furthermore, a number of Space Based Augmentation Systems (SBASs) and Regional Navigation Satellite Systems (RNSSs) will add extra satellites and signals to the multi‐constellation GNSS/RNSS ‘mix’. How will this proliferation of satellites and signals impact on the spatial information disciplines? Is the concept of a ‘system of systems’ receiver, and associated ground infrastructure, to support high accuracy positioning feasible, or even desirable? This paper explores some of the issues of multi‐constellation GNSS/RNSS from the perspective of high accuracy users such as geodesists, surveyors and mappers, in Australia.

GPS satellite and receiver instrumental biases estimation using SVD algorithm

The category I precision approach (CAT I PA) requirements can be achieved by augmenting the GPS. The space- based augmentation system (SBAS) in India is known as GPS aided geo augmented navigation (GAGAN). One of the prominent errors in GAGAN that limit the positional accuracy is instrumental bias. The calibration of these biases is particularly important in achieving the CAT I PA landings. In this paper, singular value decomposition (SVD) based least mean square (LMS) algorithm is proposed to estimate the instrumental biases. The algorithm uses the values of one month period corresponding to three GAGAN stations. The experimental results indicate that the estimation precision of the satellite biases is of the order of plusmn0.6 ns. The estimated receiver bias for Hyderabad station using the proposed algorithm is -4.21 ns. The algorithm is validated with both fitted receiver bias method and hardware calibrated value. It is found that results are good and consistent over the observation period and confirm the validity of the developed algorithm.

GPS satellite and receiver instrumental biases estimation using least squares method for accurate ionosphere modelling

The positional accuracy of the Global Positioning System (GPS) is limited due to several error sources. The major error is ionosphere. By augmenting the GPS, the Category I (CAT I) Precision Approach (PA) requirements can be achieved. The Space-Based Augmentation System (SBAS) in India is known as GPS Aided Geo Augmented Navigation (GAGAN). One of the prominent errors in GAGAN that limits the positional accuracy is instrumental biases. Calibration of these biases is particularly important in achieving the CAT I PA landings. In this paper, a new algorithm is proposed to estimate the instrumental biases by modelling the TEC using 4th order polynomial. The algorithm uses values corresponding to a single station for one month period and the results confirm the validity of the algorithm. The experimental results indicate that the estimation precision of the satellite-plus-receiver instrumental bias is of the order of ±0.17 nsec. The observed mean bias error is of the order −3.638 nsec and −4.71 nsec for satellite 1 and 31 respectively. It is found that results are consistent over the period.

Symmetric Overbounding of Correlated Errors

: In aviation navigation systems such as the Ground-Based and Space-Based Augmentation Systems (GBAS and SBAS) for GPS, it is critical for users to compute a conservative navigation-error bound during precision approach and landing. This paper describes a method for overbounding outputs of linear systems (i.e., conservatively describing their error distributions). Whereas earlier overbounding methods apply only to sets of independent samples, the methods developed in this paper handle samples drawn from correlated time series, so long as those samples have a probability distribution which a linear mapping can make spherically symmetric. Conservative error bounds constructed with these methods are useful in modeling the worst-case performance of filters under a wide range of environmental conditions. An example illustrates how symmetric bounding could enable aircraft to apply user-customized filtering to differential GPS corrections in GBAS.

Mapping and Survey of Plasma Bubbles over Brazilian Territory

Ionospheric plasma irregularities or bubbles, that are regions with depleted density, are generated at the magnetic equator after sunset due to plasma instabilities, and as they move upward they map along the magnetic field lines to low latitudes. To analyse the temporal and spatial evolution of the bubbles over Brazilian territory, the mapping of ionospheric plasma bubbles for the night of 17/18 March 2002 was generated using data collected from one GPS receiver array, and applying interpolation techniques. The impact on the performance of Global Navigation Satellites System (GNSS) and on the Space Based Augmentation System (SBAS) in the tropical regions of the GPS signal losses of lock and of the signal amplitude fades during ionospheric irregularities is presented.

First observations of SBAS/WAAS scintillations: Using collocated scintillation measurements and all‐sky images to study equatorial plasma bubbles

The first observations of amplitude scintillations on a Space Based Augmentation System (SBAS) satellite signal are presented. The scintillations occurred on the signal transmitted by a Wide Area Augmentation Satellite (WAAS) on 8–9 September 2004 from 2250–0045 LT. The GPS receiver that measured the scintillations is located on Haleakala, Hawaii (geomagnetic: 21.3°N, 271.4°E). With a maximum S4 = 0.35, corresponding to a peak‐to‐peak SNR variation of 8 dB, the scintillations are relatively weak, which is to be expected for a site poleward of the equatorial anomaly during declining solar conditions. Using a collocated all‐sky imager, features of the irregularity structuring in the equatorial plasma bubbles are resolved. The satellite signals scintillate when the ray path intersects the three main bubbles. The scintillation intensity tends to peak near the walls, and decreases slightly in the interior of the bubbles. In this case, the bubbles' leading (east) walls contain smaller‐scale‐size irregularities than the trailing (west) walls.

GEO Orbit Determination via Covariance Analysis with a Known Clock Error*

ABSTRACT: Geostationary earth orbit (GEO) satellites serve as relays to transmit messages for space-based augmentation systems (SBASs). GEOs can act as ranging sources for SBAS users as well, adding to the availability and continuity of a number of SBAS services. The SBAS determines the GEO orbit error from covariance analysis of satellite dynamics and station-to-satellite geometry. However, separating the orbit determination errors from the clock errors is difficult, and GEO range errors tend to be much larger than GPS range errors. This paper proposes a technique for making the clock state of a “bent-pipe” GEO a known quantity, then considers the effect on the derived GEO range through a simulation of the orbit determination extended Kalman filter. The study indicates that this technique reduces the GEO orbit error by a factor of ˜22 and implies that GEO orbit determination errors can be made comparable to those of GPS satellites.