HandHeld ComputIng researCH

This work presents a new efficient positioning module that operates over client-server LBS architectures. The aim of the proposed module is to fulfil the position information requirements for LBS pedestrian applications by ensuring the availability of reliable, highly accurate and precise position solutions based on GPS single frequency (L1) positioning service. The positioning module operates at both LBS architecture sides; the client (mobile device), and the server (positioning server). At the server side, the positioning module is responsible for correcting user’s location information based on WADGPS corrections. In addition, at the mobile side, the positioning module is continually in charge for monitoring the integrity and available of the position solutions as well as managing the communication with the server. The integrity monitoring was based on EGNOS integrity methods. A prototype of the proposed module was developed and used in experimental trials to evaluate the efficiency of the module in terms of the achieved positioning performance. The positioning module was capable of achieving a horizontal accuracy of less than 2 meters with a 95% confidence level with integrity improvement of more than 30% from existing GPS/EGNOS services. to mobile users. LBS represent integration between position determination technologies, mobile communication and location related contents. LBS are currently being deployed for different civilian applications such as contextual advertising, user warning and alerting, transport, gaming, dynamic objects tracking and DOI: 10.4018/jhcr.2010070101 2 International Journal of Handheld Computing Research, 1(3), 1-18, July-September 2010 Copyright © 2010, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. mobile guidance (Rapera et al., 2007; Filjar et al., 2008). Mainly LBS can be implemented in two architectures, one as stand-alone, where the mobile unit is equipped with on board positioning devices, maps and geographical information, which are used to provide the user with required service locally. The second architecture is described as client-server based, in which services are remotely delivered to users either on demand or consecutive dependent on the application type being implemented. LBS application’s performance mainly depends on the capability and reliability of its components. This includes the positioning technology performance in terms of the achieved service availability, position accuracy and integrity. Also, the mobile network’s latency, available bandwidth, and data rates along with the mobile handsets memory capacity and processing power plays an important role in delivering the required service to the user. LBS applications require up-to-date and accurate location related information, such as maps, images, voice and video records, transport and weather updates, and so forth (Tsalgatidou et al., 2003; Aredo et al., 2003). Generally, LBS applications deliver sensitive information services related to the user’s location. Therefore, a critical aspect of LBS implementation is identifying a suitable positioning technology that is capable of efficiently determining where (user accurate location) and when the required services are delivered. Currently, there are several positioning technologies available for navigation purposes in different LBS applications. However, this work focuses on the Global Positioning System (GPS) as the most widely deployed positioning technology. PoSItIonIng technoLogIeS BAckground Generally, the positioning technologies are divided into two major categories. The first one is described as network-based which involves different types of implementations such as mobile network positioning, in which mobile signals and the network infrastructure are used to locate the mobile device utilising several methods such as Angle of Arrival (AoA), Time of Arrival (ToA) and Enhanced Observed Time Difference (E-OTD). Also, this category includes wireless local network (Wi-Fi) and Radio Frequency Identification (RFID) based positioning, these methods are mainly used for position determination in local scales and indoor environments (Esmond, 2007). However, the network based positioning techniques are still not widely implemented as stand alone solutions because of its accuracy limitations. In addition, network operators still are not considering that LBS applications are typically to be utilised by all mobile phone users. The second main category is known as satellite-based positioning, in which satellite signals are received by handheld receivers and used to position the mobile device based on a triangulation process of three or more different signals. This technology is known as the Global Navigation Satellite Systems (GNSS) such as GPS which has been widely utilised for a variety of air, land and sea applications. GPS is considered as the cornerstone of positioning in LBS applications because of its simplicity of use, inexpensive implementation, and global availability (Filjar, 2003). However, the positioning performance provided by a single frequency GPS receiver has proved to be insufficient for some precision and accuracy demanding applications (Kaplan & Hegarty, 2006). The performance degradation of GPS is due to several error sources such as poor satellite geometry, satellite orbital shifting, clock errors, multipath effects, atmospheric delays and GPS receiver internal processing errors. These limitations escalate in urban environments and densely areas as there is a significant possibility for the signals to be jammed and blocked due to high obstructing buildings and difficult landscapes. A considerable attention has been carried out during the last decades trying to augment GPS positioning services among multiple signal error sources. As a result, different methods have emerged such International Journal of Handheld Computing Research, 1(3), 1-18, July-September 2010 3 Copyright © 2010, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. as Differential GPS (DGPS) systems allowing GPS signal errors to be reduced or eliminated based on pseudo-range or carrier-phase differential correction procedures. DGPS systems are available with different coverage ranges, construction, augmentation data formats and data deliverability means, (Kaplan & Hegarty, 2006). Mainly, two types of DGPS systems are available. Local Area DGPS (LADGPS) systems providing limited coverage to the users based on their distance to the DGPS reference station (<100 m). Wide Area DGPS (WADGPS) systems implemented by a network of DGPS reference stations covering a wide region and being interconnected at a centralised location, this is known as multi-reference DGPS systems. Additionally, Satellite Based Augmentation Systems (SBAS) such as the Wide Area Augmentation System (WAAS) and the European Geostationary Navigation Overlay Service (EGNOS) are considered as WADGPS systems. In SBAS, the differential data are collected from a group of interconnected monitoring and differential stations and then broadcasted to the users using GEO satellites covering a whole region such as Europe and North America. EGNOS is the European version of SBAS, developed by the European Tripartite Group; the European Space Agency (ESA), the European Commission (EC) and EUROCONTROL. EGNOS is the European’s contribution to the first generation of GNSS (GNSS-1) and a primary step towards Galileo. EGNOS provides augmentation service for GPS, GLONASS and future Galileo. EGNOS infrastructure consists of four Mission Control Centres (MCC), six navigational Land Earth Stations (NLES), and thirty-one Reference Stations, described as Ranging and Integrity Monitoring Station (RIMS). Three EGNOS geostationary satellites (Inmarasat-3, IND-W, and ARTEMIS satellites) are successfully transmitting EGNOS augmentation signals consisting of orbit and clock corrections of all GPS satellites, ionospheric delays and integrity information of the GPS system (Gauthier et al., 2006) In the same concern, the increase of Internet capability and accessibility has made it possible to use the network as an alternative method for transmitting augmentation data for real time GPS users. The Signal in Space via the Internet (SISNet) was developed to allow access to the wide area differential corrections and integrity information obtained from EGNOS in the Radio Technical Commission for Aeronautics (RTCA) format, (Torán-Martí et al., 2002). SISNet data is accessible free of charge to internet users and only requires valid privileges from the European Space Agency (ESA). Additionally, the Network Transport of RTCM via Internet Protocol (Ntrip) was introduced for broadcasting GPS differential corrections in Radio Technical Commission for Maritime (RTCM SC-104) format over the internet to authorised users (Dammalage et al., 2006). This technology is currently being utilised by several national and regional DGPS networks, such as the Ordnance Survey GPS Network (OS Net) in UK. OS Net is a network of several DGPS reference stations providing real time L1 DGPS and Real Time Kinematics (RTK) correction for users in Great Britain (Ackroyd & Cruddace, 2006). The OS Net DGPS corrections can be received in real time from the OS Net Ntirp caster via the Internet, radio or mobile channels. In this work, the concept of using the internet network as a source of GPS augmentation data is utilised by the proposed positioning module at the server side (positioning server). This includes the reception of EGNOS data from SISNet (GPS/EGNOS-SISNet solution) and networked-DGPS corrections from the OS Net Ntrip caster (GPS/DGPS-Ntrip solution). The use of these two solutions has offered a guaranteed availability of correction data which is required to augment the roving user’s GPS measurements for improved positioning performance. This has also reduced the amount of processing power and memory space required to perform the correction calculations at the mobile device. The concept of EGNOS integrity monitoring was also utilised and implemented at the mobile device, to identify situations were additional GPS assisted data and accurate position solutions are needed from the positioning 4 International Journal of Handheld Computing Research, 1(3), 1-18, July-September 2010 Copyright © 2010, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. server. The following section describes details of EGNOS integrity calculations. egnoS IntegrIty MonItorIng Utilising the EGNOS service allows the user’s receiver to get the following types of information (RTCA, 2001): ➢ Satellite information such as the ephemeris data of the tracked satellites and associated corrections. ➢ Ranging information, including GPS satellite clock and ephemeris errors corrections, and ionospheric corrections. ➢ Measurement integrity information, provided in the form of variances related to two types of error corrections; the UDRE for the satellite clock corrections and ephemeris, as well as the variance for Grid Ionospheric Vertical Error (GIVE). This information is carried by the following EGNOS message types: 1. Message types 2-5 contain the fast corrections in pseudo-ranges and UDRE values for each satellite. 2. Message type 6 might be transmitted containing all UDRE’s in case of a system alarm. 3. Message type 7 specifies the fast correction degradation factor indicator for computing the degradation of fast corrections. 4. Message type 18 and 26 contain ionospheric correction information and the corresponding GIVE values. 5. Message type 24 is a mixed fast and slow correction message. 6. Message type 25 provides error estimates for slow varying satellite ephemeris and clock errors. The GPS receiver combines satellite and user geometry information with EGNOS corrected pseudo ranges to compute the user’s position. Moreover, the use of EGNOS integrity data allows the calculation of useful integrity factors, such as the Horizontal Protection Level (HPL SBAS ) and Vertical Protection Level (VPL SBAS ) corresponding to the horizontal and vertical position solutions respectively (RTCA, 2001; Walter, 2003). Every time a protection level is calculated it should be compared with its identified Position Error (PE) upper bound, known as the Alert Level (AL). The Misleading Information (MI) situations or the integrity failure events are determined based on samples with HPL SBAS >HAL and VPL SBAS >VAL for the horizontal and vertical position solutions, where VPL and HAL are the vertical and horizontal alert levels. An integrity failure event can be caused by several reasons such as equipment breakdown and measurement noise. EGNOS integrity monitoring method is based on the mathematical expressions which were originally developed for aviation navigation purposes (RTCA, 2001). However, this concept can be modified for the needs of pedestrian’s and vehicle navigation with respect to the integrity multipliers (Kfactors) that should be adjusted according to the application requirements (Abwerzger et al., 2004). EGNOS integrity monitoring is based on the estimation of the variances in pseudo-range measurements for all tracked satellites; this can be presented by (RTCA, 2001):