Mobile Navigation Unit (MNU) Operational Method

To find a particular user‟s location, the MNU largely focuses on the attached dual frequency RTK receiver with DGNSS capability. In case the positioning information and augmentation data at the MNU are unavailable, then an alternative source of direction is utilised, which consists of the NSC and the integration with live streaming video and voice. In Figure 5.9, a flowchart of the MNU operational method is shown, which depicts the order of procedures taking place at the MNU to find out the essential positioning coordinates.

Start

Figure 5.9: The MNU operational method.

As shown in Figure 5.9, the operational method executed at the MNU includes several processes. This starts with connection of wireless communication, which is accountable to search and connect to an existing wireless communication network (e.g., HSDPA or UMTS). Another process is data acquisition, which is involved for acquiring the augmentation data from the dual frequency RTK receiver and video, and the voice data from the intergraded components at the MNU. As mentioned already, two types of positioning data are acquired for the dual frequency RTK receiver: the code and carrier from L1 and L2 of the GPS and GLONASS signals (Chapter 4). The data update rate is 20 times per second (20 Hz). The memory of MNU accumulates these data types and then they are retrieved on the biases of the correction data update time intermissions.

Subsequently, two important processes are performed to verify the validity and integrity of the augmentation data and to select the applicable DGNSS reference stations. Based on the user‟s initial estimated position, this allows only valid and reliable augmentation data to be utilised.

In situations, for example, if inadequate positioning data is available at the MNU or the GPS and GLONASS satellites cannot be seen properly and no applicable DGNSS stations exist, the system gives an alternative directional solution based on video and voice communication. This is called the manual guidance method, and is the second position guiding method proposed by the navigation model system. The MNU would function in a server-based positioning mode, in which a communication session is opened with the NSC to transmit a video image of the immediate environment of the visually impaired person, carry out one-way communication, and to receive voice data from NSC in the bi-directional communication, in order for the visually impaired to avoid obstacles in the route.

The video camera built into the user‟s terminal (MNU) and the video display in the guide‟s terminal (NSC) make up the system‟s remote vision facility – utilised to assist in micro-navigation. Both the macro- and micro-navigational assistance (together constituting remote sighted guidance) are provided as verbal macro- and micro-navigational instructions (the guidance instructions). The use of the manual guiding method is not restricted on the GNSS signals availability constraints, only on video and voice communications limitations. However, it can be utilised for

reliable and continuous positioning if the user continuously indicates in-door or densely urban environments.

The above descriptions belong to the first and second scenarios. In the third scenario, when NRTK positioning, video, and voice data are unavailable at the MNU, the system has an option to activate a different system for guidance, demonstrated as symbol B in Figure 5.9.

The alternative system is the already established Adaptive, Reliable, and Accurate Positioning Model for Location-Based Services model, applying GPS augmentation techniques, such as EGNOS/SISNET by Al Nabhan (2009).

The model is based on the single frequency GPS Standard Positioning Service (SPS). The positioning model operates over a client-server architecture, including two main components, described as the Localisation Server (LS) and the Mobile Unit (MU), and the positioning model operates in two position determination modes. The stand-alone mode is used if enough navigation information is available at the MU using its local positioning device (GPS/EGNOS receiver).

Otherwise, the server-based mode is utilised, in which the LS intervenes and starts providing the required position solutions. At the LS, multiple sources of GPS augmentation services are received using the Internet as the sole augmentation data transportation medium. The augmentation data is then processed and integrated for guaranteeing the availability of valid and reliable information required for the provision of accurate and precise position solutions (Al Nabhan, 2009).

In Al Nabhan‟s (2009) research, the results showed that the LS, through the developed position computation methods, was able to provide position samples with an accuracy of less than 2 m, with high precision at the 95% confidence level. This was achieved in urban, rural, and open space (clear satellite view) navigation environments.

The last position solution used in navigation service provision is received from the NSC using the RTK positioning corrections and the video and voice communication received from MNU. Alternatively, as already mentioned in this

chapter, in the absence of a positioning correction or unavailable satellites, the navigation service provision will be obtained from the NSC using the visual image captured and transmitted by the user at the MNU; therefore, employing the manual guiding method solely.

The MNU performs the position calculation by using one advanced method:

coordinate domain positioning. In this work, real-time navigation guidance has been used; therefore, the coordinate domain positioning method has been operated. The real-time augmented positioning data is significant for the visually impaired user at the MNU. A detailed description of the processing procedures for real-time GNSS positioning is presented in the following chapter.