The Potential of a Ground Based Transceivers Network for Water Dam Deformation Monitoring

The Global Navigation Satellites Systems (GPS, GLONASS and the future GALILEO) has proven to be a useful tool for precision deformation monitoring applications in structural engineering. For continuous structural deformation monitoring on an epoch-by-epoch basis it is desirable for a measurement system to deliver equal precision in all position components, all the time. However, the quality of GPS position solutions is heavily dependent on the number and geometric distribution of the available satellites. Therefore, the positioning precision varies significantly and is three times less in vertical than horizontal coordinates. This situation becomes worse when the line-of-sight to GPS satellites is obstructed to structures, as it is the case for the big water dams when trying to access other interesting parts than only the crest, reducing the number of visible satellites often to less than four. A new positioning technology developed by Locata Technology Australia, that uses a network of ground-based transceivers that cover a specific area of a water dam with strong signals is becoming part of Leica Geosystems solution. This paper discusses the technology and assesses its suitability for use in structural deformation monitoring applications. is the requirement for differential corrections or measurements from a single reference station or Continuously Operating Reference Station (CORS) Network. Acceptable performance from GPS in structural deformation monitoring type applications is therefore heavily dependent on the reliability of the wireless data link used, and on a relatively unobstructed sky-view, where there are at least five satellites with good geometry available. To address these significant limitations of the GPS Locata has developed a novel positioning technology. 2 LOCATA’S POSITIONING TECHNOLOGY The Locata approach to GPS positioning in challenging environments is to deploy a network (LocataNet) of ground-based transceivers (LocataLites) that cover a local area with strong ranging signals. Figure 2 illustrates conceptually how a LocataNet can be deployed to allow positioning both indoors and outside in an urban environment. Importantly, the LocataNet positioning signals are time-synchronized, which allows single-point positioning in the same manner as GPS. However, unlike GPS the sub-centimeter level of synchronization between LocataLites allows single-point positioning with cm-level GPS accuracy without the use of a reference station. There are several other innovative characteristics of a LocataNet that will be included in the final design including: autonomous installation, ad hoc capability, expansion and coupling, and scalability, that have been discussed previously (Barnes et al. 2003c.). In July 2003 Locata came out of ‘stealth-mode’, by publishing the first results of a prototype system (Barnes et al. 2003a). Over the past two and a half years, proof-of-concepts for core aspects of the Locata technology have been verified, and trials in applications ranging from industrial machine (Barnes et al. 2004a) guidance to structural deformation monitoring (Barnes et al. 2004b) have demonstrated stand-alone cm-level point positioning. Over the past two and a half years, the proof-of-concept of this prototype system has clearly been demonstrated, but not without a number of significant limitations, including: • Interoperability with GPS – the prototype LocataLite transmitted a GPS L1 C/A code signal. • Known point initialisation – the prototype Locata receiver was required to visit a point with known coordinates before accurate cm-level positioning was possible, in order to resolve carrier phase ambiguities. • Limited multipath mitigation – as a result of using a GPS L1 C/A code signal structure. • Limited transmitter range and penetration – limited transmission power allowed in GPS L1 band, to mitigate interference with GPS signals. Figure. 1 GPS satellite availability in Sydney (4/11/05), 15 degree cut-off mask: PDOP (lower-line), GDOP (upper-line), number of satellites (bar-chart). Figure 2. Conceptual LocataNet installation providing outdoor and indoor coverage. 2.1 Locata’s Current System Locata’s current system (the next generation design) has been built to address the limitations of the prototype system. This current system incorporates Locata’s own proprietary signal transmission structure that operates in the 2.4GHz ISM band (license free). With complete control over both the signal transmitter and receiver comes enormous flexibility. This has allowed the limitations in the old system to be addressed with a completely new design for both the LocataLite (transceiver) and Locata receiver. Core aspects of the new system design are summarised in Table 1 and discussed in the following sections. 2.1.1 Signal Structure The first generation Locata system transmitted using the same L1 C/A code signal structure as GPS. Using the GPS frequency for signal transmissions has significant limitations for several reasons. The rules for transmitting on L1 vary throughout the world, but there is no doubt that a license for wide deployment of a ground based system on L1 would be 4extremely difficult (if not impossible) to obtain. If a license was granted, ensuring there was no GPS signal degradation or interoperability issues would be of paramount importance. As a result this would limit the LocataLite’s capability in terms of transmitter power and therefore operating range and penetration into buildings. It would also place a practical limit on the number of LocataLites in a LocataNet to ensure that no interference or degradation of the GPS signal quality occurred. Therefore Locata’s new design incorporates a proprietary signal transmission structure that operates in the 2.4 GHz Industry Scientific and Medical (ISM) band. The 2.4 GHz ISM band has a bandwidth of approximately 80Mhz (2.4–2.4835 GHz), and, for direct sequence spread spectrum signals, FCC regulations (Parts 15 & 18) allow a transmit power of up to 1 watt. It is anticipated that this transmit power will allow line-of-sight LocataLite signals to be received from over 10 km away. Within the ISM band the LocataLite design allows for the transmission of two carrier signals. The exact frequencies at this stage are proprietary information, but within the ISM band equates to a carrier wavelength of between 12.49 to 12.07 cm. These two carrier signals are modulated with a proprietary PRN code with a chipping rate of 10MHz (giving a chip length of approximately 30 metres). This new signal structure is beneficial in a number of areas in comparison to Locata’s first generation system including: • Interoperability with GPS and no licensing requirement. • Capability for On-The-Fly ambiguity resolution using dual frequency measurement data. • Better multipath mitigation on code measurements due to higher 10Mhz chipping rate, and theoretically less carrier phase multipath than GPS due to the higher frequency used. • Transmit power of up to 1 watt giving line-ofsight range of 10km. Table 1. Specification summary of Locata’s first and current generation systems. First Generation System (prototype since 02) Current Generation System (commercial deployment Q1/06) Current Status