# Background and Recent Advances in the Locata Terrestrial Positioning and Timing Technology
## 1 Introduction  
The field of Positioning, Navigation, and Timing (PNT) has been an active research and development focus for many decades. The Global Navigation Satellite System (GNSS) is undoubtedly the most widely used PNT technology today, with user equipment and techniques having evolved over the past few decades to their current highly-developed forms. Despite this success, GNSS cannot satisfy the PNT requirements in many commonly traversed environments, such as in urban areas and indoors. A variety of technologies have been developed to compensate for such limitations, which include short-range Radio Frequency Identification (RFID), Ultra-Wideband (UWB), Zigbee, WiFi, Bluetooth, ultrasonic, new ranging systems that are based on pseudolites, image-based technologies, inertial sensors for dead-reckoning, magnetic-based, proximity sensors, mobile telephony, etc. However, there is no technology that can be used independently, or together with GNSS and other systems, which has the advantages of GNSS, and in particular can deliver accurate PNT solutions that are similar to advanced and augmented GNSS techniques, such as Differential GNSS (DGNSS) or Precise Point Positioning (PPP).  
A number of researchers have claimed that pseudolites can be used to enhance PNT availability, reliability, integrity, and accuracy in applications, such as aircraft approach and landing [1, 2], deformation monitoring applications [3], Mars exploration [4], and others [5, 6, 7, 8, 9]. However, extensive research and testing has concluded that pseudolites have fundamental technical problems that, even in a controlled or lab environment, are extremely difficult to overcome [7, 8, 10, 11]. The challenges of optimally siting pseudolites, controlling transmission power levels, overcoming \"near-far\" problems, trying to ensure extremely high levels of time synchronisation, configuring special antennas, and designing the \"field of operations\", such that GNSS and pseudolites can work together (or at least not interfere with each other), have been largely insurmountable in the real world [12].  
Other solutions have been sought, due to these difficulties. One is the \"Locata\" concept that is based on pseudolite principles but with significant enhancements. Locata Corporation developed the Locata technology, and it has evolved since 1995 with academic collaboration of the University of New South Wales (UNSW Sydney) and the University of Nottingham (UK). Locata is a terrestrial Radio Frequency (RF)-based system that has several unique characteristics which make it superior to conventional pseudolite-based systems [13; 14; 15]. This paper presents a literature review of the Locata technology and its applications, while also discussing its main challenges. The rest of the paper is structured, as follows. Section 2 describes the core elements of the Locata technology. Section 3 presents the development history and mathematical models for Locata precise positioning. In Section 4, some PNT applications for which the Locata technology has been implemented are reviewed. Section 5 discusses the main challenges that may restrict its future application for navigation in urban environments. Finally, Section 6 presents concluding remarks and recommendations.