# A Single RF Emitter-Based Indoor Navigation Method for Autonomous Service Robots
## 1 Introduction  
As the prevalence of autonomous vehicles and robots (collectively referred to as robots in this paper) increases, the challenge of navigation in new and unknown environments (both indoor and outdoor) is a critical barrier to widespread adoption of these systems [1; 2]. There are two key components that allow a robot to know where it should go--some form of mapping is required to know the positions of boundaries and obstacles to be avoided, and localisation is needed to allow the robot to know where it is relative to those movement constraints and how to best reach the target location. Simultaneous Localisation and Mapping (SLAM) techniques enable robots to achieve both of these tasks at the same time while operating within the environment [3]. Given a distant objective or goal location, using SLAM enables path planning and overall navigation of the system by dynamically improving the quality of the path over time to account for obstacles and other constraints [4; 5]. The majority of existing indoor navigation systems [6; 7] rely on vision-based approaches, which demand high computational power, costly equipment, and a stable environment without significant environmental changes over time. In cases where these obstacles cannot be visuallyseen using cameras or human vision due to a lack of line-of-sight, methods involving wireless signals need to be investigated instead, such as the use of the radio spectrum [8], sonar [9], or fixed points as virtual landmarks. However, this generally means that we need to deploy significant infrastructure to support these methods, such as multiple powered beacons with known positions. This is sometimes not possible in real-world applications, for instance, in factory or warehouse automation where raw materials or products may be placed at random locations and need to be delivered to other locations according to dynamic production schedules. In these ad-hoc cases, we still need to allow autonomous robots to perform some level of localisation in order to achieve tasks that otherwise may be challenging or time-consuming for humans, such as locating and fetching materials from a multi-storey warehouse or removing newly discovered hazardous waste from a construction site. Global Positioning Systems (GPS) may not always be available for localisation, particularly in an indoor context where the accuracy of GPS is much lower than in outdoor scenarios, or in a natural or human-caused disaster where satellite communications may be disrupted. In this work, we propose a new approach of using a single emitter as a reference position in a global map, building on our previous work [10]. Such an emitter could be battery-powered and attached to a target object, giving an autonomous robot a target destination from its current location. In comparison to traditional RF-based approaches, we process the broadcasted messages to extract direction estimates relative to the robot's current position in addition to the distance estimates. This is achieved primarily through a novel method that utilises multiple directional antennae to determine the direction of a single emitter beacon, relative to the robot, based on the Received Signal Strength (RSS). The direction estimates compensate for the inaccuracy of traditional RSS-based approaches that rely only on distance estimates. When combined with SLAM and our motion control algorithm, this enables a robot to manoeuvre towards that target without prior knowledge of the environment or any additional infrastructure deployment. Importantly, the emitter may not be in a fixed position, such as in a factory automation [11] or indoor delivery [12; 13; 14; 15] context where targets may be mobile. We use a Pioneer 3-DX mobile robot [16] as a testbench for our method, with the goal of enabling the Pioneer robot to perform navigation by mapping a cluttered or unknown environment while localising itself relative to a single emitter of non-fixed position.