# A Single RF Emitter-Based Indoor Navigation Method for Autonomous Service Robots
## 4 RF-Based Target Localisation  
### Related Works  
In order to effectively determine the location of the RF emitter as a navigation goal, a method of RF ranging was chosen. Traditionally, RF-based ranging includes Received Signal Strength (RSS), Time Of Arrival (TOA), Time Difference Of Arrival (TDOA), Angle of Arrival (AOA) and Phase Difference Of Arrival (PDOA) [23; 24]. Each of these has its own advantages and disadvantages. RSS works by using the amount of power lost over a distance by an RF signal. It has the advantage of giving an estimate in almost all cases and being easy to implement, but it is known to fluctuate and is heavily affected by multipath effects and interference [25; 26]. Time of Arrival is implemented by measuring the time taken for a signal to propagate between two nodes. This is typically more accurate than RSS when the two nodes have a clear line-of-sight, but accuracy decreases significantly without line of sight [27]. TDOA compares the time of arrival between a node with a known location and the node to be localised. This is accurate if well calibrated, but requires a sufficiently large distance between the two nodes for the difference in arrival time to be perceptible for fast moving signals [9]. The phase difference of arrival measures the phase at the node to be ranged and a known node, and uses the difference between these two to calculate the distance [23]. In this work, we use AOA, which is measured by using one of these methods of ranging over an array of antennae, and then apply basic trigonometry to work out the angle of origin [24]. The number of antennae needed can be reduced through the use of directional antennae. When these antennae are oriented in different directions, the difference in RSS between them can then be used to find the AOA. Using directional antennae is only needed when using RSS to determine range, as the other ranging methods are not affected by the type of antenna [26]. Summaries of various localisation technologies with their measurement techniques and accuracy are available in [23; 28].  
There are a number of existing studies that use RF media for localisation. One example provided in [29] uses the RSS of Wi-Fi signals to track a moving object by detecting changes in the signal strength. In [30], three systems that use Radio Frequency Identification (RFID) tags for 3D real-time localisation are presented. In [8], a pseudo-Doppler method is used to locate RF emitters using a sensor platform that detects the line of bearing (LOB) to the emitter. Rapid switching between multiple antennae is used to find the phase difference which is then converted into a directional angle. By using multiple autonomous platforms, they can communicate and send information to each other in order to optimise their paths and determine the location of the emitter. This project aims to achieve a similar goal; however, multiple robots can be very costly and unsuitable for some applications. Therefore, we aim to achieve the goal of determining the angle to the emitter using a single robot by measuring the RSS in multiple directions and using the strongest, and therefore most likely, signal as the direction of origin.