# A Single RF Emitter-Based Indoor Navigation Method for Autonomous Service Robots
## 4 RF-Based Target Localisation
### Angle-Based Differential RSS Localisation Validation  
A test prototype, shown in Figure 5, was constructed for the experiment. The prototype was able to be rotated through a full 360\({}^{\circ}\) of motion, with points marked at 30\({}^{\circ}\) increments for testing RSS in different directions. Mounting points were provided for the reflector dish and receiver such that the receiver was both in the focal point of the dish and the centre of rotation. The parabolic reflector dish was constructed from an aluminium sheet of 0.5 mm width, and aluminium foil was added to some tests to provide shielding on the reverse side. Tests were carried out for a reflector of length 300 mm and height 200 mm, shaped to radii of 80 mm, 130 mm, and 150 mm. Additional tests were also conducted without a reflector in order to compare the improvement in directionality against a control.  
Figure 4: Angle-based differential RSS localisation algorithm.  
Figure 5: Test prototype for testing the gain of a single receiver node which is mounted on a rotational platform at a fixed position relative to the transmitter, but allowing varying orientation.  
The transmitter (i.e., the emitter) was positioned in a fixed location relative to the receiver. This allowed the test prototype to be rotated while measurements were taken and values recorded. A number of tests were performed, varying the reflector radius as well as the positions of both receiver and transmitter. For each test, the RSS results were tabulated and graphed against the angle. In addition to the monopole antenna shown in Figure 6, tests were also carried out with a Splatch ISM antenna to demonstrate the feasibility and effectiveness of this method.  
Our test results indicate that adding a parabolic reflector improved the directionality of the antenna, as can be seen in Figure 6, which illustrates an example case. As expected from the supporting theory, the reflector with the largest aperture size provides better performance and was able to achieve an error of \(\pm 15\) degrees. However, this error is also due to the fact that the step size of the angle increments was selected to be 30 degrees, so this is essentially the minimum precision.  
Once these tests were completed, a second node was added for the purpose of testing two paired receivers in order to measure the difference in signal strength. Testing of the paired receivers included trials with no reflectors and tests where each antenna had a reflector of 130 mm radius similar to the above. By using paired receivers, this test was more similar to the intended use of the antennae. When reflectors were used, the absolute differences at opposing orientations (i.e., the difference between the two receivers aligned approximately with the transmitter) indicated a significant difference in power received. This difference could then be used to isolate the direction of origin of a signal, and thus the direction of the target emitter.  
Using the RSS signal, we are also able to derive an approximate measure of distance to the target emitter. However, this is very unreliable due to the presence of noise and multipath effects, so we only use this for the purposes of path planning for setting a target destination. In the case where no path  
Figure 6: Comparative analysis of the Received Signal Strength (RSS) for an antenna with and without a reflector. The RSS (normalised as a ratio of the mean value) shows improved directionality, with the strongest RSS in the direction of the true angle when using a parabolic reflector.  
planning is used, the robot will simply navigate in the target direction, until the distance is sufficiently and reliably small to give us confidence that the robot has arrived at the destination.