## 6 Summary and Outlook  
In this paper, we presented a compact, low-cost localization architecture based on the EM wave attenuation principle. The system provides satisfying performance with an accuracy in the centimeter range at a fraction of the cost and size of existing systems. Our main contribution is the extension of the system introduced in [3; 4; 5; 6; 7] to the \(\mu\)AUV domain. The main system components are anchored beacons emitting EM signals in a frequency band centered around 433 MHz and a passive mobile receiving unit, which measures the RSS values to estimate its position relative to the beacons. The hardware part of the signal processing chain was realized with an off-the-shelf USB DVB-T dongle capable of sampling parts of the EM spectrum and an SBC, in this contribution a Raspberry Pi Zero. An FFT was implemented on the SBC to determine the RSS from each beacon within the received power density spectrum. Calibration allows us to fit the two parameters of the URSM to the RSS values and the distances between the mobile receiver unit and the corresponding beacons. The measured RSS values were fused to estimate the position of the receiver unit based on the RSM by deploying Bayesian filtering algorithms, such as EKF. Since the receiver unit is passive, only one-way signal transmission is required, and the system is not affected by the number of fleet members. We demonstrated the performance of our architecture in experiments for static and dynamic applications and analyzed the EM field characteristics of the test tank. The experiments have shown an accurate and reliable localization of the receiver unit. However, the accuracy depends highly on the receiver's position within the test tank. The results show that within the convex hull of the beacons, the localization is very accurate, whereas the accuracy decreases with larger distances from beacons and also in the vicinity of the tank walls. As the static position measurements have shown very small variances, we assume that the localization accuracy would improve significantly in a larger test environment, as inhomogeneities of the EM field that are mostly due to reflection from the tank walls would be reduced. Further investigations should be conducted on the EM field characteristics. This includes the influence and improvement of antenna design. In order to analyze the system performance independently from the tank characteristics, we plan to deploy our system in a larger water tank with characteristic lengths of approximately 5 to 10 m to achieve a better homogeneity of the EM field. Moreover, we plan to extend the system capabilities to enable 3D localization, which is challenging due to the directivity of the antennas. Therefore, a possible option is to use the approach of [6] and extend it to \(\mu\)AUVs. It adds more beacons to the system and stacks them in multiple horizontal planes. The received measurement signals are to be fused together with the actual depth measurement of the \(\mu\)AUV to obtain the absolute position in 3D space. The goal is to embed the system in \(\mu\)AUVs and perform closed-loop position control. This work was partially funded by the German Research Foundation (DFG) under Grant Number Kr 752/33-1. The support is greatly acknowledged. The authors would like to thank [PERSON] (Pohang University of Science and Technology) for his inspiration and the fruitful discussions on the topic of EM underwater localization. [PERSON] (Hamburg University of Technology) provided valuable insides to antenna theory and RF instrumentation. The authors would also like to thank [PERSON], [PERSON] and [PERSON] for their support with the hardware setup. This publication was supported by the German Research Foundation (DFG) and the Hamburg University of Technology (TUHH) in the funding programme \"Open Access Publishing\". [PERSON] and [PERSON] initiated and coordinated the project. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] participated in the definition and development of the software and hardware architectures.