## 2 Background and Related Work
### Underwater Communications  
The ability to transmit data is necessary for certain AUV missions. In many cases, a command and control entity may want to provide additional instructions to an AUV based on what the AUV has sensed in the environment. Facilitating this command and control requires two-way communications, including while the AUV is submerged. Existing technologies rely on sonar [18], radio frequency (RF) [19], and blue-green lasers to communicate [20]. Sonar uses pulses of sound to propagate through water as waves, is relatively slow compared to blue-green lasers and RF [20; 21], and can dissipate rapidly or be impeded by underwater structures. Radio waves propagate via electromagnetic fields and can pass through almost any matter or in a vacuum, but they are significantly attenuated at most frequencies by water, which limits their effective C2 range and bit-rate [19]. While sonar and RF C2 links have been deployed for some time, and blue-green lasers have been proposed and demonstrated for many decades [22], blue-green lasers are relatively new to underwater applications and have not been widely fielded. Furthermore, unlike Extremely Low Frequency (ELF) antenna transmissions, blue-green lasers require a surface or airborne asset (often a satellite) to enable communications back to shore [23].  
In spite of the limitations of RF transmission underwater, it is sometimes the preferred form of communication with the surface and other underwater assets, and it is the proposed communication method for the WIEVLE [4]. Many different antenna designs have been proposed and implemented for underwater C2 links [24; 25; 26]. Antennas that can deploy and be retracted have been proposed and implemented for a variety of purposes [27]. To enable the WIEVLE and other AUVs that operate passively for extended periods in a given space to communicate as a mobile network and receive external signals via RF link, an antenna that can be autonomously stowed and deployed is desired. Fixed antennas external to the WIEVLE hull would interfere with the ability to maneuver in locations with a high risk of entanglement. An internal antenna may interfere with onboard electronics and intelligence, surveillance, and reconnaissance (ISR) sensors, have undesirable transmission characteristics, and take up valuable space that could be used for other critical components within its relatively small profile. Thus, it is desirable to integrate the WIEVLE's antenna with an anchor system.  
Underwater radio C2 links can operate at various frequencies and power levels based on the required bandwidth and transmission distance. Various antenna types, such as dipoles, j-poles, and loops, have been used [28] from high frequencies (3-30 MHz) to low frequencies--especially extremely low frequencies in the 3-30 Hz range, using long monopole antennas (sometimes many kilometers long) that are constructed out of a conducting material that can be unrolled and trailed behind a moving AUV [29]. Higher frequencies transmit at shorter distances underwater but often have higher data throughput capabilities [30]. Higher-power transmissions can propagate farther, although in small AUVs, the additional power consumption may be too high for the system to support the load.  
A variety of protocols have been developed specifically for underwater data transmission to handle low-bandwidth and high-interference environments. The JANUS protocol was developed by NATO and is available for anyone to use [31]. It primarily operates on sonar (acoustic) C2 links [32]. Other protocols, such as the SDRT, have been developed for underwater sensor networks [33]. Many more proposed protocols exist in the literature, with many focusing on sonar C2 links, but some using RF C2 links [34; 35; 36]. Only protocols designed to work via RF transmission are applicable to the proposed RAATM integrated antenna.