## 3 RLS Design
### Transmitting Phase  
This phase proposes a new message exchange mechanism which is based on _event-driven_ reporting. All previous literatures trigger the localization process with broadcasting beacon signals, periodically. While these periodic messages are severely damaged due to the very low bandwidth of acoustic channels and highly increased overhead, the RLS scheme proposes a reverse transmitting phase to avoid excessive overhead and significantly reduce the number of messages exchanged  
In terms of transmitting a message, two different types of reports are available such as _event-driven_ or _demand-driven_[23]. In _event-driven_ based transmitting, one or several sensor nodes detect an event and report it to a monitoring station, while, in _demand-driven_ reporting sensors remain silent until they receive a request from the monitoring station. Figure 3 illustrates this concept more clearly.  
Figure 3: **(a)** Event-driven report; **(b)** Demand-driven report.  
Figure 2: RLS architecture.  
RLS is a Reverse Localization Scheme, whereby localization is triggered from a/some detector sensor(s) in deep water and broadcast a localization applicant message toward the surface anchors. The first step in the transmitting phase starts whenever a phenomenon is observed or detected by an underwater sensor, it immediately broadcasts a message toward anchors on the surface using acoustic signals.  
The signal reaches the surface anchors via the most direct path in order to reduce the delay and also decrease the number of message exchanged in the network. The multiple nondirect path signals will always arrive at the receiver antenna later than the direct path signal [22]. It is practical solution while most of acoustic modems have large range values because they are designed to work in applications where the distance between nodes are in kilometers [2], as addressed in Table 1. Moreover, the packet size of RLS is significantly reduced to transmit directly. The localization packet format is given in Figure 4, where _Node-ID_ is the unique identification number of the message sender; \(T_{S}\)(sending time) is the time when this message is sent and _Depth_ is related \(z\) coordination of node location which is available from pressure sensor.  
\(T_{S}\) field is employed to estimate the distance between detector sensor and receiver anchor. The distance is measured with the Time of Arrival (TOA) technique, which will be described in Section 3.3. It is obvious that the size of the sent message is significantly reduced compared to existing localization algorithms such as ML, SLMP and ARTL [7,8]. In sensor networks, energy consumption is related with several parameters: a significant portion of energy is spent during packet transmission. Transmission is an unavoidable activity in both periodic and event-driven localization processes. Therefore, energy consumption is related with the number of transmitted bits [2]. As mentioned, the small size of packet in RLS reduces the energy consumption when an acoustic channel with low bandwidth is utilized.  
The second step in the transmitting phase starts as soon as the surface anchors receive the localization request message from detector sensor(s) in deep water. They insert \(\mathrm{T_{R}}\) (receiving time of message) and also their location, where they equipped with GPS. After that, all receiver anchors forward completed message to the sink station. Figure 5 illustrates the completed localization request message which is forwarded by receiver anchors to the sink station.  
These messages have some more data bits compared to the localization request message which is sent by sensors. Anchors utilize RF signals to forward the message to the sink through the air with high bandwidth and more energy.