Conventional localization is used on a daily basis to navigate to destinations, provide directions in real time and discover nearby attractions. Localization is traditionally achieved through Global Position System (GPS) technology. GPS works by using satellites to send out electromagnetic signals to a receiver whose position is unknown. Based on the propagation time required for the signal to reach the user, the user’s position can be computed geometrically using a method known as triangulation. GPS can accurately give your location on the surface of the Earth but implementing localization in aquatic environments comes with significant challenges. In commercial GPS electromagnetic waves are transmitted with a center frequency of 1575.42 MHz. Electromagnetic waves work well as a means of signal transmission because the conductivity of air is low varying from 3×10 −15 to 8×10 −15 Siemens per meter so signal degradation is negligible. Fresh water however has a conductivity of 2×10 −1 Siemens per meter and salt water has a conductivity of 4 Siemens per meter so electromagnetic waves quickly lose strength as they traverse through this type of medium. Additionally in water the absorption coefficient of a signal is directly proportional to its frequency. At high frequency such as those used by GPS, electromagnetic waves cannot propagate useful distances.
To circumvent the problems faced by electromagnetic signals acoustic signals can be used as a means of transmission instead. Acoustic signal are better suited for use in aquatic environments because their signal strength does not depend on the conductivity of the medium that they propagate in. Using underwater sensor networks that transmit acoustic signals various transmission schemes can be used to implement localization. There are four main commercial based localization methods that have been developed for commercial use as well as two research based methods. The goal of our project is to study the research based localization topics and build working models so that they can be implemented for practical use in the field such as on AUVs and for sea floor navigation. To create the best possible positioning system we first studied all the localization set ups that are commercially available. In the next section we will discuss the advantages and disadvantages of each commercial method as well as the two research based methods.

Long Baseline
The first commercial solution to underwater network localization we look at is the Long Baseline (LBL) system. Node placement and calibration for the LBL system takes place on the sea floor. Each node is placed in a known location so localization can be achieved. When an autonomous underwater vehicle
(AUV) wants to locate its position it will send out an acoustic signal to the first node. When node 1 receives the signal it will then send a response to the AUV. This process will continue for all nodes in the
AUVs’ range as shown in Figure 1.
Figure 1. A typical schematic and timing diagram of the Long Baseline system.
As you can see, for one AUV in Figure 1 a total of eight signals are sent between the AUV requesting localization and the nodes on the sea floor. Because of this, LBL is not a good system for servicing multiple AUVs as the number of signals required to localize even one addition AUV would double. Aside from this, LBL has one other downside which is the cost. Due to the fact that the system needs to be fixed to the sea floor installation of the system will be expensive. It also requires skilled operators to maintain the system costing even more. The LBL system has the advantage from other systems that will be discussed in its accuracy and range.
Short Baseline
The next commercial solution to be discussed is the Short Baseline (SBL) system. In the SBL system node placement is on the side of a boat or dock. It differs from the LBL system in that it will travel with you wherever you go and does not rely on nodes installed on the sea floor. SBL can work in one of two ways; the first is displayed in Figure 2:

Figure 2. One of the standard schematics and two timing diagrams of the short baseline system.
Method one for the SBL system acts much like LBL in that a signal is relayed individually between each node and the AUV. The second method enables the AUV as a pinger. A pinger is a device which constantly emits a signal on a known frequency and interval. Method two is better for tracking an AUV because it limits the number of signals required to localize the AUV. However in the second method the
AUV will not be sent its position, but instead is monitored by the boat carrying the equipment. The ability to use two different methods in this system allows for different needs to be met, where method one is not good for servicing multiple AUVs because of the number of signals required (same as long baseline) but with the advantage of the AUV knowing its position. Method two can be used to service multiple AUVs with only one additional signal per AUV added but with the AUV position being monitored by a boat.
The SBL system is much cheaper than the LBL system, node calibration and installation occurs right on the boat making it a much more simple system overall. SBL allows for portability with a loss in effective range. If we examine the two systems we see that LBL can localize all the way to the sea floor, in an SBL system it is limited to a depth of about 30 meters.
Ultra Short Baseline
The Ultra Short Baseline (USBL) positioning systems implement localization by having a fixed array of receivers attached to a boat. The boat sends out an initiator signal to the AUV. The AUV receives the message and responds. The response time is recorded by the receivers on the boat as well at the angle of arrival (AOA) that each receiver received the signal at. Overall the time equation can be given as: 𝑡 = 𝑡
2
− 𝑡
1

Figure 3. USBL System Timing Diagram.
Where 𝑡
1
is the time at which the boat sends out at a signal to the AUV and 𝑡
2
is the time at which the
AUV responds. A geometric picture representation can be given:
Receiver 𝑑 𝜃
AUV
In USBL the value d is obtained from a time of arrival scheme where 𝑑 = 𝑐 ∗ 𝑡 . The angle and the distance represent the position of the AUV in polar coordinates. Converting to Cartesian coordinates, the x and y values for that AUVs position can be obtained. 𝑥 = 𝑑 cos 𝜃 𝑦 = 𝑑 sin 𝜃
The advantage of USBL is that it is a low complexity system so setup does not require deployment of nodes on the seafloor or buoys. The disadvantage of USBL is measurements are highly dependent on the angle of arrival. Slight variation could greatly affect the position accuracy. Additionally calibration of the receivers and AUV require that their clocks be synchronized when using time of arrival estimates.

Figure 4. USBL System Diagram.
GPS Intelligent Buoys
Another commercial localization method is using GPS Intelligent Buoys, or GIBs. This system consists of an Autonomous Underwater Vehicle (AUV), four surface nodes, and a surface vehicle. The AUV sends out an acoustic signal, which is received by each surface buoy at a different time, depending on the distance between the AUV and each buoy. Each surface node records the time at which it received this signal. These nodes are also equipped with GPS receivers. This allows them to know their position. Each node sends a radio signal to the radio receiver on the surface vehicle that contains the time of arrival of the acoustic signal ( 𝑑 = 𝑐 ∗ 𝑡 ) and its position from the GPS. The surface vehicle is equipped with an onboard computer which can compute the position of the AUV using this information.
Figure 5. GIBs Signal Timing Diagram.

Some advantages of this system are that the buoys are easy to deploy and recover, and there is no calibration of the buoys needed other than the communication with the satellites. The disadvantage is that the buoys can drift if the water is too deep. This could potentially ruin the array, thus making the system less accurate. The position of the buoys would have to be controlled to maintain the array.
Figure 6. GIBs System Diagram.
Synchronous
The synchronous localization system is one of the research methods which we will be testing in our design project. In this system, we have nodes placed at the surface of the water which are equipped with satellite-based GPS receivers. The clocks of the nodes are synchronized using the internal pulse provide by the GPS device. Once the clocks are synchronized, the nodes send signal to the AUV which is placed underneath the water surface. The nodes send the signals to the AUV after a certain delay. Figure
1 below shows the schematic of this system with its signal time diagram.

Figure 7. Synchronous Localization schematic and signal time diagram.
One of the biggest advantages of the synchronous localization system is that it is able to service multiple
AUVs at once and it also does not require a continuous GPS signal to synchronize the clocks on the nodes. However, a disadvantage of this system is that the nodes must be on the surface of the water initially to receive GPS signal. Any missed node signal means that position cannot be computed if working with the minimum node schematic.
Asynchronous
The asynchronous localization system is other research method that we will be testing in our design project. In this system, we have the nodes at the surface of the water and the AUV underneath the water. Unlike the synchronous localization system, the GPS signal is not required in this system because the clocks do not need to be synchronized. In this system, the AUV sends a pinger signal to the nodes.
Once all the nodes have received the signal, they send a signal back to the AUV which then computes the position. Figure 2 shows the schematic of this system with its signal time diagram.
Figure 8. Asynchronous Localization schematic and signal time diagram
The advantages of asynchronous localization system is that the clocks on the nodes do not need to be synchronized together and extra timing measurements sent from other nodes can be factored into

calculations to provide better position accuracy. On the other hand, this system also has some disadvantages. The initiator signal must send out a delay factor long enough so no nodes send out signals at the same time. This system has also never been field tested so the actual accuracy improvement is unknown.
Our group needs six digital processing boards, four anchor nodes, one tracking node and one active node. We also need six hydrophones and six transducers. Our project requires underwater housing for all components. We also need four GPS tracking devices.
Figure 9. DSP Boards in Waterproof Housing.
The Department of Electrical and Computer Engineering (ECE) at the University of Connecticut has provided us with a budget of $1000. Currently, we will be obtaining all hardware and software from the
University of Connecticut Underwater Sensor Network Laboratory for designing and testing. Since we have already been provided with everything that we will need, we do not have a plan to utilize our budget.

September
• Project
Statement.
•Localization research.
October
•Project specifications.
•Additional localization research
•Coding DSP C
November
•Coding DSP C
•Finalize implementation plans.
December
•Hardware setup of nodes.
•Coding DSP C algorithms.
January
•Hardware setup of nodes.
•Field testing of algorithms .
February
•Field testing of algorithms.
•Algorithm comparison analysis.
March
•Field testing of algorithms.
•Algorithm comparison analysis.
April
•Integration of localization with other groups.
•Algorithm comparison analysis.
May
•Complete integration of localization.
Shown above is an overview of our progress so far and our future objectives for the project. We have already conducted extensive research on the commerical and research localization methods. In
November we plan to begin learning how to code the main component of the sensor nodes, the digital signal processing board. In December we will conduct pool testing to establish the sound speed of the acoustic signal in the pool water and get variance measurements corresponding to the water conditions of the pool. In Jaunary we will set up the required nodes for testing and begin field testing in the pool and if results are favorable, possiblly in the lake. Finally at the end of our timeline we plan to have a working accurate localization solution that can be integrated with an AUV.