The Pressure Vessel:

The pressure vessel's primary goal was to safely protect vital subsystems from the compressive pressures at up to 304.8 m (1000') water depth. A 5.1 cm (2”) inner diameter (ID) design made from PVC SCH 80 was determined as the system's main pressure vessel. This decision was made over a 7.6 cm (3") ID design due to both sponsor request and because, from analysis and research, lower inner diameters bring greater resistance to compressive pressure. Despite a gravitation toward the 5.1 cm (2") pressure vessel design, the 7.6 cm (3") design was still used to provide sufficient housing for the slightly larger FM surface beacon component. This decision was justified by testing of the 7.6 cm (3") pressure vessel design at the Scripps Institute of Oceanography, which found it could resist up to 7.584 MPa (1100 psi) - far greater than the 3.061 MPa (444 psi) at 304.8 m (1000') water depth.

The Battery Pack:

Ultimately, it was decided that while disposable batteries are wasteful and could be polluted into the environment, they were best suited for this design use case. This is because fishermen value ease-of-use, simplicity, and often don’t have time to wait for cells to charge back up. Furthermore, due to the lengthy amount of time between cell replacements, they  do not incur significant negative impact to the environment. The communication device on board also does not demand robust power, so AA cells are sufficient to supply power for over six months. Battery holders can be sourced from off the shelf electronic component suppliers such as Digikey, and each holder has a unit cost of roughly four dollars. Primary battery cells are also inexpensive, and after building both prototypes it was determined that that primary battery system is three times more inexpensive when it comes to cost and manufacturing time.

The Localization Component:

The localization component must be made such that it can work with an Android app to record the deployment/retrieval location of the "ropeless" fishing trap. The fisherman starts the deployment process by clicking “mark location” in the app. Then, the program needs to capture the moment when the trap enters the water (deployment) and, later exits the water (retrieval) while placing markers on an on-sea map. The final decision is to use an 18mm x 45mm Arduino Nano ESP32 microcontroller board along with a NEO-6m GPS module. The Arduino Nano ESP32 microcontroller has built-in Bluetooth® LE v5.0 capability from the ESP32-S3 microcontroller with an output power at up to 10 dBm EIRP, -98 dBm conducted sensitivity, and up to 2 Mbps data rates, which is enough for transmitting two float numbers for the latitude and longitude of the device. The NEO-6m GPS module can connect to 50 different channels, with -161dBm sensitivity, achieving a 2.5m GPS horizontal position accuracy [4]. Moreover, it only needs about 27s for a cold warm start [4], which is very useful when the device resurfaces. 

The Surface Beacon:

The surface beacon is involved with expediting the trap retrieval process. When a fisherman wishes to retrieve their trap, they send out an acoustic signal that triggers the release of the retrieval unit’s buoys to the surface; the surface beacon’s purpose is to help a fisherman locate these now-surfaced buoys. It operates under the assumption that ocean currents produce a significant distance between the initial drop location and surfacing location. The final surface beacon design involves direct integration with the aforementioned tracking tag. The tag is equipped with a LoRa radio transceiver transmitting at the standard 915 MHz for North America, and a 3 Axis MPU6050 Accelerometer for waking up the tag from deep sleep mode during retrieval. LoRa radio is commonly used in urban IOT applications and usually has a range of about 4.5 km, or up to about 16 km with line of sight. To read data from the LoRa radio, a receiver was made to display the latitude and longitude of the tracking tag up to 8 decimal places, the ID of the tag for multi-tags scenario, and the signal strength of the received radio signal. Signal strength was included since GPS could take longer than expected to lock on and find its position, in which case the this value could indicate the approximate distance between the receiver and tag.

As a precaution, a secondary, FM radio based surface beacon was also developed. It involves a Retekess FT11 FM Radio Transmitter and an acoustic signal transducer. The transmitter operates with frequencies of 76 MHz to 108 MHz, transmits up to 200 meters (656.2 feet) away in unobstructed conditions, and has a built-in 2000 mAh battery. Additionally, it is very light at 0.077 kg (0.17 lbs) and compact, with dimensions of  99.1x55.9x15.2 mm (3.9x2.2x0.6 in). It is housed in a separate 7.6 cm (3") PVC vessel along with the transducer. Acoustic signals sent from nearby fishing boats can activate the transmitter through the transducer to assist with trap retrieval.

Fig.5: Schematic of the initially planned, one-pressure vessel internal layout.