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Communications
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Surface-Habitat
Intra-Habitat
Habitat-Vehicles
The Cable
References

Communication between the Habitat and the Vehicles

Acoustic Modems

Acoustic modems will be the method of communication between the habitat and the vehicles. We chose it because it is the most practical way of communication under water without cabling (radio waves attenuate too fast under water), which the robotics teams decided not to do except for the tether between the manned vehicle and the ROVs.

The modems in both the habitat and the vehicle will have the ability to rotate so they face the modem that they need to communicate to. The vehicles will know where they are with the Inertial Navigation System and the periodic check with information from the auxiliary positioning system. The habitat will know where the vehicle is through the use of auxiliary positioning system or with a preset mission schedule or by requesting the information from the vehicle. The auxiliary positioning system will be described in a later section.

If two vehicles go on a mission at the same time and both need to communicate to the habitat, then the communication problem becomes a little more complicated. If the two vehicles are more than about 45 degrees apart, then separate modems from the habitat could be used to communicate to the two vehicles. If the two vehicles are close together, within about 60 degrees of each other, then the two vehicles could communicate to the same modem in the habitat, sharing the bandwidth.

Product Specifications from LinkQuest
The UWM4000:

Working range: 4000 meters
Maximum depth: 3000 or 6000 meters
RS-232 data rate: 4800 bits/second
Payload data rate: 3200 bits/second
Acoustic link: 8500 bits/second
Bit error rate: less than 10
Transmit mode power consumption: 8 Watts
Receive mode power consumption: 1 Watt
Sleep mode power consumption: 8 mW
Beam width of transducer: 70 to 80 degrees
Operating Frequency: 12.75 to 21.25 kHz
RS-232 Configuration: 9600 baud, 1 start bit, 1 stop bit, no parity bit, and no flow control
RS-232 input data buffer: 900k bytes
Voltage: 12 to 28 volts
Operating temperature: -2 to 45 °C
Storage temperature: -5 to 75 °C
Overall length: 286 mm
Housing diameter: 144 mm
Weight out of water: 7.6 kg
Weight in water: 4.1 kg
Optional Higher Data Rate: 9600 bits/second

In the chart above we have the modem that is most ideally suited for our purposes. In this chart, the actual rate of data transmission is the payload data rate, which is 3200 bits per second. It could be pushed up to about twice that much under good operating conditions.
On the other hand, under excessive noise, the configuration automatically switches from 9600 baud to 1200 baud. The modems could be configured to sleep and wake up regularly to check for incoming signals. We will configure the habitat modem to be always awake, but the configuration of the vehicle modems depends on the situation. The weight of the modem does not include the power supply and the habitat and the vehicles will supply the power.

We do not yet know how much these would cost. We have sent LinkQuest an email, but have not received a reply.

In the case that vehicles needs a longer range, the following modem could be used instead. However, while this modem has longer range, it has smaller bandwidth, consumes more electricity in transmit mode, and is substantially heavier.

The UWM7000:

Working range: 7000 meters
Maximum depth: 6000 meters
RS-232 data rate: 2750 bits/second
Payload data rate: 2000 bits/second
Acoustic link: 5500 bits/second
Bit error rate: less than 10
Transmit mode power consumption: 25 Watts
Receive mode power consumption: 1 Watt
Sleep mode power consumption: 8 mW
Beam width of transducer: 70 degrees
Operating Frequency: 8.25 to 13.75 kHz
RS-232 Configuration: 9600 baud, 1 start bit, 1 stop bit, no parity bit, and no flow control
RS-232 input data buffer: 900k bytes
Voltage: 18 to 28 volts
Operating temperature: -2 to 45 °C
Storage temperature: -5 to 75 °C
Overall length: 580 mm
Housing diameter: 150 mm
Weight out of water: 21 kg
Optional Higher Data Rate 5500 bits/second

Connection of the Modems

The modems will be stationed horizontally on top of the vehicles at the height of about a foot, with the ability to turn around 360 the height of about a foot, with the ability to turn around 360 degrees horizontally, and to turn up 90 degrees from horizontal.
Since the modems have directional transducers, and the vehicles may also block the signals, communication may not be possible when the vehicle is at some particular angles. On the manned ship (which carries more weight), this can be partially avoided if we choose to equip two modems, one in the front and one in the back.

The modems for the habitat will be similarly stationed except they will be positioned at a height dependent on the topography around the habitat, and that they will have the ability to turn downward as well as upward. (They will be positioned higher than the projected working range of the vehicles.) They will be stationed next to the habitat instead of right on top of it.

The Modems for Habitat

There will be six acoustic modems total for the habitat. Two constantly point in the direction of specific vehicles each is assigned to. The four other ones serve as an auxiliary positioning system aside from the Inertial Navigation System, which will be used on the vehicles. They also serve as emergency replacements if either or both of the two main modems malfunctions.

The four modems for the positioning system will be positioned in a tetrahedron, each side of which will be two meters long. The tetrahedron will have a vertex (a modem) pointing up. Right over that modem will be one of the two main modems, and right under it will be the other. The whole tower will be secured and will be taller than the projected range of the vehicles.

The vehicles that are out on missions will be scheduled periodic checks of their position. During each of those checks the vehicle sends the beacons signals, and the computers in the habitat calculate the position of the vehicle with the different times each modem receives the signal. The modems in the beacon formation will also be the ones that periodically search for help signals that malfunctioning sends the beacons signals, and the computers in the habitat calculate the position of the vehicle with the different times each modem receives the signal. The modems in the beacon formation will also be the ones that periodically search for help signals that malfunctioning AUVs may be sending out.

Inertial Navigation System

All the vehicles will be equipped with inertial navigation system so that they would be able to determine their position when the help of the auxiliary positioning system is not available. Here is the product description from Litton Guidance and Control Systems. This is a product designed for missiles and air planes, as the ones used for submarines and ships are typically a lot heavier (Around 20 kilograms). The Litton LN-200 family of inertial equipment uses fiber optic gyroscopes (FOGs) and silicon accelerometers (SiAc's) for measurement of vehicle angular rate and linear acceleration. Studies conducted during the late 1980's, revealed the need for a low-cost, small, lightweight 1 degree/hr IMU to satisfy tactical missile and guided projectile guidance requirements and aircraft flight control systems. Trade studies were conducted to determine the most cost-effective IMU for these applications. These studies resulted in the selection of FOG and SiAc technology for the development of the LN200 Core IMU. The core unit consists of an instrument-sensing module with FOGs and SiAc's and supporting electronics. The basic core unit can them support a wide variety of missile and aircraft applications.

Significant features of the LN-200 IMU include the following:
Low cost - makes extensive use of qualified high reliability industrial parts and advanced technology FOGs and SiAc's and sensors, which results in a simple and producible product with a low parts count.
High reliability - uses solid-state instruments, has no moving parts
Two level maintenance - high reliability combined with extensive BIT; only organizational and depot-level maintenance is required
Conduction Cooling - external forced air or liquid coolants are not required
Small size - 3.5 inches (8.9 cm) diameter by 3.35 inches (8.5 cm) high
Low weight - 1.54 pounds (700 grams)
Rapid reaction - the LN-200 IMU is fully functional within 0.8 seconds after application of power
Environmental ruggedness - there are no resonances below 2,000 Hz in the sensors or sensor assembly. The solid state sensors are inherently rugged
Low Power - 10 watts
AHRS software has been developed for the LN-200 and is available as an option
Processing Power is available for expansion tasks.

Summary of LN-200 IMU Characteristics

Physical:
Weight: 1.54 pounds (700 grams)
Size: 3.5 in. (8.9cm)
Diameter: 3.35 in. (8.5cm high)
Power: 10 watts (steady-state) (nominal)
Cooling: Conduction to mounting plate
Mounting: 4
Mounting bolts: M4
Activation Time: 0.8 sec (5 sec to full accuracy)
Performance:
Gyro Bias Repeatability: 1/hr to 10/hr (1 sigma)
Random Walk: 0.04 to 0 0.1 deg/r.t.-hr
Power Spectral Density: (PSD) level
Scale Factor Stability: 100 PPM (1sigma)
Bias Variation: 0.35/hr 1 sigma with 100-second correlation time
Nonorthogonality: 20 arcsec 1 sigma
Bandwith: 500 Hz
Performance-Accelerometer:
Bias Repeatability: 200 micro g to mili-g, 1 sigma
Scale Factor Stability: 300 ppm 1 sigma
Vibration Sensitivity: 50 micro g/g2 1 sigma
Bias Variation: 50 micro g 1 with 60-second correlation time
Nonorthogonality: 20 arcsec 1 sigma
White Noise: 50 micro g/Hz PSD Level
Bandwidth: 100 Hz
Angular rate: 1000/sec
Angular Acceleration: 100,000/sec/sec
Acceleration: 40g
Velocity Quanitization: 0.00169 fps
Angular Attitude: Unlimited
Operating Range: 23,345 hours
MTBF (30C missile launch environment)
Input/Output: RS-485 Serial Data Bus (SDLC)
Data Latency: 1msec Environmental
Temperature: -54deg. C to +85deg. C
Operating Vibration: 11.9g
Rms-performance: 17.9g rms-endurance
Shock: 90G ms terminal sawtooth

List of Equipments and Estimated Costs:

Inertial Navigation Systems: About 6 * $100,000 = $600,000
Acoustic Modems: About 12 * $16,000 = $192,000
Robotic arms for each modem: About 12 * $5,000 = $60,000
Constructing and setting up the modem tower near habitat: About $20,000
Total Cost: $872,000

Problems and Solutions

The specifications say that the modems cannot operate at temperatures higher than 45 degrees Celsius. Even though the temperature in the vent could be as high as 300 degrees, we do not know whether the temperature would ever be so hot near the modems that are placed on top of the vehicles. The large amount of dust particles in the water around the smokers may also cut off the communication completely.

One solution would be to just cease the communication. However, the ROVs that are not tethered to the manned vehicle will not work without the controls from the habitat. To prevent losing the ROVs in that case, they can be programmed to either go up or go backwards, tracing their recent trajectory. Then the habitat can resume control after the vehicle regains communication.

Another solution for cases like this would be to tether a ROV to the manned vehicle or tether two ROVs together, and have one of them communicate with the habitat while the other performs the work.

If the water temperature gets higher than 75 degrees then there would be risk for breaking the modems. However, since the vehicles are unlikely to stay in that kind of temperature for long, there shouldn't be a problem. Otherwise, we could either implement a heat resistant casing around the modem that could open and close (costing the vehicle some mobility), or simply use the tether method, making sure that the vehicle doing the work does not have a modem equipped at all.

Communication between the Manned Vehicle and the Remotely Operated Vehicle

In our computer and information technology age there is increasing demand for faster and better communication links. Fiber optic is a key factor in meeting this demand and has become a natural choice when high speed communication links are required. Fiber optics hold many advantages over copper based cables. These advantages can be summarized as large bandwidth, low transmission losses, no electromagnetic interference, freedom from crosstalk and are electrically non-conductive. Fire optics are also free from sparking and fire hazards and are also low in weight compared to copper based cables. The only main disadvantages with fiber optic is that it is more difficult to terminate and more care needs to be taken when it is handled, but it is possible to overcome this problem to some extent. We decided to use fiber optic transmission medium to and from the tethered ROV due to the nature of the data to be transmitted. Although acoustic modems would provide a wireless link between the Manned Vehicle and the ROV, they do not have the bandwidth to provide the option of transmitting live video from the ROVs. Hence we decided to provide the communication link via a tether between the two.

In using fiber optic cable for data transmission to the tethered vehicle a number of factors have to be considered. Our choice of cable will depend on the flexibility and strength of the cable and the flexibility of the cable to vehicle connection. To give the cable strength and flexibility simultaneously we decided to use a cable sheathed in a high grade polymeric material such as polyurethane or polyethylene due to their superior abrasion resistance, flexibility and reliability. We finally chose a Kevlar Braid with a waterblock compound in a polyurethane sheathing.
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