Operational Subsystems

Altimeter

For accurate close-to-the-bottom soundings, a 200 KHz Benthos (ex-Datasonics) Model PSA-900D altimeter is mounted in the bow about 20 cm above the skids. Its scale is 1-300 meters, and an accuracy of one meter may be expected. Typical maximum range is to 150-200 meters; maximum range will depend on bottom type. Digital readout is in meters and is updated every second. Resolution is 0.1 meter.

Sonars

Search Sonar

The Sunwest SS300 sonar is a medium range FM sonar suitable for bottom navigation and search. It is particularly useful for providing range and bearing to natural or pre-planted sonar reflectors, specially on flat bottoms. Moreover, the sonar is invaluable for rough bottom navigation in canyons, etc. Large ridges, steep slopes and similar features (+25 db targets) may be detected at ranges of 1,000 meters; small boulders (oil drum size) and sonar reflectors (0 db targets) 400-500 meters; and smaller objects (gallon can size) at 100-200 meters. The sonar has a maximum range of 1,000 meters (+50 db or greater targets) and minimum is about 1 meter, with 360-degree sweep, range resolution 2% of range scale and 3 degree bearing resolution.

A visual display appears on a 15” TFT flat panel monitor. The sonar display can be viewed on one of the Alvin video channels when that computer screen is selected. Five range rings are marked on the display screen; these can be used with any one of the five range scale settings 3000, 2000, 750, 200, 75, and 20 feet) for quick range estimation. A range/bearing feature can be used to determine a more precise range and bearing to specific targets. In addition, acoustic pingers from 20-50khz may be received and bearing to the pinger indicated.

In unusual circumstances when additional payload is required, the sonar head and electronics bottle can be removed for a net gain of approximately 50 lbs. Basket space is not impacted.

Profiling Sonar

The Imagenex 881 profiling sonar is standard Alvin equipment. It is used primarily for precision bathymetric mapping at altitudes from 5 to 15 meters above the sea floor. The single pencil-beam head is positioned to scan from side to side in a 90-degree arc directly below the submersible keel. This produces a “zipper”-like contour profile of the sea floor as the submersible moves forward. Data is stored in the in-hull computer and later downloaded post-dive for science user processing. Water weight is included in the submersible payload. The unit is mounted aft and requires no basket space.

Depth Measuring Systems

The primary depth measuring system aboard Alvin uses a temperature compensated quartz oscillator pressure transducer to monitor ambient pressure. This system provides a digital display of depth resolved to 0.1 meter and is accurate to 1.0 meter. Depth data from this system is available to the Alvin computer for display and logging. A second quartz transducer and a strain gauge (of lower accuracy) transducer provide backup depth to the computer. Additionally, an external Bourdon tube dial gauge in a pressure-proof housing may be read through the viewport. Finally, the underwater telephone may be used as an upward beam echosounder to measure depth acoustically. This system may be activated by the pilot but must be used sparingly, as it interferes with normal communications and tracking.

Prior to May 1994, a constant of 0.6838533 m/psi was used for conversion of pressure to meters in Alvin data systems. Since May 1994 the algorithm for pressure to depth conversion has been based on the "Algorithms for Computation of Fundamental Properties of Seawater" by N.P. Fofonoff and R.C. Millard Jr., endorsed by Unesco/SCOR/ICES/IAPSO in 1983.

The Alvin calculation of this algorithm assumes a latitude of 30 degrees, a salinity of 35ppt, and a temperature of 0°C. According to the above publication, "the correction for the actual density distribution would be 2 meters or less".

The following table can be used to refine the observed and logged depth by applying a correction for the actual latitude of the dive.

Depth Corrections for Latitude

The datalogger will log two depth values:

1. Depth -- Calculated depth in meters as determined by the algorithm described above.
2. Pressure -- Raw sea water pressure in psig

Gyrocompass, Magnetic Compass and Attitude

An IXSEA “Octans” fiber optic gyrocompass is provided to indicate the submersible’s true heading. The gyro heading is fed to the Alvin computer for display and logging. Accuracy of the gyrocompass is +/- 0.1 degrees from true north with a resolution of 0.01 degrees. The gyro also provides pitch and roll information with a resolution of 0.01 degrees.

An RDI Navigator doppler velocity log provides flux-gate magnetic heading with an accuracy of +/- 2.0 degrees as well as pitch and roll information with an accuracy of +/- 0.5 degrees.

Hydraulic System

The Alvin hydraulic system provides power for Alvin’s manipulators, trim system, and other accessories. The system is also equipped to supply hydraulic power for user science equipment. There are presently six hydraulic valves available to control user equipment. The hydraulic valves are usually controlled by panel switches and controls in the sphere. Detailed information on use of the hydraulic system for science-supplied equipment is provided in the Science Equipment Interfaces section.

Manipulators

Alvin is fitted with two jettisonable, hydraulically powered manipulators. The figures below illustrate the working range of these manipulators superimposed on the horizontal field of view from Alvin’s viewports, and the manipulators in elevation view with information on their geometry.

The starboard manipulator has six degrees of movement: shoulder pitch and yaw, elbow pitch, wrist pitch and rotate, and jaw open and close. The arm can be fitted with a hydraulic actuator that can be used as a trigger mechanism to operate devices held by the jaw. Details are shown below. The manipulator has a maximum extension of 69 inches and a lift capacity of 100 pounds at maximum extension. Remote operation is controlled by a switch panel in the personnel sphere. The arm may be viewed from either the front or starboard side viewports during operation.

The port manipulator has the same six degrees of movement with the additional capability of wrist yaw. Maximum extension is 74 inches, with a fully extended lift capacity of 150 pounds. Wrist torque is rated at 30 ft/lbs and has a maximum rotational speed of 65 RPM. A second wrist assembly can be installed having half the torque but twice the speed. This arm is controlled by a position feedback master/slave mechanism, with the spatially correspondent master located in the personnel sphere allowing viewing and control through the front and port side viewports. This manipulator can also be fitted with an auxiliary hydraulic ram to trigger special equipment.

The jaws of both manipulators are functionally equivalent and consist of opposing overlapping finger pairs. They are specifically designed to grip instruments which are fitted with a standard “T” handle (see below). The user should align the “T” with the vertical load. The user is cautioned not to assume compatibility between your tools and Alvin’s manipulators, even if the tools are fitted with T handles. It is best to seek the advice of the Alvin Group on instruments which have not been previously used with the manipulators, regardless of how dependable they may seem. If it is discovered that the equipment is incompatible with the manipulators in its current state, alterations rendering it acceptable can probably be developed by the Alvin Group given adequate advance notice. Many biologically and geologically oriented tools, including a variety of pry bars and other rock breaking tools, soft and hard sediment corers, box corers and a current meter have been adapted for use in conjunction with the manipulator hands and the actuator mechanism.

Navigation and Tracking Systems

 There are three major methods for navigating the submersible:

The first and simplest method consists of surface tracking and vectoring, using an ultra-short baseline Nautronix acoustic system mounted on the support ship. In this system, the submersible emits an acoustic pulse every 3 seconds. This pulse is received by Atlantis from a tracking transducer array, yielding bearing and depression angle to the submersible. The position of the submersible relative to the ship is calculated using the bearing, depression angle and manually input depth. This information may be integrated with GPS navigation to log the submersible’s geographic position. Accuracy of about 100 meters is the best that can be expected.

The second system uses the fiber optic gyro heading and distance traveled, as determined by the doppler velocity log, to estimate position. The position is determined relative to a given starting point. This point may be a manually input position, or a position determined by the long baseline navigation system. This system has the advantage of giving continuous position information, but the position is subject to accumlated errors over time. These errors can be minimzed when used in conjuction with the long baseline navigation system to update the dead-reconed position. For more information on this system, go to the DVL Nav home page.

The third system, long baseline navigation, utilizes a net of two or more acoustic transponders deployed from Atlantis and allows tracking of Alvin’s position from the support ship and from within the submersible. Alvin is equipped with an in-hull navigation transceiver which allows the submersible to utilize the long baseline transponder net to navigate independently of Atlantis and thus more accurately (<5 meters). This transceiver is capable of transmitting on any one frequency and receiving on any four frequencies between 5.0 and 15.0 KHz in 100 Hz steps. The acoustic travel times measured by this transceiver are fed to the Alvin computer for position computation and display.

The acoustic transmitter/receiver (Benthos 455 ASP) on the Atlantis can provide precision navigation timing of the support ship with an accuracy of within 10 meters relative to the transponder net. The uncertainty is primarily due to errors inherent in the system such as imprecise sound velocity information and instability of the transponder moorings. However, since these errors are relatively constant for a particular net, the precision of the system is quite good and a repeatability of 5 meters can be expected. The preferred baseline distance between transponders is approximately 1.5 times the site depth, and reliable fixes may generally be obtained at distances up to one baseline length away from any two transponders.

If a long baseline acoustic net is to be used, some diving time may be lost due to the need for deploying, surveying, and recovering the transponders; however, it is possible to make a dive before the deployed transponders have been surveyed for true position. Navigation data collected on such a dive can then be reprocessed to determine Alvin’s true position during the dive once the transponder positions have been determined. Thus, with proper planning it is possible to arrive at a dive site in the early morning, deploy the transponders, conduct a reconnaissance dive and survey the net in the evening.

If the ship arrives on station the evening before a dive, and there are at least three hours of daylight remaining, it is possible to deploy and survey a long baseline navigation net. This process requires about six hours of ship time. Due to the need for navigation computer hardware and software familiarization, as well as experience to recognize valid acoustic travel times, this task will generally be done by Atlantis SSSG technicians or Alvin Group personnel.

At the end of each dive during which long baseline navigation has been used, the investigator will be presented with a chart of Alvin’s track during the dive. The system may also be used for other purposes (i.e., navigated camera tows, dredging, coring, etc.) while Alvin is not diving. The long baseline navigation system is available at no extra cost with the following restrictions:
1. Four transponder deployments per cruise for each 15 days on station (DOS). A cruise of 16 or greater DOS is entitled to four additional deployments.

2. Use during normal Alvin diving hours.

3. Adequate prior notice of intended use.

Pressure Hull

The Alvin pressure hull has an outside diameter of 82 inches and an inside diameter of 78 inches. The hatch opening is 20 inches in diameter; any equipment which users wish to bring on board must be capable of passing through the hatch with its sealing surface protection ring in place (19” maximum diameter suggested). Four conical acrylic plastic viewports, each 3.5 inches thick with a 5-inch inside diameter and a 12-inch outside diameter, are set at different points in the hull. A fifth, smaller viewport is located in the hatch. The arrangement is shown below, along with photographs of the hull interior. Note that the bottom viewport is always covered by the floorboards and is not useful for observation.

Propulsion System

Maneuvering of the submersible is accomplished with six electric thrusters. Three thrusters on the stern provide forward/reverse motion, two thrusters mounted amidships provide vertical motion and a final thruster mounted athwartships in the stern turns the submersible about an axis just aft of the sphere center line. Two of the aft-mounted thrusters can be rotated downward to provide additional vertical thrust at the expense of fore/aft thrust and some pitching. Each thruster provides up to 150 pounds of thrust. The battery power consumed by the propulsion system is a strong function of the submersible’s speed; the greatest range and endurance (bottom time) will be achieved by moving slowly (1.5 km/hr (0.8 kts) or less).

The pilot has various means to control the thrusters, the most commonly used being a joystick to effect fore/aft, vertical and turning motions. The submersible has only a limited ability to move sideways without turning. A “heading hold” autopilot is available to maintain a fixed heading under the control of Alvin’s gyrocompass.

Trim System

The trim system allows the pitch angle of the submersible to be adjusted by pumping as much as 500 pounds of mercury between tanks located in the bow and stern. Normally, this system is used to maintain a level attitude as the load in the science basket varies during a dive; but it also permits the submersible to intentionally pitch bow up or bow down. The pitch range is approximately 15 degrees, depending upon the loading of the submersible.

Underwater Telephone and Echosounder

Alvin’s underwater telephone, which is compatible with Navy systems, can be used for both voice and code communications with Atlantis. Moreover, it has built-in echosounder, pinger, and transponder interrogation capability. The identical unit on Atlantis can thus interrogate the submersible for ranges, and Alvin may interrogate the mother ship as well. Since Alvin has both “up”- and “down”-looking transducers, the phone may be used to echo range to the bottom to determine altitude, or to the surface to determine depth.

Variable Ballast System

The variable ballast system allows the pilot to adjust the buoyancy of the submersible while at depth. Salt water is added or removed from spherical titanium ballast tanks to effect a weight change of up to 10 pounds per minute. This system can be used to neutralize the submersible’s rate of vertical motion, thus allowing it to hover in the vicinity of a cliff or underwater structure. Vertical travel may then be accomplished using the lift thrusters. For routine, near-bottom transits, the submersible is ballasted to be near neutral buoyancy. In cases where the bottom consists of light sediment, the pilot may ballast about 10 pounds “light”, thus allowing the vertical thrusters to be consistently used in a manner which directs their water jets upwards. To prevent drift caused by currents, the pilot may use the variable ballast system to get 50 to 100 pounds heavy. The amount of variable ballast weight available on each dive is not a fixed number and is affected by the payload on the dive. With advance notice, the ballast system can be configured to provide as much as 400 pounds of variable ballast. An additional negative force of up to 600 pounds can be applied by downward thrust of the lift thrusters for short periods of time. The VB system is both power and time consuming, and numerous trim changes will reduce bottom “work” time.

VHF Radio

A VHF radio with all marine channels is used for communications when Alvin is on the surface. An all-channel portable VHF marine radio is carried as a backup and Atlantis is equipped with a marine VHF radio direction finder. The direction finder receives only marine VHF frequencies; users of VHF beacons should confirm reception capability before planning use of the Alvin Group’s direction finding equipment.