134 



from the surrounding ocean. The hydrogen gas and excess electrolyte are rejected 

 to the ocean. The battery weight 23kg (50 lbs) including all the subsystem 

 components necessary for self-sustained operation. The cell stack uses 10 cells 

 and operates at 12.5 V (For one specific application, a DC-DC converter is 

 used to provide the 50 V output required by the system. ) It generates sufficient 

 power and energy to delivery up to 500 W and 4000 Whr to the load as well as 

 run its pumps and activate its valves. The overall system energy density for 

 this small power source is seen to be 80 Whr/lb of system weight, energy density 

 will double with the addition of hydrogen peroxide oxidizer. 



5.7 NAVIGATION 



The navigation systems described in Section 2.1.12 all rely upon acoustic, 

 thru-water transmission. An inertial navigation system, called HASINS (High 

 Accuracy Submersible Inertial Navigation System) , has been developed by Ferranti 

 Ltd. , which has been demonstrated aboard a manned submersible and is under 

 consideration by several ROV operators as having application to remote vehicles. 

 The system is attractive because it does not rely upon acoustics and, therefore, 

 has potential for working within a structure and not be subject to the effects 

 of reverberation as are the acoustic-based systems. The manned submersible 

 demonstration, conducted in early 1977, involved the measurement of seven 

 positions along a 259m (850 ft) line and demonstrated a total error of 20cm 

 (8 in. ) . 



The system and demonstration tests performed are described by Stankoff and Tait 

 (1977). The following description of the system and its operation is taken 

 from their report. 



"Essentially the Inertial Navigation System (INS) comprises three 

 gyroscopes and three accelerometers. These instruments are held 

 on an orthogonal triad, with one accelerometer and one gyroscope 

 pointing north-south (N/S) , another set (E/W) and the third set 

 vertical. This triad, called a cluster is mounted on gimbals. With 

 a motor and synchro on each gimbal, the total arrangement has, with 

 an appropriate servo system, the ability to be gyro-stabilised. 

 This implies that the cluster will remain pointing in one direction 

 irrespective of the motion of the gimbals. 



At switch-on, an INS must perform two processes. The first is levelling: 

 here the cluster is aligned to the local vertical. Output signals 

 from the accelerometers become error signals in servos which drive 

 the gimbal motors. Only when N/S and E/W accelerometers detect zero 

 gravitational acceleration is the cluster level. The cluster is 

 levelled to within a few seconds of arc from the horizontal plane. 

 The second process is gyrocompassing alignment - the object is to 

 point the N/S accelerometer to True North. This is effected by 

 measuring a component of the earth's rotation detected by the E/W 

 gyroscope. With a knowledge of the latitude at the point of align- 

 ment, this rate is a measure of the misalignment of the cluster from 

 True North. The cluster is then precessed until this error is zero. 

 Alignment to North is accurate to within a few minutes of arc. Both 

 these processes are automatic and require about 10 minutes to complete. 



