54 MARINE SCIENCE 



It was found that the abyssal plains were so flat that with the instruments 

 available it was not possible to detennine how flat they were. This led to the 

 development of the precision depth recorder, now called the PDR, in order to 

 determine how flat the abyssal plains really are. 



They have proved to have features less prominent than about 1 foot and in 

 many cases to be more level than the best of billiard tables. This instrument 

 has proved to be ideal for many defense problems, and over 50 of these have 

 been put into operation on classified projects. Soundings have, of course, been 

 used for navigation since before Revolutionary days. 



The new accuracy and detail available with the PDR makes the ocean depths 

 even more useful for navigational purposes. Needless to say, it is only useful 

 to the accuracy to which it is originally located. The private laboratories have 

 done the greatest part of the precision sounding work to date and unfortunately 

 they must rely on celestial navigation and dead reckoning and loran (within 

 about 500 miles of the United States) to locate these features. It is unfortu- 

 nate that the more accurate navigation systems available to the Defense De- 

 parment have not been available for these primary studies. An outgrowth of 

 the PDR is the SDR (sub-bottom depth recorder) which records echoes from 

 beneath the sediments of the ocean floors. This work is quite new and its use- 

 fulness for defense applications is still imperfectly known. It does promise to 

 find features in the sub-bottom where the bottom is too featureless to use in 

 navigation. These studies will aid in designing the foundations for Texas tow- 

 ers, and other structures to be built in the ocean for defense purposes. They 

 have already been used very successfully in the foundation studies of bridges 

 and tunnels and in the search for petroleum in water-covered areas. 



Magnetic detection techniques were introduced in World War II for the de- 

 tection of submarines. This equipment has been modified so that it is now used 

 to measure the earth's magnetic field continuously wherever many of our 

 oceanographic research vessels travel. We have found that there are clear 

 magnetic signatures near the margins of continental shelves and over seamounts 

 and other structures. Needless to say, these are potentially useful for naviga- 

 tion. In some areas there are features which can easily be confused for sub- 

 marine signatures. In such areas defense groups could potentially waste much 

 time chasing ghosts. It is obviously urgent to locate each of these areas and 

 delineate them. Once again the limitation for the use of these features for 

 navigation is the accuracy of their original position determination. 



Gravity observations have been made at sea on submarines since 1954. De- 

 spite many efforts it only became possible to make measurements at sea on a 

 surface vessel in November 1957. Since topography is of primary importance in 

 making a gravity signature, the gravity measurement is potentially the secure 

 sounder. This is so since it makes no signal that anyone can detect, and it is 

 unjammable by man as yet. In addition, there are features of the gravity field 

 caused by the sub-bottom structure in regions where topography is featureless. 

 The potential use of gravity measurements for navigation is obvious. Gravity 

 measurements are also useful to determine the precise shape of the world — ■ 

 approximately an oblate ellipsoid. Gravity measurements provide the only 

 means to obtain the absolute deflection of the vertical at a location. Unfortu- 

 nately, almost all systems for position determination depend on the direction of 

 the vertical. In many places, particularly near continental borders, islands, 

 oceanic ridges, and deeps, the vertical may be deflected enough to mislocate 

 oneself by a mile or more. 



Better navigation in the broad reaches of the ocean is urgently needed for 

 all oceanic operations. The ability to use topographic, magnetic, gravity, or 

 other features for navigational purposes is limited by the accuracy of their 

 original location. For this purpose it is not important that the system could be 

 jammed in a conflict or that its use betrays the user. Once the other features 

 are located the jammability or self -betrayal of the parent system no' longer is 

 vital. Probably the system most likely to succeed is the use of Sofar. TO' date, 

 every attempt to check the accuracy of the Sofar system has yielded the errors 

 of the system against which it is checked. Logistically, it is impractical toi jam 

 a Sofar system for a long period of time. 



It is probable that it will be necessary to use a combination of the various 

 possible navigation systems for defense purposes. In some cases, one will be 

 superior and in others another will be superior. In general, more than one will 

 be required. The inertial and electronic systems will have to be checked and 

 restarted in the event of electronic failure. At sea the topographic and geo- 



