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Gravimetric Gradiometry: The topography of the ocean floor can be mapped by an entirely separate approach that also relies on local variations in the force of gravity, caused by the same density difference between water and rock (and mud and silt) that satellite altimetry uses. This technique measures local three-dimensional gravity fields directly, and in real-time. The raw data is obtained by one or more sets of accelerometers on small rotating platforms, a mobile set-up emplaced amid or over the area that needs to be mapped. This data is fed into complicated mathematical models that work backwards to calculate the unique configuration of nearby terrain that would produce the gravity fields actually measured. This terrain map is then rendered for viewing by the user as a 3-D see-through image on a video display screen. To watch a gravimeter demonstrated for the first time is impressive, as I can testify myself. It seems like science fiction, but it's real, and in fact has become routine.
The method, civilianized after the Cold War, is a valuable tool in commercial mining and oil and natural gas exploration: In one mode it can identify deeply buried geological formations without the need to take core samples -- or conduct much cruder mapping via seismic echo-sounding with explosives set off at ground level. In this context, airborne versions of the gravimeter systematically overfly areas of interest to search for untapped mineral wealth and find new fossil-fuel energy reserves.
Public information from around 1999 indicated that a next-generation gravimeter installed on a nuclear sub could at that time clearly "see" underwater terrain out to about thirty miles, and at close range had a resolution -- sharpness -- of under ten meters. (Improvements in these specs since then seem likely). The hardware and software available in '99 refreshed the imagery several times per minute while the submarine moved with no limitations as to her depth, course, and speed. Because gravity reaches through solid rock, the 3-D gravimeter display can also see through solid rock -- it's possible to known in advance about one seamount that's hidden behind another from the submarine's perspective. It's also possible to detect the legs of off-shore drilling rigs, and gain warning of the presence of unmarked shipwrecks. Metal, like rock, is much denser than water, grist for the mill of the gravimetric gradiometer.
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One big advantage of this approach is that it's completely passive -- it generates no signature that might compromise the submarine's stealth. (The Navy originally developed the technology in secret, for SSBN strategic-missile deterrent subs to be able to obtain precision navigation fixes while remaining totally covert.) In applications for the rather different mission roles of SSN fast-attacks, the system isn't dependent on any stored data or charts, or even on the wildest guess about the sub's location, to develop a very good picture of the topography surrounding the vessel. You merely switch it on, and voila. Unlike active and passive sonars, it can't be spoofed or decoyed, its performance isn't degraded by the distracting sounds a submarine makes itself when it goes very fast, and the system is completely immune to outside loud noises (grinding ice caps, exploding torpedoes) that can "blue out" sonar hydrophones.
One disadvantage of the gravimeter is that its algorithms are unable to track moving objects -- such as other nuclear submarines whose reactor compartments, with their massive shielding, thick containment-vessel walls, and super-dense uranium core(s), make for a substantial discontinuity in micro-scale gravity fields. So a nuclear sub had better not sit still, hovering, when being hunted by another sub equipped with a gravimeter. This raises interesting questions about offensive and defensive tactics, particularly under ice, which are beyond the scope of this essay. Ditto for antisubmarine weapon homing sensors in an era of ever-increasing miniaturization. (The unjammable multi-use technology, by the way, can also identify sub-soil voids -- empty chambers -- with very low false-positive and false-negative rates. It thus might even some day be mounted in recon drones and cruise missiles: to locate, map, target, and destroy enemy cave hideouts and deep hardened bunkers with ultra-high confidence in success on the first try, and with the least possible collateral damage.)
Returning to our main theme, another disadvantage of gravimeters on naval submarines is that each system installation is very expensive. Reportedly, the Navy, the Department of Defense, and Congress -- in some combination thereof -- decided that these powerful navigational tools were too costly for continuing widespread use. Reliance for terrain avoidance, during stealthy ops where sonar pings are precluded, was to be placed instead on traditional nautical charts. Again one may conjecture that if San Francisco had had a gravimeter aboard, she'd have gotten ample warning of the uncharted seamount ahead of her, to maneuver to avoid it.
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