3 ms·

Applying the reverse of this is actually really fascinating as well. A friend of mine used to work on "navigation systems for submarines". According to him some

Applying the reverse of this is actually really fascinating as well. A friend of mine used to work on "navigation systems for submarines". According to him some submarines measure the pull of gravity to help keep track of their location.

snowwrestler 11y ago
In the movie The Hunt for Red October, a sonar technician trying to track another submarine reports detecting "milli-gal anomalies." This caused a bit of a stir at the Navy because at the time, the use of gravitational sensors on submarines was thought to be a secret.
breatheoften 11y ago
Some of these shipboard gravimeters originally developed for subs during the cold war ended up on research ships -- and are still on them. In my old life I would tend to these instruments (still in operation). Finicky, old, entirely analogue systems ... but they keep "working" 35+ years later.
- ajmurmann 11y ago
  And here I thought this was cutting edge technology...
  - jofer 11y ago
    We've been doing gravity surveys for over a century. All you need is to 1) know the elevation precisely, and 2) have a very well calibrated spring. In fact, the really old Lacoste & Rhomberg style gravimeters are often thought to be more accurate than their modern counterparts because the springs used to be very meticulously hand-crafted instead of mass-produced. Even Sandwell & Smith's method for inverting for bathymetry from the Free Air Anomaly (i.e. local sea level) has been around for a long time at this point. (Still damned neat, though.) The cutting edge stuff these days is in satellite methods that can measure gravity anomalies over land (e.g. GRACE and GOCE). They're still much lower spatial resolution than the sea-surface based methods, but you can use them over the entire globe. GRACE, for example, measures the distance between two satellites ~100km apart to an accuracy of ~1 micrometer. Pretty impressive! GOCE has multiple very precise accelerometers at different ends of the satellite. Because acceleration of the craft will affect accelerometers equally, the differences in acceleration must be due to local changes in the geoid. (In other words, GOCE is gradiometry based.)
qubex 11y ago
If a submarine is zero-buoyant it follows it is the same average density as the water it is suspended within — and therefore has the same gravity when observed from an external reference... or am I missing something?
- jofer 11y ago
  It has the same average density, but doesn't produce the same gravity anomaly. If the submarine were a perfect spherical shell of denser and lighter sub-shells, everything would cancel out. However, they're not. They're essentially really dense blobs attached to much less dense blobs. You get a gravity anomaly due to the lack of spherical symmetry.
  - qubex 11y ago
    Thanks for the clarification. I presume the really dense blob is the reactor and the less dense blobs are the air-filled crew compartments?

snowwrestler 11y ago

In the movie The Hunt for Red October, a sonar technician trying to track another submarine reports detecting "milli-gal anomalies." This caused a bit of a stir at the Navy because at the time, the use of gravitational sensors on submarines was thought to be a secret.

breatheoften 11y ago

Some of these shipboard gravimeters originally developed for subs during the cold war ended up on research ships -- and are still on them. In my old life I would tend to these instruments (still in operation). Finicky, old, entirely analogue systems ... but they keep "working" 35+ years later.

ajmurmann 11y ago
And here I thought this was cutting edge technology...
- jofer 11y ago
  We've been doing gravity surveys for over a century. All you need is to 1) know the elevation precisely, and 2) have a very well calibrated spring. In fact, the really old Lacoste & Rhomberg style gravimeters are often thought to be more accurate than their modern counterparts because the springs used to be very meticulously hand-crafted instead of mass-produced. Even Sandwell & Smith's method for inverting for bathymetry from the Free Air Anomaly (i.e. local sea level) has been around for a long time at this point. (Still damned neat, though.) The cutting edge stuff these days is in satellite methods that can measure gravity anomalies over land (e.g. GRACE and GOCE). They're still much lower spatial resolution than the sea-surface based methods, but you can use them over the entire globe. GRACE, for example, measures the distance between two satellites ~100km apart to an accuracy of ~1 micrometer. Pretty impressive! GOCE has multiple very precise accelerometers at different ends of the satellite. Because acceleration of the craft will affect accelerometers equally, the differences in acceleration must be due to local changes in the geoid. (In other words, GOCE is gradiometry based.)

qubex 11y ago

If a submarine is zero-buoyant it follows it is the same average density as the water it is suspended within — and therefore has the same gravity when observed from an external reference... or am I missing something?

jofer 11y ago
It has the same average density, but doesn't produce the same gravity anomaly. If the submarine were a perfect spherical shell of denser and lighter sub-shells, everything would cancel out. However, they're not. They're essentially really dense blobs attached to much less dense blobs. You get a gravity anomaly due to the lack of spherical symmetry.
- qubex 11y ago
  Thanks for the clarification. I presume the really dense blob is the reactor and the less dense blobs are the air-filled crew compartments?