Can GPS work underwater?

[wgoldfarb]

Could you use GPS navigation underwater?

I know that GPS devices require line-of-sight to the satellites. But can the signals penetrate water and still work? Even if they can penetrate water, would the refraction of the signal as it passes the air/water surface prevent the GPS device from functioning properly?

 

[PottedMeat]

i dont believe so, water probably attenuates ~1.5Ghz considerably or reflects most of it. it's why subs use extremely low frequency rf ( tens to 100's of khz ) for communications.

 

[Born2bwire]

Originally posted by: PottedMeat
i dont believe so, water probably attenuates ~1.5Ghz considerably or reflects most of it. it's why subs use extremely low frequency rf ( tens to 100's of khz ) for communications.

If you were in fresh water you might be able to get a signal a few feet under the surface. But definitely not in seawater due to its conductivity. Probably 1 foot would kill off the signal if even.

 

[wgoldfarb]

Ok, thanks for the replies.

 

[Kreon]

How do submarines navigate?

I always thought they used GPS, though I could definitely be wrong.

Sorry to get slightly off topic OP

 

[NikolaeVarius]

They surface from time to time to get an update. When they can't, I think they just estimate using speed and time values.

 

[Kreon]

Originally posted by: tenshodo13
They surface from time to time to get an update. When they can't, I think they just estimate using speed and time values.

Thank you

 

[ScottMac]

Originally posted by: tenshodo13
They surface from time to time to get an update. When they can't, I think they just estimate using speed and time values.

A sub on patrol usually doesn't surface until the end of the patrol (~90+ days), and then only within the channel / near path of their destination, never in the patrol zones.

Navigation uses some flavor of inertial guidance ... (something like a laser ring gyro ?) that remains sufficiently accurate for long periods of time.

Boomers can't give away their position for anything less than complete loss-of-boat disasters or they'd be a target. Boomers don't generally cruise, they go to a station for some periodof time and sit there, then move again after a while ... for the duration of the patrol.

Fast Attack boats can't give away their positon for anything less than loss-of-boat disasters, because the boats / subs they track aren't supposed to know where they are.

FWIW

Scott

 

[Indolent]

I'm pretty sure I was just reading something about this in Popular Science. They're basically going to sink a bunch of sonar beacons in known locations in the ocean. I can't seem to find anything on it at the moment though. This doesn't answer the original question anyway.

 

[Leros]

Haven't we mapped the ocean floor rather extensively? Couldn't the submarine just follow its way along the floor, comparing its depth and estimated location to the ocean floor map?

 

[ScottMac]

Originally posted by: Leros
Haven't we mapped the ocean floor rather extensively? Couldn't the submarine just follow its way along the floor, comparing its depth and estimated location to the ocean floor map?

I'm pretty sure it's well-mapped, but to use it for navigation would (probably) require active components (like a SONAR ping) ... which would be a Bad Thing, since emissions can be tracked and used for locating.

Passive is the name of the game; do your best to sound like a patch of water. Too noisy (or too quiet) and you can be located.

FWIW

Scott

 

[Cattlegod]

Originally posted by: ScottMac

Originally posted by: Leros
Haven't we mapped the ocean floor rather extensively? Couldn't the submarine just follow its way along the floor, comparing its depth and estimated location to the ocean floor map?

I'm pretty sure it's well-mapped, but to use it for navigation would (probably) require active components (like a SONAR ping) ... which would be a Bad Thing, since emissions can be tracked and used for locating.

Passive is the name of the game; do your best to sound like a patch of water. Too noisy (or too quiet) and you can be located.

FWIW

Scott

Why too quiet?

 

[BladeVenom]

Originally posted by: Cattlegod
Why too quiet?

I don't know about subs, but if a stealth boat is too stealthy it leaves a blank spot on the radar. Water does offer some radar reflectivity, and they need to match that. I guess it would be similar with sonar. I imagine if a sub completely absorbed a sonar ping that would be just as much a dead giveaway.

 

[AgentJean]

Originally posted by: Kreon
How do submarines navigate?

I always thought they used GPS, though I could definitely be wrong.

Sorry to get slightly off topic OP

Navigation charts and the stars.

 

[NikolaeVarius]

Originally posted by: ScottMac

Originally posted by: Leros
Haven't we mapped the ocean floor rather extensively? Couldn't the submarine just follow its way along the floor, comparing its depth and estimated location to the ocean floor map?

I'm pretty sure it's well-mapped, but to use it for navigation would (probably) require active components (like a SONAR ping) ... which would be a Bad Thing, since emissions can be tracked and used for locating.

Passive is the name of the game; do your best to sound like a patch of water. Too noisy (or too quiet) and you can be located.

FWIW

Scott

Actually, I read somewhere that we mapped the terrain of planets better then we mapped the sea floor.

 

[aj654987]

Originally posted by: Born2bwire

Originally posted by: PottedMeat
i dont believe so, water probably attenuates ~1.5Ghz considerably or reflects most of it. it's why subs use extremely low frequency rf ( tens to 100's of khz ) for communications.

If you were in fresh water you might be able to get a signal a few feet under the surface. But definitely not in seawater due to its conductivity. Probably 1 foot would kill off the signal if even.

could you explain further please?

 

[f95toli]

Radiowaves are attenuated very rapidly once they enter a conductor, the typical length scale over which this happens is known as the Skin depth.
The better the conductor the smaller the distance the waves will travel.

 

[oynaz]

Originally posted by: AgentJean

Originally posted by: Kreon
How do submarines navigate?

I always thought they used GPS, though I could definitely be wrong.

Sorry to get slightly off topic OP

Navigation charts and the stars.

Yep, they navigate using the well-known underwater stars ;-)

 

[Born2bwire]

Originally posted by: aj654987

Originally posted by: Born2bwire

Originally posted by: PottedMeat
i dont believe so, water probably attenuates ~1.5Ghz considerably or reflects most of it. it's why subs use extremely low frequency rf ( tens to 100's of khz ) for communications.

If you were in fresh water you might be able to get a signal a few feet under the surface. But definitely not in seawater due to its conductivity. Probably 1 foot would kill off the signal if even.

could you explain further please?

Pretty much what f95toli said. When electromagnetic waves impinge upon a conductive material, currents are induced on and inside (If it has a finite conductivity) of the material that work against the incident electromagnetic wave. This causes an attenuation of the wave as it travels through the material. There is also a power loss going into the material due to reflection. Going from one medium to another of a different conductivity, permittivity, and/or permeability causes some of the wave to be reflected back. The skin depth, which is the distance that the wave must travel to be reduced by e^-1 in amplitude, is dependent upon frequency and conductivity. So a very low frequency signal can penetrate the earth several hundred meters while the high frequency GPS cannot penetrate a few feet. Specifically, sea water has a conductivity of 5 S/m (if what I get from Google is to be believed). GPS has a frequency of around 1.3 GHZ. So a GPS signal has a skin depth of 6 mm. That means that the signal loses 36.7% of it's amplitude after only 6 mm. So I was wrong, GPS can't even make anywhere close to a foot. The signal is reduced to 1% of its amplitude after only 28.7 cm, or a little over an inch.

Perfectly pure water has a very low conductivity, practically zero. However, almost all water has various impurities and ions that raise the conductivity. So if we assume that fresh water has a conductivity of 0.005 S/m, then the skin depth of GPS is about 20 cm. So the amplitude is reduced to 1% after 90 cm or about 3 feet.

 

[Fallen Kell]

Originally posted by: Kreon
How do submarines navigate?

I always thought they used GPS, though I could definitely be wrong.

Sorry to get slightly off topic OP

ummm... same way everything navigated 20 years ago before GPS even existed... maps, charts, sensitive gyroscopes/compasses, speedometers, computers, and/or a clock.

Sorry to sound like a jerk, but subs have been around for over 150 years (documented...although there is evidence that they existed before that even back possibly thousands of years to the heyday of Greece). Satellites have been here for 50-60.

 

[nismotigerwvu]

if it means anything the gps on my phone poops out in the Chesapeake Bay tunnel

 

[RossGr]

Back in the '70's a good buddy of mine served 4yrs as a Navigation aids tech (ETN) aboard a boomer. He bragged of thier ability to come to periscope depth and pick out a star for verification of thier postion. He claimed that the star they needed would always be in the field of view for the check. In other words they knew where they were from thier initerial navigation systems (gyros) well enough that the star check was usually a verifcation, more then a correction. I am not sure of the frequency of such a check.

 

[pradeep1]

For boat navigation, dead reckoning is still a valid way of navigating, even with the use of a GPS. I figure a submarine would use that to find its location in the world.

 

[dkozloski]

Nuclear submarines have the ability to locate their position within a few feet if not inches at all times. Navigation equipment includes inertial navigation, star trackers, VLF radio, GPS, underwater beacons, bottom mapping, and other means. After patrolling submerged for six months a missile can be launched at the pitcher's mound in Yankee Stadium from a distance of 2500 miles and it will land inside the bases.

 

[NikolaeVarius]

If a sub is on a stealth mission, they resort to a timer and a hopefully accurate map.