Slashdot Mirror
Navy ELF to Be Scrapped
engywook writes "National Public Radio and The Daily Press of Ashland, Wisconsin (among others, I'm sure) are reporting that the US Navy plans to scrap the Extremely Low Frequency (ELF) system for communication with its fleet of nuclear submarines, both in Wisconsin and Michigan. The report states that the Navy no longer feels that ELF is necessary, and that they will now rely on 12 VLF systems. The system has been in operation since October 1989. The system has been protested nearly the whole time, both as a part of a Weapon of Mass Destruction and as a potential health hazard."
The ELF system was designed so that it didn't have to rely on satellites or even the earth's atmosphere reflecting radio waves. ELF waves travel through the earth; the ELF system would be used in the even of a global nuclear war, so that if all other communications means go down, submarines can still be in contact naval command. A laser would be totally inappropriate for this; it needs line-of-sight to work.
The news article doesn't really have any technical information on ELF, so here's the obligatory Wikipedia article.
Of course, the first haphazard search I tried came up with this.
It would be cool if it didn't suck.
A quick Google search revealed the following: ZEVS, THE RUSSIAN 82 Hz ELF TRANSMITTER. Located near Murmansk. The article has some nice maps, screenshot of the spectrum, etc.
When we do exercises with foreign nationals, we have to mask our true sound signature, so as not to give away valuable data on HOW quiet we are, exactly what noises we normally make, and how best to detect us. Also, a submarine exercise where nobody can see anyone else isn't very good training - it's just driving around in circles. So sometimes we have to give away our position on purpose to get the show on the road.
That means we have to run all kinds of noisy gear that we normally don't (or only do when we're sure nobody is around) when non-US subs are about.
In addition, we frequently have "prospective" commanding officers play captain-for-a-day during the exercises to get some experience before we let them loose by themselves on a sub where, essentially, they're an absolute ruler.
The interesting thing is, we don't lose EVERY time, or even MOST of the time.
There are currently 29 GPS satellites in orbit, 24 functional and 5 spares to be used as backups. They are in 6 12-hour orbits to ensure that there are at least 4 satellites available from every position on earth at all times.
Basically, in order to locate you in three dimensions, the GPS receiver needs to be able to see four satellites. (It's actually locating you in four dimensions: X, Y, Z, and time).The satellites, however, not being in different times, cannot triangulate on your time, but instead assume that you are in the same time that they are in (A reasonable assumption). In fact, only three satellites are needed to triangulate on your 3-D position, but due to various atmospheric conditions that vary the amount of time for transmission, and therefore vary the distance estimates, a fourth satellite is always used for error checking. With four satellites, you have four combinations of three satellites, with each combination giving you a 3-space estimate. You can then average these four estimates to get a more correct position. The more satellites, the more pieces of information you can apply to the problem, and the more correct your estimate -- which is why seven is often the prefered number for most civilian applications. Even with seven, though, there are errors, so all data collected with GPS units should then be corrected by referencing it to a nearby base station with a known exact location on the earth's surface.
As for the other post that suggests the reason 3 can be used is because the system knows you are on the Earth's surface -- this assumption would be true if, 1) the system made such as assumption, and 2) the Earth's surface were smooth. Since the system locates the position of mountain climbers, forest rangers, cars on roads, and airplanes in the sky, neither assumption can be made.