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Comment: Re:How can you screw up a power cord? (Score 1) 71

by Phil Karn (#48580039) Attached to: Lenovo Recalls LS-15 Power Cords
It's a little hard to believe it's insulation degradation despite that .au recalls website entry. When insulation degrades, you tend to get short circuits that trip circuit breakers rather rapidly. It seems more likely to be an undersized or underprotected conductor, e.g., a multistrand conductor in which flexing from improper strain relief can break most of the strands, increasing the local series resistance and heat dissipation and possibly leading to a complete conductor failure with series arcing. Only an arc-fault protector would trip on a failure like this, and those breakers are still uncommon in the US even though they're required in much new construction. It would also seem that cord failures would be more likely in North America, Japan and other 100-120V countries because a universal switching supply producing a given amount of power will require twice the line current draw and produce 4x the heat dissipation (I^2 R) in a high resistance section of cord as it would in a country with a 230-240V supply voltage.

Comment: Re:LMFAO (Score 1) 139

by Phil Karn (#48291031) Attached to: World War II Tech eLoran Deployed As GPS Backup In the UK
Because of inherent drift, inertial navigation is inherently suited only to fast vehicles that get to where they're going in just a few minutes or hours, e.g., planes and missiles. Cargo ships do not qualify. It is best combined with GPS to "flywheel" through outages (e.g., vehicles in tunnels) and so it can be automatically recalibrated whenever GPS is available.

Besides LORAN-C, there used to be another low frequency radio navigation system even better suited for global shipping: Omega. It operated on even lower frequencies, in the 10-14 kHz (yes, kHz) range, and had worldwide reach unlike LORAN-C which was only regional. It was shut down in 1997.

Comment: good to have backups (Score 1) 139

by Phil Karn (#48291001) Attached to: World War II Tech eLoran Deployed As GPS Backup In the UK
I certainly wouldn't bet that GPS satellites couldn't be destroyed, but most anti-sat weapons demonstrated so far work only on low altitude orbits. The US systems consist essentially of lobbing a small suborbital missile up in the path of the target satellite. Destroying a GPS satellite in a 20,000 km orbit takes a much bigger launch vehicle and considerably more time, and would be much harder to conceal from US space sensors.

Jamming and spoofing are the much bigger threats.

Comment: are you sure? (Score 3, Informative) 139

by Phil Karn (#48290979) Attached to: World War II Tech eLoran Deployed As GPS Backup In the UK
LORAN-C would probably be rather resistant to EMP. Like just about everything military, the transmitting equipment would be designed to be EMP-resistant, and receiving equipment on vehicles would not be particularly susceptible. It's stuff with long cables that picks up EMP. LORAN-C is certainly much more jam-resistant than GPS. The transmitter power levels are/were enormously higher, some in the megawatt range, to overcome natural background noise and antenna inefficiency. Even the large towers used are only a small fraction of a wavelength (3 km). Also, LORAN-C operates by groundwave propagation (that's why the frequency is so low) so it's not very sensitive to solar activity.

Comment: Re:Meanwhile, in the U.S. (Score 1) 139

by Phil Karn (#48290615) Attached to: World War II Tech eLoran Deployed As GPS Backup In the UK
Actually, the US military has a very simple way of selectively shutting down GPS: they locally jam the L1 frequency. The satellites also transmit on a second frequency, L2, with an encrypted, high precision "P(Y)" code for which the keys are closely controlled. They have receivers that can work with just the P(Y)-code, so it doesn't matter to them if L1 is jammed.

Comment: Re:No need to read TFA ... (Score 1) 346

Yeah, to have local governments build and maintain networks that serve all comers, commercial and private, while recovering all costs from usage-based user fees would be, dare I say it, socialism! Next thing you know, the socialists will even propose to have local governments build and maintain roads for the public good!

Comment: Re:How do they get around the altitude limit? (Score 2) 48

by Phil Karn (#48089469) Attached to: Send Your Own Radiosonde 90,000 Feet Into the Sky (Video)
The problem is ITAR - International Traffic in Arms Regulations. The idea was to keep cheap civilian receivers from being used in ICBMs. For that reason, there's an altitude and velocity limit, but the language was ambiguous. Some manufacturers interpreted it as an altitude AND a velocity, others interpreted it as an OR. The latter create the problem.

Comment: Re:Ion Thruster (Score 1) 48

by Phil Karn (#48089455) Attached to: Send Your Own Radiosonde 90,000 Feet Into the Sky (Video)
Your explanation is pretty much correct. But getting higher with a balloon is literally exponentially more difficult because that's exactly how the density of the atmosphere decreases with height. Your balloon has to expand exponentially as it climbs, and exponentials are not functions to be trifled with. The vertical distance over which the atmospheric density decreases to 1/e of its starting value is the "scale height", and for the earth it is an average of 7.6 km (it varies with temperature). But you can see that just getting to 30 km (100,000') is already about 4 scale heights, with your balloon expanding by a factor of e^4. Even that much is harder than it looks because the balloon expands as rises, and the gas inside cools adiabatically, causing its density to increase. Even in thermal equilibrium with the air outside, that air is awfully cold, which doesn't help decrease the lifting gas density. I think 100 km is completely out of the question. That's the Karman line, and it was chosen as roughly the altitude where an airplane could not generate enough lift to hold itself up even if it was going at orbital velocity. That's not a lot of air.

Comment: Re:10,000 MPH to get into orbit (Score 1) 48

by Phil Karn (#48089425) Attached to: Send Your Own Radiosonde 90,000 Feet Into the Sky (Video)
Not quite. Assuming "100k" means "100 km", conventionally chosen as the edge of space, getting there going straight up from 30 km (an easy weather balloon altitude) requires an upward force greater than the weight of the payload. Anything less and you'll just fall back to earth. If you do this with a rocket (what else would you use?) you will find that doing it slowly is *very* expensive in propellant. In rocketry this is called "gravity loss", and it's one of the reasons rockets don't just go straight up to space even when the intent is to escape the earth. They fly an arched path known as a 'gravity turn': just enough altitude is gained to reduce air drag to an acceptable level while you try to build up horizontal velocity as fast as you can. The less time you spend with your rocket anything but horizontal, the lower your gravity losses will be and the more hard-earned rocket impulse you can devote to getting orbital velocity and *staying* in space.

Comment: Re:Jamming unlinced spectrum is illegal? (Score 1) 278

by Phil Karn (#48060289) Attached to: Marriott Fined $600,000 For Jamming Guest Hotspots
Sorry, but the rules make no distinction between licensed and unlicensed spectrum. If you deliberately interfere with someone else's radio communications, you are breaking the rules.

Marriott's reply is laughable. It might work on unsophisticated readers but not anyone who knows anything about WiFi. They said they wanted to "protect" their guests against "rogue" access points. Well, if those "rogue" access points were spoofing Marriott's own SSID, they might have a point. But I certainly don't set my own portable hotspot SSID to that of any hotel. It's set to something quite unique, and it's encrypted. Nobody is going to mistake it for a hotel's network, much less actually associate with it.

Comment: Re:If the Grand Ayatollah's against it.... (Score 1) 542

by Phil Karn (#47801121) Attached to: Grand Ayatollah Says High Speed Internet Is "Against Moral Standards"
The Christian fundies' fear of Sharia Law is one of the most ironically amusing things to come out of them in recent years. If they didn't spend so much time railing against our consitutitional separation of church and state, maybe, just maybe they might realize that it's exactly what protects them from such an (unlikely) threat.

Comment: Re:Civil Unrest (Score 1) 191

by Phil Karn (#47771569) Attached to: New NRC Rule Supports Indefinite Storage of Nuclear Waste
And civil unrest becomes vastly more likely in a future with runaway global warming and the climatic changes, floods, draughts, food shortages, rising sea levels, mass extinctions, habitat destruction, economic upheavals and the like it will bring. Nuclear power, wind, solar, hydro and geothermal are ALL essential to combat it.

CO2's atmospheric lifetime is something like 1,000 years. How come those who fret about the longevity of nuclear waste never seem to talk about this? With fast reactors that burn the actinides (including plutonium) as fuel, the remaining fission products decay to the level of the original uranium ore (while being considerably more compact) in only a few hundred years, much less than the atmospheric lifetime of CO2.

The hype about "carbon capture" is just that -- hype. But it serves one useful purpose: its utter impracticality shows just how minor the nuclear waste "problem" is by comparison.

Comment: Re:Ridiculous (Score 4, Insightful) 191

by Phil Karn (#47771537) Attached to: New NRC Rule Supports Indefinite Storage of Nuclear Waste
Not far from Yucca Mountain you will find hundreds if not thousands of craters under which are buried the fission and activation products of decades of US nuclear testing. They're not reprocessed and contained in silica glass, they were simply mixed (quite violently) with the soil and rock. And yet they don't seem to go anywhere. There is no need for Yucca Mountain to contain reactor waste for even a hundred years because it will surely be removed and burned as fuel in fast reactors. Once people wake up to the fact that global warming is a vastly greater threat than nuclear power, and that nuclear power is just as essential as wind, solar, geothermal and hydro in combating it, people will realize that "spent" fuel from light water reactors is far too valuable to just throw away.

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