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Comment: Line printers (Score 3, Interesting) 790

by Phil Karn (#48785225) Attached to: Ask Slashdot: Sounds We Don't Hear Any More?
Others have already mentioned the dot-matrix printer, but there was a big one before that: the high speed line printer. They were too expensive for individuals, but they certainly were a familiar sound to 1970s programming students like me.

There were two main types: the drum printer and the chain printer. The drum printer was cheaper and therefore much more common. The drum, which contained all the characters in a given font, rotated once for each row printed. An entire row was printed simultaneously; a separate solenoid-driven hammer in each column fired at the right instant to print the desired character in that column. You could easily tell from across the room whether your program had failed to compile or if execution ended with a core (!) dump. The burst pages between jobs had their own highly characteristic sound.

A related sound is that of ripping fanfold line printer paper to separate jobs. Who uses any kind of fanfold paper these days? Or even paper...?

Oh, and let's not forget the sound of the Hollerith (IBM punch card) reader...

Comment: Are they still down? (Score 4, Insightful) 360

by Phil Karn (#48657641) Attached to: North Korean Internet Is Down
Is NK still off the net? About a half hour ago I had no trouble reaching the sites www.kcna.kp - 175.45.177.74 / 175.45.176.71 naenara.com.kp - 175.45.176.67 / 175.45.177.77 According to https://www.northkoreatech.org..., both sites are physically hosted inside North Korea. I see that both are in the 175.45.176.0/22 block that whois says is assigned to North Korea, and traceroute shows an extra latency (satellite hop?) for that network past China. Is that their only net block? A /22 is 1024 addresses, which I keep hearing is the total number for the entire country.

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.

The 11 is for people with the pride of a 10 and the pocketbook of an 8. -- R.B. Greenberg [referring to PDPs?]

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