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Comment Frequency Hopping vs Direct Sequence (Score 4, Informative) 67

I don't mean to denigrate her contribution in any way, but Lamarr's frequency-hopping spread spectrum is not the one used in CDMA mobile phones. It is used in Bluetooth.

Lamarr invented "frequency hopping" while CDMA cellular and GPS use "direct sequence". Frequency hopping is just what it sounds like: a narrowband transmitter is continually retuned to a different radio channel. Unless the receiver tuning follows the same sequence at the proper times it cannot receive the transmission.

Direct sequence XORs a narrowband signal with a high speed pseudorandom "chip" sequence, and the receiver undoes this operation by XORing it again by the same sequence properly synchronized in time. It closely resembles a keystream-type encryption system, though the "keystream" is not necessarily secret. The main difference is that direct sequence is a wideband signal while, at any instant, a frequency hopped signal is still narrowband.

Each method has advantages. Frequency hopping can be especially resistant to strong narrowband jamming, so it's a favorite of military systems (Lamarr's intended use). Direct sequence is easier to use with coherent modulation so it tends to use transmitter power more efficiently, and it can often provide precise timing and positioning as a side benefit. Or, in the case of GPS, as its primary purpose.

While CDMA mobile phones were very important in the 1990s and 2000s, it is now being replaced with LTE (Long Term Evolution), which uses OFDM - Orthogonal Frequency Division Multiplexing. So do many other modern terrestrial digital communication systems including DSL, HD Radio, DVB-T (but not ATSC), WiFi and DRM (Digital Radio Mondiale).

Comment No more tourist attraction (Score 1) 336

My guess is that the Metropolitan Police is far from gone from the area; they simply got tired of being an overt tourist attraction. The Ecuadorian Embassy is right around the corner from Harrod's and also the hotel where we stayed as tourists last summer. I got the definite impression that the police on duty were photographed a lot...

Comment Re:Noocular (Score 1) 298

Yes. I think cooler heads will eventually prevail, and they'll reverse their knee-jerk decision to phase out nuclear. One only has to look at German CO2 emissions over the past few years to see why. Of course, until they do I'm sure the French and Czechs will be happy to sell Germany their surplus nuclear power.

Comment Nuclear power phobia (Score 1) 69

Speaking of the horrific consequences of nuclear weapons testing in the Pacific, a big one that's still with us today is the knee-jerk phobia of nuclear power, often by people who can't distinguish between the two. Along with wind and solar, nuclear power is one of our chief tools to mitigate global warming, which will in the long term prove to be far worse than weapons testing. It sure doesn't help that the US government lied through its teeth about atmospheric testing. I've been trying to find a copy of Joseph Rotblat's paper deducing that most of the yield of the Ivy Mike and Castle Bravo tests came from the fast fission of the U-238 tamper, revealing as a lie the government's claim that fusion bombs were inherently clean. Anybody know where I can find a copy?

Comment Line printers (Score 3, Interesting) 790

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

Is NK still off the net? About a half hour ago I had no trouble reaching the sites - / - / According to, both sites are physically hosted inside North Korea. I see that both are in the 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

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

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

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

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.

Mommy, what happens to your files when you die?