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Slashback: Privacy, Spectrum, Location
Sir, you just need to trust us. geekee writes "An article on CNN claims that the proposed passenger-screening system for air travelers is much more innocuous than previously claimed. Now it is claimed that the Transportation Security Administration "will not view credit records, traffic violations or other personal data", according to Admiral James Loy. He also claims records of travel will not be maintained. "Airline reservation agents would provide a traveler's name, address, phone number, date of birth, and travel plans to the TSA, which would then check that information against a variety of commercial databases and an FBI watch list.", according to TSA spokeperson Heather Rosenker."
Thinking of the children means more than hiding their eyes. Jim Tyre writes: "You pointed out that my censorware.net piece ["CIPA Before the Supreme Court"] provided a nifty link to where the official supreme court oral argument transcript would be when available online. It's now available."
What's good for the mercantilists ... wait, no doesn't have the same ring. Lawrence Lessig says that the current radio spectrum is vastly underutilized, and that new technology can extract much more use from it, creating a true radio commons. Zo writes to point out that many Salon readers disagree: "Radio waves, bandwidth, the spectrum. . .Don't we know *anything* for sure?
Sir, these books smell fine ... what's the catch?
silentbozo writes "Avid Slashdotters will remember the Baen Free Library, which puts up free web versions of Baen titles for ANYONE to download and read without having to mess around with encryption and DRM. They went a step further with this experiment last fall with the release of David Weber's War of Honor which had a bunch of novels in html, rtf, doc, palmdoc, and othe formats on CD (bound into the hardcover), which you could copy and give away to anyone. Well, they're at it again. In May, they'll have another CD for those of you who didn't get War of Honor, bound into John Ringo's Hell's Faire.
I got hooked reading John Ringo's books after browsing through my copy of the War of Honor CD... and it's a great way of catching up on the previous books in the series. Hell's Faire looks really good - I personally am looking forward to finding out what happens to the O'Neals as they fight the Posleen on Earth, and to the crew of Bun-Bun... Eat anti-matter Posleen-boy!"
As secure as ... well, you pick. Anthanos writes "pGina [http://pgina.xpasystems.com], a modular authentication framework for Windows, has come a long way since it was last noted on /. nearly a year ago. Since then a full-fledged LDAP plugin, PAM plugin, and chaining have all become part of the feature set. The kicker is the recently released Slashdot plugin, which allows authentication of Windows clients with... yup you guessed it, Slashdot Accounts! XPA Systems has even begun offering services revolving around this GPL product. Seems this may be the solution for people looking to merge authentication of Windows clients with MacOSX, Solaris, and other *nix boxen."
Let's see a handheld that uses both, please ... Mattias Östergren writes "Well aware of the risks with dependency of GPS the European Space Agency (ESA) have developed their own satellite navigation system, EGNOS. EGNOS is more accurate than GPS and the signal also tell you how much it could be off.
The first reference station have just been installed on the roof of the Land Survey in Gävle, Sweden. There is a Swedish press release about it."
At any rate, there's a lot of good Galileo information on the web. Here are some links:
These links are from a file I have of info on Galileo. Hopefully no link rot.
EGNOS is the European answer to WAAS, folks, not GPS.
EGNOS is only available in Europe at the moment because it's only being transmitted from one geostationary satellite that's sitting over Europe. WAAS is currently being transmitted from two geostationary satellites over the Americas.
Neither system is what I'd call new - they've been in a sort of beta test phase for years, and there are already consumer receivers on the market that support EGNOS/WAAS.
For AM transmissions, theoretically a single, exact frequency can suffice. Assuming the transmitter is truly on the expected frequency, all you need is a very narrow bandpass filter.
If you try to send an AM signal across a 1 Hz band, you will get a 1 Hz bandwidth signal out at the other end. Not very useful if you were trying to play music. Definitely not useful if you were trying to transmit data.
The number people are interest in is data rate. Data rate is bandwidth times the log to base 2 of the number of levels you can distinguish. Different encoding schemes (FM, wide-spectrum coding) express the relation differently, but the same limit applies.
You can narrow the bandwidth, but as soon as you hit noise limits, your data rate starts going down too. *That's* the problem. Low-noise electronics doesn't help if the noise is from other users.
The only way to avoid user clutter is to switch to something other than a broadcast system, which involves either large dishes or short-range transceivers and hubs connected to a _wired_ backbone.
I believe the claimed accuracy is 5 meters.
Cheers!
Where does that 5cm number come from? It says 2m in the swedish text, and 5m in the english text..?
(1m = 100cm, for those who find the decimalness of the metric system confusing)
Currently there are three ways to partition the available spectrum:
FDMA (Frequency Division Multiple Access): The standard technique of TX/RX on different frequencies (or colors if you read the analogy on Slashdot a few days ago). Ho-hum, it's the first thing I would have tried too. Our predominate and most wasteful technique.
CDMA (Code Division Multiple Access): A set of spread-spectrum techniques that use a sort of RF kung-fu to manipulate previously considered undesirable properties of radio waves to advantage. On the coolness factor the engineers that designed these technologies should be in the nonexistant Engineering Hall of Fame. The scuttlebut is that some of this technology was invented by Qualcomm as early as WWII but was highly classified until recently, so Qualcomm still holds most of the patents to this today.
TDMA (Time division multiple access): This involves standard unix-like time splicing, except using radio signals. GSM works like this by partitioning groups of eight consecutive time slots to form a TDMA frame with a duration of 4.615 ms. Each transmitter (cell-phone) in the area gets one burst period (a slot) of duration 15/26 ms (approx. 0.577 ms) to use the channel. This is an immensely powerful technique, and one that is infinitely scalable. It's only limitation is the speed of our electronics, which can and should maintain it's exponential speed curve. This is why the spectrum is underutilized.
EGNOS is the European version of WAAS, a system that enhances GPS accuracy by providing differential corrections (like DGPS, only from a satellite instead of a ground-based transmitter).
It's currently in testing, and is expected to be turned on for real soon.
See http://gpsinformation.net/waasgps.htm
The statement about AM is flat-out wrong.
Do the fscking trig.
Consider a sinewave modulating signal. Let c be the carrier frequency, and m be the modulating frequency. Recall that cos(u) varies between -1 and 1. We want the modulating control signal to vary between 0 and 1, so the modulator is 1/2(1+cos(m)).
We use cos(u) because it simplifies the key trick in the derivation. OBVIOUSLY, it is just a phase shift to do it in sin(u).
Then the fundamental equation of AM is
f(t) = 1/2(1+cos(m))cos(c) (1)
= (1/2 + 1/2cos(m))cos(c)
= 1/2 cos(c) + 1/2 cos(m)cos(c) (2)
The first term is the carrier wave. Observe that it carries half of the input power and NONE of the modulating signal.
Recall from basic trig
cos(u+v) = cos(u)cos(v) - sin(u)sin(v)
and
cos(u-v) = cos(u)cos(v) + sin(u)sin(v)
Then
cos(u+v) + cos(u-v) =
(cos(u)cos(v) + cos(u)cos(v)) +
(sin(u)sin(v) - sin(u)sin(v))
which simplifies to
cos(u+v) + cos(u-v) = 2 cos(u)cos(v)
Or
cos(u)cos(v) = 1/2(cos(u+v)+cos(u-v))
That looks familiar. Recall (2)
f(t) = 1/2 cos(c) + 1/2 cos(m)cos(c) (2)
Substituting
f(t) = 1/2 cos(c) + 1/2(1/2(cos(m+c)+cos(m-c)))
= 1/2 cos(c) + 1/4 (cos(m+c) + cos(m-c))
And there you have it. You have a carrier wave, and you have two sidebands, and the bandwidth of the whole thing is twice the modulating frequency.
The next step is to observe that the Fourier theorem applies and is carried straight through, and so ANY modulating signal will generate two sidebands, one above and one below the carrier wave, each preserving the harmonic content of the modulating signal, but with one reversed in frequencies.
Your explanation of FM is just as bad. I'm not going to do the derivation, because it is MUCH messier, involving very ugly Bessel functions, and I don't have my textbook handy.
You can reduce the bandwidth of an FM signal, but you lose fidelity.
You can reduce the bandwidth of an AM signal by band-limiting the input audio information, which is routinely done in voice communications gear: the full audio spectrum goes up to NOMINALLY 20 kHz, but the useful speech formants are pretty much all found between 300 Hz and 3 kHz.
You can suppress the AM carrier wave, and you can suppress one of the sidebands. This is also routinely done, in single sideband communications. This involves loss of redundancy and loss of easy tuning, which in turn makes careful tuning much more important: any mistuning comes out as distortion.