GFSK

GFSK is an acronym for Gaussian frequency-shift keying.  It's a type of frequency shift keying (FSK) modulation that uses a Gaussian filter to smooth positive/negative frequency deviations. Gaussian filtering is a common way to reduce spectral width. It's also used in GMSK. I got into some of this in my posts on Digital modes here.

Gaussian filtering, like Gaussian elimination and Gaussian functions come from linear algebra. In 1810,  Johann Carl Friedrich Gauss invented a lot of what we now call linear algebra. I won't get into this in too much detail.  The Gausian filters in GFSK have more to do with Gaussian functions, it's more related to geometry than algebra. A function reveals the probability that any real measurement will fall between any two real limits (a low and high value) and the probability curve approaches zero on either side of those limits. a comparison of probability density functions allows us to assign values within that rage with confidence. Here it's used outbound to shape the pulses to reduce spectral width.

In the case of GFSK the Gaussian filter is used to smooth positive and negative frequency deviations, which represent a binary 1 or 0. remember the amplitude is not varying (much), the frequency is shifting, just over a relatively narrow frequency band, the minimum deviation is only 115 kHz. In a technology like Bluetooth, where the frequency hops 1600 times a second, the pulses might be a bit messy. So GFSK has the advantage of reducing out-of-band spectrum.

Titan Is Watching You

Nobody involved could possibly think this is a good idea.  They probably enjoy being paid very well but you can't participate without thinking it might not end well. The headline on Buzzfeed read as follows:

"A company that controls thousands of New York City’s phone booth advertising displays has planted tiny radio transmitters known as “beacons” — devices that can be used to track people’s movements — in hundreds of pay phone booths in Manhattan"

Sadly it's true. Titan, the outdoor media company installed about 500 of the beacons in New York City. Their idea was to track users and serve them yet more advertisements. The problem was that they are physically tracking them.. you know like a creepy leery stalker guy. Apparently they have installed them in other major cities s well including San Francisco and L.A. among others. Supposedly, New York City’s Department of Information Technology and Communications (DoITT ) was only told by Titan that they were intended "for maintenance purposes only."  If that statement is true... then they should be expecting a tersely worded letter from the NY attorney general. Only weeks ago the U.S. Justice department arrested the CEO of InvoCode Hammad Akbar, for selling mobile apps designed to spying on peoples' smart phones including their location. More here.

How is this possible?  I'm not asking about oversight. I'm talking physics.  In a word—Bluetooth.  Invented by telecom vendor Ericsson in 1994, it was originally intended as a wireless alternative to RS-232 data cabling. Today it's the duct tape of the internet-of-things. Bluetooth operates in the UHF band between 2.4 to 2.485 GHz. Different standards can slice that into 40-79 channels that are either 1 or 2 MHz wide. The devices can change frequencies at up to 1600 hops per second, GFSK (Gaussian frequency-shift keying). it can move a lot of data quickly, but it only works over short distances.

For Titan, this meant extrapolating the movements by tracking the movements of multiple people via the wireless devices they carried as they passed near their beacons. they then tracked their movements like the old Decca air navigation system in reverse. [I wonder if they violated any patents doing that?] For the record the devices are made by Gimbal, Titan invented none of this RF technology. Previously before embarking on a career of crime, Titan was just selling ad space on the thousands of panels in phone kiosks around the city. After the story broke last week, New York City ordered the removal of the devices.