A Brief History of NTP Time: Confessions of an Internet Timekeeper (2003) [pdf]

I've long been impressed by how many key contributors to the IETF were not only among the top practitioners in their fields but also fantastic writers, able to describe their work and necessary next steps with just the right balance of simplicity and complexity and always with clarity.

Many of the more important RFCs are almost a joy to read, and this article - written by the author of many of the NTP-related RFCs and predecessor docs - is a gem: Clear, concise, to the point, balanced, informative.

Section 4 ("Radios, we have Radios") really dates this article.

DARPA purchased four of the Spectracom WWVB receivers [...] The radios were redeployed in 1986 in the NSF Phase I backbone network, which used Fuzzball routers [26]. It is a tribute to the manufacturer that all four radios are serviceable today

Those receivers almost certainly aren't in use anymore.

There's a real problem with WWVB reception here on the east coast of the United States, especially in urban areas. NIST investigated several possibilities for improving reception, including building a second WWVB-like transmitter site in Alabama. (That plan ended up dying because of a dispute with NASA that never got resolved before the funding expired.) So they went with Plan B, a public-private partnership with a company named Xtendwave, where Xtendwave worked with NIST to design a phase modulated signal format for WWVB, NIST implemented the new phase modulated signal, and Xtendwave filed for and received patents on it all.

So NIST is currently broadcasting two signals on WWVB's carrier: the old amplitude modulated signal, which is prone to interference, and a new phase modulated signal that's more resistant to interference and has error-correction built-in. Unfortunately, when the phase modulated signal appeared, all of the "laboratory grade" carrier tracking WWVB receivers in the world (like Dave Mills' Spectracom receivers and a Kinemetrics/TrueTime receiver I had at ${PREVIOUS_JOB}) became instantly obsolete — they can't lock onto a carrier that's constantly, unexpectedly going 180° out of phase.

It's worth noting that Xtendwave has been saying that their line of receiver chips for the new phase modulated WWVB signal will be available Real Soon Now™ since at least late 2012, but you still can't buy them. Also, no one seems to know if you can build your own receiver for WWVB's phase modulated signal without running afoul of Xtendwave's patents.

So, yeah, NIST obsoleted working equipment actively used in industry in order to provide a "better" time signal that nobody can actually make use of. Yay.

These four radios, together with a Heath WWV receiver at COMSAT Laboratories and a pair of TrueTime GOES satellite receivers at Ford Motor Headquarters and later at Digital Western Research Laboratories

GOES is NOAA's geostationary weather satellite program, and NOAA used to broadcast a publicly documented timecode from those satellites. NOAA finally killed the time service a decade ago because GPS existed, there was almost no funding to keep the GOES timecode going, and the ground hardware used to drive the service was obsolete.

There's a nice GOES retrospective at http://tf.nist.gov/general/pdf/2013.pdf