[time-nuts] Re: What everyday uses are there for accurate clocks?

[John Sloan]

Someone else probably brought this up already but here I am anyway. I've spent decades working in the telecommunications industry, working as a software/firmware engineer on distributed PBXes that used ATM as a cross connect (sometimes across international boundaries), cellular base stations, and satellite communication systems (Iridium and Inmarsat) for aircraft. Synchronization (same time) and syntonization (same frequency) were always an issue.

The higher bandwidth the communication link, the more vital that each end agrees on its clock rate, which is fundamental for the conversion between pulses and bits. Some technologies, like Ethernet, use a mechanism to encode the clock right onto the wire with the signal, so it is self-clocking. But that inevitably costs a largish portion of the available bandwidth. Copper land lines used by the big carriers dedicate a wire just to clocking, sourced from one end. At one time AT&T had several cesium atomic clocks in their network just for this purpose, their stratum 0 clock sources. Again, there is a cost to this, where that wire could be used to transmit data (and hence book revenue).

GPS - especially cheap GPS receivers - changed all that. Now virtually every thing we care about - not just telecommunications, but the stock market, public utilities, air travel, etc. - depends on GPS, from which both timing and frequency is derived. Everything, directly or indirectly, derives its frequency from a GPS receiver, in effect distributing the stratum 0 responsibility into cost effective GPS receivers at virtually every major end point. The U.S. Department of Homeland Security (DHS) considers GPS a critical infrastructure.

The use of code-division multiplexing in the GPS air interface makes it trivial to jam the satellite transmissions. Even though such jamming is illegal in the U.S., GPS jammers can be had for tens of dollars. And state actors, like Russia, routinely jam GPS over wide areas. The U.S. DOD does this too, for example when testing at White Sands, and announces it in advance. This makes GPS resilience a big issue, big enough it has been discussed in the U.S. Congress, and the RAND corporation has published a report (sponsored by DHS) on GPS alternatives.

A year or so ago, I volunteered to be part of IEEE’s “capitol visit day” where we zoomed with staffers for U.S. senators and representatives, and this is in part what I used my brief time for, to discuss the critical nature of GPS, and push for continued funding for the DOT (where the responsibility has been placed) to develop alternatives. (The other thing was continued funding for NIST for post-quantum cryptography.)

--
J. L. Sloan             Digital Aggregates Corporation
+1.303.489.5178         3440 Youngfield Street
mailto:jsloan at diag.com  #209
http://www.diag.com     Wheat Ridge CO 80033 USA