Local “Truth” and enhanced timing
In part 1, we explored the concept of Coordinated Universal Time (UTC) and how it is disseminated globally by various means, including satellite constellations such as GPS, Galileo, and the new-for-2023 Iridium/Satelles Low Earth Orbit (LEO) Satellite Time & Location (STL). Now in part 2, we’ll look at other ways of generating the nanosecond accuracy needed in today’s networks.
Are you assured?
Achieving resilience in timing and synchronization is not like heaving arrows at a dartboard, but the result of evaluating risks and informed deployment of appropriate strategies. Rather than wondering if the bases are covered, more and more operators of critical network infrastructure are turning to solutions like the Oscilloquartz Assured Position, Navigation and Timing (aPNT™) platform for 100% reliable network timing and synchronization. With OSA aPNT™, you can rest assured that single or even multiple GNSS failure events will not cripple your network. Our multi-point backup strategies lock in dependability and accuracy to shield you from network failures.
By modern standards, legacy communications systems and networks have much simpler requirements for timing and synchronization. A GNSS-referenced local clock with a rubidium holdover oscillator could maintain the required accuracy for nearly a month. However, that same setup couldn’t meet the new packet network timing requirements for more than several hours.
Therefore, new timing and synchronization strategies and components are required. One aspect of legacy sync designs is the concept of multiple inputs for a local clock. For example, a local clock may utilize GNSS for primary reference, but also have a T1 digital signal from a distant GNSS- (or STL-) referenced clock or local cesium-beam reference to use as a backup to local GNSS. These are in addition to whatever quality internal oscillator is installed. Therefore, when GNSS is lost, the clock automatically switches to whichever additional external reference is available and has been qualified as reliable, such as STL. This illustrates the concept of “local Truth.”
The concept of “local Truth” means the local clock is smart enough that, once warmed and normal timing signals are applied, the internal brains (processors, programs, algorithms, etc.) can monitor and evaluate every potential timing source, from GNSS to other signals for local or distant references. Once the clock evaluates all potential timing references, the internal oscillator is disciplined to a state where it becomes the “local Truth” against which all references are evaluated. The local clock can then either manually (by user selection) or automatically determine a priority list for selecting timing references. If one or more of the references is unstable as compared with the “local Truth,” the clock will remove that source from the priority list.
Now that we have established a “local Truth,” if GNSS is jammed or spoofed in a manner that impacts timing, the local clock will remove GNSS as a viable reference until such a time as it returns to stable performance. It is notable the simple GNSS jamming devices have little or no impact on STL signals. This scheme worked well for older networks over decades of dependable service. However, it falls short of the needs for modern networks.
The timing and synchronization needs for modern networks require accuracies down to the billionths of a second. In order to reliably meet those needs in the absence of GNSS for up to 4-5 months, a new solution called the enhanced primary reference time clock (ePRTC) is required. The ePRTC is almost like plucking a GNSS satellite from the sky and installing it at your site.