Time underpins everything! So much of our inter-connected modern life has come to rely on a certain type of timing distribution that comes from the sky (Satellite or GNSS time) and it remains at the heart of most, if not all, modern technologies, processes and systems. The telecom networks, the power grids, the utilities sector, radio & TV broadcast, financial services and so many more. And they all rely on this magic ‘clock from the sky’.
GPS and GNSS applications
Since the mid-1990s, when GPS was first declared fully operational, it has had a second use other than navigation: as a time reference clock (typically for Network Time Protocol (NTP) master clocks. The GPS system inherently needs all elements of the ground control segment and all satellites in the space segment to be synchronised for the receivers to compute a navigation fix.
Luckily, GPS time is also synchronised and traceable back to the civilian global timescale known as UTC (otherwise known as Coordinated Universal Time). However, the radio signals transmitted by these GNSS satellites can be blocked (or ‘jammed’) by a high-power noise source as said signals are inherently low power in nature. For instance, any radio equipment located near the signal can cause this kind of interference.
Generating a false copy of the GNSS radio signals is becoming ever easier, too, thanks to ever more powerful computers and SDRs (Software Defined Radios). Add some open-source software from GitHub and a few ‘Googled’ commands and you can have a fully functional GPS signal simulator for a few hundreds pounds and less than an hour’s work. Malicious intent here might be to fool GNSS receivers to believing their position, time or both are away from their true values. This would cause errors or failures in the systems that are relying on satellite timing.
GNSS Constellations
GPS is one of four GNSS (Global Satellite Navigation System) constellations, the others being Galileo, GLONASS and BeiDou. The latest Samsung Android phone reports that it can see 67 separate satellite signals from these four constellations, and it’s using 30 of those signals to locate itself with an accuracy of +/- 3m. The stuff of science fiction only a few decades ago is now the norm.
GPS’s greatest strength is also its greatest weakness: “It’s free and it works everywhere.” Phenomenal system uptime and availability since the mid-1990s has led to a certain complacency in its use. Having critical infrastructure (e.g. the utility sector) rely on these vulnerable signals from space is obviously less than ideal.
Timing Distribution and the GNSS Firewall
To address the cyber security concerns from both jamming and spoofing of the GNSS radio signals, a new type of product has emerged – the GNSS Firewall. Just like a traditional IT Network firewall, the GNSS Firewall sits between your protected assets/network and the wild, open interface – which in this case is the GNSS radio antenna. The GNSS Firewall employs sophisticated algorithms to analyse the GNSS RF signal structure and detect spoofed signals, looking for obvious signs (e.g. large changes in transmitted power, time or position) along with more subtle markers carried in the navigation message data that the radio signal has not in fact originated from the space-based constellation of satellites.
If it decides that the GNSS signal is not authentic, it can disconnect the sky signal from its output, meaning that any clocks downstream that are synchronising to GNSS will go into holdover. However, that’s not the limit of the GNSS firewall’s defences. It also contains a GPS signal simulator of its own and from a physically separate output it can synchronise downstream clocks to this signal. If we pair the GNSS firewall with an atomic clock (either internal Rubidium- or and external Caesium-based oscillator), it can supply highly precise, stable and accurate time for the duration of any GNSS interference.
Separation from the Sky
Other developments in ITU specifications for ePRTCs (Enhanced Primary Reference Time Clocks) contained in G.8272.1 are harnessing the best of both GNSS signals and Caesium atomic clocks. These use the long-term accuracy of the UTC-traceable GNSS signal to effectively calibrate/measure the tiny residual frequency offset present in all atomic clocks. After a few weeks of continued operation, the ePRTC will maintain less than 100ns of error to UTC without further GNSS input (e.g. under extended jamming) for the next 14 days.
High performance PTP boundary clocks (HP-BCs) based on the ITU’s Class D specification (better than 5ns error) that are connected via direct fibre/lambda connections can transport the ePRTC’s signals anywhere the network can carry them with minimal error. The network operates like one huge, distributed ePRTC (known as the virtual-PRTC or vPRTC).
Using ePRTCs and High-Performance (Class D) Boundary Clocks (HP-BCs) we can supply UTC traceable time around an optical network with just a handful of nanoseconds error. Wherever there is network, there is clock. Imagine the telecom network supplying accurate, traceable resilient timing to every 61850-based substation via the PTP (Precision Time Protocol). NTP reference clocks at the edge of networks in the energy sector are now far easier to synchronise.
Summary
Protect vulnerable GNSS receivers with an extra layer of cyber security defence and utilise the optical networking assets you already have in place to distribute your own time. This enables the present and correct GNSS signals to synchronise your atomic clocks. Distribute the atomic vPRTC (Virtual Primary Reference Time Clock) to every point in the network safe in the knowledge that any GNSS RF attacks will have no impact on your network’s time. Applications in the energy sector and others can now benefit from reliable, available and assured synchronisation thanks to advances in cyber security technologies as applied to frequency, phase & time generation and timing distribution.
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