Time from the sky

When Journal contributor Charles Curry first started using the US-based Global Positioning System (GPS) in the mid-1980s in the oil industry, there were only seven GPS satellites giving just one hour per day when you could get a fix. Nowadays, nearly every smart phone and tablet has a multi-constellation 30-channel Global Navigation Satellite System (GNSS) receiver embedded.

GPS was conceived in 1973 by the US Department of Defence to address defence navigation and the first experimental satellite launched in 1978. Today there are 31 operational GPS satellites orbiting with 24 needed for full coverage. But GPS is not alone. The Russians created their equivalent as did Europe, China, Japan and India with varying degrees of coverage and maturity.

So how does a navigation system disseminate precise time? All GNSS satellites have atomic clocks on board. These are continuously monitored by ground stations and provide the triangulation capability and accuracy for navigation and positioning. (Consider that light travels approximately 30cm in a nanosecond and then one has the basis for slaving a local oscillator to visible GNSS satellites.)

By the mid-1990s GPS started to be used as a timing reference for telecoms. In the UK, BT were the first carrier to adopt GPS to frequency stabilise Rubidium atomic and quartz oscillators at major switch sites. GPS was not the primary reference source; this was, and still is, a cluster of Caesium atomic clocks.

Timing performance can be measured quite easily and displayed using an ITUstandardised metric known as Maximum Time Interval Error – i.e. the time interval error over varying observation periods and relative to a higher stability reference.

GPS is indeed a great achievement and it became somewhat taken for granted. In 1996, when GPS was starting to become accepted as the solution for frequency stability in telecom networks, the time aspect had yet to emerge. However, in the early 2000s, 2G mobile phone technology was emerging which, for some 2G standards, needed precise time at the base station as well as frequency stability. 3G, 4G and 5G also need precise time. Clusters of small cells need to communicate with each other in a synchronous manner which requires precise time at the edge. This need coincided with the transition from Synchronous Digital Hierarchy to Carrier Ethernet-based networks and the development of Synchronous Ethernet to provide traceability to the central reference clocks. An alternative to Synchronous Ethernet is the use of IEEE Precision Timing Protocol as the mechanism for transporting time and frequency over Ethernet networks. Although this is proving a successful technology for frequency, it is not quite so effective for time; it may yet be necessary to deploy GPS at the edge.

GPS at the edge seems the ideal solution but there are a number of issues.

  • The cost of deploying roof antennas is a major concern.
  • Reliability and continuity are critical for mobile networks but, in order to reduce the price of GPS receivers to meet the edge of network cost model, holdover stability is the first casualty, resiliency the second.
  • There is an emerging threat from lowcost GPS jammers which are readily available although their use in the UK contravenes the Wireless Telegraphy Act.
  • Space weather can disrupt the GPS service with unpredictable results.
  • Spoofing is another threat – a concept based on rebroadcasting the GPS signal with different time and position information.

One potential solution, at least for fixed infrastructure, is the terrestrial transmission of a complementary Positioning Navigation and Timing service known as eLoran. It works indoors and is not vulnerable to the same jamming and spoofing threat as GNSS. It does, however, have geographical limitations. eLoran is at a different technology readiness level to GPS and can’t yet be relied upon for synchronisation and timing.

Where will we be in another 10 or 20 years? If the lessons of the past including cost reduction, miniaturisation and technology hybridisation are to be learned we will have eLoran-type receivers embedded in all fixed infrastructure applications. We won’t have to worry about roof antenna installations and the whole thing will be less than a few dollars.

This is an executive summary of the full article by Prof Charles Curry, BEng, CEng, MITP, FIET Charles, Managing Director of Chronos Technology, which appeared in The Journal, Volume 10, Part 1 – 2016. 

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