Establishing “Truth” with Satellite Time & Location (STL) service using Low Earth Satellites 

Have you ever stopped to think about what time it really is? We may check our watches or phones and see a specific number, but in reality, that's just a designation we've assigned. In past eras, we were satisfied simply to have a common calendar date, but as timekeeping technology advanced, we were able to coordinate our clocks to within minutes of each other. Now, we can synchronize clocks worldwide to within billionths of a second. 

The ability to synchronize a clock anywhere in the world with high accuracy is made possible by  a concept called Coordinated Universal Time (UTC). If you’re a PhD in physics, you will argue multiple timekeeper scales exist, but for the rest of us dealing with local network synchronization principles, UTC is basically an approved group of world clocks with exceptional accuracy that contribute their time measurements to a central authority that averages the readings to arrive at what is generally accepted as “Truth” relating to exactly what time it is at any moment. That “Truth” is then transmitted globally by various means, which include a method to account for transit time for each transmission medium.

In most cases, Truth is disseminated globally by several Global Navigation Satellite Systems (GNSS) such as:

• USA: GPS – and new for 2023- Iridium/Satelles Low Earth Orbit (LEO) Satellite Time & Location (STL)
• Europe: Galileo
• China: Beidou
• Russia: Glonass

GPS, Galileo and Iridium/Satelles satellite constellations

It is notable these satellite constellations do not generate Truth; they merely transmit Truth to all corners of the earth. As a reminder, Truth is what is generally accepted as Coordinated Universal Time (UTC) or what the world experts agree is the exact time at any moment, delivered by GNSS within +/- 30ns 90% of the time.

Once Truth is transmitted over satellite constellations it can be recovered at nearly any point on earth using a satellite receiver. The receiver then decodes the satellite messages to provide data elements to resolve latitude, longitude, altitude and time. This method is utilized to discipline, or tune local oscillators that can transfer UTC frequency and phase (time) to a local “clock.”

The official designation for the metrics shown above are position, navigation and timing (PNT). These three entities: position (where you are), navigation (where you’re going) and timing (exact time of day) are used together and separately depending on your needs. For example, when you use GPS for travel routing you rely on P&N (Where am I and where am I going?) Likewise ships and airliners use P&N to determine exactly where they are and the proper route forward to their intended destinations.

However, power, telephone, finance, cellular and cable TV/broadband providers use the timing aspect to synchronize their vast networks. Since most of their equipment is installed in buildings that don’t move, they care less about position and navigation (with the exception of California and Florida where buildings may move during earthquakes and hurricanes).

Time is of the essence

Regardless of GNSS use case, without accurate time there would be no position and navigation. The accuracy of time is of utmost importance in making Truth available globally. 

Humans are now overly dependent on GNSS in nearly every aspect of life from driving a car, making financial transactions, receiving emergency services (fire, police, ambulance), enjoying reliable electrical power, traveling commercially and much more.

Since GNSS impacts nearly everything we do daily, it has also become a tempting target for signal jamming and spoofing, leading to disruption of societies and neutralizing critical military advantages. 

Imagine a world without GNSS

It’s difficult for the average person to consider loss of GNSS availability. However, GNSS jamming occurs around the globe on a daily basis. In some cases, GNSS jamming is unintentional as myriads of new, potentially interfering radio frequency technologies are deployed. However, the majority of GNSS jamming is intentional by individuals and governments.

There are growing numbers of people who do not want their location to be trackable, so they purchase low-cost GNSS jamming devices to obscure the location of their automobiles. This is highly illegal and leads to disruption of critical GNSS reception as they drive by power utilities, telephone and cable company buildings and especially airports.

Likewise, GNSS reception is often erased on a broader scale during armed conflicts, provocations between countries, while ships at sea are lured off-course by GNSS spoofing.

Are you resilient?

Since jamming and spoofing have become everyday occurrences, governments around the globe have warned critical service providers they may no longer rely on GNSS alone for PNT accuracy. The concept of safeguarding critical infrastructure from GNSS jamming and spoofing is called “resiliency.” Therefore, all critical service providers (communications, financial institutions, power utilities, emergency services/E911, transportation, broadcast, military) MUST design resilience into their timing and synchronization schemes.

Historically, GNSS has been the prime distribution method for Truth. Since we are all now informed that vulnerabilities can make GNSS untrustworthy, how do we then arrive at dependable Truth for timing and synchronization?

The good news is that Truth is transferable from one media to another. In some cases, it can also be made portable. Therefore, Truth is available from more than one source, so long as that source can trace back to UTC. To understand this, let’s see how local network synchronization systems make use of the Truth they receive from GNSS.

 

Once GNSS signals are decoded, the local clock can process the information to derive two distinct types of timing and synchronization outputs: analog and digital time-domain (electrical) signals, and Ethernet/packet messages. However, both types must be traceable back to a UTC reference or standalone cesium reference clock.

Resilience through additional timing sources

For many years GNSS was the only practical source of PNT for network timing and synchronization. However, additional PNT services are now available from Satelles, using the Iridium constellation of Low Earth Orbit (LEO) satellites. As shown previously, these satellites are very near the earth, providing up to 1,000 times more radio frequency power than GNSS satellite signals at the surface of the earth. The Satelles service is called Satellite Time & Location (STL).

The LEO satellite signals can penetrate buildings and also fill in gap areas not currently served via GNSS such as canyons and deep forests. Since these signals penetrate buildings, they eliminate the requirement for an outdoor antenna which counts for a significant portion of cost for GNSS clock installations. Therefore, a small antenna may be installed at the top of the equipment rack housing the STL receiver.

STL signals are encrypted, so spoofing is not an issue, and the signal levels are up to 1,000 times stronger than GNSS/GPS power levels so in live tests GPS jammers do not impair the STL signals.

Adtran has partnered with Satelles to include STL receivers in many Oscilloquartz synchronization systems (clocks).

Worldwide coverage

Polar orbits of the Iridium/Satelles constellation

Satelles’ STL service has been tested and analyzed by the European Commission, US National Institute of Science & Technology (NIST) and the US Department of Transportation, with each reporting the service exceeded all expectations in rigorous testing scenarios.

STL service is available today and Oscilloquartz is an enabler for utilizing STL as a reliable timing and synchronization source for your network. Our embedded STP receivers allow dual operation of GNSS and STL in the same clock system, or use of either GNSS or STL separately.

To be continued …

We’ll talk more in part 2 about the challenges facing modern networks and the new timing and synchronization strategies required to meet their needs. We'll explore the concept of “local Truth” and the enhanced primary reference time clock (ePRTC) as a potential solution for maintaining accuracy in the absence of GNSS. Along with formulas for ensuring resilience in timing, we’ll provide the recipe for achieving assured PNT (aPNT™) for 100% reliable network timing and synchronization, including:

• Jamming/spoofing detection and mitigation
• GNSS backup via multiple qualified timing sources
• Global network monitoring, diagnosis, and management of timing via Ensemble Network Controller (ENC)
• Monitoring both new and legacy clocks
• Intelligent clocks with built-in probing capabilities utilizing patented Syncjack™ technology