Let’s start with a discussion of the different sources of precise and accurate timing.
Timing systems utilizing "time from the sky" rely on more than one time source, and continue to evolve. The major systems today are:
GPS
GPS is available worldwide and offers up to 25 nanoseconds accuracy to UTC. It is traceable to UTC and serves a primary time reference for countless applications worldwide, across numerous industries. It is sensitive to jamming and spoofing, though, due to its relatively weak signal strength and unencrypted signal.
Other GNSS
Other Global Navigation Satellite Systems have been made available by different nations over the last 20+ years, some of which offer global coverage (such as Glonass and Galileo), and others that are designed for regional usage only.
These systems complement GPS and, for many applications, offer some degree of resilience through additional or redundant signals, operated by independent entities. Some critical applications are already relying on these additional systems.
Although these systems use slightly different frequencies and other signal formats, the underlying technology is similar, and as a result, they are also susceptible to jamming and spoofing.
Timing systems can also use other common signals or technologies to provide the required level of accuracy:
Signals of Opportunity
Depending on the type of PNT application, signals of opportunity can be utilized to augment GNSS. These include truly opportunistic signals such as.WiFi and cellular signals as well as augmentation signals designed into a PNT solution, such as inertial sensors, Radar or other RF signals.
Oscillators
GNSS-based time servers almost always utilize crystal, rubidium or other oscillators as a holdover frequency source. If carefully chosen and disciplined, quality oscillators can maintain timing accuracy within millionths of a second and can even smoothen the effects of a spoofing attack.
These frequency sources therefore have become an integral component of timing systems and on more than on one occasion saved the day when things went wrong.
Today, there is also a new satellite based timing signal, called STL, or Satellite Time and Location.
What is STL?
STL is a new satellite-based GNSS augmentation system: Originating from the Iridium® constellation of 66 low-earth-orbiting satellites, STL is 1,000 times stronger than GPS/GNSS, reaching deep into buildings and preventing GPS/GNSS jamming without the aid of local infrastructure. STL signals are further protected by cryptographic security features that are much more difficult to misdirect or “spoof.”
STL offers several advantages over traditional timing systems.
- >30 dB stronger than GPS
- Higher jamming and interference resistance
- Operates indoors
- Encrypted signal
- Inherently anti-spoof
- Subscription based service
- Available for civilian use
To ensure maximum system integrity, the STL receiver must be integrated into the time and frequency system in a way that allows detection of threat events that render GPS signals unusable or unreliable. After the system detects such events, it should actively switch the input reference to STL, which allows uninterrupted performance and limits the degradation of the system’s accuracy. Here are some handy comparisons:
Timing Signal
Repurposed paging channels
1620 MHz band, 24-kHz channel, QPSK
Spread spectrum coded signal, 90 msec frame, about 1.4 second burst on average
UTC synchronized timing
500 nsec specified
0-200 nsec typical
Timing updates to a precision local oscillator of continuous time and frequency sourcing
Low Earth Orbit (LEO): Strong Signal with Spot Beams
Iridium: about 100-minute orbit (vs. GPS: 12-hour orbit)
- Encrypted signal with subscriber keys
- Positioning determination
- Spot beam location (gross)
- Range and Doppler updates
- Convergence over minutes
Geolocation security
In a world where we have come to expect GNSS to be operational at all times, STL provides the safest, strongest, most dependable backup to our GPS/GNSS infrastructure today.
For more information about STL, I recommend the following articles:
Orolia Fortifies Resilient PNT Solutions with Satelles STL Signal Technology
About Lisa Perdue
Lisa Perdue is a world-leading expert in testing critical GPS and GNSS systems. She has trained hundreds of engineers and technicians who are responsible for high-reliability positioning, navigation and timing (PNT) applications. She took a lead role in the development of the first GNSS Vulnerability Test System and speaks widely on the topic at many industry conferences.
Lisa Perdue is currently the simulation product line director at Orolia, directing the organization’s GNSS simulation activities and contributing to its entire portfolio of resilient PNT solutions. She has more than 15 years of navigation and RF systems experience, which includes 10 years of service with the U.S. Navy, where she was a certified master training specialist.