Iridium PNT – NIST

NIST submitted a report to DHS entitled An Evaluation of Dependencies of Critical Infrastructure Timing Systems on the Global Positioning System (GPS). Based on testing of STL conducted in 2019, the report was not published until November 2021.

The President of the United States issued Executive Order 13905 entitled “Strengthening National Resilience through Responsible Use of Positioning, Navigation, and Timing Service.” The multiple directives of the EO emphasize PNT resilience, diversity, and urgency to ensure the survivability and resilience of our national critical infrastructure. The directive in Section 4, Part (i) is for NIST to make available a GNSS-independent source of Coordinated Universal Time to support critical infrastructure owners and operators in the private and public sectors.

NIST and Satelles entered into initial discussions about a potential cooperative agreement that would benefit both parties.

NIST conducted a 50-day test of STL in which a GPS-disciplined clock (GPSDC) and a Satelles EVK-2 evaluation unit with a quartz oscillator comparable to that in the GPSDC were both compared to UTC(NIST). In this evaluation, the GPS device received its signal from an outdoor antenna whereas the Satelles device was connected to an indoor antenna in a deep indoor environment where GNSS signals did not reach.

NIST submitted a report to DHS entitled A Resilient Architecture for the Realization and Distribution of Coordinated Universal Time to Critical Infrastructure Systems in the United States. The report was not published until November 2021.

NIST and Satelles executed a cooperative agreement on “Satellite Ground Monitoring Station Received Signals to UTC(NIST).” Satelles’ objective is to seek an independent and operational timing reference to UTC(NIST). NIST’s objective is to seek a potential way to indirectly distribute UTC(NIST) via a correction broadcast by STL, thereby providing NIST with a broadcast signal distributing UTC(NIST) at sub-microsecond-level accuracy.

Satelles delivered a Satellite Time and Location (STL) Ground Monitoring Station (GMS) at NIST’s Physical Measurement Laboratory (PML) in Boulder, Colorado. This lab is home to an ensemble of high-precision cesium beam and hydrogen maser atomic clocks that maintains the Coordinated Universal Time scale known as UTC(NIST), the national standard for time and frequency in the United States.

Dr. Elizabeth Donley, Chief of the Time and Frequency Division at NIST, presented the results of a study at the Workshop on Synchronization and Timing Systems (WSTS) conference organized by the Alliance for Telecommunications Industry Solutions (ATIS). Dr. Donley referenced a then-unpublished report that categorized STL as one of the non-GNSS sources of UTC.

As a follow-on to Dr. Donley’s presentation at the WSTS conference, Satelles issued a press release announcing that NIST had confirmed STL as an accurate and reliable source for the wide-area delivery of UTC. Specifically, time deviation (TDEV) calculations estimated the stability of the two signals with respect to the UTC(NIST) time scale. Based on one day of averaging, the GPS instability was less than two nanoseconds (< 2 ns), and the STL instability was only slightly higher at under three nanoseconds (< 3 ns). These measurements demonstrated that STL delivers stability that is comparable to GNSS and does so in an indoor location where GPS signals usually cannot penetrate.

Following a successful series of preliminary tests with an onboard GPS-disciplined clock, NIST directly connected the STL GMS to the PML’s clock ensemble.

An Evaluation of Dependencies of Critical Infrastructure Timing Systems on the Global Positioning System (GPS) was published as NIST Technical Note 2189.

In its report, NIST identified STL as a non-GPS public access time distribution technology capable of microsecond accuracy that is independent of GPS. In confirming this finding, NIST states that “Due in part to the success of GPS, which has at least indirectly led to the demise of eLoran and other systems, only a small number of free public access time distribution systems remain that are under U.S. control.”

Referring to STL as a commercial alternative, the report goes on to say that of the alternatives listed in the report, “All but one of these systems have at least one caveat when considered for critical infrastructure usage, they are either not independent of GPS, not capable of microsecond-level accuracy, or both.”

A Resilient Architecture for the Realization and Distribution of Coordinated Universal Time to Critical Infrastructure Systems in the United States was published as NIST Technical Note 2187, and it revealed previously unpublished details of NIST’s evaluation of Satelles’ premier timing solution.

The report fulfilled an important part of the agency’s obligation under the executive order to make available a GPS-independent source of Coordinated Universal Time (UTC) to support critical infrastructure owners and operators in the private and public sectors. The comprehensive architecture defined by NIST comprises technical descriptions and recommendations for bolstering national resilience by having multiple ways to realize and distribute UTC in the U.S.

NIST’s architecture classifies STL as an indirect distribution source for UTC(NIST), meaning that critical infrastructure-protective applications can obtain accurate and reliable timing without using GPS by incorporating STL into a plan that comports with the responsible use of PNT. The agency based its findings on a thorough technical evaluation showing that STL is a reliable source of timing that is highly consistent with UTC(NIST) and is based on a signal that is independent from GPS.

Satelles issued a press release about how two reports from NIST cast Satellite Time and Location (STL) in a favorable light on the alternative PNT landscape. Technical Note 2189 classifies STL as an indirect distribution source for UTC(NIST) within NIST’s resilient timing architecture for the United States.

Technical Note 2187 characterizes STL as a resilient public access time distribution technology capable of microsecond accuracy. NIST confirmed that STL is highly consistent with Coordinated Universal Time (UTC) and is based on a signal that is independent from GPS and other GNSS. The report provides the full details behind the claims made by Satelles in an April 2021 press release.

Satelles issued a press release about a cooperative agreement with the U.S. National Institute of Standards and Technology (NIST) that led to a direct connection between an STL Ground Monitoring Station (GMS) and NIST’s collection of extremely accurate atomic clocks in Boulder, Colorado, that maintains the official time scale for UTC(NIST).

This was the first public announcement describing not only how Satelles delivered and configured an STL GMS at NIST’s Time and Frequency Division the previous year (see timeline above) but also how after conducting a series of successful preliminary tests, NIST then directly connected the STL GMS to its primary clock ensemble in June 2021. Comparing timing provided by STL to UTC(NIST), testing that concluded in early 2022 confirmed STL’s long-term stability of better than 25 nanoseconds with short-term time deviation of 50 nanoseconds.

Satelles and NIST presented a jointly written paper at the Institute of Navigation (ION) Precise Time and Time Interval Systems and Applications (PTTI) meeting. The paper, entitled “Measuring the Timing Accuracy of Satellite Time and Location (STL) Receivers,” was co-developed by time experts at Satelles and NIST and demonstrated that a typical STL receiver with an oven-controlled crystal oscillator (OCXO) can provide a stable output pulse with an average time offset near 10 ns and a maximum time offset of less than 200 ns with respect to UTC(NIST). The paper also presented measurements of an STL receiver with a local rubidium oscillator that demonstrated improved short-term stability and a maximum time offset of less than 75 ns.

Satelles and NIST presented a jointly written paper at the Institute of Navigation (ION) Precise Time and Time Interval Systems and Applications (PTTI) meeting. The paper, entitled “The Long-Term Timing Performance of Satellite Time and Location Receivers Utilizing Signals from Low Earth Orbit Satellites,” was co-developed by time experts at Satelles and NIST and included data showing that an STL receiver with an oven-controlled crystal (OCXO) or a rubidium (Rb) oscillator provides a stable timing solution and maintains an average offset less than one nanosecond (ns) to UTC(NIST) after calibration. The paper also described how STL timing receivers with a high-quality oscillator can maintain a Maximum Time Interval Error (MTIE) less than 100 ns for long durations, meeting the ITU-T G.8272 PRTC-A requirement for a primary reference clock.

Iridium acquired Satelles, making STL — now known more broadly as Iridium PNT — an end-to-end Iridium service.

Iridium engineers upgraded the configuration of performance monitoring equipment that was first deployed at NIST’s Physical Measurement Laboratory (PML) in Boulder, Colorado, in February 2021. The enhancements establish an authoritative truth for Iridium PNT time offset and stability and give experts at Iridium and NIST real-time feedback on the latest Iridium PNT receiver timing performance compared to the UTC(NIST) time scale.

Iridium’s team brought in the latest generation of its PNT receiver technology and installed a rubidium-based miniature atomic clock to provide an optimum frequency source and timing reference for the new receivers. New data collection appliances were connected to enable deeper analysis of various performance metrics, and time interval counters were incorporated into the setup to directly measure the time offset between Iridium PNT and UTC(NIST). Additional computing and networking hardware to improve remote data collection and support ongoing system optimization rounded out the upgrades.