NIST’s Technical Evaluations of STL
Satelles has supported NIST’s various technical evaluations of Satellite Time and Location (STL). One of the tests conducted by NIST involved comparing a GPS-disciplined clock (getting its signal from an outdoor antenna) and an STL receiver (with an indoor antenna) to UTC(NIST) for 50 days. The study showed that 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).
This was just one of the tests that led NIST to conclude that STL is a reliable source of timing that is highly consistent with Coordinated Universal Time (UTC). As a result, NIST determined that STL is capable of indirectly distributing UTC(NIST), thereby making STL a vital element of NIST’s resilient timing architecture.
Cooperative Agreement Between Satelles and NIST
Satelles and NIST are working together under the terms of a Cooperative Agreement. One aspect of the agreement is for Satelles to provide STL to NIST so that it may explore options for using STL to distribute UTC(NIST) at sub-microsecond-level accuracy. The agreement also allows Satelles to deploy an STL Ground Monitoring Station (GMS) within NIST’s primary time lab, with a direct connection to the atomic clock ensemble that will allow Satelles to propagate UTC(NIST) throughout the STL network.
The following table lays out the chronology of the various engagements between Satelles and NIST.
|February 2020||NIST submitted a report to DHS entitled An Evaluation of Dependencies of Critical Infrastructure Timing Systems on the Global Positioning System (GPS). The report was not published until November 2021.|
|February 12, 2020||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.|
|May 2020||NIST and Satelles entered into initial discussions about a potential cooperative agreement that would benefit both parties.|
|June 25 – August 13, 2020||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 were not able to reach.|
|August 2020||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.|
|October 27, 2020||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 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.|
|February 2021||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.|
|April 1, 2021||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 categorizes STL as one of the non-GNSS sources of UTC.|
|April 21, 2021||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.|
|June 2021||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.|
|November 1, 2021||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.”
|November 3, 2021||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.
|March 2, 2022||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.
|March 30, 2022||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 early last 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.
|January 25, 2023||Satelles and NIST present 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. (NOTE: A link to the paper will be included here once it is published by ION.)|