Government authorities in the U.S. (NIST, DHS, DOT) and Europe have subjected STL to rigorous evaluation, and other entities have also studied the performance specifications of the service. This report provides in-depth background information and summarizes the key findings.
U.S. National Institute of Standards and Technology (NIST)
Satelles has supported NIST’s multiple technical evaluations of STL. An important test conducted by NIST involved comparing a GPS-disciplined clock and an STL receiver to UTC(NIST) for 50 days. 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).
Additionally, 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 permitted 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.
- NIST identified STL as “a resilient public access time distribution technology capable of microsecond accuracy” and referred to STL as a commercial alternative that exists today to provide an essential capability that protects the operations of PNT-dependent critical infrastructure and ensures the survivability and resilience of our nation.
- NIST concluded that STL is a reliable source of timing that is highly consistent with Coordinated Universal Time (UTC) and is based on a signal that is independent from the Global Positioning System (GPS) and other Global Navigation Satellite Systems (GNSS). 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.
- Testing conducted by NIST compared 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).
- Testing by NIST also confirmed that “[t]he timing accuracy specification for STL is ±500 nanoseconds (0.5 microseconds) which meets critical infrastructure requirements.” In fact, NIST acknowledged previously published measurements indicating that STL has timing accuracy better than 200 nanoseconds.
- After connecting an STL GMS to NIST’s primary clock ensemble — an achievement that is very rare for a commercial entity — NIST compared timing provided by STL to UTC(NIST) and confirmed STL’s long-term stability of better than 25 nanoseconds with short-term time deviation of 50 nanoseconds.
Satelles and NIST have also collaborated on papers presented at the two most recent Institute of Navigation (ION) PTTI conferences:
- “Measuring the Timing Accuracy of Satellite Time and Location (STL) Receivers” (January 2023) – The underlying research demonstrated that a calibrated STL receiver can achieve an average time offset better than 18 ns with respect to UTC(NIST) with a peak-to-peak variation of 325 ns for a typical OCXO receiver and better than 80 ns with a rubidium-based receiver.
- “The Long-Term Timing Performance of Satellite Time and Location Receivers Utilizing Signals from Low Earth Orbit Satellites” (January 2024) – Based on 66-day and 100-day testing conducted at Satelles and NIST facilities, the study reached a number of conclusions relevant to civil government and commercial entities with timing synchronization requirements, including:
- STL timing receivers, utilizing only signals originating from a LEO satellite constellation, provide a globally available time reference that’s a viable backup or alternative to GPS.
- Measurements verified that typical STL timing receivers can provide a very stable and accurate timing solution with an average offset within a few nanoseconds of UTC(NIST).
- 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.
Need more information? Visit our NIST page for additional details.
U.S. Department of Homeland Security (DHS)
The FY 2017 National Defense Authorization Act (NDAA), required DHS “to address the needs for a GPS backup by identifying and assessing viable alternate technologies and systems.”
Following an in-depth assessment of PNT systems currently used in the United States conducted by the Homeland Security Operational Analysis Center (HSOAC) on behalf of DHS and DOT, the Cybersecurity and Infrastructure Security Agency (CISA) summarized and analyzed HSOAC’s assessment and provided “recommendations for the Federal Government’s next steps in efforts to increase the resilience of US Critical Infrastructure to disruption of GPS services.”
The CISA report stated that “DHS could not identify generic specifications for a national backup” because “[t]he position and navigation functions in critical infrastructure are so diverse that no single PNT system, including GPS, can fulfill all user requirements and applications.” However, as DHS explains, “a minimal acceptable precision of anywhere between 65-240 nanoseconds […] supports all critical infrastructure requirements.”
The report states that this range “is expected to meet future requirements, including 5G.” Based on the precision of timing references used by receivers, STL currently delivers timing accuracies between 50 to 240 nanoseconds, proving that it is ready to meet timing requirements that strengthen the resilience of critical infrastructure.
Specifically, the report confirmed that STL is able to meet the precision timing synchronization requirements of financial services (50 milliseconds), phasor measurement units used in the electrical grid (1 microsecond), 4G-LTE wireless networks (1.5 microseconds), and 5G wireless networks (~240 nanoseconds). Furthermore, of the technologies evaluated by DHS, the report confirmed that STL is a nationwide service that is commercially available today.
Need more information? Visit our DHS report page for additional details.
U.S. Department of Transportation (DOT)
In March 2020, the U.S. Department of Transportation organized a demonstration of positioning, navigation, and timing (PNT) technologies from multiple providers. On January 14, 2021, DOT published an in-depth report for Congress entitled Complementary PNT and GPS Backup Technologies Demonstration Report as required by the FY 2018 National Defense Authorization Act (NDAA).
In addition to confirming that STL was the only solution in its study that is nationally and globally scalable without incremental infrastructure expense, DOT echoed many of the sentiments expressed by DHS in the reports detailed in the previous section. In its statement from the Conclusions and Recommendations passage of the report (p. 194), DOT clearly expresses its alignment with DHS:
“The demonstration indicates that there are suitable, mature, and commercially available technologies to backup or complement the timing services provided by GPS. However, the demonstration also indicates that none of the systems can universally backup the positioning and navigations capabilities provided by GPS and its augmentations. The critical infrastructure positioning and navigation requirements are so varied that function, application, and end-user specific positioning and navigation solutions are needed. This necessitates a diverse universe of positioning and navigation technologies.”
The high-level summary of the 457-page report is as follows:
“Again, suitable and mature technologies are available to owners and operators of critical infrastructure to access complementary PNT services as a backup to GPS. To achieve the parallel objective of resilience, as described in Executive Order (EO) 13905, that path should involve a plurality of diverse PNT technologies. Promoting critical infrastructure owner/operator use of those technologies that show strong performance, operational diversity, operational readiness, and cost-effectiveness is worthwhile. Based on this demonstration, those technologies are LF and UHF terrestrial and L-band satellite broadcasts for PNT functions with supporting fiber optic time services to transmitters/control segments.”
The mention of “L-band satellite broadcasts” is a specific call-out of STL as an integral part of a multi-technology approach for PNT resilience. This is also highly consistent with the resilient timing architecture concept recommended by NIST (see earlier section).
STL was one of the 11 technologies subjected to a range of testing scenarios during the demonstration for DOT, and it emerged as one of the top performers. The solutions were evaluated based on various measures of effectiveness (MoE), each with its own detailed rubric. For example, Satelles earned top-tier scores in categories such as Service Synchronization (MoE-7), PNT Signal Robustness (MoE-8), and Service Resilience (MoE-9).
The two most important top-level takeaways from the report are as follows:
- DOT applied weighting factors and technology considerations to produce scoring and ranking in six aggregate groupings. These high-level values were rolled up even further to generate combined scoring and overall ranking. In addition to being evaluated favorably in many individual underlying test scenarios and evaluation categories, STL was ranked #2 overall for timing and positioning in both the performance-sensitive weighting and cost-sensitive weighting classifications, and in key categories Satelles performed better than the #1 overall provider.
- STL has some important distinctions compared to other solutions demonstrated, such as being the only solution that is nationally and globally scalable without incremental infrastructure expense. STL is available today across the entire United States — including urban centers, rural areas, and navigable waterways — as well as offshore locations. This ability to provide timing and 3 D positioning coverage in the U.S. and worldwide is possible due to the multibillion-dollar investment that has already been made by Iridium to launch and operate its commercial satellite network.
Visit our DOT report page for additional details about how STL was rated by DOT in several more categories.
European Commission
Satelles was awarded a contract in October 2021 to participate in a demonstration and evaluation of alternative PNT technologies conducted by the European Commission (EC) Directorate-General for Defence Industry and Space (DEFIS). STL was subjected to a range of performance and operational tests in a technical evaluation conducted in late 2021 and early 2022 at the EC’s Joint Research Centre (JRC) in Ispra, Italy.
As stated by the EC, the aim of this project was to “analyse the technologies which could deliver positioning and/or timing information independently from GNSS, to be effective backup in the event of GNSS disruption, and if possible, to be able to provide PNT in the environments where GNSS cannot be delivered.”
| European Commission AltPNT Performance Requirement | STL Performance |
| Act independent of GNSS and with no common points of failure; resilient to GNSS failure modes and vulnerabilities | STL is resilient to the loss of GNSS, without common modes of failure in the event of GNSS disruption or in environments where GNSS cannot be delivered. This was (1) verified by third-party testing conducted for the U.S. Department of Transportation (later published in the aforementioned report) and (2) confirmed in a study conducted by the U.S. National Institute of Standards and Technology. |
Minimum performance for at least one day upon GNSS loss:
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STL surpassed the EC’s expectations in tests performed in the absence of GNSS:
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| Able to cover the EU European territory including inland waters | As a global service that can be broadcast to any point on the planet, the worldwide footprint of STL delivers service to the entire EU European territory and can also provide coverage to the EU’s overseas countries and territories (OCT). |
| Technology Readiness Level (TRL) > 5 for positioning/navigation OR > 6 for timing services | Testing confirmed what customers that have been using the commercially available service for years already know — which is that the STL service and STL user equipment are both rated at TRL 9 (the highest technology readiness level).
Substantiating the full maturity of the technology, STL receivers demonstrated true plug-and-play operation, with on-site calibration neither required nor applied in advance of any testing, taking less than 10 minutes to set up the equipment and under one minute to output an accurate position and time with an indoor antenna. Whereas other technologies in the EC study need to secure spectrum rights from government regulators, STL is a fully deployed global service that currently operates with all the necessary spectrum rights in Europe. |
| Extend PNT provision to the environments where GNSS cannot be delivered (optional) | Satelles demonstrated that STL operates within indoor environments during tests with an antenna positioned in the center of the ground floor of a large building at the Ispra facility. STL operated flawlessly whether placed on top of or inside a metal filing cabinet. |
Looking for more in-depth coverage? Check out our European Commission study page for more detailed performance results.