4
Concluding Thoughts
The long history leading to the April 2020 Authorization of the Ligado Networks, LLC, low-power mobile satellite services (MSS) with an ancillary terrestrial component (ATC) stands as clear evidence of both the complicated technical, business, and operational challenges involved and of the complex administrative process that the Federal Communications Commission (FCC) has executed to reach its conclusions. The tasks assigned to the study committee are similarly complex, and the responses depend on the physics of the relevant electromagnetic emissions and reception, the use cases of that spectrum in a very busy bandwidth geography, and the implications of the long history of assessing and assigning spectrum rights for new entrants without generally assigning performance requirements on receivers.
In this context, the committee summarizes its responses to the task questions assigned as follows:
Task 1: [Consider] which of the two prevailing proposed approaches to evaluating harmful interference concerns—one based on a signal-to-noise interference protection criterion and the other based on a device-by-device measurement of the Global Positioning System (GPS) position error—most effectively mitigates risks of harmful interference with GPS services and U.S. Department of Defense (DoD) operations and activities.
Conclusion 1: Neither of the prevailing approaches to evaluating harmful interference concerns effectively mitigates the risk of harmful interference.
Task 2: [Consider] the potential for harmful interference from the proposed Ligado network to mobile satellite services including GPS and other commercial or DoD services, including the potential to affect DoD operations, and activities.
Conclusion 2: Based on the results of tests conducted to inform the Ligado proceeding, most commercially produced general navigation, timing, cellular, or certified aviation GPS receivers will not experience significant harmful interference from Ligado emissions as authorized by the FCC. High-precision (HP) receivers are the most vulnerable receiver class, with the largest proportion of units tested that will experience significant harmful interference from Ligado operations as authorized by the FCC.
Conclusion 3: It is within the state-of-the-practice of current technology to build a receiver that is robust to Ligado signals for any GPS application, and all GPS receiver manufacturers could field new designs that could co-exist with the authorized Ligado signals and achieve good performance even if their existing designs cannot.
Conclusion 4: Iridium terminals will experience harmful interference on their downlink caused by Ligado user terminals operating in the UL1 band while those Iridium terminals are within a significant range of a Ligado emitter—up to 732 m.
DoD has evaluated the impact of FCC Order 20-48 on department devices and missions and has asserted (see slides dated March 15, 20221), “The terrestrial network authorized by FCC Order 20-48 will create unacceptable harmful interference for DoD missions. The mitigation techniques and other regulatory provision in FCC Order 20-48 are insufficient to protect national security missions.”
Additional information on the test results and analysis as they related to DoD systems and missions is discussed in a classified annex to this report.
Task 3: [Consider] the feasibility, practicality, and effectiveness of the mitigation measures required in the FCC order with respect to DoD devices, operations, and activities.
Conclusion 5: Although the mitigation procedures proposed in the order may be effective, in many cases such mitigation may be impractical without the extensive dialog among the affected parties presumed in the Order. In some cases, mitigation may not be practical at operationally relevant time scales or at reasonable cost.
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1 These slides, along with other materials provided to the committee, were placed in the project’s public access file and are available on request from the National Academies’ Public Access Records Office.
This report concludes with several additional observations related to the processes employed in the kinds of proceedings that led to FCC Order 20-48. Spectrum real estate is a living asset and approaches must allow not only for a degree of confidence that a deployed system will not be compromised by future, unforeseen entrants, for a period of time, but also must recognize that capabilities will evolve. Some form of more definitive receiver standards and establishment of set time periods where adherence to those receiver standards will ensure successful operation for a frequency band’s incumbents and new entrants seem to be important tools in this regard.