Summary

BACKGROUND

On April 19, 2020, the Federal Communications Commission (FCC) adopted order 20-48 authorizing Ligado Networks, LLC, to move forward with the deployment and operation of a low-power terrestrial nationwide radio network. The order authorizes Ligado to provide terrestrial base station to mobile earth station downlink transmissions in the 1526–1536 MHz band within the mobile satellite services (MSS)-allocated 1525–1559 MHz band and mobile earth station to terrestrial base station uplink transmissions in the 1627.5–1637.5 and 1646.5–1656.5 MHz bands of the 1626.5–1660.5 MHz band. The order comes after a process that began 17 years earlier when Ligado’s predecessor-in-interest (AMSC Subsidiary Corporation) originally obtained certain MSS licenses. This process has involved multiple application modifications and the commissioning of numerous test-and-evaluation and analytical studies. The complexity of this region of the spectrum, including its adjacency to Global Positioning System (GPS) and other satellite services and the variety of commercial, economic, and national security interests involved has led some parties to question the analyses and assumptions underlying the FCC order. To help shed light on the dispute, the U.S. Congress requested a study by the National Academies of Sciences, Engineering, and Medicine.

The motivation for this study and report is the possibility that emissions by the Ligado system as it would be deployed in accordance with the terms authorized in FCC Order 20-48 might disrupt GPS services or MSS. At issue are two simple facts: (1) radio-frequency transmitters do not operate with arbitrarily sharp cutoff frequencies and thus, depending on how rapidly (as a function of frequency) their emitted power spectrum falls off, may emit power beyond their authorized spectral bands; and (2) receivers of

electromagnetic spectrum do not “listen” only within a band defined with arbitrarily sharp boundaries and thus may receive power from frequencies outside their designed band. The FCC’s goal is to enable as much productive use of valuable spectrum resources as possible, balanced against the deleterious impacts that may arise when adjacent signals cause receivers to experience interference.

GPS services are vital to the modern economy and to national defense operations. Standard receivers provide positioning information at the meter scale and precision timing at the tens-of-nanosecond scale. High-precision receivers, such as those used in surveying, geodesy, and infrastructure and farming applications, can deliver positioning services at the centimeter or smaller scale. Interference of the GPS receivers can potentially lead to degraded performance or loss of operation.

This study was not charged with considering whether the FCC reached a correct outcome in authorizing the Ligado system. That is the purview of the FCC’s processes involving materials and testimony from a wide array of interested parties over the course of the proceedings. Instead, the committee was asked to consider three specific tasks, the response to which make up the bulk of this study. The three principal tasks for the committee, paraphrasing the statement of task (see Appendix A), are as follows:

• Task 1: Assess which of two commonly used approaches to evaluating interference that might cause harm to GPS services would most effectively mitigate the risks of harmful interference to GPS services and U.S. Department of Defense (DoD) operations and activities.
• Task 2: Assess the likelihood that the authorized Ligado service will create harmful interference to GPS, MSS, and other commercial or DoD services and operations.
• Task 3: Assess the feasibility, practicality, and effectiveness of the measures in the FCC order to mitigate harmful interference effects on DoD devices, operations, and activities.

In providing responses to these tasks, the committee stresses three important points:

• Throughout this report, the term “Harmful Interference”—when capitalized—is a defined term used by the FCC and is specifically not the same thing as the general term “interference,” which describes what happens in a receiver when some other signal affects the intended received signal in a way that

• reduces the effective received signal-to-noise ratio (SNR). This report uses the uncapitalized term “harmful interference” in a more general sense to imply degraded receiver operations without assessing whether such degradation is actually causing a degradation of function or whether the receiver is operating in accordance with FCC rules. The committee assumed that in the statement of task, “harmful interference” meant the defined term.
• The legal definition of “Harmful Interference” also includes whether affected devices are or are not “operating in accordance with [the International Telecommunication Union] Radio Regulations” and “operating in accordance with this chapter.” However, the committee proceeded under the assumption that it has been charged with determining whether existing radionavigation satellite services (RNSS) or MSS would be harmed by Ligado interference independent of any legal ruling and concentrated on the physics and engineering questions of harmful interference.
• FCC Order 20-48 authorizes Ligado’s use of spectrum in the United States only. The committee did not assess and reached no conclusion regarding the potential for interference caused by emissions in the bands in question outside the United States.

ANSWERS TO THE THREE ELEMENTS OF THE STUDY TASK

Task 1: Approaches to Evaluating Harmful Interference Concerns

Task 1 asks which of the two prevailing proposed approaches to evaluating harmful interference concerns—one based on a signal-to-noise interference protection criterion (IPC) and the other based on a device-by-device measurement of the GPS position error—most effectively mitigates risks of harmful interference with GPS services and DoD operations and activities.

Conclusion 1: Neither of the prevailing approaches to evaluating harmful interference concerns effectively mitigates the risk of harmful interference.

Neither approach provides analytical, repeatable, or straightforward criteria to evaluate new entrants. Both approaches have a role to play in evaluating harmful interference to existing receivers. The signal-to-noise approach is inflexible as a determinant or threshold, providing what in some circumstances may be an overly conservative emission limit because no single value for signal-to-noise degradation determines when the various types of possible harm to receiver performance will become significant. The

position measurement approach is dependent on the test sampling approach and is too narrow in its applicability to the many and varied uses of the GPS.

Of the two approaches, the one based on the SNR, when done properly, is the more comprehensive and informative. By indicating the degradation in link margin, this approach can be used to predict harmful impact across a broad set of use cases. However, the commonly advocated 1 dB SNR loss criterion has not been linked to the FCC’s definition of Harmful Interference. Although adherence to a “1 dB criterion” may generally prevent Harmful Interference, the vast majority of GPS use cases do not experience Harmful Interference at that level. As such, the 1 dB criterion is prophylactic, but conservative.

The determination of Harmful Interference is dependent on the characteristics of the transmitter and the receiver as well as the particulars of each specific use. There is a wide array of GPS use cases—for example, car navigation, network timing, precise surveying and farming, geophysical monitoring, and aircraft ground and flight operations—all of which may be relevant to DoD operations. These use cases have different failure modes, which result in varying interference tolerance. The most appropriate approach to evaluating potentially harmful interference must be mapped to each relevant use case. For example, some applications are harmed when code-lock is lost, while other applications are harmed from loss of carrier phase lock. As such, the use of a single, fixed SNR-based IPC is not practical when applied to device-by-device performance.

The question posed in the task does not directly address the bigger challenge: regardless of which approach is applied, drastically different conclusions can be reached. There are numerous test design particulars that must be considered, including determining which path loss model to use, the appropriate stand-off range to use, antenna coupling, the degree of insensitivity of a particular receiver’s design to adjacent-band power, and performance thresholds. Even for a given use case, these issues are not easily resolved. Furthermore, a per-device SNR threshold may create a downstream hazard—receiver manufacturers may be incentivized to keep using outdated designs that preclude higher-value use of nearby spectrum or to design adjacent-band-susceptible receivers to claim spectral easements.

Ultimately, both proposed approaches are cumbersome, owing to the intensive, device-by-device testing required. They do not provide an engineerable, predictable standard that new entrants can readily use to evaluate impact. As such, these approaches impede progress in making more efficient and effective use of the spectrum. A new applicant for emissions in a new adjacent channel will have great difficulty in determining the emitter power levels and stand-off distances that will be guaranteed not to cause Harmful Interference to the installed base of GPS receivers. A GPS receiver designer will be unable to design a receiver that will be guaranteed to tolerate unknown potential future allowed levels of adjacent-band power.

Task 2: Harmful Interference to GPS and Mobile Satellite Services

Task 2 asks about the potential for harmful interference from the proposed Ligado network to GPS, MSS, and other commercial or DoD services, including the potential to affect DoD operations and activities. This potential is evaluated across several different use cases, each restricted to operation in the United States.

For GPS, several sets of interference tests have been performed that span many representative GPS devices drawn from many different receiver classes and suppliers. The tests evaluated various scenarios and advocated for different metrics to determine the onset of harmful interference. Despite these differences, the results consistently indicate that a majority of devices do not experience harmful interference. This is discussed in more detail in Section 2.2.

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 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.

The committee also reached the following conclusion regarding the state of the art in GPS receivers:

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.

Turning to impacts on MSS, the committee concluded that the Globalstar system is unlikely to experience harmful interference, because only its uplink is in the L-band and it uses code-division multiple access (CDMA) signals. However:

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 meters.

Additionally, DoD has evaluated the impact of FCC Order 20-48 on department devices and missions. The following summary points were provided to the committee in

a set of slides dated March 15, 2022.¹ It is important to note that the following conclusions were asserted by DoD without providing publicly available supporting data and were not discussed by the committee in a public session:

• DoD and interagency partners conducted testing to determine the impacts to GPS (captures FCC Order 20-48’s authorized deployment). The tests demonstrated that the proposed signal introduces harmful interference to critical national security mission capabilities.
• The terrestrial network authorized by FCC Order 20-48 will create unacceptable harmful interference for DoD missions. The mitigation techniques and other regulatory provisions 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: Feasibility, Practicality, and Effectiveness of Mitigation Measures in the FCC Order

Task 3 asks about the feasibility, practicality, and effectiveness of the mitigation measures required in the FCC order with respect to DoD devices, operations, and activities. The FCC order enumerates several potential mitigations when a receiver experiences harmful interference, including but not limited to enacting exclusion zones for Ligado emitters; replacing components (e.g., antennas or filters) or full receivers; enabling a “kill switch” mechanism for Ligado to turn off emitters in some geographic locations; and additional negotiated mitigations between Ligado and the affected government agency for Ligado to reduce emissions to an acceptable received power level over certain installations.

For the question of harmful interference with GPSs, the effectiveness and practicality of any of the foregoing potential mitigations depends on the type of receiver and the application. One must also distinguish between two types of equipment:

1. DoD-authorized/compliant devices approved for weapons and weapons delivery systems and other national security certified devices.
2. Commercial GPS devices when used in national security applications with an express waiver or navigational warfare (NAVWAR) compliance determination

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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.

1. per DoD Instruction 4650.08 and CJCSI 6140-01 or commercial devices that are used in other DoD operations or missions such as emergency response or partner operations.

For DoD authorized/compliant devices, GPS receivers, or systems that incorporate such GPS receivers used inside the United States, replacing older devices with newer versions of such devices that are protected from harmful interference and already qualified may provide a plausible solution. However, where such replacements are not immediately available this study concludes that this is not likely a satisfactory mitigation. These systems typically must pass very long and expensive operational test certification; generally, mitigations that include replacing or augmenting older devices would involve unsatisfactorily long delays.

Regarding commercial devices used by DoD, the committee concludes that several of the mitigation procedures may be effective but only to the extent that they are timely, affordable, and practicable. (The question of affordability may, in the end, be a question for Ligado and not DoD because Ligado has asserted that they have committed to bear the cost of mitigating such DoD interference problems.) Such actions are highly application specific. These include replacing antenna subsystems; full-scale replacement of older commercial GPS receivers with newer models; and negotiated extended exclusion zones in which no Ligado emitters are placed.

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.

FCC Order 20-48 sets forth procedures for Ligado and those claiming interference to engage in dialog to determine if Ligado is, in fact, responsible for causing harmful interference to a DoD-operated receiver. As noted in this report’s analysis of inference and in commentary on Tasks 1 and 2 (see Sections 1.4, 2.1, and 2.2), interference is not simple, and receivers live in complicated electromagnetic environments. As a result, mitigation will not be practical without extensive dialog as intended within the order. Even if such dialog takes place, mitigation may in some cases not be practicable within operationally relevant time and financial parameters.

ADDITIONAL OBSERVATIONS

The study committee’s statement of task (see Appendix A) also provides that the committee may address “other related issues the study committee determines relevant.” In the course of its work, the committee has concluded that there are several important issues surrounding the technical and administrative processes used in the long saga that has led to the authorities granted in FCC Order 20-48. Chapter 3 provides observations with the hope that future proceedings might provide more streamlined and optimized approaches to balance protection of incumbents with maximizing the economic and operational benefits provided by new entrants into a given spectrum region.

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.