NASEM Study of FCC’s Ligado Decision Contains both Basic Truths (-1 dB C/No is Dead) and Fundamental Distortions (FCC’s Rules Don’t Matter)

On 9 September the National Academies of Sciences, Engineering, and Medicine (NASEM) released a study that many hoped would definitively resolve the long running and for the past two-and-a-half-years, intense debate between some government and commercial parties and the Federal Communications Commission (FCC) on the use of Ligado’s spectrum for terrestrial cellular service.?As background for those who may be less familiar with the issue, this debate has raged for nearly 20 years, with some government and commercial parties adamantly declaring that any terrestrial use of the Ligado spectrum located in the general vicinity of the GPS band would cause enormous harm to GPS devices.???The FCC on the other hand has ruled that no harm would accrue to nearly all GPS devices, or to Iridium devices for that matter, and has further worked with Ligado to make various accommodations to ensure this would be the case. ?In April of 2020, the FCC, in a carefully reasoned and documented decision, granted Ligado’s application and authorized terrestrial use of its spectrum with a variety of restrictions to ensure that GPS devices wouldn’t experience Harmful Interference.

In response to the FCC’s decision, the Department of Defense (DoD), and companies argued that the decision could affect GPS devices because the FCC had not used their “-1 dB C/No metric” and urged Congress to adopt legislation requesting NASEM to analyze the decision. This resulted in the recently released study entitled “Analysis of Potential Interference Issues Related to FCC Order 20-48.”??

Basic Truths

The NASEM panel, in response to the questions posed to it, explicitly did not address the fundamental question of whether the FCC’s decision was appropriate.?Unfortunately, despite NASEM’s explicit statements, some parties are spinning this detailed, lengthy report as NASEM concluding that the FCC Order should be rescinded. ?If one reads the NASEM report, as opposed to the press releases and commissioned articles, you will observe that this “spin” is dead wrong.??So, let’s consider what the NASEM Report actually says:

1)?????The NASEM agreed with the FCC that the -1 dB C/No interference protection criteria on which the GPS Community has repeatedly touted is not a valid means to determine the presence of harmful interference to GPS Receivers.

2)?????Most current GPS devices will not be affected by the Ligado signal.

3)?????DoD Authorized/Compliant GPS receivers are unlikely to experience degradation owing to Ligado emissions.

4)?????Designing GPS Receivers for any GPS application that are completely compatible with the Ligado’s spectrum use has been the norm for GPS device manufacturers for over a decade and is completely consistent with the current design state-of-practice.

As discussed in detail below, the NASEM Report provides a comprehensive technical review of the elements of the issue and analysis of the tests that were performed in an attempt to resolve the issue.

Fundamental Distortions

While the NASEM Panel carefully addressed in a serious analytical manner the tests submitted to the FCC and reached the sound Basic Truths drawn above, I am quite concerned about their description of the “way forward”.?This stems from the fact that the Panel seems to create an “alternative universe” that presumes an approach to spectrum management and policy that is divorced from the FCC’s current rules, the Table of Allocations (which reflects the global allocation decisions made by the ITU), and its legally mandated mode of operation.?The Panel thus created its own construct of “harmful interference” which is acknowledged by the Panel to be quite different from the FCC’s legal definition (using capital letters) of Harmful Interference.?I suppose that simply assuming away the laws and rules-based expectations and obligations of the FCC is viewed as acceptable for a Panel operating under the National Academies, but from an FCC perspective – and from the perspective of its tens of thousands of licensees and those who rely on the services they provide – the FCC’s rules-based obligations are real and the FCC’s definition of Harmful Interference and Table of Allocations are concrete and must be enforced to determine the compatibility of devices and device class acceptability.

The NASEM Panel made this very point in its press release:

“The committee evaluated “harmful interference” as related to relevant physics and engineering questions, using the term in a general sense to imply degraded receiver operations. It did not assess whether “Harmful Interference” as defined by U.S. or international regulations, which include language pertaining to assessments of the consequences of the interference, might occur.”

This is a huge issue since the FCC and virtually all international regulatory bodies use a harmonized definition for Harmful Interference which the Committee acknowledges and even quotes the formal definition, then chose to ignore it.?This is something the FCC cannot do, and no responsible spectrum licensee would want them to do. Our entire spectrum system depends on adherence to the FCC’s spectrum rules. ?The DoT’s ABC Study ignored the FCC’s rules and thus carried no weight in the FCC’s deliberations.?The NASEM Panel noted this fact in their evaluation of the three recent studies.

“This study [DoT ABC Study] appears to be discounted by the FCC because the 1 dB C/No metric was not linked to harmful interference for GPS.”

Harmful Interference, i.e., demonstrated impact to the function of a device or service, is the “coin of the realm” for the FCC.?This is why the other two studies both included the measurement of impacts to the functionality of the GPS devices (e.g., location error, timing error, time to first fix, time to reacquisition, etc.) in addition to observing the C/No approximations discretely reported by most (but not all) GPS Receivers tested.?

But the Committee concluded that it was within its prerogative to both ignore and to replace long standing FCC rules and regulations as it deemed appropriate.?This includes the Table of Allocations, primary and secondary usage allocations, well considered power transmission masks, etc. which have been built up and canonized over many decades of time.?The FCC is legally bound to follow and enforce adherence to these regulations.?This is what provides regulatory certainty to the companies who invest vast sums of money to build out the infrastructure that we all are so dependent on as citizens of this great land.?While it may be acceptable for a purely academic organization to muse about how things might be, these musings cannot be the basis for the FCC, the designated Regulatory Body for the United States, to make the critical decisions it must regularly produce.

Basic Truths Explained

The following elaborates on these Basic Truth points made above drawing on quotes directly from the Report to illustrate the Committee’s thinking on these topics.

1)?????The NASEM agreed with the FCC that the -1 dB C/No interference protection criteria on which the GPS community has repeatedly touted is not a valid means to determine the presence of harmful interference to GPS Receivers.

The Report is comprehensive in presenting this conclusion with numerous commentaries on the issue to ensure that the point is solidly made.?The following self-explanatory quotes illustrate this point:

“The commonly advocated 1 dB SNR [signal to noise ratio, aka C/No] loss criterion has not been linked to the FCC’s definition of Harmful Interference.”,

or more directly stated later in the Report,

“a single hard IPC [Interference Protection Criteria] threshold for ??/??o is inadequate.”

or in another more descriptive rendition:

“Although all tests looked at the ?? /?? [Signal / Interference] (or C/No) metric, this metric is self-reported from the device, and the measurement is non-standardized in concept and in hardware implementation, making it a problematic metric.?Variations of up to 62 dB[1]1 in device-reported ??/?? were measured between devices under the same conditions in the ABC study.”

These “variations” illustrate the inconsistency in the reported C/No levels where degradation occurs.?

Finally, the Report points out that:

“Almost all GPS receivers operating in open-sky conditions, including most HP [High Precision] receivers, tolerate noise equivalent to a 4 dB ?? ???o signal degradation, or six times the equivalent of a 1 dB ?? ???o signal degradation before the onset of harmful interference.”

The NASEM Report contains many other direct and indirect references to the complete inadequacy of the -1 dB C/No based Interference Protection Criteria.?This includes the fact that various GPS devices reported greater than 1 dB changes in their estimated C/No value even when no Ligado signal was present.?All this effectively points out that as far as the NASEM Committee is concerned, the GPS Community’s proposed -1 dB C/No Interference Protection Criteria is essentially worthless because it is not a useful or usable metric.?Since as noted in the Report, this metric is the only metric measured in the Department of Transportation’s Adjacent Band Compatibility Assessment [DoT ABC Study], this means that Study is also not valid or useful.?Since this Study is the cornerstone of the Federal Government Agency’s concern about Ligado’s use of its spectrum (the Department of Defense and the Department of Transportation in particular stated that to the FCC), their concerns also seem to be baseless. Thus, the NAS report clearly supports the FCC’s conclusion in the April 2020 Ligado Order to not give any credence to the DOT Study.?

2)?????Most current GPS devices will not be affected by the Ligado signal

This point is captured numerous times in the Report as well but is also stated as the second of their five Conclusions which is repeated below.

“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.”

It also concludes that:

“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.”

To understand this Conclusion and especially the High Precision reference, the Report later explains that the Committee’s concerns are only related to devices that are at least ten years old. ?Unfortunately, given that the tests being reviewed were performed five years ago, many of the devices that were available were designed before 2012 (i.e., now ten years ago). ?One needs to carefully review the Report’s Conclusion 3, to understand that any issues that old GPS Receivers might have, were conscious cost-based design decisions by GPS manufacturers and were not issues related to either physics-based limitations or even design capability-based limitations.

3)???????????DoD Authorized/Compliant GPS receivers are unlikely to experience degradation owing to Ligado emissions.

The NASEM Report concludes that devices designed for the DoD to survive in harsh environments with intentional jammers and interference should ensure that they are immune to the level of Ligado emissions authorized by the FCC.?The exact performance of the DoD devices is classified and is therefore not reflected in the report.?The following summary quote describes this position in the context of the Committee’s views on all GPS Receivers.

" In its response to Task 2, the committee assesses that harmful interference with commercial devices is substantially unlikely with respect to CEL, GLN, and TIM devices as well as HP devices sold after circa 2012 The committee also believes that it is reasonable to assume that DoD Authorized/Compliant Devices are expected to withstand willful interference under substantially higher power than is authorized under FCC 20-48. However, no specific information in this regard was presented or made available as part of the committee’s public study.”

4)????????????Designing GPS Receivers for any GPS application that are completely compatible with the Ligado’s spectrum use has been the norm for GPS device manufacturers for over a decade and is completely consistent with the current design state-of-practice.

This extraordinarily powerful Conclusion is made in the Report’s third Conclusion which states:

“It is within the state-of-the-practice of current technology to build a receiver that is robust to Ligado signals for any [emphasis added] GPS application, and all GPS receiver manufacturers could field new designs that could coexist with the authorized Ligado signals and achieve good performance even if their existing designs cannot.”

In other words, if a GPS device is incompatible with the FCC’s approved transmission levels in Ligado’s spectrum, it is because the GPS device manufacturer made the decision to design the GPS Receiver to be incompatible.?So, why might a GPS Receiver have been designed to have a problem with the spectrally distant Ligado out-of-band signal? ?First, for old designs, the GPS companies chose to ignore clearly established FCC precedents and orders going back to 2003 and instead chose to assume that only very low power satellites would ever be located in the frequency bands near GPS.?As a result of this decision to ignore the FCC’s orders, some of the designs had no protection for a signal higher in power than the existing very low power satellite signals.?This was especially true for high precision receivers. ?Most currently fielded GPS devices are less than ten years old (when the GPS industry started to fully acknowledge the FCC’s decade-old order), and so the vast majority of GPS receivers in use today, and literally all smartphone devices (by far the largest GPS platform), have no compatibility problem with the terrestrial use of Ligado’s spectrum.

It also bears mention that the tests that led to the FCC decision were conducted at least five years ago, which means many of the devices tested were designed more than five years before the tests and therefore are greater than ten years old now.??If the tests were re-run today with newer devices, presumably the results for each of the test would have yield even more positive results than they did simply based on the technology progress in the intervening years.?The authors of the NASEM Report specifically noted the huge range (see related quote below) in interference performance among the receivers tested which relates to their conclusion that there are no “laws of physics” or design expertise constraint that limits receivers from being designed to be compatible with the Ligado authorized signal.

“An important takeaway is that, depending on the receiver design, the levels of power that get translated from adjacent-band to GPS in-band can vary by 60 dB or more”.?

Summary:

While high profile attempts are being made to suggest otherwise, when read in its entirety, the NASEM Report supports the technical basis for the FCC’s decision.??It specifically and convincingly repudiates the notion that a -1 dB C/No metric is a valid basis for asserting harmful interference.?It further explains why this form of metric is inconsistent with the FCC’s (and indeed all International Regulators) position that Harmful Interference is the basis for making these kinds of decisions.?Since this is the foundational basis for objections made to the Ligado decision, the objections themselves are rendered specious.

Importantly, the Report clearly states that there is no technical reason why GPS devices can’t be designed to be completely compatible with Ligado’s use of the spectrum for terrestrial transmissions.?It further says that the vast majority of GPS devices are already compatible with the FCC approved Ligado transmission levels.?

After convincingly analyzing recent test results and weighing and ruling on their value, unfortunately, the Committee also exercised its scientific freedom to invent an alternative regulatory universe.?This was done by making the conscious decision to ignore many of the current rules and the realities of the FCC’s legally mandated role and procedures in establishing and maintaining certainty in the allocation of spectrum and the protection of services from Harmful Interference. ?For me, this unfortunately clouded the clear and decisive technical analysis performed by this outstanding Panel and the otherwise outstanding service that the Panel performed for the Nation.

[1] For those not conversant in “dBs” and the logarithmic scale that underlies this measurement approach, 3 dB represents a doubling in power and 62 dB represents a difference of more than one million times the power that a robust receiver can tolerate verses another much less robust GPS receiver.