Analysis of Potential Interference Issues Related to FCC Order 20-48 (2023) / Chapter Skim
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1 Introduction and Background
1 Introduction and Background
Pages 9-49
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1.1.1 Allocations Under Consideration Spectrum is divided into frequency bands that are allocated for specific radio services. The allocations under consideration in FCC Order 20-48 primarily pertain to two radio 9
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Department of Commerce, September. 5 Federal Communications Commission, 2020, FCC Order 20-48, Order and Authorization, April 22; hereafter, FCC Order 20-48.
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Letter Regarding NAS Committee Members [DEPSCSTB-21-02] -- Review of FCC Order 20-48 Authorizing Operation of a Terrestrial Radio Network Near the GPS Frequency Bands," letter submitted to the Committee to Review FCC Order 20-48, Washington, DC: National Academies of Sciences, Engineering, and Medicine.
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This is termed OOBR and is generally undesirable depending on the adjacent or nearby band activity. 1.1.4 In-Band and Out-of-Band Considerations Specific to the efforts under FCC Order 20-48 are emissions from Ligado's equipment in its licensed bands (in-band emissions)
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More sophisticated use of GPS signals allows commercial services and scientists to locate ob jects with better than centimeter precision using carrier phase measurements.11 Last, GPS is a critical technology employed in the defense of the United States and its allies. GPS services consist of a space segment of approximately 32 satellites (space ve hicles, SVs)
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Pseudo-range is a combination of true range, transmitter and receiver clock offset effects, effects from signal slowing in the Earth's ionosphere and neutral atmosphere, and multipath-induced spreading of the signal arrival time. A navigation fix first corrects for each satellite clock offset by using SV transmitted clock calibration data and for ionosphere and neutral atmosphere delay by using models.
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signal on L1. Early military GPS receivers were not capable of acquiring the encrypted wideband signals directly given the limits of receiver electronics in those days.
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b. Use of civil, commercial, or foreign sources to obtain PNT information for non-combat operations is authorized, subject to successful NAVWAR compli ance determination.13 1.2.3 GPS Satellites and Transmitted Signal Structure GPS signals are broadcast in three frequency bands.
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. The chipping code provides noise immunity so that the navigation code can be reliably received and demodulated at GPS receivers; in addition, the chipping code is SVN dependent, so individual SV signal streams can be distinguished.
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L1 Carrier 1575.42 MHz C/A Code 1.023 MHz NAV/System Daten 50 Hz P-Code 10.23 MHz L2 Carrier 1227.6 MHz FIGURE 1-5β Modulated GPS signals for the original system design.
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code, modulation technique, data rates, data format, and information content were developed in the early 1970s based on the receiver and information system technology of that era as well as projections of how GPS military receivers might operate given those limitations. Twentyfive years later, in the mid-1990s, when DoD initiated the military GPS modernization program, to include M-Code, the GPS Joint Program Office (JPO)
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Space Force shows M-Code, L2C, and L5 IOC as the first quarter of FY 2024. 1.2.4 GPS Receivers The design of a notional GPS receiver is depicted in Figure 1-6.
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Ligado interference in adjacent channel is limited to β85 dBW/MHz near the co-channel. With appropriate filtering, the adjacent power, at the license specified levels, in the vast majority of existing GPS receivers does not have a material adverse effect on receiver circuits processing GPS signals; in addition, practical receiver performance characteristics of the sort recommended in later sections of this report can ensure that this is the case.19 The increase in effective noise density is often calculated as (I + N0)
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Under test conditions, many GPS CEL receivers tolerate20 noise equivalent to an 8 dB C/N0 signal degradation. Almost all GPS receivers operating in open-sky conditions, including most HP receivers, tolerate noise equivalent to a 4 dB C/N0 signal degradation, or six times the equivalent of a 1 dB C/N0 signal degradation before the onset of harmful interference.
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. generates Fromtwo reference asignals: GPS receiver samplesππB# β2 ππππππ(2ππ theβsignal isaccurate randomly estimate distributed of (Ο, with respect to the incoming signal phase.
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A good GPS receiver accurately tracks both C/A code align ment and carrier phase alignment. These are the two "tracking loops" in a GPS receiver.
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The satellite clock offset (Ξ΄s) is also provided in the broadcast ephemeris a this problem could be mitigated by using two GPS receivers and estimating "differential" of similar precision.
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with a horizontal precision of around several millimeters. The atmospheric delays on GPS signals (and on those of any other Global Navigation Satellite System [GNSS]
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After considering "the concerns raised regarding potential harmful interference to adjacent band operations," including general location and navigation de vices, certified aviation GPS devices, non-certified GPS receivers, other U.S. government devices, as well as MSS operations, the FCC order concludes that, subject to the various conditions it imposes on Ligado's ATC authority, granting the license "will promote the efficient and effective use of our nation's spectrum resources." The FCC thus granted the license subject to certain conditions, detailed in Section 1.3.2 below.
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This grounding is important as a precursor to the committee's responses to the questions one and two of the study charter. 23 FCC Order 20-48, paragraph 34, p.
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1.3.2 Requirements and Conditions on the Authorization FCC Order 20-48 approved the Ligado amended application to operate subject to numerous requirements and conditions, which are summarized in Box 1-1. 25 Letter from Douglas W
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β85 dBW/MHz in the 1541β1559 MHz and 1610β1650 MHz frequency ranges, ii. β100 dBW/MHz in the 1559β1610 MHz frequency range.
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d. Based on the base station and technical operating data made available to it, if an affected agency determines that Ligado's operations will cause harmful interference to a specific, identified GPS receiver operating on a military installation and that the GPS receiver is incapable of being fully tested or replaced, Ligado shall negotiate with the affected government agency to determine an acceptable received power level over the military installation.
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This section explores further how an out-of-band signal can interfere with a spe cific receiver. The discussion illuminates the mechanism of harmful interference and in forms the analysis of impact of signal characteristics on existing GPS receivers.
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These are also known as point constellations in the signal processing literature. Figure 1-7 is typical of a GPS receiver that processes a signal that carries navigation data that have been modulated onto the signal using binary-phase shift keying.
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. This quantity is nominally thought to give a good raw measure of the signal power relative to the average power spectral density of the received noise and to be independent of the signal processing within the receiver.
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The effects of interference depicted in Figure 1-9 are shown as reductions in the signal power A2 while the noise power 2Ο 2 remains constant. This is typically the case at the output of a GPS receiver's RF front end, where the effect typically manifests itself as an apparent reduction of A2, consistent with the figure.
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in a situation where the noise power dominates the signal at the output of the RF front end owing to the spread spectrum nature of GPS signals. As noise-plus-interference power increases, the receiver's AGC decreases its gain in order to hold the power constant at the output of the RF front end's analog-to-digital converter (ADC)
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Considerations from this viewpoint, however, do not allow the prediction of harmful interference by a single value for a tolerable amount of SNR loss or, equivalently, C/N0 loss. One problem that can occur for a GPS receiver is an inability to correctly decode the navigation data that are broadcast by the GPS satellites on the signals and that are needed by the receiver in order to compute GPS satellite locations and clock calibration offsets, which are necessary inputs to a navigation solution.
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, then Ξfi measurement errors translate directly into beat carrier phase errors in this loop's feedback path, and they also affect its estimation of carrier Doppler shift. The measure ment precision of carrier Doppler shift translates directly into range-rate measurement 38 A N A LY S I S O F P OT E N T I A L I N T E R F E R E N C E I S S U E S R E L AT E D TO F C C O R D E R 2 0 - 4 8
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Thus, the RMS value of the velocity error component or its high-precision position error component is proportional to 1/βC/N0. Furthermore, if the RMS frequency or beat carrier phase measurement errors become too large, then the receiver can lose frequency lock or phase lock on the signal, in which case it becomes unusable for navigation.
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A-GPS receivers can operate at very low C/N0 values. Other receivers have hard lower limits of about 20β25 dB-Hz for proper point-cloud association for the GPS L1 C/A codes that form the foundation of much of the currently used civilian GPS.
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GPS receivers cannot mitigate this component of interference unless it has special properties, such as lying in a narrow frequency band or in a narrow time window. If it is too large and does not have such special properties, then all GPS receivers' performance will degrade significantly.
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Under these circumstances, the aliased energy is likely to interfere with the GPS signals. LNA compression and intermodulation, being nonlinear effects, likely will not exhibit a one-to-one relationship to the adjacent-band signal power.
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SOURCE: U.S. Department of Transportation Volpe Center, 2022, "Response to National Academies of Sciences, Engineering, and Medicine Questions," Document submitted to the Committee to Review FCC Order 20-48, Washington, DC: National Academies of Sciences, Engineering, and Medicine.
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Thus, there is a 60 dB variation of receiver susceptibil ity to interference from Ligado towers as measured using a 1 dB C/N0 loss threshold. This wide variation of sensed interference at each receiver is not owing to Ligado's out-of band emissions from the tower that fall within the GPS band, because that mechanism of interference is virtually identical for all GPS receivers.
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What is clear is that the best receiver does a much better job relative to the worst receiver of attenuating the Ligado signal before it can affect the GPS in-band C/N0. 1.4.6 Summary Regarding Harmful Interference Harmful interference of a given GPS receiver owing to an adjacent-band signal occurs when that signal causes the effective C/N0 values of enough signals to degrade by large enough values to degrade the receiver's performance.
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It is possible to design L1 GPS RF front ends so that the degradation of the effec tive C/N0 is negligible for the permitted Ligado signal power characteristics even at close ranges. Not all existing GPS receivers have RF front ends that do this, although almost all receivers tested before the Ligado order experienced no harmful interference.
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Therefore, it is known that the technology currently exists to design GPS receivers for some functions, perhaps some of the most demanding functions, that will not experience harmful L1-band interference from Ligado downlink signals. This has been achieved through proper design of the RF front end to attenuate the Ligado signals sufficiently before they can negatively impact the effective in-band C/N0 to a noticeable extent.
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From page 48... ...
This is to allow regulators to balance the relative benefits of competing systems and technologies. In many, but not all instances, once competing and complementary interests are reconciled, technical rules and regula tions can be written to define the boundaries of "Harmful Interference" as purportedly accomplished in FCC Order 20-48.
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From page 49... ...
Therefore, the committee made no effort to assess whether any RNSS or MSS -- for example, GPS and Iridium -- were or were not "operating in accordance with [the ITU] Radio Regulations" or "operating in accordance with this chapter." Instead, the committee proceeded under the assumption that it has been charged with determining whether existing RNSS or MSS would be harmed by Ligado interference independent of any legal ruling about whether they were "operating in accordance with [the ITU]
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