Analysis of Potential Interference Issues Related to FCC Order 20-48 (2023) / Chapter Skim
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Pages 6-36
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From page 6... ...
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.
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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.
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ADDITIONAL OBSERVATIONS The study committee's statement of task (see Appendix A) also provides that the com mittee 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.
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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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as "a radio communication service between mobile Earth stations and one or more space stations, or between space stations used by this service; or between mobile Earth stations by means of one or more space stations."1 RNSS is defined as "a radiodetermination-satellite service used for the purpose of radionavigation."2 Radiodetermination is defined as "the determination of the position, velocity and/or other characteristics of an object, or the obtaining of information relating to these parameters, by means of the propagation properties of radio waves."3 1.1.2 Frequency Bands Under Consideration MSS space-to-Earth downlinks are from 1525 to 1559 MHz, and MSS Earth-to-space uplinks are from 1610 to 1660.5 MHz, with a downlink allocation also from 1613.8 to 1626.5 MHz. The L5 band is centered at 1176.45 MHz.
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, which permits MSS licensees to operate, if approved and licensed, terrestrial base stations and mobile terminals for the purpose of filling gaps in service areas.8 Ligado was granted authorization for ATC opera tions in its MSS-licensed bands.9 6 M McLaughlin, 2021, "Iridium Communications Inc.
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. Regulatory limits on power levels have been established for Ligado emissions both in its licensed bands and beyond its licensed bands.
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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 1-5 Modulated GPS signals for the original system design.
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realized that receiver technology and connectivity had progressed to the point that a static 37,500-bit message, transmitted at 50 bits per second, and taking 12.5 minutes to receive, was inefficient, was militarily ineffective, and made military receivers vulnerable to exploitation because L1 C/A is unencrypted. M-Code adopted a signal structure that employed modernized modulation techniques, data rates, data format, information content, and flexible messages that were 1,000 times shorter and could be changed by the GPS ground control system so that receivers could receive the information needed much faster.
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and high-precision (HP) receivers that use carrier phase mea surements to improve accuracy.
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Ligado interference in adjacent channel is limited to −85 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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The signal is demodulated, and it is here that the C/A code tracking and carrier phase trackingprocessing processing for the is performed. Thisnavigation/timing for the navigation/timing signal processingsolutions for thesolutions is described is described briefly navigation/timing below.
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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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not Abegood GPS worried receiver about accurately downloading tracks both C/A c alignment and carrier phase alignment. These are the two "tracking loops" in a GPS receiver.
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GPS data are also now used to provide early warnings of such hazards as tsunamis and earthquakes. Scientific users of GPS signals ignore all models transmitted by DoD (orbits, clocks, ionospheric models)
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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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From page 28... ...
It is often the case that a −1 dB ΔC/N0 is used as the threshold for interference. As noted previously, these are both measurements of interference and must be interpreted based on the FCC's definition of Harmful Interference: Interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocom munication service operating in accordance with [the ITU]
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belief that the Commission "cannot reasonably reach" a conclusion that harmful interference concerns have been resolved. Section 2.2 of this report, in providing the committee's response to Task 2 of its charter, discusses the results of the various studies noted above.
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−85 dBW/MHz in the 1541–1559 MHz and 1610–1650 MHz frequency ranges, ii. −100 dBW/MHz in the 1559–1610 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 2σ 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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From page 36... ...
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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