Analysis of Potential Interference Issues Related to FCC Order 20-48 (2023) / Chapter Skim
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2 Analysis Regarding the Three Study Tasks
2 Analysis Regarding the Three Study Tasks
Pages 50-77
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approach was too inflexible, and the position approach was too narrow in its applicability. Yet, both approaches do have a role in evaluating harmful interference to existing receivers.
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As such, these approaches impede spectral progress. 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 existing GPS receiver base.
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Their strengths and weaknesses are noted. 2.1.2 Considerations Regarding an SNR IPC The SNR IPC approach consists of testing GPS receivers at various interference levels that could be attributed to Ligado transmissions and measuring the resulting degradation in reported C/N0.
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In the Ligado downlink band from 1526–1536 MHz, there is a 60–65 dB variation of the emitted out-of-band power levels that result in a 1 dB reported loss of C/N0 for various classes of receiver. It is reasonable to infer that these large differences in reported C/N0 degradation stem from large differences in the various receiver classes' ability to tolerate adjacent-band signal power.
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Adjacent Band Compatibility Assessment, Washington, DC, Figure 3-22. 2.1.3 Considerations Regarding a Receiver Position Error IPC At least for some use cases, position error is a reasonable metric in measuring harmful interference.
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The Roberson report applied an ambiguous approach to determining harmful impact based on position error. The implementation of the position error metric also requires careful consideration.
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Decisions regarding the proper path loss model, or the acceptable impact region size, or which thresholds to use, or which receiv ers should be guaranteed to maintain acceptable performance would still need to be settled. How much SNR margin do receivers need?
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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)
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." Certified aviation receivers will not experience harmful interference outside of this exclusion zone, and fixed-wing manned aircraft operating under standard flight rules will never enter these cylinders of exclusion.2 Other aviation users, however, cannot be guaranteed to remain outside these cylinders. Public safety helicopter operations, which are conducted by medevac providers, law enforcement agencies, firefighting departments, the U.S.
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The accuracy of high-precision applications is complicated and cannot be reduced to a simple assessment of the impact of carrier-to-noise density ratio, or the impact of a 1 dB reduction in that ratio. 2.2.3 Test Results To assess interference with adjacent channel Ligado signals on GPS performance, key tests were performed by four different groups: • Roberson and Associates (RAA)
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Those GPS receivers selected must be tested under fair, realistic conditions. Metrics depend on what needs to be tested and how that metric relates to harmful interference.
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GPS In-Band Interference Noise levels in the GPS band are approximately −174 dBm/Hz, so at a distance of 10 meters from a Ligado transmitter limited to −130 dBm/Hz in the GPS band (56.5 dB free space path loss at 1584.5 MHz) , the interference from the Ligado user or base station will be at least 12 dB below the noise floor.
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Devices 80 GPS receivers were tested 14 in different categories such 27 devices of various receiver Tested simultaneously. Cellular, as general navigation, high- classes.
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position errors. metric was not linked to harmful interference for Unclear if these receivers have Demonstrated mitigation strategies GPS.
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In existing GPS receiver designs, aliasing of out-of-band signals into the GPS band is typically not a problem. However, the large power discrepancies between the Ligado base station/user and the GPS signal make it possible for aliased power to be folded into the GPS band and may adversely affect receivers not designed for that environment.
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GPS receivers operating in a quiet environment will require less attenuation, while receivers that use corrections delivered via an MSS signal will need to receive those frequencies as well. The current Ligado downlink band (1526–1536)
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There are, however, many GPS receivers that show 1–5 dB degradation even at ranges beyond 100 m. FCC Order 20-48 allows a deployment with 433 m between base stations or a maxi mal range of 250 m to a base station.
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The RAA study evaluated 2D position error, and the NASCTN study evaluated 3D position error. Figure 2-5 shows the mean and mean +3 standard deviations of the data overlaid on a scatter plot of the 3D position error versus the A n a l ys i s Re g a rd i n g t h e Th re e S t u d y Ta s k s 67
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In addition, vertical lines indicate power levels that correspond to ranges between the receiver and the interferer of 100 m, 10 m, and 1 m as calculated using free space path loss. As an example, a GLN device (DUT2)
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, sample error distribution, etc.) in the evaluation of harmful interference to capture the increased spread in position error with increased interference.
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2.2.5 Mobile Satellite Services The MSS band supports uplinks and downlinks between mobile users and satellite relays. (See Figure 1-2.)
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Similar arguments can be made for the Iridium uplink as for the Globalstar uplink and despite the higher interference power levels at the satellite, no claim of interference has been made. Satellite to Earth The Iridium downlink shares the same frequencies as the uplink and is subject to the same interference.
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.7 On the lower end of the band, the interference level is −102 dBm/Hz, which will require 68 dB of attenuation to be reduced to the noise floor. This will occur at a distance of 40 m (free space path loss)
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One can even find commercially available but illegal GPS jammers on the market that are used to defeat tracking systems. Military system GPS receivers are more difficult to jam.
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The database will be helpful in ruling out Ligado's influence in harmful interference in many cases and help identify potentially interfering Ligado emitters, but attributing harmful interference as experienced by identified receivers to one or more Ligado emitters will be difficult. In the case of harmful interference with GPSs, the effectiveness and practicality of any of the foregoing potential mitigations depends on the type of receiver and the ap plication.
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While such DoD Authorized/Compliant GPS receivers are unlikely to experience degradation owing to Ligado emissions, the committee did not have access to concrete information about the actual prospect of harmful interference in these devices. If any DoD Authorized/Compliant Devices GPS receivers or systems that incorporate such GPS receivers used inside the United States experience harmful interference, there are relatively few satisfactory mitigations because these systems must pass very long and expensive operational test certification; generally, actions that include replacing antennas or electronics to provide mitigation would involve unsatisfactorily long delays.
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When asked by the committee if DoD could describe any engagement with Ligado to resolve any specific or general interference scenarios or cases since the April 22, 2020, Order and Authorization, DoD responded, "Per NTIA direction there has been no engagement with Ligado since the April 22, 2020, Order and Authorization pending resolution of the petition for reconsideration." The previous discussion addressed the potential harmful interference from Ligado emissions on DoD operations as it relates to GPS receivers. This analysis now turns to po tential harmful interference from Ligado's uplink emissions in the band between 1627.5 and 1637.5 MHz.
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From page 77... ...
First, it may be possible to require much sharper filtering by Ligado uplink transmitters operating near 1627.5 MHz. Second, it may be possible to bar Ligado uplink emitters within, say, 1,000 meters of potentially affected DoD operations inside the United States.
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