The National Academies Press

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Pages 37-59

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From page 37... ... 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. Read the entire page →
From page 38... ... , then Δfi 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 Read the entire page →
From page 39... ... Thus, the RMS value of the velocity error component or its high-precision position error component is proportional to 1/√C/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. Read the entire page →
From page 40... ... Cold-start acquisition involves a brute-force search for signal power above the noise floor in a two-dimensional space of possible carrier Doppler shifts and PRN code delays. For low values of C/N0, the required amount of coherent integra tion time, Taccum, increases markedly, and this increase forces the use of a finer grid in the Doppler shift search space. Read the entire page →
From page 41... ... This is referred to as compression. This gain reduction affects the inband GPS signal, thereby reducing the value of the received signal power C in the ratio 28 C.J. Read the entire page →
From page 42... ... 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. Read the entire page →
From page 43... ... If designed properly, the impact of adjacent-band power can be reduced by many 10s of dB. A good RF front-end designer considers the anticipated adjacent-band power levels and designs the RF front-end LNA, FIGURE 1-11 C/N0 versus received power from a Ligado tower power for two identical receivers and two tests. Read the entire page →
From page 44... ... 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. Read the entire page →
From page 45... ... The amount by which C/N0 actu ally gets degraded is highly dependent on receiver design. Thus, any standard purporting to predict harmful interference must first charac terize the power of a potentially interfering signal in specific frequency bands. Read the entire page →
From page 46... ... 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. Read the entire page →
From page 47... ... 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. Read the entire page →
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. Read the entire page →
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] Read the entire page →
From page 50... ... 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. Read the entire page →
From page 51... ... For example, some applications are harmed when code-lock is lost, while other applications are harmed from loss of carrier phase-lock. The commonly advocated 1 dB SNR loss criteria has not been linked to the definition of Harmful Interference. Read the entire page →
From page 52... ... 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. Read the entire page →
From page 53... ... As such, receiver-reported C/N0 degradation provides insight into how a given GPS receiver's performance will be impacted across a variety of use cases. Harmful interference must be tied to the available margin a receiver has for its intended task. Read the entire page →
From page 54... ... 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. Read the entire page →
From page 55... ... However, it would not reveal harmful interference in all use cases. To provide an example, interference might not affect average position error very much, but it might affect continuity of operations, which can be highly problematic for aviation devices or high-precision devices. Read the entire page →
From page 56... ... 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? Read the entire page →
From page 57... ... 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) Read the entire page →
From page 58... ... ." 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. Read the entire page →
From page 59... ... Ligado interference may occur exactly when GPS is in forming a critical safety service, such as GPS-based terrain avoidance after an unexpected transition to low visibility conditions. Supplementary navigation instruments, including radar and fixed beaconing, may mitigate the occasions when visual flight rules cannot be relied on and proximity to a Ligado tower results in loss of GPS, but there is presently no guarantee of this. Read the entire page →

From page 37...

... 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.