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--> Appendix H Signal Structure Options Ten signal structure enhancement options were considered by the committee, as shown in Table H-1. Each involves possible changes to L1 or L2, as well as a possible signal transmission on a new frequency. The options are listed in priority order.
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--> Table H-1 Signal Structure Options Option L1 L2 L4 Advantages Relative to the Current Configuration Disadvantages Relative to the Current Configuration Earliest Possible Implementation 1 Y C/A Y Pa-like code wide-band signal Ionospheric correction; improved accuracy; anti-jam; 10-dB improvement over narrow- band in interference rejection; faster cycle ambiguity; fast acquisition; easier direct Y- code acquisition; can track to lower elevation angles than codeless receivers Must jam two bands; satellite and receiver costs increase; satellite power requirements increase; frequency allocation considerations IIR 2a Y C/A Y C/A-like code narrow -band signal Ionospheric correction; improved accuracy, anti-jam; 10-dB improvement over narrow- band in interference rejection; faster cycle ambiguity; fast acquisition; easier direct Y- code acquisition; can track to lower elevation angles than codeless receivers Must jam two bands; satellite and receiver costs increase; satellite power requirements increase; frequency allocation considerations IIR 2b Y C/A Y with C/A-like code added to null of L2 narrow-band signal Ionospheric correction; improved accuracy; anti-jam; 10-dB improvement over narrow- band in interference rejection; faster cycle ambiguity; fast acquisition; easier direct Y- code acquisition; can track to lower elevation angles than codeless receivers Must jam two bands; satellite and receiver costs increase; satellite power requirements increase; frequency allocation considerations IIR 3 Y C/A Y C/A C/A- or P-like code narrow or wide band signal Improved accuracy, improved anti-jam for civilians; ionospheric correction; cycle ambiguity More difficult to deny signal by jamming; more satellite power required IIF 4 Y C/A Y C/A Y-like code(military only) wide-band signal Improved anti-jam for the military ionospheric correction for civilians; improved cycle ambiguity; improved direct acquisition of Y-code Military receiver costs may increase; must jam two bands; may require more satellite power; frequency allocation considerations IIF
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--> 5 Y C/A Y — Baseline Baseline Baseline 6 Pa C/A Y — Improved accuracy, improved anti-jam; some codeless receivers will have improved performance current military dual-frequency receivers won't work; some current civilian codeless receivers won't work; must make changes to satellite IIF 7 Y C/A Pa — Improved accuracy, anti-jam; civil ionospheric; correction cycle ambiguity More difficult to deny signal by jamming, current military dualfrequency receivers won't work; must make changes to satellite IIF 8 Y C/A C/A — Civil ionospheric correction; improved cycle ambiguity, some jam resistance Military receiver costs increase; must jam two bands; satellite power may increase; no dual-frequency military ionospheric correction (Current) II/IIA 9 Pa C/A Pa Y-like code(military only) wide-band signal Precision; improved anti-jam; provides ionospheric correction for civilian users; improved cycle ambiguity Military receiver costs increase; must jam two bands; satellite power may increase; possible frequency allocation difficulties; no dual-frequency military-only ionospheric correction IIF a. "P" refers to the unencrypted code
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--> Options 1 And 2 Options 1 and 2 provide the optimal balance between civilian and military utility. These options were selected by the committee for further study and are discussed in Chapter 3 of this report, along with specific recommendations. Options 3 And 4 Options 3 and 4 include two variants. For both, a C/A-code is added as soon as practical to L2 transmissions. This would be relatively easy to implement on Block IIR spacecraft. With either option, a new civilian or military signal could be added when practical. In the near term, civilian users would benefit in terms of interference reduction, ionospheric error reduction, and improved reliability of cycle ambiguity wide-laning. With the later enhancement of an additional civilian signal, many of the advantages of Option 1 would be obtained. However, enabling C/A-code on both L1 and L2 raises potential difficulties for military local access denial. Under Option 3, the military would need to jam three separate civilian frequencies, two of which overlap the military frequencies. Both L1 and L2 would be affected simultaneously, which could have undesirable consequences for the existing inventory of military receivers. Under Option 4, a new dedicated military wide-band signal with an encrypted code would be added to provide increased military capability and better segregation of military and civilian services. Option 5 Option 5 is the baseline case. As pointed out earlier in this report, the civilian community currently has many applications where the narrow-bandwidth C/A-code structure is detrimental. Furthermore, the lack of a second frequency with known codes has substantial impact upon precise differential applications as well as on stand-alone applications. Since the Block IIF constellation lifetime could extend into the year 2020 or beyond, it follows that an acceptance of this option could render GPS obsolete. Option 6 Option 6 eliminates encryption on L1, which allows full civil access to the wide-band P-code, with many potential performance benefits. Anti-spoofing remains on at the L2 frequency. While enhancing civilian performance, it negatively impacts some existing civilian receivers and most military receivers. Civilian codeless receivers of the cross-correlation variety will need modification to handle processing of P-code and Y-code together. The
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--> widely deployed military P-code receiver "Plugger" will loose its anti-spoofing capability, and intentional jamming of L1 will inhibit two-frequency ionospheric corrections for the military. Option 7 Option 7 provides civilian access to a wide-band signal format, as well as excellent dual-frequency wide-laning and ionospheric corrections. As in Option 6, some changes to military software and hardware will be required to handle the mixed P/Y-code situation on L1 and L2. However, this change is compatible with single-frequency military receivers such as the Plugger. Local denial will entail selective jamming and/or C/A-code spoofing on L1, as well as complete jamming/spoofing on the L2 band. In a geographic region of denial the military might be without a dual-frequency capability. Option 8 Option 8 emphasizes civilian dual-frequency operation, as well as military A-S operation. Civilians would obtain very good wide-laning capability, but would not get enhanced wide-bandwidth features. Also, the availability of widely spaced frequencies would offer some interference reduction. On the military side, ionospheric correction might be lost in denial-jamming/spoofing situations unless careful cross-aiding from L1 were employed, and the military would not have a signal solely for their purposes. Option 9 Option 9 essentially gives to civilians the wide-band, dual-frequency capabilities of the military. Clearly, this option would be highly beneficial to the civilian sector, but it would leave most of the military receiver inventory vulnerable to spoofing or even outright loss of navigation capability in denial environments. The most critical military users would have available a new Y-code signal, perhaps of much wider bandwidth and operating on a higher carrier frequency. Such a Y-code signal upgrade is for the late/post Block IIF time period.
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