The Global Positioning System: A Shared National Asset (1995)
Chapter: OPTION 6
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
