The Global Positioning System: A Shared National Asset (1995)
Chapter: Option 2: Narrow-Band L4 Signal
discussed guidelines, the NRC committee determined that 2 of the 10 options should be seriously considered. These two options are discussed below in order of preference.
Option 1: Wide-Band L4 Signal
The optimal scenario for an enhanced civilian GPS signal would entail the provision of a new wide-band frequency, termed L4, that would be broadcast unencrypted to allow for universal access. The wide bandwidth sufficiently offset from the current L1 signal would allow for ionospheric delay correction, wide-lane ambiguity resolution, improved interference rejection, and faster accuracy recovery in multipath environments.
The pseudorandom noise chosen for the L4 wide-band signal should have a bandwidth similar to the present P-code, but with a sequence length chosen for rapid acquisition by low-cost civilian receivers. 22 Although not needed for acquisition purposes, the signal could have C/A-code in phase quadrature, which would allow manufacturers to get the most benefit from the new signal without significant changes to their investment in application-specific integrated circuit (ASIC) correlators.23
Based on the previously mentioned frequency allocation analysis, it appears that several options may exist for a wide-band L4 signal. The first option would be to place the center of the wide-band L4 signal at 1258.29 MHz. If the Russian Federation follows through on plans to move GLONASS L2 transmissions to the lower portion of their frequency allocation (1242.9-1251.7 MHz by 1998 and 1242.9-1248.6 by 2005), even a wide-band signal placed at 1258.29 MHz would cause little frequency overlap. Therefore, the possibility of interference with GLONASS would be low. The second option would be to place the wideband L4 signal at 1841.40 MHz. Again, the feasibility of receiving a frequency allocation in this area of the spectrum would require further investigation.
Option 2: Narrow-Band L4 Signal
If a wide-band frequency allocation proves impossible to obtain for L4, a narrow-band signal should be considered as the second best option. Several potential frequencies have been identified that have sufficient spacing from L1 to allow for the correction of ionospheric delay. These include 1237.83 MHz (which is the upper null of the existing L2 frequency); 1258.29 MHz; and 1841.40 MHz. A narrow-band signal placed at any of these frequencies would carry a C/A-type code.
