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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 90 Additional Considerations Regardless of which frequency, bandwidth, and type is chosen for the new L4 signal, its relative utility to a number of different user communities also will be affected by the type of data superimposed on the signal. For example, the inclusion of integrity information in a data message would be useful to aviation, maritime, and land transportation users concerned with safety. A navigation message also would be useful because it would allow the L4 signal to be used for navigation without access to an additional frequency (that is, L1 or L2). Users employing codeless techniques, who are interested in improved correlation between the L4 signal and the L1 signal, would benefit from having the same data transmitted on each signal. However, if a navigation message were broadcast unencrypted, potential adversaries of the United States also could take advantage of an L4 signal in a theater of war, unless L4 is jammed along with other radionavigation signals. Thus, an L4 signal with no data would probably be most acceptable from a military perspective. The rate at which data are broadcast on the L4 signal also is important. A high data rate would increase the amount of information that could be sent to a user and would allow the information to be sent very quickly. High data rates, however, generally make a signal more susceptible to jamming. Conversely, a signal with a low data rate is more jam resistant, but also is limited in its ability to get information to a user in a timely manner. Data rate also may have an impact on the power level required for a new L4 signal, which is an important consideration because of its effect on required satellite power. Because of these many considerations, the committee believes that it is premature to suggest a specific data message or broadcast rate for the L4 signal, but believes that it should be designed with the flexibility to add the data considered most critical to the GPS user community when the first L4-capable satellite is launched. Improvements Anticipated from Adding L4 Increased Accuracy The new L4 signal, which would be available to civilian users, would reduce the typical ionospheric error of 7.0 meters to 0.01 meters (la), regardless of the option selected, as shown in Table 3-5. This would result in a stand-alone accuracy as low as 21.2 meters (2 drms) compared with approximately 30 meters (2 drms) with L1 alone. With the addition of the L4 signal, several DPGS accuracy requirements could be met with the stand-alone GPS accuracy, including those for surface surveillance and autonomous vehicle location and interrogation. The addition of an L4 signal also assists short- and long-baseline differential users (e.g., Category III approach and landing, mapping, surveying, precision farming, and Earth science applications) by calibrating the spatially uncorrelated components of the ionosphere seen across the baseline, and by speeding up ambiguity resolution to get accuracies of a decimeter or better. Even in the presence of SA, dual-frequency civil receivers that operate in a codeless mode would benefit from an additional, unencrypted, signal.
