National Academies Press: OpenBook

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


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Suggested Citation:"SIGNAL STRUCTURE MODIFICATIONS TO REDUCE ATMOSPHERIC DELAY ERROR." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 86

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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 86 SIGNAL STRUCTURE MODIFICATIONS TO REDUCE ATMOSPHERIC DELAY ERROR If SA is turned to zero, as recommended above, the next largest contributor to the civilian SPS error budget is the atmospheric error consisting mainly of ionospheric delay as discussed in Appendix C and as shown in Table 3-1. Since the military normally has access to two frequencies, military users can correct the ionospheric error,14 but civilian users cannot.15 In order to compensate for the ionospheric error, the civilian community has been able to develop innovative techniques for recovering components of the encrypted Y-code signal. The chief limitation on the use of these somewhat expensive receivers is that to function effectively, the signal-to-noise ratio required for the L2 signal must be considerably higher than that required by a military PPS receiver. While this is achievable in stationary situations, there are many circumstances in which these conditions do not apply. For example, when the receiver is in a moving vehicle and/or there is ionospheric scintillation present (which corrupts the phase of the received signals), the receiver can lose lock. Several minutes may be required to recover the tracking ambiguity cycle needed for precise positioning.16 The same is true when the receiver must view some of the satellites through foliage or in the presence of multipath signals. Unfortunately, an ideal signal reception environment is the exception rather than the rule. As the number of more demanding real-time civil applications increases, users are seeking ways to improve GPS performance. With the current GPS signal structure, civilian designers and users must confront impediments such as non- trivial levels of electrical interference and strong and rapidly changing multipath reflections from buildings and nearby vehicles. Civilian access to an additional frequency would enable improved accuracy through ionospheric corrections, multipath rejection, and single-frequency operation when interference jams one of the two civilian frequencies. 14 As mentioned in Appendix C, the tropospheric portion of atmospheric delay cannot be eliminated through the use of two frequencies. 15 Because the ionosphere is a dispersive medium, the ionospheric delay is frequency dependent. The existence of two frequencies allows the time of arrival of each to be compared by a receiver, calibrating the error caused by signal delay through the Earth's ionosphere. PPS users have access to both L1 and L2, whereas SPS users have access only to L1. 16 Ionospheric scintillation of the GPS signals occurs when two or more paths are taken between the satellite and the receiver. This is caused by fluctuations in the free electron content and therefore, the refractivity of the ionosphere. When these paths carry signals of about the same amplitude, they cancel as the differential delay of the paths vary by integer plus one-half wavelengths, or they add as the differential delay of the paths vary by integer wavelengths. This scintillation is analogous to optical delays in the neutral atmosphere, which cause stars to twinkle in the visible spectrum.

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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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