National Academies Press: OpenBook

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

Chapter: Cross-Correlation Type Y-Codeless Receivers

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Suggested Citation:"Cross-Correlation Type Y-Codeless Receivers." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 126

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TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 126 the radio frequency probe could be transmitted to all other satellites in the constellation every 36 seconds. This would not be timely enough to meet many stringent integrity (time-to-alarm) requirements, but would provide much better integrity than is currently available. Increased L2 Signal Strength One enhancement to the existing signal structure (C/A-code and Y-code on L1, and the Y-code on L2) that would improve performance for both civilian and military users is an increase in L2 signal strength. Currently, civilian receiver manufacturers attempt to correct for ionospheric errors through innovative codeless tracking techniques, with varying degrees of success. The chief limitation in the use of these somewhat expensive receivers is that, in order to function effectively, the carrier-to-noise ratio needed on the L2 signal must be considerably higher than that required by a military PPS receiver. In many environments codeless receivers work very well. However, both L2 pseudorange measurement precision and tracking margin for these receivers are considerably worse than for PPS receivers. In vehicle applications where there is signal attenuation due to foliage, the codeless receivers are more prone to signal loss. After loss of signal, the codeless receivers take a longer time to reach a given level of accuracy than a well-designed PPS receiver would. In order to increase the L2 signal strength by 6 dB, some spacecraft modifications must be made.7 If an additional signal is added to the GPS satellites, as recommended in the previous chapter, then civilians would have access to another frequency for ionospheric corrections, so enhancements such as increasing the signal strength of L2 would not be as vital. However, the military benefits obtained with an increased L2 signal strength would not be addressed by adding another frequency, so such an enhancement should be considered on this basis alone. As shown below, the performance of codeless receivers can be improved significantly if the transmitted power of the L1 signal is increased by 6 dB. Cross-Correlation Type Y-Codeless Receivers This receiver recovers the L2 observables by correlating the L2 and L1 Y-codes. A 6-dB increase in the GPS L2 signal causes a 6-dB signal-to-noise ratio increase in the reconstructed L2 carrier phase and pseudorange. This can be exploited in three ways: 7 According to Martin Marietta Astro Space Division of Lockheed-Martin, an increase of 6 dB to 12 dB would require several spacecraft modifications. None are major except for DC power and thermal control, and these changes only become important at end-of-life when specification-to-performance margins will be lower than normally required on U.S. Air Force programs. Other factors such as harnessing, re-balancing, and panel re-layouts need to be assessed in detail but should not be significant problems. If an L4 signal is also added to the Block IIR spacecraft, power sharing will be required, decreasing the amount by which the L2 signal could be strengthened.

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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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