For kinematic survey and precise control of aviation, marine, and land vehicles, reliable GPS carrier phase tracking is critically important. In commercial GPS receivers, carrier phase tracking functions for individual satellites are carried out in a decoupled fashion, ignoring intersatellite path correlations seen at the receiver antenna. In contrast with traditional GPS signal processors, the performance of the new coupled tracker is geometry-dependent. Simulations for the complete tracking loop system illustrate how the coupled structure takes advantage of path correlations to greatly reduce phase tracking errors during periods of signal attenuation and blockage. The coupled processor is compared with a traditional processor during simulated aircraft turning maneuvers. Cycle tracking continuity is explored for overdetermined and minimal geometry scenarios. Differential correction options and computational load are also addressed.
Discussed is a timing emulation and operating system which predicts loading for microprocessors and enables efficient generation of software found in such applications as the Global Positioning System. The timing emulator is applied to two classes of user equipment, a conventional fast-multiplexed design and a fully integrated processor featuring continuous state feedback into the spread-spectrum demodulation and tracking loops. Timing data are developed for five representative microprocessors ranging from 818 to 32132 register/bus widths. The advanced integrated processor requires augmentation of the 32132 processor with a math coprocessor. The emulator is used to develop speed requirements on the co-processor. Lastly, the software development system is described. This contains a multi-tasking operating system and matrix library written in the language "C" which may be easily adapted to a range of radio-navigation problems.
Real‐time differential GPS is fundamentally limited by a) signal estimation errors, b) satellite ephemeris errors, c) propagation decorrelation errors, and d) local multipath errors. In this paper we address the latter two error sources. An understanding of these is needed to establish an error budget for any monitor station network that the U.S. Coast Guard would consider for a nationwide system.
To estimate these error sources, extensive two‐frequency P‐code and integrated Doppler data have been collected over 150 kilometer and 1500 kilometer baselines. A processing scheme is developed which employs both code and carrier observations. Applicable to real‐time differential systems, as well as to post‐collection data analyses, the estimation scheme treats multipath errors and carrier‐code offsets as state‐variables. The resulting Kalman filter yields the very precise carrier observation, corrected for carrier‐code offset.
Day‐to‐day repeatability of multipath errors is illustrated. The decorrelation of ionospheric errors across short and long baselines is evaluated by combining the integrated Doppler estimation scheme with the standard two‐frequency ionospheric measurement technique. Also discussed are the impact of multipath and ionospheric error sources on Differential GPS.
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