Timing Evaluation of Iridium Satellite Time and Location Signal: Measurement-Level Implementation and Receiver Hardware Time Interval Comparison
Type of DegreeMaster's Thesis
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This thesis assesses the accuracy, stability, and convergence rates of receiver timing solutions with the Iridium Satellite Time and Location (STL) signal through two studies. In the first experiment, an Extended Kalman Filter (EKF) and a Weighted Least Squares (WLS) solution are used to estimate the clock states of a static receiver at an antenna location which is known and unknown, respectively. In the second experiment, a 1-PPS (Pulse-Per-Second) time interval study is conducted with two, commercially-available Jackson Labs Technologies STL-2600 receivers, which are both provided a precise position. Both tests are conducted using an on-board Temperature Compensated Crystal Oscillator (TCXO) and external rubidium oscillator. Nanosecond-level timing solutions from Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), are integrated into many personal and industrial systems, including transportation, communications systems, electrical power grids, and financial institutions. However, due the orbital altitude of the satellites, the received signal power of the end user is critically low, resulting in vulnerable timing solutions. The Low-Earth Orbit (LEO) Iridium constellation orbits significantly closer to Earth’s surface, ensuring higher received signal strength. While the system was originally intended for communications, the satellites have been updated to broadcast the STL message, which can be used for navigation applications. The results of these experiments indicate that the Iridium STL signal is capable of providing GNSS-independent, nanosecond-level timing accuracy for stationary receivers. Throughout the 120 hours of data collected, the receiver timing accuracy was maintained to within a mean timing error of less than 205 nanoseconds. The timing state estimation experiment demonstrates a substantial improvement in timing performance when the receiver position is provided to the estimator, resulting in standard deviations of less than 1 nanosecond per second. The 1-PPS time interval experiment shows the off-the-shelf capabilities of the STL receiver to be accurate to within 110 nanoseconds of deviation and approximately 500 nanoseconds of error at all times. The second experiment also indicates the maximum timing error and deviation can be reduced when a high-fidelity, rubidium oscillator is integrated into the receiver hardware.