Iridium TOA Estimation and Positioning Based on Carrier Tracking and Beam Decoding

To address the challenge of unknown signal emission time, this article proposes a method to estimate the time of arrival (TOA) observation from Iridium signals. It leverages correlation and multiframe joint estimation techniques via minimum mean square error (MMSE) criterion, analyzing local time references and antenna beam temporal emission patterns. A third-order phase-locked loop (PLL) is introduced to track Iridium carrier signals, enhancing accuracy in Doppler frequency of arrival (FOA) and phase synchronization, thereby facilitating beam and satellite identification (ID) decoding. Moreover, a mapping table associating satellite IDs with decoded information and two-line element (TLE) ephemeris is constructed. This streamlines the acquisition of visible Iridium satellites, alleviating the need for exhaustive searches and reducing computational burden. Theoretical performance analyses are conducted for multisatellite and multiepoch positioning based on TOA and FOA measurements using least squares (LS) estimation. Experimental results indicate that TOA-based 3-D positioning achieves an accuracy improvement of approximately 48.51% compared to FOA-based positioning. Furthermore, by employing differential operation on both TOA and FOA positioning, the errors in 3-D positioning can be further reduced by eliminating the effects of inaccurate clocks and orbital influences in time difference of arrival (TDOA) or frequency difference of arrival (FDOA) measurements.