# Positioning Using IRIDIUM Satellite Signals of Opportunity in Weak Signal Environment
## 7 Conclusions  
This article studies the positioning using Iridium satellite signals in weak signal environment. The Iridium satellite signals here are treated as the non-cooperative SOPs for positioning. The key difficulty of the Iridium satellite SOPs positioning in weak environment is the Doppler-shift estimation. The new method termed QSA-IDE algorithm is proposed to estimate the Iridium satellite SOPs Doppler-shift in weak environment after detailed analysis of the Iridium satellite orbit and signal characteristics. The new algorithm mainly includes signal detection and signal acquisition, the former is used to find Iridium burst signals and the latter is used to estimate Doppler-shift. The quadratic square processing in the new method and the whole Iridium burst signal (including tone signal, unique signal, and information signal) is used to estimate that Doppler-shift are the core of the new algorithm. The mathematical model is created after introducing the principle of the QSA-IDE algorithm. We use the MLE to search Iridium signals and obtain the Doppler-shift fine estimations. The maximum correlation of MLE is not affected by the modulation mode of Iridium signal after quadratic square processing. For the Iridium SOPs positioning, the satellite position and velocity are calculated by the satellite TLE data and SGP4 prediction model, the instantaneous Doppler positioning technique is used after estimating Iridium satellite signal Doppler-shift. The new concept is tested using real collected data. The real IRIDIUM NEXT satellite data is collected under a dense forest where the Iridium signal is weak due to the occlusion environment. An Iridium antenna and a RF front end are used to obtain the Iridium IF data, and the software in the PC is used to realize Doppler-shift estimation and Iridium SOPs positioning. The Doppler-shift estimation results show that the new algorithm can effectively measure Doppler-shift in weak signal environment compared to the traditional method. It can see enough satellites and increase the number of the Doppler-shift measurement for every satellite. As a result, the new algorithm can used for Iridium SOPs positioning in weak signal while the traditional method is not. By continuously measuring the Iridium SOPs Doppler-shift and combining the satellite orbit information, the receiver positioning results based on real collected data in weak signal environment are carried out. When the Iridium ring alert signal is used, the positioning error mean values of east-direction and north-direction can be 147 and \(-\)162 m, while the value is only 58 m corresponding to the up-direction. The positioning error RMS value is larger for east-direction than for north-direction and up-direction. When the height aiding is used, the error mean value of east-direction is only 21 m and the value corresponding to north-direction is \(-\)108 m. The error RMS value of east-direction and north-direction is 144 and 137 m, respectively. In this case, the 2D positioning error RMS is 198 m. After performing the same step for the primer message signal, we got the approximate positioning results. In this case, the positioning error mean value of east-direction is \(-\)30 m and the value corresponding to the north-direction is \(-\)113 m, the 2D positioning error RMS is 163 m. The Kalman filtering is used to average the solutions from different Doppler-shift measurement combinations. As a result, the 2 Dpositioning accuracy in both cases can achieve \(110\) m. The receiver solutions in two cases are both located at the south of real receiver position, the positioning error in east-west direction are more optimistic than for south-north direction in two cases. The common features of positioning error are caused by the Iridium satellite orbital error. Writing--original draft preparation, Conceptualization, Data curation, Investigation, methodology, Software, Writing--review and editing, Z.T. ; Conceptualization, Funding acquisition, Resources, Writing--review and editing, H.Q., L.C. ; Investigation, C.Z.