# Positioning Using IRIDIUM Satellite Signals of Opportunity in Weak Signal Environment
## 6 Discussion  
We introduce the technique of receiver positioning based on Iridium SOPs in weak signal environment in this paper. Since its point is the Iridium SOPs Doppler-shift estimation in weak signal environment, we present a new method, termed QSA-IDE algorithm, to obtain Doppler-shift after analyzing the Iridium orbital and signal characteristics, and we show the principle and mathematical model of the QSA-IDE algorithm. The concept and principle of Iridium SOPs positioning in weak signal environment also appeared. Finally, we show the test results using the real collected IRIDIUM NEXT data, and the corresponding result analysis.  
The OpNav and COpNav have emerged for many years. Using the Iridium satellite to offer location service is very popular in recent years. The Iridium satellite LLC, in 2016, announced that the service termed satellite time and location (STL) based on the Iridium satellite network can provide solutions for time and location. It can aid GPS or replace GPS. Recently, the scholar from Saint Petersburg State University also conducted the research about Iridium Next LEO satellites as an Alternative PNT in GNSS denied environment. These techniques treat the Iridium satellite signals as the cooperation signals instead of SOPs. Many scholars also study the technique of Iridium assisted GNSS, such as improve the GNSS satellite geometry distribution, decrease the time of integer ambiguity solution of the GNSS carrier phase positioning. Our work about Iridium satellite signal positioning treat the Iridium signal as the SOPs and realize positioning with the Iridium satellites alone. Our early work studied the concept of Iridium satellite SOPs positioning in open sky environment. We show the research result of Iridium satellite SOPs positioning in weak signal environment in this paper based on the early work. We show the test results based on real data. The Doppler-shift estimation results suggest the new QSA-IDE algorithm can effectively measure Iridium SOPs Doppler-shift in weak  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **East-Error/m** & **North-Error/m** \\ \hline Mean value & \(-30\) & \(-113\) \\ RMS value & \(121\) & \(110\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Error statistic results of positioning with the primer message signal and height aiding in weak signal environment.  
signal environment. The positioning results show, in weak signal environment, that the receiver 2D position accuracy can achieve between 100 and 200 m. The positioning accuracy is reasonable since many error influences are not removed, such as satellite orbital information is calculated with TLE data. Even so, we still realize the positioning under the occlusion environment using Iridium weak SOPs.  
Our work in this paper also has shortcomings. The positioning accuracy cannot match GPS. Treating the Iridium signals as the SOPs, the result is that only partially known characterizations, such as lack of satellite ephemeris. However, the satellite orbital errors and other errors should be estimated with the receiver position. The positioning accuracy will be improved. On the other hand, in this paper, only Iridium SOPs are used to realize receiver positioning. There are plentiful satellites operational today, and many of them also have potential for positioning.  
Further work on this topic can cover the improvement of positioning accuracy by orbital error correction or the differential positioning technique. Other LEO satellite SOPs are also considered to be used for positioning.