# Positioning Using IRIDIUM Satellite Signals of Opportunity in Weak Signal Environment
## 1 Introduction  
The new methods for receiver positioning based on abundant and various frequency signals have emerged in recent years. This new positioning technique is termed opportunistic navigation (OpNav) [1, 2] and collaborative opportunistic navigation (COpNav) [3]. The navigation via signals of opportunity (NAVSOP) program made by the UK defense firm BAE Systems in 2012 is the typical system. The US Defense Advanced Research Projects Agency (DARPA) also presents a new program, all source positioning and navigation (ASPN), to research the correlation technique. These systems treat the same signals used by mobile phones, TVs, and other radio systems as the radiation sources instead of navigation satellite signals to realize receiver positioning. It can get rid of dependence on the Global Navigation Satellite System (GNSS), or as a new method for overcoming the situation that GNSS does not work. Space-based low earth orbit (LEO) communications satellite signals, as the new SOPs, have some important advantages in terms of more availability and better coverage than for ground-based SOPs. Plentiful non-GNSS satellites are operational today; IRIDIUM [4], GLOBALSATR [5], and ORBCOMM [6] are typical representatives. The related institutions plan to launch a large number of low earth orbit satellites [7].  
The Iridium SOPs positioning can still provide location services when GNSS does not work due to interference and other effects. It also can be treated as a full standalone backup of the GNSS. The Iridium signals are generally stronger than the GNSS satellite signals. However, it also encounters signal attenuation caused by interference and signal occlusion. In a weak signal environment, the traditional method of positioning based on Iridium satellite SOPs may fail to capture the signals. As a result, the receiver cannot offer a location service since failing to get the positioning observationinformation. This situation limits the field of application of this technology. Therefore, this paper mainly studies the method of positioning using Iridium satellite SOPs in weak signal environment.  
Before realizing receiver positioning based on Iridium satellite SOPs, we need to measure the positioning observation information, Doppler-shift. Therefore, the difficulty of positioning using Iridium satellite SOPs in weak signal environment lies in accuracy estimation of signal Doppler-shift. The main reason is that Iridium satellite SOPs have a partially known characterization, which means the Doppler-shift estimation in weak signal environment only bases on the limited information. On the other hand, unlike traditional GNSS signals, Iridium satellite transmits discontinuous time-division signals, which will also affect the design of the Doppler-shift estimation in weak signal environment.  
The early work of our team researches on the concept of positioning based on the Iridium satellite SOPs and the tests and demonstrations of positioning based on Iridium satellite SOPs in open sky were presented [8]. Based on the previous work, this paper mainly studies the Iridium satellite SOPs positioning technology under the condition of weak signal in the occluded environment.  
In Section 2, the Iridium satellite orbit and Iridium signal characteristics are discussed. It can provide a basis for the acquisition of positioning observation information in weak signal environment. In Section 3, a new algorithm of quadratic square accumulating instantaneous Doppler estimation (QSA-IDE) is proposed and its mathematical model is given. In Section 4, we describe the principle of the positioning using Iridium satellite SOPs in weak environment. Experimental results based on real Iridium signals are described in Section 5. Finally, the discussion and conclusions are presented in Sections 6 and 7.