# Pseudorange Measurement Method Based on AIS Signals
## 1 Introduction  
As ships sailing on the sea lack reference, a dependable technological means of position, navigation, and timing (PNT) is an important guarantee of ship navigation safety. Nowadays, global navigation satellite systems (GNSS) are the main source of PNT information in the maritime domain. Unfortunately, there are certain risks in using only GNSS, which are vulnerable to jamming or interference [1; 2; 3]. Thus, the International Maritime Organization (IMO) called on the research institutions which are engaged in maritime information technology to study innovative technology in alternative backup navigation systems [4].  
In addition to GNSS, shore-based radio navigation systems (SRNS) [5; 6], inertial navigation systems (INS) [7; 8; 9], and celestial navigation systems (CNS) [10; 11; 12] have been used in the maritime domain. INS and CNS are used mainly in military applications due to cost factors. The Loran-C system (one of the SRNS) is widely used in the civil field [13; 14] and was first developed successfully by the United States, with China and Russia developing corresponding systems (Changhe-2 and CHAYKA) [15]. However, due to low user usage, the United States shut down the system in 2010 [16]. Therefore, subsequent shore-based backup systems should meet reliability and economic requirements.  
There have been two main development directions in SRNS. The United States has discussed the feasibility of an eLoran system since 2015 [17]. In order to avoid the tragedy of Loran-C, the new generation system plans to provide time service to land users based on ships [18]. The European Union (EU) and China are interested in using the existing AIS shore station to build new backup location systems, with a related technique called automatic identification system range (AIS-R) mode.  
AIS is the most widely-used system in the maritime wireless communication field. According to the IMO correlative standards, vessels are mandated to be equipped with automatic identification system (AIS) ship-borne terminals [19], and the IMO's major member states have to locate a series of AIS base stations along coastlines to realize data exchange between ship and shore. Currently, AIS plays an important role in the field of maritime safety management, therefore the study of AIS-R mode without increasing maintenance and owner costs has a feasible market application value as a backup positioning system.  
In the EU, research of AIS-R mode is an important part of the Accessibility for Shipping, Efficiency Advantages and Sustainability (ACCSEAS) project. Current progress includes a completed technical feasibility study [20], but has not yet provided corresponding technical solutions. In China, the research team of Dalian Maritime University has developed prototypes and carried out various field tests [21]. In this paper, the key technology of the pseudorange measurement method is described in detail.  
The rest of the paper is organized as follows. Section 2 introduces the basic principle of pseudorange measurement, including the time of arrival (TOA) measurement method and timestamp option. Section 3 discusses the detective feature of timestamp, including amplitude characteristics and differential characteristics, and analyzes the influence of adjacent bits in digital message sequence on the timestamp detection characteristic. Section 4 compares the detection accuracy of zero-crossing detection and differential peak detection under different signal-to-noise ratio (SNR) conditions. Section 5 studies the optimal estimation algorithm based on minimum mean square error and the fusion algorithm based on \(\alpha\) difference filtering. In order to validate the proposed measurement scheme, field tests in Xinghai Bay of Dalian (China) are provided in Section 6. Finally, some conclusions are put forward in Section 7.