# GNSS Real-Time Precise Point Positioning in Arctic Northeast Passage
## 1 Introduction  
The Arctic Sea ice is undergoing unprecedented decline as a result of global climate change [1]. [PERSON] and [PERSON] [2] predicted that, by 2030, sea ice will be greatly reduced in the Arctic regions, improving several Arctic routes, including the Northeast (NEP) and the Northwest Passages (NWP), especially in summer. Arctic routes can save up to tens of days and come with large cost reductions, promoting huge commercial potential [3]. Furthermore, the development of several industrial fields has increased in Arctic Sea lanes, including fishing, scientific research, mining, and tourism [4, 5]. To ensure the safety of the growing amounts of human activities in the Arctic, it is necessary to develop a navigation system with high positioning accuracy. Commonly usednavigation and positioning methods at sea include satellite, inertial, magnetic, and gravity-aid navigation systems. However, the feasibility of the strapdown inertial navigation system (SINS) is constrained because the fictitious particules change from rectangular in low latitudes to triangular in high latitudes [6]. Magnetic and gravity sensors are not suitable either, because magnetic and gravity forces are nearly constant in the polar region [7]. GNSS has global coverage, a high sampling rate, and high-precision positioning results; therefore, it is utilized as a preferred navigation method in the Arctic [8]. A number of high-precision GNSS navigation and positioning algorithms have been developed, including kinematic (RTK), satellite-based augmentation systems (SBASs), and precise point positioning (PPP). The RTK method requires building base stations in the Arctic; therefore, it is not suitable. SBAS services need the GEO satellite to broadcast precise correction information; however, this cannot be appropriately received in the Arctic due to the low satellite altitude angle [9]. The PPP concept was proposed in 1997, and has been widely used for its higher positioning accuracy (centimeter-level or higher) with a single GNSS receiver [10; 11]. As a result, PPP boasts huge advantages over other algorithms and is preferred for use in the Arctic regions. Little work on the GNSS PPP has been conducted in the polar regions due to the harsh environment. [PERSON] and [PERSON] [12] installed a GNSS receiver on the Antarctic Amery ice shelf to analyze its movement trend using PPP technology. Through simulation calculations, [PERSON] and [PERSON] [7] simulated and analyzed BDS-2 and BDS-3 constellations for navigation and positioning services in the polar regions, as well as the coverage of integrated GPS/BDS constellations. Similarly, [PERSON] et al. [13] analyzed the positioning accuracy of BDS PPP and combined GPS/BDS PPP. [PERSON] et al. [14] evaluated the performance of both single- and multi-GNSS PPP solutions in high latitudes, based on an Antarctic GNSS station. Traditional post-processing PPP methods with several-day latencies for the precise ephemeris do not meet the requirements for real-time positioning. Meanwhile, IGS provides services via Radio Technical Commission for Maritime Services (NTRIP), whose real-time capability can be further improved by increasing the number of analysis centers (ACs) and real-time products. These ACs, e.g., BKG, CNES, DLR, ESA/ESOC, GFZ, GMV, NRCan, and WHU, can provide both GPS and multi-GNSS combination products. Thus, the RT-PPP method has a wide range of applications spanning different fields, e.g., earthquake early warning, weather forecasting, and sea-level measurement systems [15; 16; 17; 18]. In polar regions, real-time data transmission may produce a bottleneck. According to The International Convention for Safety of Life at Sea (SOLAS) and The International Code for Ships Operating in Polar Waters (Polar Code), both Immarsat and Iridium equipment must be installed on the ship when sailing in the Arctic, especially in areas above 76\({}^{\circ}\) N [19].