# A Joint Dual-Frequency GNSS/SINS Deep-Coupled Navigation System for Polar Navigation
## 1 Introduction  
The polar area has attracted much attention from the world for its abundant natural resources and important strategic position. The combination of undiscovered natural resources and normal shipping is driving a dramatic increase in the demand for navigation in polar areas [1; 2]. The polar areas refer to the northern resting on the Arctic Ocean and the southern on the continent of Antarctica, which are located in high latitudes (generally higher than 84\({}^{\circ}\) N or 84\({}^{\circ}\) S). The geomagnetic navigation system (GNS) cannot work well in polar areas due to the unreliable magnetic field. Besides this, the landmark navigation system (LNS) and celestial navigation system (CNS) also have their limits for polar navigation because of meteorological factors such as severe weather, visibility, and so on [3]. The strapdown inertial navigation system (SINS) is highly autonomous and self-contained, and does not need to depend on external environments, which has potential advantages in polar areas. However, the directional reference will be a problem when the SINS works in high-latitude areas, especially for the traditional SINS based on a north-oriented geographic frame due to the meridian convergence [4; 5]. The wander frame is a traditional solution in high-latitude areas; however, it cannot provide position and orientation information near the pole. A grid frame was introduced and the grid SINS algorithm was designed in [6; 7] to solve the problemof meridian convergence by setting an available reference line. However, the single SINS navigation output still contains periodic oscillation errors and accumulated errors. The global navigation satellite system (GNSS) can provide real-time observation data globally and in all weather [8; 9]. Besides this, more and more GNSS frequency bands are available for users, which will benefit from the development of dual-frequency or multifrequency receivers [10; 11; 12]. However, in polar areas, GNSS signals often suffer low elevation problems of satellites, signals with poor carrier-power-to-noise-density ratio (C/N\({}_{0}\)), ionospheric scintillations, and even dynamic requirements, which may cause the navigation performance to decline and even fail [13; 14]. The above discussion indicates that it is hard for a single navigation system to provide precise and reliable navigation information in polar areas. On the contrary, the integrated navigation system is a popular solution for many applications which need a precise and reliable navigation output. Some grid-SINS-based integrated systems with the star tracker [6] and Doppler velocity logger (DVL) [7; 15] have been designed, but they have limits of application such as weather conditions or underwater/shipping applications only. Although a loose-coupled GNSS/grid SINS is proposed in [16], neither the coverage and usability of satellites, the quality of GNSS signals, nor the dynamic requirements have been considered, which may greatly affect the navigation performance in polar areas. It is noted that the deep-coupled GNSS/SINS structure has the deepest information complementation and will provide preferable solutions under challenging environments [17; 18; 19], which will be a good solution for polar navigation. Therefore, with the improvement of the coverage and usability of the GNSS constellations in polar areas and the development of grid SINS, this paper proposes a deep-coupled navigation system composed of dual-frequency GNSS and grid SINS for polar navigation. The coverage and visibility of the GNSS constellation in polar areas are briefly reviewed firstly. Then, the dual-frequency vector-tracking-based architecture of GNSS is designed with the aid of grid SINS information, which can adaptively optimize the tracking band, sharing tracking information to aid weak signal channels with strong signal channels and meet the dynamic requirements to improve the accuracy and robustness of the system.