# Integrated GNSS/IMU-Gyrocompass with Rotating IMU. Development and Test Results
## 1 Introduction  
In a wide range of problems involving mapping, exploration of North areas of the World Ocean, active application of robotic technology, unmanned aerial vehicles and automobiles, and determination of the vehicles attitude is required [1; 2; 3; 4]. Traditionally, data from inertial measurement units (IMU) and strapdown inertial navigation systems (SINS) are used for solution of this problem, which are completely autonomous but unfortunately have an error accumulating with time [5; 6; 7]. Recently, devices contains at least one baseline formed by two global navigation satellite systems (GNSS) antennas have been widely used for attitude determination of mobile objects. [8; 9; 10; 11], the so-called GNSS compasses. These measurements contain data on the angle between the satellite direction and the vector formed by GNSS antennas [8; 11; 12; 13]. GNSS compass advantages include no error accumulation, acceptable accuracy, and relatively low cost [14]. However, their accuracy and reliability are greatly degraded due to satellite signal outages, unfavourable observation geometry, and multipath propagation, which is inevitable particularly with highly dynamic object and in signal degraded conditions such as urban canyons [15; 16; 17; 18; 19]. To overcome these drawbacks, attitude determination systems integrating IMU and GNSS compass data are developed [6; 7; 20], we will call them as integrated GNSS/IMU gyrocompass or simply GNSS/IMU gyrocompass. IMU rotation is important for improving the performance of these integrated systems, because rotation provides observability of IMU errors and finally enhances the attitude accuracy [21; 22; 23; 24; 25]. One of major problems in implementation of GNSS/IMU compasses using phase measurements is ambiguity resolution, i.e., finding the component equal to the integer number of carrier wave lengths [8; 9; 11; 26]. Applying two- and three-frequency receivers makes it possible to resolve the phase ambiguity [27; 28]; however, it increases the system cost and power consumption. With a single-frequency receiver, to enhance the chance of phase ambiguity the redundant number of navigation satellites from different GNSS can be observed (multi-GNSS), including the Russian GLObal Navigation Satellite System (GLONASS), the Chinese BeiDou navigation satellite system (BDS), and the European Galileo navigation satellite system [9; 15; 27; 29]. However, studies conducted by the authors [30] demonstrate that if the number of various GNSS satellites in the visibility zone is considerable, not all of them are effective for the heading determination. The most effective satellites are the ones with elevation of 15\({}^{\circ}\) to 75\({}^{\circ}\) and with azimuth within \(\pm\)45\({}^{\circ}\) to the normal of antenna baseline (green areas in Figure 1). Note that the satellites are situated in the zone satisfying the above conditions for a short time because they move with respect to GNSS/IMU compass antenna module. Thus, even in favorable reception conditions the number of satellites will be limited due to ineffective zones, not to mention the urban canyons. Therefore, in designing integrated systems it is important to reduce the number of satellites sufficient for attitude solution. As shown in the papers including those written by the authors, rotation of satellite antennas--resulting in phase ambiguity resolution--helps to reduce the number of required satellites [31; 32; 33]. References [30; 31] contain the research results for a compact (antenna baseline of 0.3 m) integrated GNSS/IMU compass with single-axis antennas and IMU rotation. Experimental data reveal heading accuracy better than 3\({}^{\circ}\) (3\(\sigma\)) and roll and pitch angles' accuracy, better than 0.3\({}^{\circ}\) (3\(\sigma\)). Note that the algorithms proposed in [32; 33] use the double phase differences, whose generation requires simultaneous observation of min two satellites [17; 18; 32; 34].