## 4 SINS/GPS Ultra-Tightly Coupling Structure Based on a Double Loop  
### Design of SINS/GPS Ultra-Tightly Coupling Structure Based on a Double Loop  
In this section, we present a typical application of the proposed method, that is, the application to an ultra-tightly coupled SINS/GPS navigation system of a hypersonic vehicle.  
In the literature [34], the federated ultra-tightly coupled structure is divided into three types. In essence, three kinds of structures are implemented only in signal tracking error estimation, which use three different ways to estimate code and carrier correlation error. Other parts are essentially identical.  
The realization of satellite signal tracking uses a closed loop mode of an external loop, which is a typical vector tracking structure, and this kind of structure enhances the mutual assistance of satellite signal tracking between channels. However, at the same time, there is a problem that when the number of unstable channels is large, if the carrier to noise ratio of multiple satellite signals is reduced due to strong interference, the unstable channel dominates in the signal tracking process. The accuracy of the integrated navigation filter will be reduced, and using this single information to adjust the local signal generator of all channels, the tracking performance of a few normal channels will be affected, and the performance of integrated navigation system will further deteriorate. Therefore, the design of this kind of single regulating loop has a certain \"inverse\" effect on the tracking process of the satellite signal under special circumstances. In addition, the GPS tracking loop of this kind of structure is an open loop [35; 36]. If the integrated navigation filter, for some reason, such as failure, cannot work normally, the entire system will be paralyzed, and the reliability of the system will reduce.  
In order to reduce the influence of the \"inverse\" action mentioned above, we take the advantages of the conventional scalar tracking structure and design the ultra-tightly coupled structure based on the double loop adjustment method shown in Figure 1. One path is aimed at the internal loop of each channel, which is like the channel tracking of the conventional receiver. We use the method of mutual auxiliary tracking between the code and carrier closed loop, and the tracking error that is estimated by the pre-filter of each channel to adjust the code/carrier NCO in real-time, in which the pre-filter adopts a non-linear filtering model. The other path is the external loop described above, which uses the position and velocity information output by the corrected SINS and per the ephemeris information to update and correct, so as to adjust the code phase and the carrier frequency information of the NCO. The structure design of the double loop determines the code/carrier NCO adjustment in two ways, one of which achieves a single internal loop adjusting mode through the pre-filter, and the other way is a combined adjustment mode of external loop supporting internal loop with SINS participating. The strategy of adjustment is to read the corrected inertial navigation information in the program design. If the information meets the accuracy level requirement of NCO adjustment, the combination adjustment method will be adopted. If not, it will depend on the internal loop to adjust. This strategy can reduce the influence of the \"inverse\" function existing in the common vector tracking method and improve the reliability of the integrated navigation system.  
Figure 1: SINS/GPS federated ultra-tightly coupled structure based on double loop.