The Design Of High Sensitivity GPS Tracking Baseband

Global Positioning System (GPS) has become the most widely used satellite navigation and positioning system in the world. With the demand for navigation services increasing, there are more and more challenges for GPS receivers to work properly in weak signal environments, such as indoors, forest, and city streets. Especially in extremely weak signal environment, the normal GPS receivers cannot operate correctly and accurately. Therefore, it is particularly important to research the key technology of high sensitivity GPS receivers. And the design of high sensitivity GPS tracking baseband becomes the core and difficulte issue.To enhance the tracking sensitivity, on the basis of the principles of GPS, the long coherence integration scheme is used to effectively suppress noise. Meanwhile, the frequency-difference-losses caused by long coherence integration become the core difficulty. In order to solve this problem and improve tracking sensitivity, accuracy and dynamic performance, an acceleration lock loop based high sensitivity tracking algorithm (ALL_HST) is proposed. Specific to the carrier tracking loop and the code tracking loop, the innovative works can be divided into two parts as following.(1) An acceleration lock loop based high sensitivity carrier tracking algorithm (ALL_HSCAT) is proposed. To solve the frequency-difference-losses, a mathematical model of long coherence integration is established. In this model, the frequency-difference-losses are analyzed quantitatively in the second order (frequency) and third-order (acceleration) levels. An acceleration compensation and frequency compensation mechanism is used to suppress the frequency-difference-losses. A two-dimensional interpolation algorithm is used to determine the accurate frequency and acceleration. Additionally, a filter of second order ALL assisting third order FLL is used to smooth frequency and acceleration value before feeding back to the carrier NCO. Therefore, a new tracking loop is constructed with ALL assisting FLL. And this leads that the carrier tracking loop has a third-order acceleration tolerance, and the frequency-difference-losses is suppressed effectively. With the ALL_HSCAT algorithm, carrier tracking sensitivity is improved to -166dBm, and dynamic performance is enhanced.(2) A frequency compensation based high sensitivity code tracking algorithm (FC_HSCOT) is proposed. Specific to the features of code tracking loop, the carrier tracking loop assisting code tracking loop scheme is used to ensure the dynamic performance. And frequency compensation based coherence integration and non-coherence integration are used to solve the frequency-difference-losses. Therefore, the SNR is improved. Additionally, a noise floor independent DLL discriminator is used to improve code phase detecting accuracy. With the FC_HSCOT algorithm, code tracking sensitivity is improved to match with the carrier tracking loop.The proposed ALL_HST algorithm is characteristic of the new ALL assisting FLL tracking loop constructure, which is superior to traditional non-coherent integration phase locked loop, vector tracking loop, FFT-based frequency locked loop and other tracking algorithms in tracking sensitivity.Finally, the GPS tracking baseband is designed based on the proposed ALL_HST algorithm. This design includes two parts, which are hardware design and software design. Then the designed tracking baseband is tested based on the FPGA platform and the GPS simulator environment. Comparison shows that the designed tracking baseband achieves a high tracking sensitivity of -166dBm. This result equals to the latest commercial GPS chip NEO/LEA-M8 of the industry-leading U-Blox company, and is 3dB higher then previous work of our research group. Meanwhile, the tracking accuracy is better than previous work of our research group in weak signal conditions. The dynamic performance is superior to the contrastive receivers. In conclusion, the GPS tracking baseband has met the design requirements.