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In a GPS-denied environment, even the combination of GPS and Inertial Navigation System (INS) cannot provide location reliably and accurately. We propose a new denoised stereo Visual Odometry VO/INS/GPS integration system for autonomous navigation based on tightly coupled fusion. The presented navigation system can estimate the location of the vehicle in either GPS-denied or low-texture environments. Because of the random walk characteristics of the drift error of the inertial measurement units (IMU), the errors of the states grow with time. To correct these growing errors, a continuous update of observations is necessary. For this purpose, the system state vector is augmented with the extracted features from a stereo camera. Consequently, we utilize the measurements of extracted features from consecutive frames and GPS-derived information to make these updates. Moreover, we apply the discrete wavelet transform (DWT) technique before data fusion to improve the signal-to-noise ratio (SNR) of the inertial sensor measurements and attenuate high-frequency noises while conserving significant information like vehicle motion. To verify the performance of the proposed method, we utilize four flight benchmark datasets with top speeds of 5 m/s, 10 m/s, 15 m/s, and 17.5 m/s, respectively, collected over an airport runway by a quad rotor. The results demonstrate that the proposed VO/INS/GPS navigation system has a superior performance and is more stable than the VO/INS and GPS/INS methods in either GPS-denied or low-texture environments; it outperforms them by approximately 66% and 54%, respectively.

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The presence of code Doppler and navigation bit sign transitions means that the acquisition of global positioning system (GPS) signals is difficult in weak signal environments where the output signal-to-noise ratio (SNR) is significantly reduced. Post-correlation techniques are typically utilised to solve these problems. Despite the advantages of these techniques, the post-correlation techniques suffer from problems caused by the code Doppler and the navigation bit sign transitions. We present an improved semi-bit differential acquisition method which can improve the code Doppler and the bit sign transition issues in the post-correlation techniques. In order to overcome the phenomenon of navigation bit sign transitions, the proposed method utilises the properties of the navigation bit. Since each navigation bit takes as long as 20 ms, there would be 10 ms correlations duration integration time between the received signal and the local coarse/acquisition (C/A) code in which the navigation bit sign transitions will not occur. Consequently, this problem can be cancelled by performing 10 ms correlations in even and odd units separately. Compensation of the code Doppler is also accomplished by shifting the code phase of the correlation results. To validate the performance of our suggested method, simulations are performed based on three data sets. The results show that the quantity of required input SNR to detect at least four satellites in the proposed method is − 48·3 dB, compared with − 20 dB and − 9 dB, respectively, in traditional differential and non-coherent methods.