Sparse arrays offer evident advantages over their uniform linear counterparts, including larger aperture and higher angular resolution. Despite their benefits, typical sparse arrays, such as nested and coprime arrays, exhibit increased complexity, and mutual coupling effects between array elements can not be neglected. In contrast, the sparse uniform linear array (SULA) features uniform inter-element spacing, where the distance between elements exceeds half a wavelength, resulting in a larger aperture and reduced mutual-coupling effects. However, estimating the direction of arrival (DOA) with the SULA may lead to inevitable angle ambiguity. Recently, FMCW radar has gained significant attention due to its capability of high-precision ranging under conditions of high signal-to-noise ratios. This study leveraged the high-precision ranging characteristics of FMCW radar to achieve unambiguous DOA estimation by utilizing absolute distance values from targets to each SULA element. Simulation experiments validated the effectiveness of the proposed algorithm.

The essence of frequency-modulated continuous wave (FMCW) radar-ranging is to estimate the frequency of the beat signal, and the frequency’s accuracy determines the range’s accuracy. This paper introduces a coherent spectral model based on the correlation between the phase and frequency of the FMCW radar beat signal. A coherent chirp z-transform (CZT) algorithm is proposed by applying the coherent spectral model to the CZT algorithm. The simulation results proved that the ranging accuracy of the coherent CZT algorithm is better than other algorithms. Generally, there are two Cramer-Rao bounds (CRB) in range estimation. The first CRB assumes that the phase is unknown, and the second CRB assumes that the phase is known. Most existing ranging algorithms can only reach the first CRB, while the coherent CZT algorithm can reach the second CRB. The efficiency of the coherent CZT algorithm has been confirmed in three radar-ranging experiments with targets of various sizes and shapes. The experimental results also showed that the coherent CZT algorithm has a higher accuracy than 50 μm, and its root mean squared error (RMSE) is significantly lower than other algorithms.

The global navigation satellite system (GNSS)-based attitude determination system has attracted more and more attention with the advantages of having simplified algorithms, a low price and errors that do not accumulate over time. However, GNSS signals may have poor quality or lose lock in some epochs with the influence of signal fading and the multipath effect. When the direct attitude determination method is applied, the primary baseline may not be available (ambiguity is not fixed), leading to the inability of attitude determination. With the gradual popularization of low-cost receivers, making full use of spatial redundancy information of multiple antennas brings new ideas to the GNSS-based attitude determination method. In this paper, an attitude angle conversion algorithm, selecting an arbitrary baseline as the primary baseline, is derived. A multi-antenna attitude determination method based on primary baseline switching is proposed, which is performed on a self-designed embedded software and hardware platform. The proposed method can increase the valid epoch proportion and attitude information. In the land vehicle test, reference results output from a high-accuracy integrated navigation system were used to evaluate the accuracy and reliability. The results indicate that the proposed method is correct and feasible. The valid epoch proportion is increased by 16.2%, which can effectively improve the availability of attitude determination. The RMS of the heading, pitch and roll angles are 0.52°, 1.25° and 1.16°.

Ambiguity resolution (AR) is crucial for high-precision indoor pseudolite positioning. Due to the existing characteristics of the pseudolite positioning system, such as the geometry structure of the stationary pseudolite which is consistently invariant, the indoor signal is easy to interrupt and the first order linear truncation error cannot be ignored, and a new AR method based on the idea of the ambiguity function method (AFM) is proposed in this paper. The proposed method is a single-epoch and nonlinear method that is especially well-suited for indoor pseudolite positioning. Considering the very low computational efficiency of conventional AFM, we adopt an improved particle swarm optimization (IPSO) algorithm to search for the best solution in the coordinate domain, and variances of a least squares adjustment is conducted to ensure the reliability of the solving ambiguity. Several experiments, including static and kinematic tests, are conducted to verify the validity of the proposed AR method. Numerical results show that the IPSO significantly improved the computational efficiency of AFM and has a more elaborate search ability compared to the conventional grid searching method. For the indoor pseudolite system, which had an initial approximate coordinate precision better than 0.2 m, the AFM exhibited good performances in both static and kinematic tests. With the corrected ambiguity gained from our proposed method, indoor pseudolite positioning can achieve centimeter-level precision using a low-cost single-frequency software receiver.

Due to the great influence of multipath effect, noise, clock and error on pseudorange, the carrier phase double difference equation is widely used in high-precision indoor pseudolite positioning. The initial position is determined mostly by the known point initialization (KPI) method, and then the ambiguities can be fixed with the LAMBDA method. In this paper, a new method without using the KPI to achieve high-precision indoor pseudolite positioning is proposed. The initial coordinates can be quickly obtained to meet the accuracy requirement of the indoor LAMBDA method. The detailed processes of the method follows: Aiming at the low-cost single-frequency pseudolite system, the static differential pseudolite system (DPL) method is used to obtain the low-accuracy positioning coordinates of the rover station quickly. Then, the ambiguity function method (AFM) is used to search for the coordinates in the corresponding epoch. The real coordinates obtained by AFM can meet the initial accuracy requirement of the LAMBDA method, so that the double difference carrier phase ambiguities can be correctly fixed. Following the above steps, high-precision indoor pseudolite positioning can be realized. Several experiments, including static and dynamic tests, are conducted to verify the feasibility of the new method. According to the results of the experiments, the initial coordinates with the accuracy of decimeter level through the DPL can be obtained. For the AFM part, both a one-meter search scope and two-centimeter or four-centimeter search steps are used to ensure the precision at the centimeter level and high search efficiency. After dealing with the problem of multiple peaks caused by the ambiguity cosine function, the coordinate information of the maximum ambiguity function value (AFV) is taken as the initial value of the LAMBDA, and the ambiguities can be fixed quickly. The new method provides accuracies at the centimeter level for dynamic experiments and at the millimeter level for static ones.