To improve simulation precision, the signal model of navigation satellite signal simulators is illustrated, and the generation mechanism and evaluation criteria of an important error source-phase jitter in baseband signal generation, are studied subsequently. An improved baseband signal generator based on dual-ROM look-up table structure is designed with the application of a newly-established concept-virtual sampling rate. Pre-storage of typical baseband signal data and sampling rate conversion adaptive to Doppler frequency shifts are adopted to achieve the high-precision simulation of baseband signals. Performance analysis of the proposed baseband signal generator demonstrates that it can successfully suppress phase jitter and has better spectral performance, generating high-precision baseband signals, which paves the way to improving the overall precision of navigation satellite signal simulators.

In order to estimate fast time-varying channels exactly, the Inter-Carrier Interference (ICI) caused by time-varying fading channels in Orthogonal Frequency Division Multiplexing (OFDM) systems is analyzed based on the Basis Expansion Model (BEM). A channel estimation and ICI cancellation algorithm with low complexity is proposed. A special pilot sequence is designed to minimize the cost of computing the channel state information in the proposed algorithm. Based on the property of channel frequency impulse matrix, the ICI can be canceled iteratively in frequency domain. The complexity of the algorithm is analyzed theoretically. Through simulation, the algorithm is shown to be effective in estimating channel state information and in cancelling ICI.

In view of the frequent and complex changes of GNSS visible satellite constellation in attitude determination system, an improved attitude signal simulation algorithm for high dynamic satellite signal simulator is proposed. Based on Software Radio architecture, elevation calculation in the antenna coordinate system and channel state control logic under the condition of carrier attitude changes are introduced into the algorithm to implement synchronous scheduling of visible satellite constellation and attitude signal simulation. This work guarantees the simulator to run constantly and stably for a long time with the advantages of high precision and low complexity. Compared with synchronous positioning results from the receiver, the simulation results show that not only can the output signals of the simulator accurately reflect the carrier's attitude characteristics, but also no step error is generated and the positioning precision is not influenced when visible satellite constellation changes.

An advanced interest point detector is proposed to improve the Hessian-Matrix based detector of the SURF algorithm. Round-like shapes are utilized as the filter shape to calculate of the Hessian determinant. Dxy can be acquired from approximate round areas, while the regions for computing Dyy or Dxx are designed with the consideration to symmetry and a balance of pixel number. Experimental results indicate that the proposed method has higher repeatability than the one used in SURF, especially in the aspects of rotation and viewpoint, due to the centrosymmetry of the proposed filter shapes. The results of image matching also show that more precision can be gained with the application of proposed detector.

This letter proposes a new transceiver for OFDM systems based on Smooth Local Trigonometric Transform (LTT). In our transceiver, the transmitter is realized by first modulating the original serial data using a constellation mapper, then feeding the results into the inverse LTT modulator. Unlike the conventional DFT-OFDM system, which always uses the roll cosine function as its window function, the proposed system needs no additional window function for the reason that LTT transform includes a bell-shaped window function by itself. Moreover, each LTT-OFDM symbol has a much more rapid attenuation rate outside of the spectral bandwidth and better spectrum convergence. In the receiver, the original data is recovered by demodulating the received data using forward LTT. Comparative simulation results from the conventional DFT-OFDM system, the system we proposed, and the recently proposed DCT based OFDM system are discussed in terms of bit error rate (BER).

Vehicle routing is an important combinatorial optimization problem. In real transport networks,the travel speed and travel time of roads have large time-variability and randomness. The study of vehicle routing problem in time-dependent network has even more practical value than static network VRP problem. This paper combines the features of time-dependent networks and gives the mathematical models of the time-dependent vehicle routing problem. On this basis, the traditional ant colony optimization algorithm is improved. A new path transfer strategy of ants and new dynamic pheromone update strategy applicable to time-dependent network are proposed. Based on these strategies, the improved ant colony algorithm is given for solving the vehicle routing problem in time-dependent networks. The simulation results show that the algorithm can effectively solve the vehicle routing problem in time-dependent network and has better computational efficiency and convergence speed.

Research of the shortest path problem in time-dependent networks has important practical value. An improved pheromone update strategy suitable for time-dependent networks was proposed. Under this strategy, the residual pheromone of each road can accurately reflect the change of weighted value of each road. An improved selection strategy between adjacent cities was used to compute the cities' transfer probabilities, as a result, the amount of calculation is greatly reduced. To avoid the algorithm converging to the local optimal solution, the ant colony algorithm was combined with genetic algorithm. In this way, the solutions after each traversal were used as the initial species to carry out single-point crossover. An improved ant colony algorithm for the shortest path problem in time-dependent networks based on these improved strategies was presented. The simulation results show that the improved algorithm has greater probability to get the global optimal solution, and the convergence rate of algorithm is better than traditional ant colony algorithm.

Inter-satellite link (ISL) is an important part of the next generation global navigation satellite system (GNSS). In this paper, key technologies of GNSS ISL ranging and time synchronization are researched. Considering that Ka frequency band is used for ISL, a fixed topology is designed and a new time division duplex (TDD) mode is proposed after analyzing the characteristics of GNSS constellations. A novel method called Non-coherent Dual One-way Measuring (NC-DOWM) is applied to this TDD mode. In addition, relevant mathematical formulas, error models and error compensation are discussed in detail. It is found that the proposed NC-DOWM method for GNSS ISL ranging and time synchronization outperforms the current method for GPS in terms of channel utilization efficiency and measuring precision. Furthermore, the presented method has excellent anti-interference capability and engineering feasibility, which can provide a strong technical support for the ISL of the next generation GNSS.

The estimation problem of structured clutter covariance matrix (CCM) in space-time adaptive processing (STAP) for airborne radar systems is studied in this letter. By employing the prior knowledge and the persymmetric covariance structure, a new estimation algorithm is proposed based on the whitening ability of the covariance matrix. The proposed algorithm is robust to prior knowledge of different accuracy, and can whiten the observed interference data to obtain the optimal solution. In addition, the extended factored approach (EFA) is used in the optimization for dimensionality reduction, which reduces the computational burden. Simulation results show that the proposed algorithm can effectively improve STAP performance even under the condition of some errors in prior knowledge.