The expressions for the frequency of satellite antenna control and work principle of GPS satellite signal simulator consists of a GPS signal generator, the computer simulation software, the GPS satellite signal simulator working principle shown in Figure 1. GPS satellite signal simulator is to generate a user terminal receives the visible GPS L1 C / a code signal, the signal [2] in AI (T) t denotes the ith satellite signal amplitude; CI (T) at time t I satellite launch of C / a code signal; Di (T) is expressed in the ith satellite signal modulation of the navigation message data; fi (T) denote the ith satellite of the instantaneous carrier frequency; Phi I (T) said carrier signal phase; tau I denotes the ith satellite signal propagation delay.

In the simulator design, the first generation of intermediate frequency GPS signal discrete, if GPS signal sampling rate is FS, the sampling interval is Ts, then the sampling time in the first k, discrete if GPS signal generated from fIFi 2 - (kTs) GPS IF signal sampling time of instantaneous frequency is k Doppler, contains the effect of frequency. In order to produce a desired intermediate frequency signal with low sampling frequency, and reduce the FPGA (large scale online programmable devices) computing speed requirements of S (kTs) 4 times zero interpolation 3, i.e. every 1 sampling points into 3 zeros, get new in insert signal S4 (kTs), S4 (kTs) by D/A into analog signal by band-pass filter to filter out the signal to the GPS signal through the frequency will produce L1 frequency. The signal intensity attenuator control simulator output, the output signal level is maintained within a predetermined range Within.

2. Software simulation and control software is the core of simulator, simulator needs to control parameters by the software calculated. It is mainly composed of the following modules: initialization module, self checking module, communication control module, satellite navigation parameters calculation module, message control module, the target trajectory calculation module, error calculation module, antenna characteristic calculation module, user input and display module and data analysis module. Initialization module and self checking module to complete the simulator to set the initial parameters and the simulator hardware self-test, ensure simulator of each part in the normal state.

The communication control module to complete the communication between the computer and the PCI card, signal generator sends commands to the navigation data and control state information, the received signal generator sends the calculation module. Choose the best parameters of GPS satellite navigation, according to the ephemeris and target state calculation time signal pseudo code state emission, carrier Doppler frequency and Doppler frequency rate of change of navigation. Satellite information message control module automatically edit the GPS satellites, various data domain and special symbol management message data, set the satellite ephemeris error, satellite including radial, tangential and transverse error. Modify the parameters for the satellite message receiver for RAIM detection. The error calculation module calculates ephemeris error, ionosphere, troposphere refraction error source all kinds of errors on the multipath effect of code phase and carrier phase calculation module. A series of complex vector trajectory The carrier motion model is complex, can generate vector trajectory. According to the type of carrier can be divided into the following modules: Satellite (spacecraft), rockets, aircraft, ship and vehicle trajectory module. In addition, this module allows the motion characteristics of user-defined vector. The GPS receiver antenna gain and phase in a certain direction, from different directions, different antenna gain and phase signal of the incident angle is different. Especially the target attitude change is large, the GPS receiving antenna to receive signals and measurement accuracy will be affected. In addition, if the target environment is blocked, causing part of the satellite signal interruption. Considering the influence of calculation module these factors characteristic of the antenna in the azimuth and elevation respectively according to a certain resolution, the establishment of the antenna, the calculation of vehicle attitude variation of the received GPS signal.

User input and display module is proposed to use an interactive graphical interface, allows the user to complete the various parameters of the simulation setup and modify, at the same time, the way to animate realistic display test process. Data analysis module based on the simulation software of simulation data and measurement data of GPS receiver, receiver acquisition time and precision measurement indexes of evaluation.