During the early stage of the development of an integrated positioning and navigation system it is necessary to test the components, assess the behaviour of the various integration strategies and verify the performance of the system algorithm. For this purpose, SNAP has developed a MATLAB-based GPS, pseudolite and INS measurement simulator based on INS and GPS Toolboxes.

The simulator comprises three components: the trajectory profile, and the Strapdown IMU (SDIMU) and GPS/pseudolite measurement simulation modules. Both the GPS/pseudolite and SDIMU data simulation require a reference trajectory (e.g. a flight profile) for the moving vehicle. The reference trajectory is defined by the time, coordinate, velocity and attitude angle values. The inputs for the software include the initial coordinates, velocity, attitude, update rate, starting and ending times. In addition, to defining the vehicleÕs movement, segment parameters, e.g. acceleration, velocity and heading changes, need also to be defined. The generated trajectory profile is output at a 1Hz rate.

GPS/Pseudolite/INS measurement simulator

In the case of the SDIMU, specific force and angular velocity are firstly generated, based on the given trajectory profile. Then, the related sensor errors, accelerometer/gyro bias, scale factor and noise, as well as effects associated with Earth rotation and gravity, are computed and added to the generated ÔtrueÕ measurements. All the data generated are stored in a binary format at a rate of 64Hz.

GPS/pseudolite data generation starts with the computation of the coordinates of the satellites. While GPS satellite coordinates are computed using an ephemeris data file (converted from an almanac file), those for the pseudolites need to be provided by the user. Subsequently, based on the coordinates of the receiver antenna and the satellites, the distances between the two points in space are computed. The biases, errors and measurement noise, defined by the appropriate models, are then added. Some points need to be considered at this stage. One is the lack of an ionospheric delay model for the pseudolite measurements. Secondly, a different tropospheric effect model must be used for each pseudolite, as the standard GPS tropospheric models cannot compensate for the pseudolite tropospheric effect. A GPS/pseudolite simulation output rate of 1Hz is used. The simulated data is output in the RINEX V2 format.