Software-defined Receiver For Pseudolite Precise Indoor Positioning

Location based services have been developed rapidly.And the demand for precise indoor location service is increasing-The application of GNSS positioning is limited as the signal is blocked in the indoor environment.Bluetooth,WIFI,RFID and IMU have been used for indoor postioning.But the precision of these positioning results are bigger than one meter.The pseudolite indoor positioning provides carrier phase observation,and achieve the centimeter level position with RTK.The pseudolite receiver has a great advantage in the popularization of hardware because of the same structure as the GNSS receiver.However,the development of the pseudolite indoor positioning is limited because of the time synchronization,near-far problem,the complexity of multipath and signals are blocked easily.This paper studied the problems in high-precision positioning of the pseudolite based on the software-defined receiver.How the carrier phase measurements are generated in receiver is introduced in detail,and the precise positioning is realized with RTK in the pseudolite indoor environment.Multipath signals are mostly close-range reflected signals in the indoor environment,and the MDR of reflected signal is large.1)When the reflected signal MDR is large and the phase difference with the direct signal phase is 180 degrees,the synthetic signal energy received by the receiver antenna is greatly reduced,which makes cycle slips in the carrier phase measurements.2)because the phase of the reflected signal is different from the direct signal,there is a certain deviation in the carrier phase.When the receiver keeps the direct signal tracking and the phase difference is 90 degrees,the carrier phase of the synthetic signal gets maximum error of 45 degrees.In order to keep signal tracking in the indoor environment,a dual antenna receiver is designed which antenna distance is half of the wavelength of the carrier to avoid the phase difference are both 180 degrees.A dual antenna interferometry receiver is designed in this paper.The receiver uses the phase difference of two antennas to achieve the interference location.The method has the advantages of no need to set up a reference station,no need for clock synchronization between pseudolites,no need to solve integer ambiguities,etc.,and can achieve submeter-level positioning accuracy after being calibrated in a small range.Studies have used the polarization characteristics of GNSS signals and bipolar antennas to detect the CNO of direct signals and reflected signals.Thes paper analyzes the disadvantages of using bipolar antennas based on commercial receivers,and designed a bipolar antenna receivers:1)Real-time aiding of reflected signals keeps the receiver tracking the weak signal in time and output CNO continuously.2)The model for multipath detecting with the difference of CNO is improved,the energy of the reflected signal is decomposed,and only the reflected signal energy that affects the direct signal in the loop is extracted.3)The aiding loop is used to estimate the time delay and phase difference of the synthesized signal.Pseudo-satellite indoor positioning can provide high-precision observation and position.Although there are many difficulties and disadvantages,it can be used for multi-sensor fusion positioning.In which it could plays a role of improving the accuracy.