Yujing Bian

High-Resolution and Multi-Channel Time Interval Counter Using Time-to-Digital Converter and FPGA

A high-resolution and Multi-Channel time interval counter is designed using Time-to-Digital Converter(TDC) and(Field-Programmable-Gate-Array)FPGA. The core of counter is the TDC chip which is interfaced to and controlled by FPGA . The Counter has 1 start channel and 8 stop channels and can be provided with four different modes, and thus four different resolutions. In the highest resolution mode, the counter demonstrates a statistical standard deviation of 18.6 ps. In the lowest resolution mode, that is 82ps, the counter can work with 8 channels and be endless measurement range by internal retrigger of start. The measurement data can be read from the two FIFOs which are part of TDC as a form of real-time transfer or block. Therefore, the counter is communicated with personal computer by serial port RS232. This configuration provides an simply and efficient way of using a computer not only to control and operate the counter, but also to store and process measured data.

Algorithm of intersatellite dynamic two-way time transfer based on GEO satellite

Two-way time transfer is a common method applied to spacecrafts and ground stations to carry out time transfer. This method improved the accuracy of time transfer by eliminating the impact of propagation paths and extra delay caused by atmosphere, so the algorithm of two-way satellite time transfer is presently one of the time transfer methods with high accuracy. But this method requires the spacecrafts and ground stations, on which time transfer is to be performed, to keep a relatively static state in time transfer. So it is mainly used in GEO satellites for satellite-ground two-way time transfer. On the basis of two-way satellite time transfer algorithm, the characteristics of the propagation delay of two-way time transfer signals between MEO satellite and GEO satellite varying with intersatellite range were analyzed, and the rule of intersatellite clock offset varying with intersatellite range obtained with this algorithm was deduced. This paper presents a dynamic two-way time transfer algorithm(DTWTTA) with which high-accuracy intersatellite clock offset is solved through the combination of intersatellite pseudo-range polynomial fitting and clock-offset polynomial fitting. Simulation data of actual satellites showed that with the algorithm the intersatellite time transfer error can be controlled within 5ns provided with minimum square fitting, the two-way time transfer periods beginning at moments basically symmetrical with the occurrence of minimum intersatellite range relatively, and the impact of other factors being taken into account. As a result the algorithm can be used for intersatellite high-accuracy time transfer.

Implementation of channel equalization of indoor positioning system based on MB-OFDM

Precise indoor positioning is becoming increasingly important in commercial, military and public service applications for tracking people and asset. Ultra Wide Band (UWB) can provide high accuracy position with strong anti-jamming and low power consumption. By combining orthogonal frequency-division multiplexing (OFDM) with multi-band, MB-OFDM systems can capture multi-path energy more efficiently than single-band direct sequence UWB (DS-UWB). MB-OFDM modulation is selected in indoor positioning system based on UWB. Channel estimation and equalization are very important and can not be ignored for indoor positioning system based on UWB. There will be a 3dB loss in signal-to-noise ratio (SNR) approximately under no channel estimation. Through the analysis of architecture, signal and channel model of MB-OFDM indoor positioning systems, we studied the channel estimation techniques based on preamble training sequences and pilot sub-carriers respectively. Further more, the linear estimations of least square (LS) and minimum mean square error (MMSE) are analyzed and compared under different UWB channel conditions. A suitable algorithm is selected and hardware implementation scheme is proposed. Some most significant modules for channel estimation and equalization have been validated with Xilinx Virtex IV FPGA. It has been shown by some modelsim simulations that this channel estimation and equalization approach has some merits, such as simple implementation and less consumption of resource. It can meet the requirement of system design and some good performances can be achieved.

UTC message broadcasting over Loran-C data channel

Although GPS has been widely used, Loran-C is still highly desirable as an alternative PRS (primary timing reference source) for an important part of a nations infrastructure. The current Loran-C signals lack of enough time information to flag the precision reference pulse, means that Loran-C has to be assisted by another time system in precision timing applications. In this paper, we present a new approach for the automatic independent TOC (Time of Coincidence) synchronization by Loran-C itself. Based on this method, Loran-C can be upgraded as an independent system in the aspect of precision timing. We also put forward a new message type - UTC message type, which is coincident with the latest ITU-R recommendation M589-3. The details of the message bit assignment and time code format are discussed. A precision transmission control of the UTC message is needed, so that the information and data in the message could be consistent with the reference pulse and denote the pulse. As an example, a control algorithm is given to determine and ensure the time order and position of UTC message frames.