列车防撞辅助预警系统，作用于ATP（Automatic Train Protection Unit）切除条件下行驶的轨道列车。系统运行时，将判断列车运行方向，实时监测前后两车间距，当两车距离小于或者处于安全距离临界值时，系统产生制动信息，实现实时辅助防撞。此系统无需安装地面设备，自动检测ATP切除信号，追踪范围大、精度高、抗地形天气等干扰能力强，可以在隧道环境下运行。     

         本系统采用2.4G无线定位测距方案，其中底层调制技术采用片上CSS核心技术。传统的无线定位技术采用RSSI信号强度检测方法，在环境障碍、遮挡等典型情况下，测距误差极大，无法满足城轨列车多隧道和弯道情况下的定位需求。而CSS技术具有极强的抗干扰、抗频率偏移特性以及抗多路径效应能力，使得测距精度大幅提高，这是传统的无线定位技术所望尘莫及的。            本系统还采用先进的TDOA定位技术，对称双边双程测距方式，利用信号在两个非同步的收发机之间往返的传输时间来测量距离，抵消了测距过程中因时间非同步带来的抖动，大幅提高了测距精度。

   URCAS is a train collision avoidance warning system, which is used under the condition of ATP (Train Protection Unit Automatic) cut off. When the system is running, direction of the train is judged, and distance between the two trains is monitored in real time. When distance between the two trains is less than the critical value of the safe distance, braking information is generated, which can realize real-time auxiliary collision avoidance. System good points : no need to install the ground equipment, automatic detection of ATP cut off signal, wide tracking range, high accuracy, anti terrain weather and other interference ability, can run in the tunnel environment.

     The system uses 2.4G wireless positioning scheme, in which the underlying modulation technology using CSS technology. Traditional wireless positioning technology using received signal strength indicator (RSSI) detection method, in obstacle、occlusion and other typical cases, deviation too large to meet the positioning requirements of urban rail train with multi tunnels and curves condition. The CSS technology has very strong anti-jamming and anti frequency offset characteristics and ability against multipath effects, the ranging accuracy is greatly improved. This greatly exceeds the traditional wireless positioning technology.

     The system adopts advanced technology of TDOA location, Symmetric bilateral two-way ranging method, using the signal round-trip transmission time between two non-synchronization transmitter and receiver to measure the distance and offset the jitter in ranging process due to the non synchronization, which increase the ranging accuracy.