Unmanned aerial vehicle （UAV） has played an increasingly important role in recent military conflicts due to its low cost and high flexibility. This attracts wide attention from countries around the world. The number of “black flights” of UAVs in China is increasing year by year， which directly reflects the challenges of the prevention and control of low-altitude airspace UAVs. The construction of urban low-altitude airspace anti-UAV is worthy of further research. As for the urban low-altitude airspace anti-UAV problem， the possible UAV threats were analyzed. The urban anti-UAV scenarios were classified， and the challenges and problems of the urban anti-UAV were explored. Additionally， five solutions to the urban anti-UAV system were proposed. In conclusion， for the construction of the urban anti-UAV system， the positive design oriented to the target and scenarios should be conducted， and special attention should be paid to the UAV target and environment characteristics， the anti-UAV point position design， the integration of the command and control system design with user operations， and the standardization of system integration. This can provide a reference for the urban anti-UAV system design.

In the TDOA target generation system， microwave photon links are used to transmit multi-channel and multi-band target simulation signals containing accurate TDOA information. To ensure the accuracy of TDOA information is enough， it is necessary to accurately measure the propagation delay of the target simulation signal through the photon link. A novel method for measuring the propagation delay of photon link is proposed in this paper， which uses a dedicated delay measurement chip based on the gate delay counting principle to achieve high-resolution and high-precision measurement. By occupying the same optical transmission channel of a single fiber through time sharing between the delay measurement signal and the target simulation signal， the measurement of the propagation delay and the transmission of the target simulation signal are achieved for a photon link， it meets the hardware requirements for accurately measuring the propagation delay of photon link mechanically. The experimental results show that this method can accurately measure the propagation delay of the TDOA target simulation signal through the photon link， with an error of less than 1ns， which is one order of magnitude higher than the TDOA measurement accuracy of the sensor， meets the measurement accuracy requirement of the system.