Frequency measurement of microwave signals has a wide range of applications in the fileds such as aerospaceand electronic system. The traditional frequency measurement scheme based on the electronic method has limited frequency rangeand bandwidth, making it difficult to measure the signals with high frequencyand large bandwidth. The microwave photonic technology can combine the advantages of opticaland microwave technologies in terms of large bandwidth, high center frequency, low loss, anti-electromagnetic interference as well as high accuracy, and can achieve microwave frequency measurement with good performance. This article reviews the microwave photonic frequency measurement methods based on frequency-to-time mapping. The basic principles are provided, and detailed introduction to the frequency-to-time mapping schemes based on dispersive mediaand different types of scanning components are given. Finally, a discussion of the future development is provided.

Tianwen-1 (TW-1) is the first Chinese interplanetary mission to have accomplished orbiting, landing, and patrolling in a single exploration of Mars. After safe landing, it is essential to reconstruct the descent trajectory and determine the landing site of the lander. For this purpose, we processed descent images of the TW-1 optical obstacle-avoidance sensor (OOAS) and digital orthophoto map (DOM) of the landing area using our proposed hybrid-matching method, in which the landing process is divided into two parts. In the first, crater matching is used to obtain the geometric transformations between the OOAS images and DOM to calculate the position of the lander. In the second, feature matching is applied to compute the position of the lander. We calculated the landing site of TW-1 to be 109.9259° E, 25.0659° N with a positional accuracy of 1.56 m and reconstructed the landing trajectory with a horizontal root mean squared error of 1.79 m. These results will facilitate the analyses of the obstacle-avoidance system and optimize the control strategy in the follow-up planetary-exploration missions.