The time synchronization system of optical remote sensing satellites generally consists of a timing system and a timekeeping system. Its accuracy is determined by both the timing and the timekeeping accuracy: the timing accuracy is determined by the accuracy of the pulse per second signal output from the global navigation satellite system, while the timekeeping accuracy is determined by the imaging time calibration accuracy of the optical remote sensing camera during operation. As a timekeeping device, the time scale accuracy of the on-board optical remote sensing camera is an important technical indicator that directly affects the geometric positioning accuracy of remote sensing images. With the increasing demand for high temporal and spatial resolution satellite remote sensing data, optical remote sensing satellites require higher accuracy in their whole time synchronization systems. We discuss the design and verification of a high-accuracy time scale system based on the imaging mechanism and process of optical remote sensing cameras and analyze the influence of time scale accuracy and image positioning accuracy, demonstrating that the time scale accuracy of optical remote sensing cameras ensures the geometric positioning accuracy of in-orbit images.

We first analyze the influence of the time scale accuracy of optical remote sensing cameras on the geometric positioning accuracy of remote sensing images. The calibration accuracy of the imaging time directly affects the orbit and attitude accuracy through the satellite orbit parameters and satellite attitude parameters in the image auxiliary data, which affects the geometric positioning accuracy of remote sensing images. To achieve high-accuracy time scale performance for linear array remote sensing cameras, these cameras utilize high-accuracy local clocks and counting homology design, along with an image auxiliary data embedding scheme based on high-accuracy pulse-per-second signals from satellites combined with corresponding satellite integer second time data. This allows the calculation of the corresponding imaging time for each line of remote sensing images. Upon activation, the camera uses a local fixed high-accuracy clock frequency for counting, with the counter width sufficient to prevent overflow throughout the entire operating period. Counting starts on the falling edge of the pulse per second and the rising edge of the image line synchronization, with these counter values latched separately. These values are then used as parameters for calculating the imaging time data. Based on the count value in the image auxiliary data, the imaging time T_H of this image line can be determined. Using error theory and data calculation processing, an analysis formula for the calibration accuracy of the imaging time is derived. According to the design specifications, the calibration accuracy of this camera is calculated to be less than ±32 μs. The imaging time (integer second time code + imaging relative time Δt) calculated from the image auxiliary data deviates from the actual imaging time of this image line. The deviation time is shown in Fig. 5, including hardware delay t_d of the pulse per second signal reaching the camera imaging circuit; relative time deviation value Δt' of the current imaging time. Among these, the relative imaging time deviation is the most influential factor.

According to the analysis, the hardware delay is a fixed delay in the hardware link of the signal, which is an accurate and measurable fixed value. Hardware delay includes three types: intra-board wiring delay, inter-board transmission delay, and device transmission delay. Analysis shows that the total hardware delay is less than 1 μs. By comparing the oscilloscope timing test values with the image auxiliary data calculation values, the calibration accuracy error Δt' relative to the imaging time can be obtained, as shown in Eq. 17. To obtain the maximum calibration accuracy error for each image line of a spatial optical remote sensing camera, several image line synchronization signal positions (T_H) must be selected several times far away from the pulse per second signal, including the farthest end, as shown in Fig. 8. In addition, to ensure completeness and calculate the maximum error, multiple data sets of data with different integration times need to be tested. To verify the calibration accuracy testing method proposed in this article, an example verification is performed on a satellite camera subsystem. According to Eq. 18, the imaging time calibration accuracy of this optical remote sensing camera is 2.8558 μs. Furthermore, based on the camera design and analysis of this test method, its time scale accuracy test error is less than ±2 μs. Combined with the principle analysis of time delay integration push-scanning imaging. Under the premise that the accuracy of satellite orbit and satellite attitude data meets the requirements, it is proved that the time scale accuracy of the optical remote sensing camera can ensure the accuracy of geometric positioning of in-orbit remote sensing images, with sufficient margin.

The optical remote sensing camera incorporates a high-accuracy time scaling function using a hardware-based second pulse that combines the satellite integer second time with the camera’s local clock count for precise calibration of the camera’s imaging time. The system achieves high-accuracy time synchronization performance through a count design from the same source and a high-accuracy local clock scheme. Theoretical analysis and practical tests verify the high-accuracy implementation of the camera’s time scale system. From the perspective of time synchronization accuracy of remote sensing satellites, the high accuracy of the optical remote sensing time scale is shown to meet the accuracy requirements for the geometric positioning of remote sensing images. The hardware delay of the time scale system of the camera subsystem ensures that the imaging time obtained through the calculation of remote sensing image auxiliary data closely matches the actual imaging time of the satellite in orbit.