Colocation in Time and Space of High-Precision Two-Way Optical and Microwave Observations for Calibration of a Microwave Ranging Link—The ACES Mission Case

Abstract

The ACES mission of the European Space Agency combines optical and microwave-based geodetic observation techniques with highly accurate atomic clocks to achieve a new level of accuracy for geodesy and fundamental physics applications. In addition, the combination of two high-precision measurement techniques provides an even more exciting insight into their errors. Fundamental physics is particularly interested in experiments that require high precision between the results of the successive passes of a satellite. An example of such an experiment is the determination of gravitational redshift. Geodesy applications, in contrast, require both high accuracy and precision. Especially for applications like precise ranging or time synchronization, all possible error influences must be characterized and determined with high precision. Therefore, electronic delays of microwave link terminals pose a challenge to achieving high accuracy. They must, therefore, be calibrated, and the stability of the electronic delays must be monitored. While optical observation techniques can be calibrated sufficiently on the ground, the calibration of microwave measurements before a launch is not precise enough, and continuous monitoring is also not possible. In this study, four calibration methods were tested, all based on colocating optical and microwave measurements onboard a satellite and on the ground. The results of two methods achieved the required accuracy of 100 ps for time synchronization, with a mean error and standard deviation of better than 4 ps and 55 picoseconds, respectively. Correlations between the measured parameters were identified, and the impact of the different approaches on accuracy was investigated. It will be shown that the satellite-based colocation of two different geodetic observation techniques has clear advantages, and the calibration results achieved the required accuracy for geodetic applications in this simulation study.