A Traceable High-Accuracy Velocity Measurement by Electro-Optic Dual-Comb Interferometry

An electro-optic dual-comb Doppler velocimeter for high-accuracy velocity measurement is presented in this paper. The velocity information of the object can be accurately extracted from the change of repetition frequency, which is in the microwave frequency domain and can be locked to an atomic clock. We generate two optical combs by electro-optic phase modulators and trace their repetition frequencies to the rubidium clock. One functions as the measurement laser and the other the reference. Experimentally, we verify its high accuracy in the range of 100–300 mm/s with a maximum deviation of 0.44 mm/s. The proposed velocimeter combines the merits of high accuracy and wide range. In addition, since the repetition frequency used for the measurement is traceable to the rubidium clock, its potential superiority in traceability can be utilized in velocity metrology.

A case study of the application of GPS to lunar laser ranging timing systems

The Hartebeesthoek Radio Astronomy Observatory is currently building a Lunar Laser Ranging station. This geodetic technique requires a good timing system to measure a round trip of laser photons from the telescope to the Moon and back to the telescope. We test the newly acquired timing system using examples of the Global Positioning System applications. Data in Receiver Independent Exchange Format was processed using GAMIT/GLOBK software. The results were compared against those derived from the Global Positioning System receivers that were integrated with a frequency standard from a hydrogen maser and a standard internal quartz. The results indicate that (i) the rubidium clock operates optimally and the clock drifted to within error margins of sub-centimetre level during the period of 2.5 seconds, (ii) the selected site for the permanent installation of the timing antenna has minimal multipath effect and (iii) we observed no improvement in Global Positioning System products derived from receivers that were integrated with different frequency standards.