In this chapter, we present a class of astrometric techniques which are not based upon analyses of electromagnetic waves received from space, but on measurements of time intervals between events one of which, at least, originates from space or is connected with it. We have already seen the importance of very precise timing in VLBI, but in this case, it is a necessary auxiliary system to the actual receiver and to the computation of the correlation function (Sect. 10.2.2). But in radar, telemetry laser, or pulsar timing techniques, the actual measurement is a time-interval measurement. This opens the way to remarkably precise astrometric results because time is the physical quantity which is at present the most accurately determined: the best clocks currently have accuracies of the order of 10−13 or 10−14. So, whenever a parameter may be determined through the measurement of a time interval, one may expect to get utmost precisions.
KeywordsMethane Quartz Dust Microwave Mercury
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