Positioning by satellites

During the previous reporting period (1971–1974) accuracies of geodetic positioning by satellites had been pushed from the level of 5 meters rms or so characteristic of the NGS Worldwide Geodetic Satellite Triangulation to the level of 1 meter rms characteristic of routine satellite Doppler positioning based on use of precise ephemerides. To be sure, within this period the threshold of 1 meter was being broached by various NASA laser ranging experiments and by various experiments with Very Long Baseline Inferferometry (VLBI). These, however, were in the category of special exploratory endeavors involving only a relatively few stations occupied for extended periods and could hardly be regarded as routine. During the current reporting period (1975–1978) accuracies have been pushed in several disciplines toward the 10 cm level, and the stage is being set for advancement toward the 1 cm level. This has been a period of extraordinary progress with particular impetus being provided by the challenge of precise monitoring of crustal movements over extensive regions of high seismic activity. More mundane, but of greater immediate impact has been the spectacular increase in the application of satellite Doppler positioning throughout the world. From the standpoint of economic significance this certainly qualifies as the most important single development in positioning by satellites during the current period. Accordingly, Doppler technology is accorded special prominence in this review. This is not to slight the impressive advances made in laser tracking and VLBI technology. However, these topics are addressed at some length in other sections of this issue and thus need not be pursued as deeply here. A similar remark applies to the Global Positioning System which is still in an early developmental stage and can therefore presently be addressed more from the standpoint of theoretical potential than from practical experience.