Book Description

The accuracy of optical distance measurements on horizontal paths is limited at present to several parts per million by uncertainty in knowledge of the average air density along the path. This report describes the development and testing of a prototype instrument capable of significant;y greater accuracy, probably as high as 0.1 ppm. The instrument makes simultaneous measurements of transit time using two superimposed beams of light which have different optical wavelengths but which are modulated at a common microwave frequency. From the difference in transit times for the two colors, plus the absolute transit for either one, the average refractive index along the path and hence the true geodetic path length can be determined. Tests over a 5.3 -km patch have given a standard error of 0.3 ppm in corrected distance, and it is believed that even better results can be obtained. The system is described in detail, including modifications that would significantly increase its accuracy and range.