Can the ratios of single‐wavelength EDM data improve the resolution of small changes in line length?: A comparison with multiwavelength EDM data

Abstract

A variety of electromagnetic techniques are now in use for high‐precision measurement of distance through the earth's atmosphere. One of the major obstacles to improving the precision of single‐wavelength electromagnetic distance‐measuring (EDM) measurements is the uncertainty in the value of the index of refraction along the path being measured. If the index of refraction were slowly varying and the relative change of the index nearly uniform, the problem would be considerably simplified; deformations, except uniform compression and extension, could be determined to high precision by analyzing the ratios of various uncorrected line lengths in an array. The development of a field‐worthy multiwavelength EDM (multiwavelength distance measuring (MWDM)) instrument has allowed a direct comparison of this ratio technique with high‐precision multiwavelength data. Three widely spaced lines in the Hollister, California, MWDM array were selected for this study. The objective was to determine whether a single observer, without access to expensive or sophisticated means of sampling the atmosphere, could collect data capable of resolving changes of less than 1 part per million (ppm) in line length. No corrections were made to the single‐wavelength data (i.e., the optical path lengths) before the ratio was formed. Results indicate that the requirement of near constancy during a particular survey is more realizable than that of near uniformity. It appears difficult to achieve a precision of better than 0.5 ppm routinely within an arbitrary radial array of substantial size. The technique does offer promise in properly designed arrays where all lines can be restricted to a reasonably small solid angle (e.g., the monitoring of some man‐made structures such as dams) or where they are between isolated mountain peaks of comparable elevation. Within such arrays a precision of 0.2 ppm should be possible.