Scale Factor Correction In The Phase-Nulling Optical Gyro

Abstract

Scale factor correction in the phase -nulling optical gyroE. Udd, R. J. Michal, and R. F. CahillMcDonnell Douglas Astronautics CompanyHuntington Beach, California 92647AbstractMeans to correct scale factor in the phase- nulling optical gyro are presented. In par-ticular, methods are described that allow the wavelength of the light source to be trackedto high accuracy. A derivative of this technique, allowing the phase -nulling optical gyroto execute an in- flight cold start, is also described.IntroductionOne of the major advantages of fiber -optic gyro technology is that it offers the poten-tial of a low -cost, all- solid -state approach with long- lifetime and high -reliability advan-tages over current gyroscopes. In order to fully exercise these advantages, it is impera-tive that these gyroscopes have scale factors compatible with strapdown applications. Forgyroscopes with sensitivity and bias drift on the order of 10 deg /hr, this often impliesscale factor correction of 100 ppm or better, although exact requirements on scale factorare dependent on the number of vehicle turns and final pointing accuracy required by theapplication in question. Scale factor correction to this accuracy implies that the linear-ity of the fiber -optic gyro is correctable to this level and that factors that change thescale factor relationship may be monitored to a corresponding accuracy. The phase- nullingoptical gyro offers an inherently linear output, the major residual problem being the abil-ity to monitor the output wavelength of the light source with an accuracy compatible withapplication requirements. Since the light source of choice used in a fiber -optic gyro iscurrently a superradiant diode with a broadband output that depends on temperature, current,and aging effects in an insufficiently predictable manner, a means to compensate or elimin-ate this scale factor error source is required.Scale factor relationships for analog and digital fiber -optic gyrosThe relationship between the output of a gyroscope and rotation rate for a mechanicalgyroscope is often expressed in the form