MODELING AND EVALUATING AN EMPIRICAL INS DIFFERENCE MONITORING PROCEDURE USED TO SEQUENCE SSBN NAVAID FIXES

Thomas H. Kerr

Peer Reviewed

Abstract:	In the past, U.S. Polaris (A-3) and Poseidon (C-3) SSBNs (ballistic missile equipped, nuclear powered submarines) took relatively frequent navigation fixes at specified but clsssified periodic rates to compensate for the degradations of gyro drift-rate. However, a randomization of the fix taking was called for to preclude enemy deciphering of the frequency of fix-taking as the enemy attempts to enhance its surveillance of U.S. SSBNs. Since warm-standby navigation configurations (such as two complete Ships Inertial Navigation Systems, SINS) are usually utilized onboard U.S. SSBNs anyway to reap the benefit of high reliability/availability through online modular repair, the so-called Difference Monitoring procedure consisting of a comparison of the outputs of both these available INSs was instituted in 1976 to randomize the fix taking for C-3 SSBNs. One contribution of this paper is to use existing results of Helstrom from level-crossing theory for the output of nonlinear operations (e.g., the RSSing of latitude and longitude error to obtain radial position error) to provide an analytically tractable theoretical model for the previous empirical procedure of Difference Monitoring. Using the parameters of RMS level and correlation-time of the underlying INS, this analytic model can be exploited to set the constant decision threshold to achieve a specified average interval between position fixes. A second contribution of this paper is to present an expression for the related variance. A comparison between actual SSBN patrol statistics and analytically predicted results for alternative threshold settings is included; however, time and navigation accuracy scales are concealed to prevent divulgence of national security information. Measures of SSBN detectability to enemy surveillance should apparently also be updated in a manner herein suggested to no longer assume SSBN fix taking at a periodic rate. Adoption of this suggestion enables use of an absolute evaluation technique (based on Pareto-optimality) already developed and utilized as described herein as a third contribution for gauging the “goodness” of either randomized or deterministic SSBN fix strategies over and specified time epoch. Two Electrostaticially Supported Gyro Navigators (ESGNs) have been postulated as a candidate configuration for D-5 Trident II SSBNs, where Difference Monitoring will again be as appropriate as in C-3 Poseidon SSBNs.
Published in:	NAVIGATION: Journal of the Institute of Navigation, Volume 28, Number 4
Pages:	263 - 285
Cite this article:	Kerr, Thomas H., "MODELING AND EVALUATING AN EMPIRICAL INS DIFFERENCE MONITORING PROCEDURE USED TO SEQUENCE SSBN NAVAID FIXES", NAVIGATION: Journal of The Institute of Navigation, Vol. 28, No. 4, Winter 1981-1982, pp. 263-285.
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