Navigation Data Reduction Enhances Plot Accuracies

ABSTRACT Data reduction of navigation parameters recorded in real-time is a key phase of satellite navigation in marine geophysical operations. The positioning system operates in a statistical predictor-corrector manner and is complemented by post processing of the shipboard data to provide the most precise postplots. INTRODUCTION Integrated satellite navigation systems are a valuable tool in today's marine exploration efforts. Such systems have proven accuracy and reliability in real-time. However, because of larger statistical data sets, that is, data before and after any given position, the post survey data reduction can further enhance the overall accuracy. Data reduction compensates 'for systematic errors and smoothes the "saw tooth effect" (see Figure 1) of real-time statistical filtering (i.e., Kalman, minimum variance, least squares, etc.). To perform the reduction task the following steps (see Figure 2) are required:Edit field dataAnalyze and recompute satellite fixesDetermine and remove systematic errorsAdjust random errorsCompute and plot auxiliary dataPlot preliminary mapsCheck and adjust mistiesPlot final maps BASIC PHILOSOPHY The basic function of the integrated satellite navigation system is to control the position of the vessel on a preplanned course or "line" during the geophysical survey (see Figure 3). Most operations are defined as "on-line" or "off-line." While "on-line" the integrated navigation system serves as a shot point counter, i.e., it determines the "on-line" course and transmits a discrete pulse, on the basis of distance traveled, to the geophysical recording system which performs the data acquisition and recording operations. The integrated system, while computing position and maintaining vessel course, records positional information on digital magnetic tape. Computations are based on inputs to a digital computer froma satellite receiver,a Doppler sonar,a gyrocompass,an inelinometer, anda velocimeter (see Figure 4). These inputs are sampled continuously to determine the vessel's position at one-half second intervals. Onboard data is filtered in a predictor-corrector (Kalman) manner to obtain optimum real-time results. It is this data recorded on magnetic tape and its post processing that is the subject of the discussion below. SATELLITE FIX COMPUTATION To compute optimum post survey positions, all navigation data, that is, satellite message data and other sensor information are recorded on magnetic tape. The first step is to edit the field reel verifying all recorded data and checking for data "drop out" (missing data, gaps in recording, loss due to parity errors) and to merge auxiliary data from external navigation aids. The most reliable data obtained by integrated satellite navigation systems are satellite fixes. Although fixes are not absolutely correct, in a stationary position accuracies of 150 feet RMS may be expected (see Table 1).