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    Re: GPS Anti Spoof based on radio sextant ?
    From: David Pike
    Date: 2020 Jan 30, 03:50 -0800

    Igor S you wrote:  Why did they place those on nuclear subs? Sake of experiment rather then for regular use ?

    To understand the logic behind some of these ideas, you must understand Cold War thinking and the equipment available at the time.  One of the main roles of a nuclear submarine was to be able to launch lethal ICBMs from a hidden location to a known location.  Lethality depended upon yield and accuracy at the destination.  Accuracy at the destination depended to a significant extent upon knowing accurately where the ICBM was being launched from plus the aligment of any inertial platforms in the missiles.  This laid down the accuracy requirement for the submarines navigation system while at sea.  If it was any thing like a V Bomber, the ‘system position’ really was a ‘system position’.  It rarely relied on just one aid as a GNSS fed system might today.  It was an automatically generated position which you carefully nursed tweaking it occasionally by comparing it to recently obtained data.  Apart from H2S fed Navigation and Bombing System fixes, one rarely reset the system completely.  It was more a case of bending it to a most probable position using laid down bands of error for incoming LOPs plus a certain amount of, horror of horrors, ‘navigator intuition’.  Therefore, a radio sextant altitude and azimuth might have been of value if fed into the system, but its influence would probably have been bounded either manually or automatically by its expected accuracy.  If that was low, then its influence would be arranged to be low.

    I’m always suspicious of magazine articles using terms such as ‘ten times more accurate’.  Ten times more accurate of what versus what?  An automatically controlled heavy mechanical system versus a human lining up a star with the horizon – unlikely.  A mathematically continuously averaged reading versus a single sextant observation in rough weather – possibly.  Similarly, when quoting machining accuracies of less than one thousandth of an inch, is that a target as specified in the initial operational requirement, or is it what was finally obtained?  Was this an absolute figure, or was it a repeatability figure between matching items such as waveguide? 

    So yes, radio sextants might once have been considered worth developing for certain limited applications in the 1950s to 70s, but they seem to have been overtaken by events, notably the arrival of GNSS.  Also, treat journalist generated articles based largely on commercial press releases from the past with a certain amount of caution.  DaveP

       
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