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    Re: Longines A-7 Avigation Hack Watch
    From: Paul Hirose
    Date: 2017 Sep 21, 22:41 -0700

    In marine navigation a "hack watch" was used for observations topside so
    the ship's chronometers could remain undisturbed below deck. There was
    no requirement that the watch be settable to the second. It was
    sufficient to record how much the hack watch was fast or slow on the
    chronometers.
    
    Shadwell ("Notes on the management of chronometers," 1861) uses the term
    "assistant-watch":
    "The practice, therefore, of taking a chronometer on deck or on shore,
    for the purpose of observation, should never be resorted to; a
    pocket-chronometer, or a good pocket-watch with a second-hand, should on
    these occasions be used as an assistant."
    https://books.google.com/books?id=1h0PAAAAYAAJ&pg=PA36&focus=viewport&dq=editions:vxpymHQeOAYC
    
    Lecky ("Wrinkles in Practical Navigation," 1884):
    "It is well to have a fourth chronometer in the second officer's room,
    as a hack-watch for general use among the officers."
    https://books.google.com/books?id=BzIAAAAAQAAJ&pg=PA26&focus=viewport
    
    Martin ("A Treatise on Navigation and Nautical Astronomy," 1899):
    "The times of making all observations are noted by a smaller portable
    chronometer termed a hack watch, which is usually constructed to beat 5
    times in 2 seconds (box chronometers generally beat half-seconds), and
    which, being more liable to change from being carried about than the box
    chronometers, is compared with their indications at as near the time of
    observation as possible."
    https://books.google.com/books?id=NP0nAAAAYAAJ&pg=PA278&focus=viewport
    
    Bowditch ("American Practical Navigator," 1914):
    "To avoid derangement, the chronometers should never be removed from the
    permanent box in which they are kept on shipboard. When it is desired to
    mark a certain instant of time, as for an astronomical observation or
    for obtaining the chronometer error by signal, the time is marked by a
    hack (an inferior chronometer used for this purpose only), or by a
    comparing watch."
    https://books.google.com/books?id=FvAqAAAAYAAJ&jtp=100
    
    Muir ("A Treatise on Navigation and Nautical Astronomy," 1918):
    "It has already been said that the chronometers should not be subject
    even to occasional removal. This is true, and in order to get the
    chronometer times of certain desired instants, as the instant of receipt
    of the noon signal, or the drop of a time ball, etc., use must be made
    of a less valuable time piece known as the hack, an inferior grade of
    chronometer, or of a comparing watch."
    https://books.google.com/books?id=h6xEAAAAIAAJ&pg=PA337&focus=viewport
    
    A modern equivalent of a hack watch is the ATTU (airborne time transfer
    unit) of the B-2 bomber. Its AINS (astro inertial navigation system) has
    an internal chronometer, but before flight it must be synchronized with
    UTC. The human eye, ear, and hand are not accurate enough. It would be
    inconvenient to wheel a rack mounted time standard out to the plane
    before each flight. Possibly the rubidium oscillator would not enjoy the
    disturbance either.
    
    The solution is to transfer the time via an ATTU. You install three 9v
    batteries, connect it to the time standard, and by manipulating a few
    switches, "hack" the ATTU. It is then carried to the B-2 and installed
    before engine start. When avionics are powered up, the AINS obtains UTC
    from the ATTU. The latter is not needed for the rest of the flight.
    
    Incidentally, the AINS has a catalog of 61 stars. I don't know their
    identities. Nor did I ever find time to inquire about how the AINS knows
    the current value of UT1-UTC. I believe it's included in the mission
    data — the waypoints, targets, etc. that are the objectives of the
    flight — which are loaded into the flight management system.
    
    

       
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