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    Re: Refraction
    From: Fred Hebard
    Date: 2005 Aug 4, 08:56 -0400

    Marcel,
    
    You can download copies of the Air Almanac from
    
    
    Fred
    
    On Aug 4, 2005, at 8:40 AM, Marcel E. Tschudin wrote:
    
    > Hello Robert
    >
    > Thank you for your fast reply. Unfortunately I am not in possession of
    > the Air Almanac; in addition I am for about an other year abroad in a
    > country where I am not able to order copies from a library. May I
    > therefore ask you, whether it would be possible for you to derive from
    > the table you indicated one or the other benchmark value? As you
    > mentioned this would at least give a clue. Thanks a lot!
    >
    >
    > Like you I am still hoping that there is some one out there who really
    > can provide a solution to this problem.
    >
    > Marcel
    >> ----- Original Message -----
    >> From: Robert Eno
    >> To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    >> Sent: Saturday, September 03, 2005 2:37 PM
    >> Subject: Re: Refraction
    >>
    >> Hello Marcel
    >>
    >> Refraction error makes the observed body appear to be higher than it
    >> actually is, ergo the same correction rules for postitive
    >> altitudes would apply to negative altitudes; only in this case the
    >> negative numbers would be added.  If, for example, your sextant
    >> altitude was minus 5 degrees, and the refraction correction was (I am
    >> just pulling a random number out of my head) minus 60 minutes
    >> (refraction correction is always subtracted from the sextant
    >> altitude), then your corrected altitude would be minus 6 degrees.
    >>
    >> Near as I can reckon, you would only experience negative altitudes
    >> from an aircraft flying at high altitudes or in the polar regions in
    >> winter.
    >>
    >> Have a look at Table # 6, HO 249, Sight Reduction Tables for Air
    >> Navigation and the correction tables at the back of the Air Almanac.
    >> These give you a bit of a clue as to what to expect. Other than these
    >> tables, I know of no other tables which provide refraction
    >> corrections for negative altitudes for observers at sea level.  If
    >> anyone out there is aware of such tables, do let me know. It is a
    >> question I have not pondered for some time now.
    >>
    >> Robert
    >>> ----- Original Message -----
    >>> From: Marcel E. Tschudin
    >>> To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    >>> Sent: Thursday, August 04, 2005 6:48 AM
    >>> Subject: Refraction
    >>>
    >>> Hello
    >>>
    >>> While searching with Google, I came across this mail list. May be
    >>> some one out here may be able to help me answering the following
    >>> question:
    >>>
    >>> How do refraction values for negative elevations have to be
    >>> calculated, such as e.g. the horizon from a plane? (I am interested
    >>> in the range of 0? to approx. ?5?.)
    >>>
    >>> Is Bennett?s approximation also valid for negative elevations? If
    >>> not, what other approximation formulae should be used, or, where can
    >>> one find some benchmark values?
    >>>
    >>> I am interested in formulae for both, refraction from apparent
    >>> position and from physical position.
    >>>
    >>> Thank you for any input to my problem.
    >>>
    >>> Marcel
    
    
    

       
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