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    Some sites for ephemerides (Modified by Fred Hebard)
    From: Fred Hebard
    Date: 2004 Mar 11, 13:47 -0500

    I saw some posts of links that might be of interest to the list.  Two
    follow.  The second is a longer discussion of space navigation that
    might be of interest.  In regard to space navigation, as best as I can
    tell, they often plot positions on an earth-centered coordinate system,
    much as we do, and get distance from the center of the earth by
    ranging, such as timing the round trip of radio signals.
    
    Fred
    
    ONE
    
    While we're seeing a burst of astronomical OS X packages getting a
    mention here, I thought I'd add that the venerable astronomical
    data reduction system, Figaro, was ported to OS X a while back.
    (If anyone is really interested, it's on the AAO's anonymous ftp
    site, ftp.aao.gov.au/pub/figaro/4.5.1.figaro.tar.gz or
    4.5.1.macosx.exe.tar.gz for a pre-built version for Panther.)
    
    
    TWO
    
    >>> On March 10, around 12 pm, EST, Nathan Strange wrote:
    
    >>> I noticed a couple of posts on the list by astronomers, and I thought
    >>> I'd mention JPL's SPICE library which can be found at:
    >>>
    >>> ftp://naif.jpl.nasa.gov
    >>>
    >>> in:
    >>>
    >>> /pub/naif/toolkit
    >>>
    >>> and:
    >>>
    >>> /pub/naif/toolkit_docs
    >>>
    >>> This library is now widely used in NASA, ESA, and I think NASDA too.
    >>>
    >>> Not only does it let you read in SPK ephemeris files for the planets
    >>>  and moons, but it has a lot of handy functions for transforming
    >>> orbit
    >>>  elements, calculation occultations, etc. And that same FTP site has
    >>> spacecraft
    >>>  ephemeris files that you can use to animate stuff like Cassini's
    >>> arrival
    >>>  at Saturn :)
    >>>
    >>>
    >>> You can do all of the calculations done on the horizons web page
    >>> , and more stuff too.
    >>>
    >>>
    >>> As far as I know this is the most complete open-source astrodynamics
    >>> library available....  but there's a few things I'd like to add
    >>> like a lambert solver :)
    >>>
    >>>
    >>> I've used the fortran version of the library on both linux and os x
    >>> to solve some fun problems.
    >>>
    >>> Anyway, that's my plug for the library... I hope some of you find it
    >>> useful...
    >>>  (Any NAIF programmers on the list feel free to correct any errors
    >>>  in this message)
    >>>
    >>>
    >>
    >
    >
    
    Nathan Strange
    Cassini Navigation
    Jet Propulsion Laboratory
    
    (The opinions expressed in this email are the opinions
    of Nathan Strange alone and do not necessarily represent
    the official position of any organization including JPL,
    NASA, and CalTech.)
    
    >> On Mar 10, 2004, at 5:12 PM, Fred Hebard wrote:
    >>
    >>> Nathan,
    >>>
    >>> Would you mind if I copied your post below to a navigation list of
    >>> which I'm a member (Nav-L).  I was going to do it without
    >>> attribution to protect your privacy, but then thought it better to
    >>> ask!
    >>>
    >>> Thanks,
    >>>
    >>> Fred
    
    > On Mar 11, 2004, at 11:30 AM, Nathan Strange wrote:
    >
    >> No problem, feel free to forward it with my name if you'd like.
    >>
    >> Is it a sea-faring navigation list?  The library was primarily
    >> designed for space-faring
    >> navigation.  It would be neat if it was also useful for sea-faring
    >> navigation.
    >>
    >> -Nathan Strange
    >>
    >>
    
    On Mar 11, 2004, at 9:35 AM, Fred Hebard wrote:
    
    > Hi Nathan,
    >
    > Thanks.  It's a sea-faring or air-faring navigation list, devoted to
    > traditional celestial navigation.  I have no idea how precise you have
    > to get for space-faring, but 0.1' of arc is plenty accurate with
    > ephemerides used at sea with hand-held sextants; in small yachts and
    > airplanes, 1.0' of arc is fine.  With GPS, celestial navigation is a
    > dying art, but the Navy, Air Force and Merchant Marine still use
    > celestial.  The Armed Forces like to have a non-emissive system of
    > navigation; I believe all the Air Force systems are automated, but the
    > Navy still uses hand-held sextants, as well as automated.  The
    > automated systems can be quite accurate, from what I understand.
    >
    > There are a number of computer systems out there for calculating
    > ephemerides and position at sea.  The Naval Observatory and the
    > British put out a package, and they also still put out printed
    > versions of the Air and Nautical Almanacs.  I just thought the list
    > might be interested in what you had.
    >
    
    Sounds like some fun stuff!  My boss, the Cassini Nav Chief, is a fan
    of Bowditch's, 'The American Practical Navigator'
    and recently found an old edition.   When I get the time, I want to
    take a celestial navigation class :)
    
    JPL is the keeper of the most accurate ephemerides in the world.  The
    most current right
    now is de405 which is on that ftp site in my email.  de405s is a
    smaller file that doesn't go as far into the future.
    
    The horizons website would be the easiest way to access the ephemeris
    data, and I think it would give you the
    information that you need for celestial navigation.
    
    
    > Some dumb questions.  Do you all use an earth-centered system of
    > coordinates for near-earth operations?  If so, do you try to determine
    > your distance from the earth using celestial observations, and how?
    > Thanks again,
    >
    > Fred
    >
    >
    
    I don't know much about near Earth navigation, but I can tell you about
    how we do deep space...
    We use a lot of coordinate systems, but mostly we use EME200
    (Earth-Mean-Equator of 2000) because
    it's the most standardized. We use a lot of different centers, so we
    always quote distances with
    stating what central body was used, and often what ephemeris was used
    to locate the central body.
    We use kilometers-kg-seconds units, but the rest of US aerospace
    industry uses feet-pound mass-seconds
    which is what caused the famous Mars climate orbiter mishap.
    
    There's two parts to the navigation.  One part is generating the
    ephemerides to predict the locations of the
    planets and moons.  The other part is solving for the spacecraft
    location relative to the Earth.
    
    The big dish antennas in the California desert, at Madrid, and Canberra
    track the spacecraft.  We get
    angular data from the antennas as to where they found the spacecraft
    signal.  But because of the distances
    involved angular data isn't accurate enough.  We also use what's called
    range data which is basically timing a
    pulse sent from the Earth that reflected from the spacecraft back to
    Earth.  Using the speed of light
    this gives a very good distance measure.  They also measure the doppler
    shift of the signal from
    the spacecraft to get a velocity measurement.  There's another type of
    data that I don't understand
    called Delta-DOR... this is something involving using two ground
    stations and a quasar for a frequency measurement
    to get better doppler data.
    
    All of this data is Earth based.... I guess this would be like the
    control tower tracking an airplane
    and radioing its position back.  But there are two more types of data
    that use measurements from the spacecraft.
    Just recently there have been enough spacecraft at Mars that we've been
    able to use radio tracking between
    spacecraft.  I think the mars rover landings were the first ones to do
    this.   More common is what's
    called optical navigation, and this is probably the closest analogue to
    using a sextant.  For OpNavs the spacecraft
    takes pictures of moons or planets and we use the location of the
    objects in the picture (i.e. which pixel is the
    center of the object) and the locations of objects relative to each
    other.  OpNavs give us information on the
    spacecraft location and on the ephemerides for the moons and planets.
    
    And finally there are observations from astronomers around the world
    and with Hubble that are used
    to improve the ephemerides.
    
    All of this data is collected a weighted and combined with models of
    the forces on a spacecraft, and
    of the gravitational fields from the planets and moons, etc...  and its
    all crunched through
    a program using a Kalman filter...  a Kalman filter is similar to a
    least squares curve fit,
    but it lets you weight some data as more precise than others and let
    you add a model of the dynamics.
    
    I think GPS devises use a Kalman filter, and I think Dennis Tito made
    his fortune by applying
    a Kalman filter to the stock market.
    
    Sorry if this explanation is too involved,  JPL has a web page that
    explains a lot of how we navigate:
       It might be easier to follow than my
    ramblings.
    
    I think for near Earth operations they mostly just use angular data and
    doppler... but I'm not sure.
    
    -Nathan
    
    
    

       
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