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    Re: How likely is a GPS shutdown?
    From: Richard B. Langley
    Date: 2000 Jan 26, 6:42 AM

    The current SPS error budget assigns an SA error of 24 metres r.m.s to the
    total user-equivalent range error (UERE).  Without SA, the UERE could drop to
    about 8 metres and would be dominated by ionosphere and multipath effects.
    With an HDOP of 2 (a bit high for a 28+ satellite constellation), this would
    lead to a 32 metre horizontal 2 d.r.m.s. (95% probability) positioning error.
    At times, this error could easily be half as much and even possibly under 10
    metres.
    -- Richard Langley
       Professor of Geodesy and Precision Navigation
    
    On Tue, 25 Jan 2000, Craig wrote:
    
    >What accuracy might we expect with SA disabled?
    >
    >Craig
    >Columbia, SC  2000.01.25 2245 EST
    >
    >-----Original Message-----
    >From- Navigation Mailing List
    >[mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Richard Langley
    >Sent: Tuesday, January 25, 2000 22:38
    >To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    >Subject: Re: How likely is a GPS shutdown?
    >
    >
    >On Tue, 25 Jan 2000, Tony wrote:
    >
    >>Richard:
    >>
    >>Thanks you. I believe I read several years ago that DOD also has the
    >>capability of dithering the time base so that SA accuracy can be degraded
    >>further if needed. Is this true?  TIA
    >
    >Yes, DoD has the ability to adjust the SA degradation to even higher
    >levels.  But SA will be switched off by 2006 under presidential directive.
    >The SA security measure will be replaced by in-theatre jamming.
    >-- Richard Langley
    >   Professor of Geodesy and Precision Navigation
    >
    >>Tony
    >>
    >>Richard Langley wrote:
    >>>
    >>> PPS uses the encrypted P-code on both the L1 (same as civilian SPS) and
    >the L2
    >>> carrier. However, there are DoD single-frequency (L1) receivers and these
    >>> also have SA and A/S decryption capability.
    >>> -- Richard Langley
    >>>    Professor of Geodesy and Precision Navigation
    >>>
    >>> On Tue, 25 Jan 2000, Rodney Myrvaagnes wrote:
    >>>
    >>> >This is not quite right, although the rest of RBE's post is
    >>> >right on. The military receivers that can get the encrypted
    >>> >(long) word get it over another carrier than that used by
    >>> >the C/A word decoded by civil receivers. There is no way
    >>> >for your Walmart gps to use the military code.
    >>> >
    >>> >
    >>> >During the Gulf War, SA was turned off because the military
    >>> >had to use civilian receivers. SA was an afterthought,
    >>> >applied because the civil receivers turned out to work much
    >>> >better than expected.
    >>> >
    >>> >On Mon, 24 Jan 2000 19:09:42 -0500, Richard B. Emerson
    >>> >wrote:
    >>> >
    >>> >> >
    >>> >> > I suspect that the military has its own encrypted version
    >>> >> > of GPS although this is simply a guess on my part.
    >>> >>
    >>> >>One of the absurdities of GPS is that during Desert Storm and Panama,
    >>> >>GPS' selective availability was turned off.  Thus, for a short period,
    >>> >>the normally encrypted data segment (it's already there and used
    >>> >>routinely) was available unencrypted.  Consider, too, the error even
    >>> >>with S/A enabled.  A fuel-air bomb (or ammonium nitrate bomb) could
    >>> >
    >>> >
    >>> >
    >>> >Rodney Myrvaagnes                   J36 Gjoa
    >>> >Senior Editor           Electronic Products
    >>> >My oyster knife is Y2K compliant
    >>> >
    >>>
    >>>
    >============================================================================
    >===
    >>>  Richard B. Langley                            E-mail: lang---.ca
    >>>  Geodetic Research Laboratory                  Web:
    >http://www.unb.ca/GGE/
    >>>  Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
    >>>  University of New Brunswick                   Fax:      +1 506 453-4943
    >>>  Fredericton, N.B., Canada  E3B 5A3
    >>>      Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    >>>
    >============================================================================
    >===
    >>
    >
    >
    >============================================================================
    >===
    > Richard B. Langley                            E-mail: lang---.ca
    > Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
    > Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
    > University of New Brunswick                   Fax:      +1 506 453-4943
    > Fredericton, N.B., Canada  E3B 5A3
    >     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    >============================================================================
    >===
    >
    
    
    ===============================================================================
     Richard B. Langley                            E-mail: lang---.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
    ===============================================================================
    

       
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