[ARC5] Old and new...

Wed Aug 9 20:48:22 EDT 2017

What about Differential GPS?  Besides screwing up a portion of the low frequency beacon band, is anyone using it?  Is it doing anyone any good?
Mike, W6MAB
  ----- Original Message ----- 
  From: Bob kb8tq 
  To: Bruce Long 
  Cc: arc5 at mailman.qth.net 
  Sent: Wednesday, August 09, 2017 5:08 PM
  Subject: Re: [ARC5] Old and new...

  Hi

  “Back in the day” GPS had SA (selective availability) turned on. That degraded the accuracy quite a bit. Since May of 2000, it’s been shut off. After that signal performance went up quite a bit. Because of the accuracy degradation, some GPS’s designed before 2000 were not quite as good as they might have been. With modern L1 gear, you can get *very* close ( < 9 feet) to a prior location > 90% of the time. With L1/L2 you can do a *lot* better than that ( < 9 inches ….). 

  Bob

    On Aug 9, 2017, at 6:28 PM, Bruce Long via ARC5 <arc5 at mailman.qth.net> wrote:

    Before wife and kids and demanding job happened i did a lot of SCUBA wreck diving, most of it offshore Delaware and Maryland.   LORAN was consistently better, for finding wrecks than GPS.  Dockside we could see 10 ft differences in the location of the boat and offshore we seemed to be getting about that good as well.

    For example we often dove the wreck of the Moonstone, a 110ft long private yacht pressed into service as a convoy escort.  One foggy Sept night she was chasing a sonar echo thinking it was a submarine when it actually was a bottom bounce echo from a freighter.  There was a collisions and the Moonstone almost cut in half by the impact sank in 30 seconds finally coming to rest upright on the bottom at a depth of 130ft.

    We had three sets of LORAN coordinates for this wreck, bow midships and stern.  We usually used the bow coordinates.  We would bring the dive boat the the LORAN coordinate as close as possible, then move about 50 feet up wind/current, drop the anchor and let the wind/current drag the anchor into the bow of the wreck.   We very often hooked either the 3inch deck gun or the adjacent stanchions.

    With GPS we could easily find the wreck ( as indicated on our video depth sounder) but could not pick and choose a prefered section of the wreck and could not consistently hook the deck gun, our preferred anchoring spot.

    I had been told that under some conditions LORAN has better repeat ability than GPS but the accuracy is not as good.   The reason for this is LORAN uses ground wave propagation and the velocity of propagation depends upon the ground resistivity.  For locations where the ground wave path is mostly sea water the repeatability can be very good.

    This is not to saying LORAN can always substitute for GPS but for a lot of positioning applications LORAN can be quite good.

----------------------------------------------------------------------------
    From: Joe Connor via ARC5 <arc5 at mailman.qth.net>
    To: "WA5CAB at cs.com" <WA5CAB at cs.com>; "arc5 at mailman.qth.net" <arc5 at mailman.qth.net> 
    Sent: Wednesday, August 9, 2017 8:57 AM
    Subject: Re: [ARC5] Old and new...

    Going over certain types of metal bridges does the same thing, Robert. My wife and I experienced that recently going form northern New Jersey into Staten Island. The GPS went completely out of whack when we were on the bridge both going and coming back. 

    Joe Connor

    On Tuesday, August 8, 2017 11:23 PM, WA5CAB--- via ARC5 <arc5 at mailman.qth.net> wrote:

    I can't comment on the performance of LF Loran but I can confirm that GPS goes off line when under something as insubstantial as a bank drive-through.  Proved it again this afternoon.  As soon as I pulled up to the pneumatic tube, The display in my Land Rover stopped telling me that I was just East of Gessner and started telling me that I was in Houston.  

    The problem with LORAN is that I can't see how it could give a fix precise enough for conventional ordnance.

    Robert Downs - Houston
    wa5cab dot com (Web Store)
    MVPA 9480

    In a message dated 08/08/2017 15:33:07 PM Central Daylight Time, kv6lee at gmail.com writes: 

      The new LORAN at 100 KHz can penetrate into buildings and basements in which GPS is useless - or so I'm told.  The new LORAN should provide a good fix, and enhance Position-Navigation-Time (PNT) capabilities in challenging environments.

      73 de Bart, K6VK ##

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