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From: George Neuner on 18 May 2010 00:18 On Mon, 17 May 2010 14:32:03 -0700, D Yuniskis <not.going.to.be(a)seen.com> wrote: >IanM wrote: > >> shallow water pass over a modern era wreck that isn't a danger to >> surface navigation, there is usually enough steel around to cause a >> significant amount of compass deviation. I've had a complete 360 deg. >> turn caused by such a wreck and innumerable S wiggles. With a little >> local knowledge you soon learn to avoid the more troublesome ones. > >Ah, that's possible! I've seen significant compass deviation passing under bridges or over tunnels, pipes, electrical cables and even over shallow net anchorages (steel nets placed to snag anchors where the bottom is too soft to hold). >It is also possible that the anomalies >in the recorded track happened when we were in "manual" >control (fishing for Blues). It could also have been an >anomaly in the LORAN receiver. <shrug> > >It's been 30+ years. I haven't heard of any *deaths* so I >don't lose too much sleep over it! :> (though I really would >have liked a resolution "back then") My Koden unit (forget which model but it was the high end one at the time) had 3 precision modes - trading accuracy for speed - but I almost always used it at highest precision because there are a lot of reefs and shoals in my area. I found that in the fast mode, the unit considered a waypoint to be roughly a 300ft circle. I had to be able to steer compass courses through channels as narrow as 30ft (lotsa fun at night in pea soup fog). At high precision my unit could repeatedly find a station set waypoint within 5ft up to about 10kt. If your unit had different precision modes and the mode was changed unexpectedly that might have affected your plot. George
From: Walter Banks on 18 May 2010 09:33 D Yuniskis wrote: > Hi Ian, > > IanM wrote: > > D Yuniskis wrote: > >> <grin> I am haunted by an autopilot (marine) I designed some > >> 30+ years ago. After returning from our test run, an examination > >> of the actual course taken showed an "S" in the plot at one > >> particular place. Did my software "divide by zero" (or > >> something similar)? Or, was this the spot where we stopped > >> to fish (which requires constantly readjusting the boat's > >> direction to keep it pointed into the swells)? > >> > >> My boss wasn't worried about it (since the rest of the > >> trip -- I think 7 legs? -- went uneventfully) but the image > >> of that "S" is burned into my memory... :-/ > > > > Not uncommon if your autopilot uses a fluxgate or other magnetic compass > > sensor as its primary heading reference. If you are in relatively > > Onto this (*my* "claim to fame") was a software servo loop > that took LORAN-C coordinates of "destination" and kept > tweeking the "motor drive" to update the "new" course > (i.e., instead of conventional autopilot that seeks to > maintain a constant heading, my goal was to reach a desired > *destination*) Fly or float pointed directly to the destination has a problem (Not a big one but a significant one) Current or wind drift forces the craft off track and the control system then corrects by turning to the destination that is good, however it is not the shortest track to the destination. It results in a track shaped like a "?". The primary reason that VOR's transmitted direction information as well as fixing a position was be able to fly a direct track. The earlier NDB's (non directional beacons) would lead to longer tracks if the drift was not corrected. Compass errors are interesting. In my poor starving student days I flew a summer in the arctic and encountered two really nasty compass errors almost daily. In Labrador there is a iron vein a mile or so wide that that runs a couple hundred miles pure havoc. The second one is compass errors near the north magnetic pole. Up to several degrees of error per flight hour. Regards, Walter.. -- Walter Banks Byte Craft Limited http://www.bytecraft.com
From: Tauno Voipio on 18 May 2010 11:43 On 18.5.10 4:33 , Walter Banks wrote: > > Fly or float pointed directly to the destination has a problem > (Not a big one but a significant one) > > Current or wind drift forces the craft off track and the control > system then corrects by turning to the destination that is good, > however it is not the shortest track to the destination. It results > in a track shaped like a "?". This is often called a dog curve, as a dog running toward his moving master will trace such a curve. > The primary reason that VOR's transmitted direction information > as well as fixing a position was be able to fly a direct track. The > earlier NDB's (non directional beacons) would lead to longer > tracks if the drift was not corrected. There is another good reason: The radials of a VOR are not dependent on the alignment of the aircraft compass system. Flying a defined direction (QDM) toward a NDB needs a correct reference direction from the compass system. It is possible to fly straight using a NDB, but it is more difficult than following a VOR radial. Also, a VOR display responds quicker the an ADF (NDB receiver): -- Tauno Voipio, (MSEE avionics and CFII) tauno voipio (at) iki fi
From: Walter Banks on 18 May 2010 15:01 Tauno Voipio wrote: > > The primary reason that VOR's transmitted direction information > > as well as fixing a position was be able to fly a direct track. The > > earlier NDB's (non directional beacons) would lead to longer > > tracks if the drift was not corrected. > > There is another good reason: The radials of a VOR are not > dependent on the alignment of the aircraft compass system. > Flying a defined direction (QDM) toward a NDB needs a > correct reference direction from the compass system. Not to mention that flying outbound from a VOR is a lot easier than from a NDB. When I was flying in the arctic the only navaids available were low powered NDB's located just a little too far apart. We got good at nailing an outbound heading. w..
From: D Yuniskis on 18 May 2010 16:12
Hi George, George Neuner wrote: > On Mon, 17 May 2010 09:52:20 -0700, D Yuniskis > <not.going.to.be(a)seen.com> wrote: >> Is there an easy/high-speed way to verify prerecorded >> media is "playable"? E.g., discs that see lots of >> circulation (e.g., "Blockbusters", public library, etc) >> that need to be verified as "undamaged" before being >> reintroduced into circulation? > > There's no way for you to check the integrity of the stamped aluminum > cookie other than to try to play it ... on the production line where Why is that? Can't you just "read" it in a DVD-R (or whatever)? Verify no read errors, etc.? > the orientation of the disc is fixed, checking of the recording is > using 2-dimensional laser imaging. No doubt because that is faster than "reading" the medium... > However, checking for scratches in the plastic coating can be done > optically. You need high resolution and low-angle offset lighting. An > undamaged disc appears as "near-black" to the camera - scratches in > the coating reflect more light into the camera. Ha! No need to check for scratches: Scratches? Yes or Hell Yes! > Probably you were looking for an answer like: "stick it in the player > and run this <obscure> software" ... Sorry. Yeah, something the functional equivalent of "playing it and 'watching' it" -- except at high speed. (e.g., a library probably loans thousands of DVDs daily. It's just not practical to "watch" every one as it is returned... |