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    Re: Polar Grid AirNav and Oceanic AirNav
    From: Antoine Couëtte
    Date: 2017 Dec 30, 01:51 -0800

    RE : Oceanic-AirNav-LaPook-dec-2017-g41072

    Dear Gary,

    Many Thanks to you for your nice and very interesting complement and references about Astro-compasses and Sky-Compasses used to check the accuracy of Gyro compasses. Yes! These were extremely useful especially in areas where it was not possible to get any reliable heading checks from the Magnetic Sensors. In vast areas of Northern Canada the Earth Magnetic Field "dives into the ground" and its horizontal component is insufficent to make any meaningful use of a Fluxgate.

    The initial post Polar-Grid-AirNav-Oceanic-AirNav-Couëtte-dec-2017-g41064 adressed Celestial Air Navigation only very briefly (as the Safety net of all navigations systems). Much more about Celestial Air Navigation could and should have been said since it deserves full chapters just on its own. Hence the special interest of your contribution.

    This initial post was rather an opportunity to address Gyroscopic Navigation in a bit of detail because it is so new to a number of us within the NavList audience.


    A complementary note to all about about Gyrocompasses (Heading Gyroscopes) which essentially worked under 2 different modes :

    • The Magnetic Heading "Slave Mode" in which they were to follow and damp out the Magnetic Heading inputs from the Magnetic Fluxgate. And :
    • All other modes where they was no real time and constant "copy-paste" external heading inputs but only constant preset precession torques. These were called "Free Modes" . Such is the "DG mode" - Directional Gyro [Free] Mode - mentioned by David in Solstice-Sun-Lines-DavidPike-dec-2017-g41027 which was the "Great Circle Following" mode. There were other various Free Modes - theoretically infinitely many - depending on the precession torque rates "fed into" the gyroscope mechanics. As an example "15° / Hour sin Latitude" - the "DG mode" - gave you a "Great Circle following compass" if you flew a constant heading in still air. A different precession torque injected into the Gyro Torquer would let you follow a Rhumb Line, another rate would give you a "Constant Bearing onto the arrival point", und so weiter, provided that you started your new Navigation Leg at the right precomputed heading to be kept constant throughout your leg

    Air Navigation between Departure and Arrival Points required new computations for each leg. As earlier said, the initial and constant "Gyro heading" (that was the interest for it) which you were to keep throughout your leg depended on which curve you intended to follow between Arrival and Departure (Great Circle, Rhumb Line, Constant bearing line, Rum line :-)  ... For all such different curves the initial departure headings were all different, nonetheless for all such curves the mid-distance tracks were parallel and equal to the Rhumbline [geographical constant] course between Departure and Arrival points. This nice feature was used into the initial heading calculation. Of course ... this was true only on paper and only if and when flying in still air with "perfect" instruments. Therefore Weather permitting, Celestial Airnav was then constantly here to help you make timely corrections.

    On the Air Navigation Charts, the pre-computed and most frequently used Great Circles showed up as a succession of straight lines. But if you were intending to fly a series of Rhumb Lines or Constant Bearing Curves (these were symmetric to Rhumb Lines on the Charts) at high latitudes, then your intended tracks on ground showed as a succession of "festoons" on the Air Navigation Charts.


    Another very important Navigation Tool unortunately left aside in the initial list : Weathers Radars. These Weather Radars have been introduced in Airliners in the early fifties and they detect water drops in the air and depict the dangerous clouds and so dreaded thunderstorms ahead of you. They have also been successfully used on the onset for Air Navigation since in "Map mode" they can feature coastlines at ranges of up to a few hundred Nautical Miles. Weather Radars are always in use and they have become so important that it is mandatory to carry 2 such equipment on board. It is forbidden to take off without at least one of them in good operating condition. They now have amazing multicolor displays to help you avoiding bad weather, essentially Cumulonimbus and Thunderstorms but they also let see quite distinctly Cities, Rivers, Mountains, and of course Shorelines. Unfortunately we do not use Weather Radars very often for ground navigation nowadays, and maybe we should use them more frequently as independent checks especially when reaching shore lines coming from the Oceans.

    If this were also an opportunity to add one last item to our list of the earlier available tools to solve these huge [Air Navigation] challenges, that would be the Doppler Radar which indicated both ground speed and drift. Unfortunately it could be quite unreliable over the oceans because of swells and waves, and it sometimes did not work well above over icy ground (insufficient returns) or uneven ground (multi returns) . However this quite promising system which came to play maybe a little late in the game and was still undergoing nice improvements did not get the fully bright future initially expected. No longer doppler radars in to-day Long Range Airliners.

    There were also other early Air Navigation tools, - maybe not onboard all long haul aircraft - and the initial list cannot mention them all. This is why contributions like yours, Gary, are so welcome, thanks again.

    I am also making one additional and final remarks about "Barometric Navigation".

    • First of all, almost only over the Oceans could Radar Altimeters be used for BaroNav since these are by nature a Zero Altitude reference surface. If Radar Altimeters could not be used at all over uneven lands for BaroNav, they always did remain and have remained excellent and mandatory height detectors and height alerting devices under all circumstances.
      • One typo in my initial post : Radar Altitude should have been listed as Zv (Altitude Vraie) - and not Zp which is the Pressure Altitude - so that the Bellamy's Correction Factor : D = (Zv - Zp) can be better understood.
    • And of sourse, such BaroNav has totally disappeared from the cockpits nowadays.

    Hybrid InertialSatNav does it all automatically for us with amazing accuracy ... as long as we have SatNav available.

    • Even without SatNav you can fly for up to 6 or 7 hours or even 8 hours over the Oceans without significant Navigation Performance degradation, which is generally quite enough to reach familiar lands and start automatic updates ground based Navaids.
      • Airbus 330/340 - and Boeing Aircarft should be [almost] the same, depending on their IRS's - certified Minima over the Oceans without any further external radio update are 6.2 hours for IRS (our 3 independent Inertial Laser Platforms) from alignment on ground, or 5.7 hours after last SatNav update.
        • If you have no choice but continue flying over remote oceans chances are that - even after 8 hours of no external IRS update as it once happened to me - visual or radar shoreline updates will show you that you are still on track to within just a very few Nautical Miles laterally.

    Thanks again to you Gary, and Happy New Year 2018 to all,

    Antoine Couëtte 

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