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    Polar Grid AirNav and Oceanic AirNav
    From: Antoine Couëtte
    Date: 2017 Dec 28, 16:42 -0800

    Dear All,

    A few recent remarks from various Contributors have prompted me into writing this new post. Here are these remarks : 

    • 1 - Solstice-Sun-Lines-Shrier-dec-2017-g41010 where Tracy triggered our renewed interest for Celestial AirNav. And :
    • 2 - Solstice-Sun-Lines-DavidPike-dec-2017-g41027 where David said QUOTE a.  We always used DG mode while observing. UNQUOTE. And :
    • 3 - Coriolis-Acceleration-general-case-DavidPike-dec-2017-g41038 where David introduced a post specifically dedicated to Coriolis Acceleration which has a direct and immediate effect onto Local vertical in an Aircraft, hence to be accounted and corrected for in Celestial AirNav. And :
    • 4 - History-Air-Navigation-FrankReed-dec-2017-g41047 where Frank indicated QUOTE   But there's certainly room here for an extended chapter or three in a more general "History of Celestial Navigation". By the way, I decided to spin this off under a new subject title since the topic of "air navigation" continues to grow in popularity, and the history of it deserves more "front page" discussion.  UNQUOTE . And finally
    • 5 - HeadingSpeed-Acceleration-Error-Card-LaPook-dec-2017-g41054 in which Gary gives us the downloading reference of an excellent AirNav Course Document. Thanks to you again Gary !

    Hence very clearly we are getting a full go-ahead on this subject of Air Navigation, which is becoming a "hot topic" ...

    *******

    I am then taking this opportunity to raise the attention of my Sea Navigator Fellows about the "Impressive Achievements" Long Range Air Navigation (AirNav) had reached by the end of the 60's, i.e. just before the introduction of the Inertial Navigation Systems coupled with long range worlwide OMEGA Navigation, not to mention even SatNav.

    By the end of WW2, some of the main Navigation challenges for long range Aircraft were essentially:

    • Flying over the [North] Polar areas which were to bring huge savings in distance between some North Hemisphere cities (e.g. Paris - Tokyo). And :
    • Flying in very isolated areas - whether over inhabited lands or over immense oceans - i.e. far away from any short range "accurate" [for that epoch] ground based navigation systems. As well as :
    • Flying in immense areas where the weather forecast were very weak if not inexistent. Note : Except for the case of wind drift determination I will almost set aside here-after the immense saga of the Worldwide Weather Service development as it is not immediately related to our strict AirNav topic..

    The only available tools to solve these huge challenges were :

    • Barometric sensors, to give both Altitude and indicated Airspeed
    • "High range" Radio Altimeters calibrated up to 25.000 ' / 30 000' (Nowadays no need for more than 2,500' or 3,000' on either Airbus or Boeing Aircraft),
    • Air Sextants,
    • Specific AirNavigation charts,
    • Superior quality Gyrocompasses, and
    • Very powerful HF Radio Beacons mainly next to the Oceans,
    • Outstanding Earth Magnetic Field Detectors, and most importantly :
    • Lots of inventive ideas, coupled with an in-depth mathematical understanding of the main physical effects encountered, such as the Coriolis Acceleration effect on local vertical deviation (pretty simple but almost unknown in our day to day life), or the very complex - and totally unexpected - Gyroscope physics, or the totally unusal also and unexpected physics of the Magnetic Fluxgates. 

    Simply stated, the Key answers to solve these AirNav challenges were :

    • Introducing the right kind of charts : Like they Seamen Colleagues, Air Navigators definitely want Charts respecting angles (Conformity) but they also require them to be "Constant Scale" and "Orthodromic", i.e. a straight line must be as close as possible to a Great Circle segment. This led to the rediscovery and extensive use of both :
      • The Lambert (tangent or secant) Conical projection charts - of various types adapted to the latitudes to be overflown - which all are fully Conform, almost Orthodromic and almost Constant Scale as well as:
      • The Polar Projection Charts which are :
        • For projections from the opposite pole : fully Conform (thanks to their well known "Geometrical Inversion" properties), almost Orthodromic and almost constant scale at high latitudes. And :
        • For Projections from the Earth Center : fully Orthodromic, almost fully Conform and almost Constant Scale for high latitudes..
    • Introducing Grid Navigation for the overflight of Polar Areas. Fast flying airborne craft can not easily accommodate fast changing "North referenced" headings when following Great Circles close to the North pole. Hence the solution came from 2 complementary areas and disciplines:
      • Covering Air Charts with "arbitrary" Square grids with one side of the squares being a Meridian : Grid Navigation was then invented. Actually, although any meridian could be a reference meridian only a few of them such 20°W, 0°, 20°E ... were actually used as references onthe printed charts. "Grid Headings" them became referenced to such a square grid so that a Great Circle - being a straight line by nature of these Charts - was also a constant "Grid Heading" track even in the immediate vicinity of the Pole. And as important :
      • Manufacturing Excellent Heading Gyroscopes able to "precession" according to chosen values to accommodate such Grid Navigation. This simple sentence could be developed into many full chapters (agreeEd and David ?) . One thing to know is that when you "force" a "perfect" Gyro into precessionning by a quantity equal to [15°/ Hour] *sin Latitude, one can demonstrate that under still air conditions, an aircraft following such a gyro constant heading will follow a Great Circle, whatever the heading or its airspeed. This was absolutely "magic" for the time. This is called the "Directional Gyro Mode", which is precisely the "DG" mode referred to by David in Ref 2 on top of this post. Hence it is quite understable that when the Vulcan Navigators needed to perform their celestial observations, their aircraft was to follow a track as close as possible to a Great Circle - hence flying DG Mode - , in order to eliminate any lateral deviation due to not flying a Great Circle. With ground speed variations expected and assumed to be as small as possible, only remained then the unavoidable Coriolis Correction to be accounted for.
        • There were even other ways to precession "more or less" such gyroscopes with decreasing or increasing Precession rates, so that flying a constant gyro heading as seen from the cockpit and followed by the autopilot resulted into the aircraft following almost any "smooth" curve between 2 chosen points, e.g. not only a Rhumb line, but also a constant bearing curve or almost any other longer or shorter curve. Obviously, choosing the right precession rate so that the aircraft would follow a Great Circle was the most frequently used option, but not always.
        • Much more could be said here ... In France and until year ... Year 2000 ! there still was a specific "Polar and Oceanic Remote Navigation" Certificate which included CelNav and GyroNavigation. It was considered as a very tough Certificate to obtain. Your could start flying as a Short Haul First Officer without it, but nonetheless it was an unavoidable prerequisite to become an Long Haul Airliner Captain or even to fly as Long Haul First Officer.
    • Obviously, Celestial Navigation was the main "Safety Net" to correct for Navigation Errors or inaccuracies in remote areas and over the Oceans. And :
    • There were also a few very powerful HF NDB's (Non Directional Beacons) on ground such as the one flanking the entire south-west Britanny coast in France. This "NAN" NDB which got decommissioned in early 1978 was replaced by the "GL" low power Locator currently in use for RWY03 ILS at LFRS. Aircraft could get bearings onto such long range NDB's which helped them updating their positions 1 or 2 hours before reaching familiar lands in due time before they could benefit from more accurate short Range NavAids (Navigation aids).
    • The Earth Magnetic Field Orientation detectors (Magnetic Fluxgates) were also extremely cleverly devised and designed. They were mounted close to the wingtips in order to be as far as possible from electromagnetic perturbations. They should never have come to life, had a stubborn Engineer not insisted that according to his own computations - a simple mathematical derivation - they would and should work. Everybody else thought that the contemplated mathematical operation was total nonsense and would lead to a dead end, to the extent that such contemplated development would be a waste of time and resources. Magnetic Fluxgates actually did work extremely well. They have disappeared on current Airbus and Boeing Aircraft. We still had them on the DC 10's.
    • Let me conclude by another very clever Navigation Method called "Barometric Navigation"  and this is where the Radar Altimeters came to play. All Aircraft obviously have always followed constant Flight Levels . When you compare your Flight Level Altitude (i.e. your Pressure Altitude "Zp") with your actual altitude "Zp" as given by your "high range" Radar Altimeter, the variation over time of the quantity "Zv - Zp" yields an excellent indication on the magnitude of your port or starboard Drift due to the wind (Dérive de Bellamy). This was an example of the demonstrations we had to be fully familiar with for another ATPL exam, i.e. the "Meteorology Certificate".

    To conclude this quite lenghty post, after starting sailing at sea for over 4 years before a French Naval Aviation Pilot course followed further down the road by an ATPL course I have been really impressed and enthralled at figuring out the amazing Navigation achievements Longhaul Airlineers had secured over a 30 year period before the introduction of our current "modern" Hybrid Inertial/satellites navigation.

    Obviously at that time (Super Constellation, DC4, Bréguet Deux Ponts, Vulcan and so many others ... ) Long Haul Air Navigation required a full time experienced and very busy Air Navigator onboard. My hat down to all of them.

    Hopefully have I been able to share this enthusiam with some of you.

    Time now for our Former NavList Air Navigators to continue on this topic, yes ?

    Best Regards to all

    Antoine Couëtte

    (Almost retired long haul pilot)

       
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