Navigation. Where are you?

[Milt]

Aerial Navigation

Where are we/you?

Presumably we aviators base all of our increasingly precise navigation on the planet's datum at Greenwich on the zero meridian. Presumably also we will continue to use that point at Greenwich as our datum even though it is itself slowly moving NE at about 30 mm per year.

Problem is that tectonic plate movements will increasingly move all of our known locations every which way relative to our declared datum.

I guess we will or have decided to keep the zero lattitude fixed relative to the axis of the planet.

So how far has our Greenwich Datum moved from its original location since it was established ? Perhaps no one knows or yet cares.

If the positions of the GPS sattelites are all Greenwich based and our receivers increase in accuracy then shouldn't we start to see maps and charts denoting positional drift as is done for magnetic variation ?

Does anyone know whether ICAO or whoever has a handle on the mounting navigational inaccuracies ?

[Pub User]

You're right, this is a serious problem.

If it continues at this rate we'll be landing 3m off the centreline in 100 years time!

[babyboeing400]

lol great sarcasm!!

[Old Smokey]

If it continues at this rate we'll be landing 3m off the centreline in 100 years time!
I guess I'm 100 years ahead of my time Pub User, that's where I land all of the time.

Does the Continental drift theory extend to continental bulging? I hope so, that'll explain why my runway impacts always seem to occur half a second before I expect them. The continental bulging theory seems most evident on Manchester 24R, I keep trying to bash it down, but mother nature is intent on winning, it's always there the next time.

Regards:O ,

Old Smokey

[Tinstaafl]

It's the rapidity of continental bulging that astounds me. I'm pretty sure my approach path is accurate, which would lead to a touchdown at the proper place & time.

But the ability of a bulge (or sometimes a tectonic sink) to happen during the flare & hold off period is truly remarkable.

[OzExpat]

No, no, no! Smokey and Tinnie, I'm surprised at the pair of you! The real problem is that you've failed to account for earth curvature in the touch-down zone! :}

[Jetstream Rider]

Its tidal really. The same way that water is pulled by the moon and sun, so the earths crust is too. When the tide is in I hit the ground before I expect to. When the tide is out I flare too high and fly by before the aircraft drops out of the sky.

Either way its a smasher.

As for datums - there are many. Greenwich is only one of them, and in fact they got it a bit wrong a couple of times. Most places in ICAO are mapped to WGS84 and they will continue to update the co-ordinates of each place as they move. That movement is so tiny that the updates are few and far between.

[Old Smokey]

Its tidal really
Actually, Jetstream Rider, in the midst of our little bit of levity, there may be a small degree of truth in your statement. Whilst doing an Airport Analysis for Mackay (Queensland, Australia), the airport civil engineer told me (with a straight face) that the South-Eastern end of the runway, which was built over tidal flats, actually 'floats' up and down slightly over the sub-terrainian water during tidal variations. Maybe just another of many aviation myths, but it sounded reasonable to me.

OzExpat, wouldn't your earth curvature proposal cause extended floats, instead of the semi-controlled crashes of which I'm far more guilty?:) Maybe I'm encountering the bulgy bits attributable to WGS84, I've only been doing the mental arithmatic for the perfect sphere during flare you see...............

Regards,

Old Smokey

[bookworm]

So how far has our Greenwich Datum moved from its original location since it was established?

Isn't there a clue in "Datum"? ;)

[Old Smokey]

A datum is a datum is a datum is a datum..............

[OzExpat]

Hi Smokey... what's that old adage that goes something like ... "Beware, lest the ground rise up and smite thee"? :D

You make a valid point about the difference between the WGS-84 spheroid and a perfect sphere! :E I'll use it the very next time I have a semi-controlled crash - I won't have to wait too long for such an event either... :}

[Piltdown Man]

But if everything else is moving NE at 30mm PA, it will cancel itself out, surely...

[Farrell]

Oh don't worry too much about it!

If it all goes wrong we can just "defer our success"

[catchup]

Another problem, in my aircraft at least, are the three independent working INS. They show three different groundspeeds. I tend to use the autopilot associated with the INS with the highest GS to make it faster and therefor cheaper for my company. My copilots don't like that. Most of them are working on overtime and prefere to use the one with the lowest GS to make some extra dollars. Always some nasty discussion about this.

regards

[Genghis the Engineer]

Unless you've got shares in the company, ask for a share of the overtime.

G

[Old Smokey]

You must take a positive attitude to all of this. Don't worry, if the present rate of 30 mm per year continues, things will be absolutely back to where they started in 1,333,440,000 years.

You just have to be patient:ok:

Regards,

Old Smokey

[enicalyth]

Polar wander, solid earth tides, tectonic movement, variable mantle-to-liquid phase buoyancy account for +/- 30 cm variations in positional accuracy per day. Nothing ever goes back to the same place twice. Fact discovered by George Darwin son of Charles, founder of Darwin City (and couldn’t find it again because it had moved) and inventor of the famous Darwin stubby, beer goggles, beer ‘flu and the beer drizabone never worn in the Wet. All bad landings are therefore attributable to the above George Darwin and also to Toohey’s who sponsor temporary unsureness of position. But because the day is getting longer by 4 milliseconds per century you also need not pay so much attention to “bottle-to-throttle” time. A lot can happen in 4 milliseconds. Ask my wife. And again the moon is moving away from the earth by 4cm per year thus changing the form of the earth through gravitation which is why Jepps are never that accurate in the first place. Except in Malta where Jepps are drawn at a scale of twelve inches to the foot. Airbi may be slower and therefore not so navigationally perfect because of spatio-temporal wander but compensate for it with a far superior autoland system.

[reynoldsno1]

OK, so what about Coriolis? There's always a coriolis effect lurking somewhere .....

[OzExpat]

Nah, Coriolis is the thing that makes the aircraft spin to the right in the southern hemisphere and left in the northern... :}