Effects of Earth's Rotation on Navigation???

[aviationluver]

Hello,

I was just reviewing a book entitled,"The Advanced Pilot's Flight Manual" by William K. Kershner. Around Page 63 of his book, Mr. Kersher talks about the effect of Earth's rotation and drift error.

As far as I understand things, the Earth's rotation has little or no effect on the navigation of the airplane because an airplane is flying in air (Ocean of atmosphere) which is also rotating with the Earth's rotation.

For example, an airplane traveling east to west is not affected or aided by the Earth's rotation and an airplane traveling west to east is not hindered by the Earth's rotation.

As for satellites (spacecraft), their speed and navigation IS affected by the Earth's rotation because space vehicles travel outside our atmosphere. In our previous example, a satellitle traveling east to west on the equator travels at their velocity plus the earth's rotational speed, whereas a satellite traveling west to east travels it's velocity minus the earth rotational speed.

With that being said, does anyone know why Mr. Kershener included the Earth's rotation in his book?

Your thoughts please.

[galaxy flyer]

While there are some infinitely more knowledgeable people hereabouts than me, here's a stab. An IRS platform needs earth rotation to align itself. Basically, when started its accelerometers measure earth rate to determine it westward drift. From drift it can find East and, hence, 90 degrees to the left is True North. Also, it can approximate its latitude. One application for earth rotation. More simply, Kershner was probably just trying to acquaint his readers with simple navigation and the basics are the arena--the Earth.

GF

[Oro-o]

With that being said, does anyone know why Mr. Kershener included the Earth's rotation in his book?

Your thoughts please.

I think he included the discussion for the reason that someone would ask about it. Thus, the likely question is addressed and the book more complete.

[extreme P]

Here's a start...

The major effect imposed by the earth's spherical
shape is somewhat similar to that caused by the
earth's rotation. As an aircraft travels across the
surface of the earth, its path becomes an arc due to
the shape of the earth. Consequently, the gyros -
particularly the pitch axis gyro - measure a rotational
rate, because traveling in a curved path always
involves rotation. This rate, called the transport
rate, does not describe rotational motion of the
aircraft with respect to the earth's surface. Therefore,
the IRS must calculate how much transport
rate is being measured by the gyros, and subtract
that value from their measurements.
INS erors continue to diverge with average amplitude
increasing exponentially

[Microburst2002]

He probably refers to the effect of earth rotation in inertial systems.
You can consider stars as being fixed in space. If you have a gyro or an IRS which keeps aligned with a star, so to speak, you can navigate. But you have to take into account earth's rate. And transport rate, too, as said above, but this effect is due to earth's shape.

Take a look at all the gyro stuff, you will find it there.

The other effect he could refer to is Coriolis force, which is also due to earth rotation. There is a recent thread about it with plenty of interesting posts. However in the end I still have some doubts...

[Wizofoz]

In our previous example, a satellitle traveling east to west on the equator travels at their velocity plus the earth's rotational speed, whereas a satellite traveling west to east travels it's velocity minus the earth rotational speed.


The two main problems with that statement are that:-

1) Velocity is a relative value. What you say might be true reference the Earths surface, but that is not the only way that velocity is measured.

2) Velocity is a vector quantity, meaning it is expressed with magnitude and direction. Stating one velocity is less than another is meaningless. You are talking about speed, which is a scalar quantity.

For extreme p, one effect of the Earths rota ion is that it is NOT spherical- it is an oblate spheroid significantly flattened at the poles as a result of its rotation when it was young and molten. This means that many route are shorter using polar routes.

Another effect of the Earths rotation is Coriolis force, which has a negligible effect on aircraft in flight, but is the route cause of weather systems, which are VERY relevant to aircraft operations.

[Mr Optimistic]

From what I recall, terrestrial navigation systems have the earths surface geometry programmed into them, eg as you travel the 'inertial platform' whether physical or virtual is rotated to keep the horizontal. Errors in the horizontal plane cause a feedback which tends to negate the error (speed too high, platform over rotates past horizontal in the belief you have traveled further round the globe, gravity leaking into the accelerometer channel not properly accounted for causing speed estimate to decrease). Called schuler tuning and has period of 80+ mins.

Everything moving that is observed from earth's surface seems to be affected by the coriolis force, including aircraft. Perhaps the nav systems compensate also. Dunno, but it is proportional to speed so an issue for artillery more than aircraft.

[Exaviator]

"This rate, called the transport
rate,"

That old ATPL Nav exam question on Transport Error. The formula is still carved in my brain........ Change Long * sin mid Lat. :ok:

[Mr Optimistic]

Sorry Exaviator, didn't read your earlier post properly (perhaps because too busy laughing about using 'g' in units of nm/hr/hr from another thread). In the horizontal plane the errors are damped, vertical errors would go exponential if not damped by other height data. Since the inertial system works in inertial space, coriolis force is not relevant.