Service Volumes of VOR's make no sense

I was at a CFI safety meeting today and the subject of VOR service
volumes came up. The AIM describes the Standard High Service Volume as
providing positive course guidance at varying distances depending on
your altitude--40nm at 1000ft., 100nm at 14,500ft, 130nm at 45,000ft, etc.

If the VOR is a "line of sight" signal device. How can there be varying
distances of service at varying altitudes? I mean, shouldn't I be able
to pick up a VOR radial from the moon as long as no obstructions intervene?


Antonio

"Antoñio" wrote in message ...
If the VOR is a "line of sight" signal device. How can there be varying
distances of service at varying altitudes? I mean, shouldn't I be able
to pick up a VOR radial from the moon as long as no obstructions
intervene?

NEWSFLASH -- THE EARTH IS NO LONGER BELIEVED TO BE FLAT !!!

(at least my *most* of us)

Grumman-581 wrote:
"Antoñio" wrote in message ...

If the VOR is a "line of sight" signal device. How can there be varying
distances of service at varying altitudes? I mean, shouldn't I be able
to pick up a VOR radial from the moon as long as no obstructions

intervene?

NEWSFLASH -- THE EARTH IS NO LONGER BELIEVED TO BE FLAT !!!

(at least my *most* of us)

I'm not sure of what that remark means.
Have you seen the "wedding cake" drawings of the service volumes?

Antonio

"Antoñio" wrote in message ...
I'm not sure of what that remark means.

The earth is round... Radio travels line of sight, which means a straight
line...Draw a large circle with a protractor... Choose a point on the
circumference at the top of the circle... Draw a line tangent to the circle
through this point... The line is horizontal... If an object is above this
line, it will be able to 'see' the original point, if it is below the line,
but above the circumference, it will not be able to 'see' the original point
since the body of the circle (i.e. the earth) is getting in the way of the
signal... The greater the distance the object is above the circumference of
the circle, the more of the circle it is able to 'see'...

Now, extend this concept into three dimensions...

Grumman-581 wrote:
"Antoñio" wrote in message ...

I'm not sure of what that remark means.


The earth is round... Radio travels line of sight, which means a straight
line...Draw a large circle with a protractor... Choose a point on the
circumference at the top of the circle... Draw a line tangent to the circle
through this point... The line is horizontal... If an object is above this
line, it will be able to 'see' the original point, if it is below the line,
but above the circumference, it will not be able to 'see' the original point
since the body of the circle (i.e. the earth) is getting in the way of the
signal... The greater the distance the object is above the circumference of
the circle, the more of the circle it is able to 'see'...

Now, extend this concept into three dimensions...


That argument would be acceptable if the service volumes were increased
at regular rates as one increased in altitude. However, according to the
AIM, they do not. In fact the configuration presented is non-linear.

Once again I say take a look at the drawings in the AIM of the service
volumes. The distances that VOR's of varying service offerings are
depicted to offer is schematically represented as sort of like stacked
tires of differing sizes! Your linear geometric model explanation just
does not hold up to what is pictured there in the AIM.

Antonio

On Thu, 12 May 2005 at 14:29:01 in message
NKJge.73931$NU4.3092@attbi_s22, Grumman-581
wrote:
"Antoñio" wrote in message ...
I'm not sure of what that remark means.

The earth is round... Radio travels line of sight, which means a straight
line...Draw a large circle with a protractor... Choose a point on the
circumference at the top of the circle... Draw a line tangent to the circle
through this point... The line is horizontal... If an object is above this
line, it will be able to 'see' the original point, if it is below the line,
but above the circumference, it will not be able to 'see' the original point
since the body of the circle (i.e. the earth) is getting in the way of the
signal... The greater the distance the object is above the circumference of
the circle, the more of the circle it is able to 'see'...

Now, extend this concept into three dimensions...

There is a very simple formula for the distance to the horizon from a
given height above the surface for a smooth sphere of 4000 miles radius.

Height (Feet) Distance (miles)
0 0.0
6 3.0
20 5.5
50 8.7
100 12.3
150 15.1
500 27.5
1000 38.9
2000 55.0
4000 77.8
8000 110.1
16000 155.7
32000 220.2
64000 311.4
128000 440.4
256000 622.8

The real earth is not of course that flat except over the oceans! Also
the further away you go the closer the horizon distance gets to being
the same as the height. It is obvious that from the moon you can almost
see the entire hemisphere.

--
David CL Francis

"David CL Francis" wrote in message
...
The real earth is not of course that flat except over the oceans!

Actually, as long as we're being pedantic, it's not even flat over the
oceans. It's much flatter, but the Earth simply is not an ideal "smooth
sphere" anywhere on its surface.

Also the further away you go the closer the horizon distance gets to being
the same as the height. It is obvious that from the moon you can almost
see the entire hemisphere.

Almost. But the difference is significant enough to matter when you
really care whether you can see the entire hemisphere (astronomy, for
example).

NEWSFLASH -- THE EARTH IS NO LONGER BELIEVED TO BE FLAT !!!
************************************************** ***********************************

Damn, and just when I was getting good at plane trigonometry...

OK, smart a** remarks aside, not only is the world round, it is very
round - in the sense that the horizon falls away so quickly that for
those who are not boaters/sailers it will be a surprise... We used to
keep our Pearson on Lake Huron, up at Tawas Bay... Every summer they
held the Laser fleet races on the bay... (These are small sail boats
with a gunnel about a foot high) It was common for me to sit on the
beach on a calm Sunday morning and look across the bay and see these
guys sailing in light air, with the sailer visible from the waist up -
the entire hull of the laser and his lower body were below the horizon
- I don't mean hidden by waves, the bay was a sheet of glass......

denny

On Wed, 11 May 2005 21:10:13 -0700, Antoñio
wrote in
::

shouldn't I be able
to pick up a VOR radial from the moon as long as no obstructions intervene?

Perhaps, but the energy of the received signal is reduced by the
square of the receiver's distance from the station.

Antoñio wrote:
I was at a CFI safety meeting today and the subject of VOR service
volumes came up. The AIM describes the Standard High Service Volume as
providing positive course guidance at varying distances depending on
your altitude--40nm at 1000ft., 100nm at 14,500ft, 130nm at 45,000ft, etc.

If the VOR is a "line of sight" signal device. How can there be varying
distances of service at varying altitudes? I mean, shouldn't I be able
to pick up a VOR radial from the moon as long as no obstructions
intervene?

Yes, if we all had extremely efficient receivers, but we don't. The FAA and
some radio guys got together and decided on applicable distances. Once they
figured that out, they had a bunch of semi-spheres. While it would have
been 'correct' to define the service volumes are a semi-sphere, it wouldn't
have been all that useful to us (pilots). So the FAA made them (mostly)
cylinders (and ensured that the cylinder lay within the semi-sphere) to make
it easy for pilots to figure out whether or not they were in the service
volume. i.e. it is a combination of radio effectiveness and pilot
usefulness that describes the service volume.

I just made that up, but it sure sounds convincing, logical, and almost as
good as if I had stayed at a Holiday Inn last night... instead of working
on software.

Hilton