Service Volumes of VOR's make no sense

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