Service Volumes of VOR's make no sense

On Fri, 13 May 2005 19:08:15 -0400, "Morgans"
wrote in ::

but a signal is never too weak, with the right receiver.

True, provided the random background noise doesn't significantly
exceed the signal strength of the source you want to receive.

Antoñio wrote:
Hilton wrote:


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

If the FAA simply depicted a cylinder of theoretical signal strength
within the actual "semi-sphere" of service, I would completely follow
the analogy.

However, the FAA has depicted cylinders of various diameters stacked
upon each other. Given that the VOR is line-of-sight, I did not
understand why, for example, a VOR would be received 130nm out at FL180
yet only be received 100nm at FL500. Doesn't it logically follow that at
the higher altitude the VOR would be able to be received further out?

No, then it wouldn't fit in the semi-sphere. Signal strength drops off
(non-linearly I believe) as you move away from its source, so the further
you go, the weaker it becomes, hence the semi-sphere. Since the sphere
tapers off at the top, so too do the cylinders.

Hilton

But Hilton, that doesn't explain the effect observed. What DOES explain it
is the antenna pattern that the FAA chose for the VORs. In their infinite
wisdom they never considered that aircraft would fly much above FL250.
Remember, this was in the late 40s and early 50s that the system was
designed.

Therefore, they "squished" the antenna pattern to squirt more signal at a
lower radiation angle than an isotropic ("all angles") radiator. Think of
it as a ball of dough (isotropic) that has been squished to become a pancake
(low angle radiation). If you are ABOVE the pancake, you receive less
signal strength than if you are in the dough, so to speak.

There are two effects here. One is "radio horizon" which limits low
altitude reception to what the antenna can "see". The equation for this is
that radio horizon (in miles) equals the square root of the aircraft
altitude above the VOR (in feet). Thus, an aircraft near San Diego
receiving SAN VORTAC (which is on an island near Pacific Beach, damn near as
close to sea level as you can get) at an altitude of FL180 will have a radio
horizon of 134 miles, almost exactly what the fellow said, and will be
almost in the dead center of the antenna "beam" pattern. However, take that
same aircraft in the same geographic spot and honk it up to FL500, the radio
horizon moves to 224 miles, but you have climbed yourself way above the beam
and the signal strength has dropped below usable..

Howzat?

(Signal strength, BTW, falls off as the SQUARE of the distance.)

Jim




"Hilton" wrote in message
.net...
However, the FAA has depicted cylinders of various diameters stacked
upon each other. Given that the VOR is line-of-sight, I did not
understand why, for example, a VOR would be received 130nm out at FL180
yet only be received 100nm at FL500. Doesn't it logically follow that at
the higher altitude the VOR would be able to be received further out?

No, then it wouldn't fit in the semi-sphere. Signal strength drops off
(non-linearly I believe) as you move away from its source, so the further
you go, the weaker it becomes, hence the semi-sphere. Since the sphere
tapers off at the top, so too do the cylinders.

"RST Engineering" wrote in message
...
But Hilton, that doesn't explain the effect observed. What DOES explain
it
is the antenna pattern that the FAA chose for the VORs. In their infinite
wisdom they never considered that aircraft would fly much above FL250.
Remember, this was in the late 40s and early 50s that the system was
designed.

The engineers may have been more farsighted than you give them credit.
Since the radiation pattern is reduced at higher altitudes, there is less
chance
of frequency congestion and receiving a signal you don't want.
With limited frequencies available, you have to depend on other
limits to prevent unwanted reception of other signals.

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).

On Wed, 11 May 2005 21:10:13 -0700, 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.

Hi...
To try to make a realllllly long explanation short....(er)

Those are the 'design' or 'advertised' service volumes. This is the
required minimum coverage they are flight checked for.

Will the FAA guarantee coverage outside of that? Nope.

Will you pick up a useable signal outside of that?
Possibly....maybe...depending on altitude/distance/terrain
But they don't guarantee xx nm at xxxx feet.

--Don Byrer


Don Byrer
Instrument Pilot Commercial/CFI Student
Electronics Technician, RADAR/Data/Comm @ CLE
Amateur Radio KJ5KB

"I have slipped the surly bonds of earth; now if I can just land without bending the gear..."

Hilton wrote:
Signal strength drops off
(non-linearly I believe) as you move away from its source, so the further
you go, the weaker it becomes, hence the semi-sphere. Since the sphere
tapers off at the top, so too do the cylinders.

Hey! I think I finally got it ! That makes sense to me now...thanks!

Antonio

RST Engineering wrote:
But Hilton, that doesn't explain the effect observed. What DOES explain it
is the antenna pattern that the FAA chose for the VORs. In their infinite
wisdom they never considered that aircraft would fly much above FL250.
Remember, this was in the late 40s and early 50s that the system was
designed.

Therefore, they "squished" the antenna pattern to squirt more signal at a
lower radiation angle than an isotropic ("all angles") radiator. Think of
it as a ball of dough (isotropic) that has been squished to become a pancake
(low angle radiation). If you are ABOVE the pancake, you receive less
signal strength than if you are in the dough, so to speak.

There are two effects here. One is "radio horizon" which limits low
altitude reception to what the antenna can "see". The equation for this is
that radio horizon (in miles) equals the square root of the aircraft
altitude above the VOR (in feet). Thus, an aircraft near San Diego
receiving SAN VORTAC (which is on an island near Pacific Beach, damn near as
close to sea level as you can get) at an altitude of FL180 will have a radio
horizon of 134 miles, almost exactly what the fellow said, and will be
almost in the dead center of the antenna "beam" pattern. However, take that
same aircraft in the same geographic spot and honk it up to FL500, the radio
horizon moves to 224 miles, but you have climbed yourself way above the beam
and the signal strength has dropped below usable..

Howzat?

(Signal strength, BTW, falls off as the SQUARE of the distance.)

Jim

Very comprehensive and "engineer like" elaboration to Hilton's answer.
I think I finally get the picture. Thank you!

Antonio

Don Byrer wrote:

Those are the 'design' or 'advertised' service volumes. This is the
required minimum coverage they are flight checked for.
Don Byrer
Instrument Pilot Commercial/CFI Student
Electronics Technician, RADAR/Data/Comm @ CLE
Amateur Radio KJ5KB

"I have slipped the surly bonds of earth; now if I can just land without
bending the gear..."


Ahhh...that angle had not entered my mind.

.....and I just loved your Sig line !


Antonio