Service Volumes of VOR's make no sense

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?

(See AIM 1-1-8)and then order some room service! ;-)

Antonio

"Antoñio" wrote in message
news

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?


Yes, it does logically follow that at the higher altitude the VOR would be
able to be received further out. Do you now understand why the distance is
less at higher altitude?

Steven P. McNicoll wrote:


Yes, it does logically follow that at the higher altitude the VOR would be
able to be received further out. Do you now understand why the distance is
less at higher altitude?


I am not sure. Is it because the radiation pattern is spherical and not
line-of-sight?

Antonio

"Antoñio" wrote in message
...

I am not sure. Is it because the radiation pattern is spherical and not
line-of-sight?


You're looking at just one VOR at a time. There are about 1000 VORs in the
US and just 100 VOR frequencies. The service volume has to ensure not only
usable reception of the desired VOR, but non-reception of undesired VORs on
the same frequency.

As an example, let's say you're flying from EAU VOR to LAN VOR at 15,500'.
They're about 320 miles apart but at that altitude line-of-sight distance is
about 180 miles so you should receive LAN before losing EAU, even though
you're well outside of the standard service volume of forty miles. When
you're about halfway you switch from EAU to LAN, but you're unable to get a
reliable signal.

The problem may be that you're closer to ESC and RFD VORs than you are to
LAN, and they all operate on 110.8.

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

Depends on the antenna radiation pattern, and interference from other
emitters.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

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.

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

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