Service Volumes of VOR's make no sense

[Antoñio]

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

[Grumman-581]

"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)

[Larry Dighera]

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]

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

[Hilton]

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

[Peter Clark]

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.
>
>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?

The way it was explained to me was that once you get above a certain
altitude you can start getting interference from other VORs with the
same frequency, so the 'valid' radius starts getting shorter again.

[Denny]

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

[Grumman-581]

"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...

[Steven P. McNicoll]

"Antoñio" > wrote in message
...
>
> 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, and every other VOR on that frequency as well. The altitude/distance
limits ensure you won't receive an unwanted VOR signal. Note that the
service volume decreases from 130 miles to 100 miles above FL 450.

[Andrew Sarangan]

Isn't that the reason certain airways have a MAA - Maximum Authorized
Altitude?

"Steven P. McNicoll" > wrote in
nk.net:

>
> "Antoñio" > wrote in
> message ...
>>
>> 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, and every other VOR on that frequency as well. The
> altitude/distance limits ensure you won't receive an unwanted VOR
> signal. Note that the service volume decreases from 130 miles to 100
> miles above FL 450.
>
>
>

[Antoñio]

Steven P. McNicoll wrote:

> Yes, and every other VOR on that frequency as well. The altitude/distance
> limits ensure you won't receive an unwanted VOR signal. Note that the
> service volume decreases from 130 miles to 100 miles above FL 450.
>
>

Exactly! And that was the reason for my confusion and original question.

Antonio

[Antoñio]

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

[Antoñio]

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
>
>

[Steven P. McNicoll]

"Antoñio" > wrote in message
...
>
> 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]

"Andrew Sarangan" > wrote in message
1...
>
> Isn't that the reason certain airways have a MAA - Maximum Authorized
> Altitude?
>

Frequency overlap is probably the most common reason but they may also exist
due to conflicts with special use airspace.

[Jose]

> 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.

[Darrell S]

The signal strength from a VOR decreases with distance. You are on a
straight line to the VOR from the moon but the signal strength would be too
low to create guidance.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"Steven P. McNicoll" > wrote in message
nk.net...
>
> "Antoñio" > wrote in message
> ...
>>
>> 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, and every other VOR on that frequency as well. The altitude/distance
> limits ensure you won't receive an unwanted VOR signal. Note that the
> service volume decreases from 130 miles to 100 miles above FL 450.
>

[Steven P. McNicoll]

"Darrell S" > wrote in message
news:4R4he.25286$tQ.4405@fed1read06...
>
> The signal strength from a VOR decreases with distance. You are on a
> straight line to the VOR from the moon but the signal strength would be
> too low to create guidance.
>

I think he was speaking figuratively, aircraft do not operate very well
outside the atmosphere.

[Antoñio]

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
>

[Morgans]

"Darrell S" > wrote in message
news:4R4he.25286$tQ.4405@fed1read06...
> The signal strength from a VOR decreases with distance. You are on a
> straight line to the VOR from the moon but the signal strength would be
too
> low to create guidance.

Signal is too weak to use with today's receivers, but a signal is never too
weak, with the right receiver. Look at the space robots; still in contact,
at the edge of the solar system. Amazing.
--
Jim in NC

[Larry Dighera]

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.

[Hilton]

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

[RST Engineering]

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.

[CryptWolf]

"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.

[David CL Francis]

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

[Peter Duniho]

"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).

[Don Byrer]

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..."

[Antoñio]

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

[Antoñio]

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

[Antoñio]

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

[Larry Dighera]

On Sat, 14 May 2005 18:53:06 -0400, "CryptWolf"
> wrote in
<1116111148.11781a37a6d5a6e2b697012478f45470@terane ws>::

>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.

The radiation pattern also puts more energy where it was needed, as
that energy that would be radiated upward is directed laterally
instead.

[RST Engineering]

Um, that's just not true. SInce the radiation pattern is reduced at higher
altitudes, the chance for frequency congestion is every bit as probable. If
all signals are reduced proportionally, then the RELATIVE signal strengths
remain constant.

Jim



>>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]

And a screw-up on the equation:

Radio horizon (in miles) equals the square root of [TWO TIMES the aircraft
altitude above the VOR (in feet)].

Jim



"Antoñio" > wrote in message
...

The equation for this is
>> that radio horizon (in miles) equals the square root of the aircraft
>> altitude above the VOR (in feet).

[David CL Francis]

On Sat, 14 May 2005 at 17:50:55 in message
>, Peter Duniho
> wrote:
>"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.
>
I did not realise that I was being pedantic!

>> 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).
>
Of course. But the point is not important when considering VORs!
>

--
David CL Francis

[David CL Francis]

On Sun, 15 May 2005 at 09:05:01 in message
>, RST Engineering
> wrote:
>And a screw-up on the equation:
>
>Radio horizon (in miles) equals the square root of [TWO TIMES the aircraft
>altitude above the VOR (in feet)].
>
[1] Jim, Doesn't that assume that the number of feet in a mile is
the same as the radius of the earth in miles?

The formula for the tangential horizon distance is

[2] d=(2*r*h + h^2)^0.5

where d is the tangential horizon distance, r is the radius of the earth
and h is the height of the object above the surface. DKr and h all in
the same units. Because the heights we are talking about are small
compared to the radius of the earth the h^2 term can be ignored leaving
what you said.

E&OE :-)
--
David CL Francis

[RST Engineering]

And you asked in another post if you were being pedantic? PEDANTIC? My
guess is that you stay up late at night worrying about whether
anal(-)retentive is hyphenated or not.

Jim



"David CL Francis" > wrote in message
...
> On Sun, 15 May 2005 at 09:05:01 in message
> >, RST Engineering
> > wrote:
>>And a screw-up on the equation:
>>
>>Radio horizon (in miles) equals the square root of [TWO TIMES the aircraft
>>altitude above the VOR (in feet)].
>>
> [1] Jim, Doesn't that assume that the number of feet in a mile is
> the same as the radius of the earth in miles?

[CryptWolf]

"RST Engineering" > wrote in message
...
> Um, that's just not true. SInce the radiation pattern is reduced at
higher
> altitudes, the chance for frequency congestion is every bit as probable.
If
> all signals are reduced proportionally, then the RELATIVE signal strengths
> remain constant.
>
> Jim

Under normal atmospheric conditions, excluding anomolies, as the frequency
or distance increases, the required transmitter power increases while the
recieved signal strength remains the same. At some point, even if you use
the same frequencies, a VOR or any radio signal will vanish into the
background noise. The reciever sensitivity is limited by background noise.
If you like, I'll look up the specific math and we can really get technical.
All but a few would understand it or care. This would be the other limit
that
I didn't explain previously.

All you have to do is space VOR's of the same frequency far enough apart
and they won't interfere. The fact that the radiation pattern is reduced for
higher altitudes seems to imply that the radiation pattern was designed
to reduce transmitted power and limit reception distances at those altitudes
where line of sight would not be a factor.

For reference you might want to find:
Schoenbeck, Electronic Communications Modulation And Transmission
It's one of the books I used when I was working on my electronics degree.

> >>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.
>
>

[Steven P. McNicoll]

"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.

[RST Engineering]

For reference you might want to find:

Hamsher, "Communications Systems Engineering Handbook" ; Jasik, "Antenna
Engineeiring Handbook"; Kraus "Antenna Design"; MIT Radiation Lab Series;
any of the ARRL publications on antennas

which is what I recommend that you use when I teach you when you are working
on your electronics degree...

Jim





> For reference you might want to find:
> Schoenbeck, Electronic Communications Modulation And Transmission
> It's one of the books I used when I was working on my electronics degree.

[Antoñio]

Steven P. McNicoll wrote:

> 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.

Ok...I think I'm with you (though feeling like I might be exhibiting
that I am a little obtuse at this point in the discussion). I assume
that your post means you think I haven't quite got it yet?

Are you saying that the service volume diagrams are artist renderings of
the reception distances that the FAA have tested and will *guarantee* to
be usable?? So the "double/inverted wedding cake" structure (which, as
you recall, was the basis of my original question) really has little to
do with the *actual* signal propagation distances of a particular VOR
but, rather, provide approximations by taking into consideration the
real world interference of other VORs, spherical wave radiations,
curvature of earth, etc. ?

So I would not necessarily loose VOR service if, while using a Standard
High Altitude Service Volume station, I were to climb above FL45, with a
depicted service distance of 130nm, to FL46 with it's depicted service
distance of 100nm?

Antonio (thinking he had it, then....)

[[email protected]]

On Sun, 15 May 2005 00:17:35 -0700, Antoñio
> wrote:

>Don Byrer wrote:
>
>> Those are the 'design' or 'advertised' service volumes. This is the
>> required minimum coverage they are flight checked for.

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

Basically "you may get more than this, but this is what the FAA
guarantee as a minimum" Usually you get a lot more...not always.

That's what I meant to say...

>"I have slipped the surly bonds of earth; now if I can just land without
>bending the gear..."
>
>....and I just loved your Sig line !

Thanks...that does still go thru my mind some days...even at 204
hours. Thought now it's sideload concerns, not slammin it down....

--Don

Don Byrer
Electronics Technician/Friendly but Sarcastic Pilot
FAA Airways Facilites/Tech Ops, RADAR/Data/Comm @ CLE
Amateur Radio KJ5KB
Instrument Pilot Commercial Student
PP-ASEL 30 Jan 2005 "-IA" 25 Mar 2005