Can GPS be *too* accurate? Do I need some XTE??

"SelwayKid" wrote in message
om...
"Icebound" wrote in message
...

In the "good old" VOR days, it must have been pretty difficult to fly
down
the centerline of an airway (or of any direct track).

....snip...

As for being difficult to fly the VOR, it was/is no more difficult
than flying a compass heading and holding it.....which many pilots
seem unable to do anymore. They would prefer that electronic gadgets
do their flying for them and no thoughts as to what happens when the
electrodes take a vacation.


Never having flown a VOR course myself... I still doubt very much that any
two pilots (OR auto-pilots), flying reciprocal headings between two VORs,
would both be able to *simultaneously* hold a course to within 10 feet of
the centre-line for the whole course, considering the receiver errors and
that the VOR radial-signal *itself* probably varies more than that.

I could be wrong.

"Icebound" wrote in message
...
[...]
Never having flown a VOR course myself... I still doubt very much that any
two pilots (OR auto-pilots), flying reciprocal headings between two VORs,
would both be able to *simultaneously* hold a course to within 10 feet of
the centre-line for the whole course, considering the receiver errors and
that the VOR radial-signal *itself* probably varies more than that.

I could be wrong.

You are wrong.

For two pilots to *intentionally* stay exactly on course center on a VOR
airway would be challenging, granted. But the airway provides an
"attractor" for airplanes, and inasmuch as the airplanes average toward the
center of the airway, eventually a couple will come along flying the exact
same distance from the actual airway (whether that's 0.0 miles off-center or
3.9 miles off-center).

Like I said before, it's happened to me on several occasions (getting close
enough to other aircraft on an airway to require evasive action, that is).
That's with me handflying. Using an autopilot, VOR navigation can
theoretically be VERY good, especially close to the station (within 10-20
miles).

GPS increases the chances of collision, by reducing the average error. But
the issue did already exist with VOR navigation. Keep in mind that GPS
error is still going to be on the order 10 to 30 meters or so, just from the
position information standpoint, and then on top of that you still have the
problem of the airplane being kept exactly at the intended position (even
with an autopilot, there's going to be some slop, and not all pilots are
using autopilots in conjunction with their GPS navigation).

The total error even in the GPS case can be much larger than the wingspan of
typical GA aircraft, and so the same kinds of factors that protect against
collisions when using VOR navigation also protect against collisions when
using GPS navigation (though to a lesser degree).

Pete

Y'All,
This entire thread seems to be totally entranced with the possible conflict
of aircraft on a heading/course. Whereas, the most likely conflict is in
altitude between IFR and VFR supposedly flying with 500 feet of FAA
separation.

Some time ago I was told that ATC figures a + - error 300 feet.
The altimeter is likewise allowed a 75 foot + - error. Not knowing for
certain but assuming it is so. Look at the following senario.

If we have an IFR and a VFR fllying in opposite hemisphereic directions in
VFR conditions we have several possible extreme conditions. Take the first
aircraft indicating 6000 feet west bound. The second aircraft indicating
5500 feet east bound. If both the transponders and altimeters have errors
to the extreme in the opposite directions, they could still miss each other.

If the first aircraft is flying 250 lower than indicated due to accumulated
instrment error, while the other is actually flying 250 feet higher than
indicated we have only see and be seen to save the situation.

To me the probability of a midair is more likely to altitude error than
heading error. The odds of having two such aircraft with hemispheric
accumulative opposite errors in altitude sufficient to cause a midair is
unlikely but more likely than an opposite heading midair. I believe this
because the distances are matters of feet rather than miles. It takes both
to actually cause the midair.so the total emphasis on course/heading is only
a part of the equation.

I haven't even mentioned GPS altitude as a factor.
Mud wrestling anyone?

Gene

"Gene Whitt" wrote in message
link.net...
Y'All,
This entire thread seems to be totally entranced with the possible
conflict of aircraft on a heading/course. Whereas, the most likely
conflict is in altitude between IFR and VFR supposedly flying with 500
feet of FAA separation.
...snip...
To me the probability of a midair is more likely to altitude error than
heading error. The odds of having two such aircraft with hemispheric
accumulative opposite errors in altitude sufficient to cause a midair is
unlikely but more likely than an opposite heading midair. .


Actually, if the two of them are not on the exact same location
horizontally, the odds of a conflict because of altitude error is zero.

While there is some remote possibility of being in the same location
horizontally at the same time while crossing tracks, the potential to be in
the same location horizontally is much greater when they are navigating
reciprocal tracks between the same two waypoints.

"Gene Whitt" wrote in message hlink.net...
Y'All,
This entire thread seems to be totally entranced with the possible conflict
of aircraft on a heading/course. Whereas, the most likely conflict is in
altitude between IFR and VFR supposedly flying with 500 feet of FAA
separation.

Some time ago I was told that ATC figures a + - error 300 feet.
The altimeter is likewise allowed a 75 foot + - error. Not knowing for
certain but assuming it is so. Look at the following senario.

If we have an IFR and a VFR fllying in opposite hemisphereic directions in
VFR conditions we have several possible extreme conditions. Take the first
aircraft indicating 6000 feet west bound. The second aircraft indicating
5500 feet east bound. If both the transponders and altimeters have errors
to the extreme in the opposite directions, they could still miss each other.

If the first aircraft is flying 250 lower than indicated due to accumulated
instrment error, while the other is actually flying 250 feet higher than
indicated we have only see and be seen to save the situation.

To me the probability of a midair is more likely to altitude error than
heading error. The odds of having two such aircraft with hemispheric
accumulative opposite errors in altitude sufficient to cause a midair is
unlikely but more likely than an opposite heading midair. I believe this
because the distances are matters of feet rather than miles. It takes both
to actually cause the midair.so the total emphasis on course/heading is only
a part of the equation.

I haven't even mentioned GPS altitude as a factor.
Mud wrestling anyone?

Gene

Excellent point Gene. This is why maintaining a good visual scan is
important, and why TCAS and TCAD systems are so valuable. I wish that
we were at the point where every airplane equipped with a transponder
also had a TCAS or TCAD system installed. It would be nice if this
equipment were affordable enough to do this.

Dean

"Peter Duniho" wrote in message
...
"Icebound" wrote in message
...
[...]
Never having flown a VOR course myself... I still doubt very much that
any two pilots (OR auto-pilots), flying reciprocal headings between two
VORs, would both be able to *simultaneously* hold a course to within 10
feet of the centre-line for the whole course, considering the receiver
errors and that the VOR radial-signal *itself* probably varies more than
that.

I could be wrong.

You are wrong.

For two pilots to *intentionally* stay exactly on course center on a VOR
airway would be challenging, granted. But the airway provides an
"attractor" for airplanes, and inasmuch as the airplanes average toward
the center of the airway, eventually a couple will come along flying the
exact same distance from the actual airway (whether that's 0.0 miles
off-center or 3.9 miles off-center).

Accepted and agreed. "eventually".

But in the GPS case, it is pretty much in "every" case that two aircraft
using those two waypoints will be pretty much in the center.

"Icebound" wrote in message
...
But in the GPS case, it is pretty much in "every" case that two aircraft
using those two waypoints will be pretty much in the center.

But "pretty much" still covers quite a bit of ground. There is only an
increased risk of a collision, not a virtual certainty.

"Peter Duniho" wrote in message
...
"Icebound" wrote in message
...
But in the GPS case, it is pretty much in "every" case that two aircraft
using those two waypoints will be pretty much in the center.

But "pretty much" still covers quite a bit of ground. There is only an
increased risk of a collision, not a virtual certainty.


Oh, for sure.

Peter Duniho ) wrote:
: GPS increases the chances of collision, by reducing the average error. But
: the issue did already exist with VOR navigation. Keep in mind that GPS
: error is still going to be on the order 10 to 30 meters or so, just from the
: position information standpoint, and then on top of that you still have the
: problem of the airplane being kept exactly at the intended position (even
: with an autopilot, there's going to be some slop, and not all pilots are
: using autopilots in conjunction with their GPS navigation).

The GPS error you have quoted is relative to a fixed point on the ground.
Most modern cheap GPS recievers in the same region looking at the same
sats with differential corrections are within meters of each other
and with good processing can produce a relative position within a few
inches.

The way to deal with this is to simply move the GPS course .1 nmi
to the right. This means if your doing a 90 degree turn over a VOR
using a GPS, you should make your turn .14 nmi away from the VOR
and you should be able to see it out the left window.

At this point it won't matter much considering the GA autopilot slop
but things could change in the future and now is a good time to
start putting these things in place.

but there are more an more aircraft flying in the skys that don't
but things could change in the future and now is a good time to
start putting these things in place.

-tim
http://web.abnormal.com

"Icebound" wrote in message ...
"SelwayKid" wrote in message
om...
"Icebound" wrote in message
...

In the "good old" VOR days, it must have been pretty difficult to fly
down
the centerline of an airway (or of any direct track).

...snip...

As for being difficult to fly the VOR, it was/is no more difficult
than flying a compass heading and holding it.....which many pilots
seem unable to do anymore. They would prefer that electronic gadgets
do their flying for them and no thoughts as to what happens when the
electrodes take a vacation.


Never having flown a VOR course myself... I still doubt very much that any
two pilots (OR auto-pilots), flying reciprocal headings between two VORs,
would both be able to *simultaneously* hold a course to within 10 feet of
the centre-line for the whole course, considering the receiver errors and
that the VOR radial-signal *itself* probably varies more than that.

I could be wrong.
*********************
Icebound
If you have never flown a VOR course, where in hell do you fly? And,
if you have never flown a VOR course, what do you know about them or
what their capabilities are? Part of the PTS for every US rating
involves VOR.
Beyond that, let me ask if you are a licensed pilot? Hmmm, well you
may be in another country, perhaps 3rd world without VOR but even
then, of the 26 countries I've worked in, all had VOR coverage of some
kind. So again, where do you fly?
Ol Shy & Bashful