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

"John R. Copeland" wrote:

"Icebound" wrote in message ...


The remainder of the article describes the 1-nm or 2-nm offsets allowed,
and only to the right of centerline, with effective date of June 10, 2004.

The name of this concept is "Strategic Lateral Offset Procedure",
so everyone can understand they've now added SLOP to the route system!
As Dave Barry might say, I am not making this up.
---JRC---

Keep in mind those offsets are in oceanic airspace where the route width is 25 or 30 miles, centerline to
edge. Along a domestic airway, offsets of those magnitude would be far too large. An offset of 1/10 of a
mile would probably be effective without creating an issue with compliance with FAR 91.189. (although the feds
might not buy that rationale ;-)

"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).

So an eastbound VFR/IFR aircraft descending from 7500/7000 to his destination, was more than likely to avoid
traffic... on the reciprocal track passing him by at 6500 or 6000... by some significant horizontal error-distance,
even if they didn't see each other (big sky theory :-) ).

GPS horizontal accuracy with WAAS is already in the order of magnitude of a Cessna's wingspan, and some are talking
about getting it down to mere inches.

So the question is: If my Westbound Cessna at 6000 feet (with the autopilot keeping it happily on the GPS-track
centerline) meets the descending Bonanza on the reciprocal track between the same two airports (using a similar
GPS/a-p combo), there is a distinct possibility that the horizontal clearance may be zero...

...so is there anything in the current crop of GPS and/or Autopilot systems that allow me to maintain a small
cross-track error of my choosing, without actually entering off-navaid-off-airport waypoints? ...or do we care; am I
overly concerned???



At midcourse, add a waypoint offset to one side or the other by a couple of miles. The enroute time would be changed
minimally...

Blueskies wrote:



At midcourse, add a waypoint offset to one side or the other by a couple of miles. The enroute time would be changed
minimally...

But, that wouldn't be a parallel offset and would result in crossing the two end points precisely on course.

Also, the offset should be much smaller than 2 miles for domestic airspace operations.

wrote in message ...


Blueskies wrote:



At midcourse, add a waypoint offset to one side or the other by a couple of miles. The enroute time would be changed
minimally...

But, that wouldn't be a parallel offset and would result in crossing the two end points precisely on course.

Also, the offset should be much smaller than 2 miles for domestic airspace operations.



But the end points are where you want to be - one is where you started and one is where you are going. If you have a
system that does not have the parallel course function, then this would be a simple solution. If you want perfect, then
plot a course using multiple waypoints...PIC chooses....

"Icebound" wrote in message
...

...so is there anything in the current crop of GPS and/or Autopilot
systems
that allow me to maintain a small cross-track error of my choosing,
without
actually entering off-navaid-off-airport waypoints? ...or do we care; am I
overly concerned???

You're certainly not alone in being concerned.

http://www.pprune.org/go.php?go=/pub/tech/MidAir.html
http://tsb.gc.ca/en/reports/air/1995...8/a95h0008.asp

Julian Scarfe

If the weather is IMC you are well advised to remain on centerline. If it is
VMC, and your equipment will support parallel track offset, then 1/10 of a mile
is probably going to mitigate your concerns. But, if operating IFR it is
essential to terminate offset operations before beginning an instrument approach
procedure.

If your equipment won't support parallel offset, then your only remaining option
is to fly the autopilot in heading mode and try to keep the XTRK error at some
value approximating 1/10 of a n.m.

Icebound wrote:

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).

So an eastbound VFR/IFR aircraft descending from 7500/7000 to his
destination, was more than likely to avoid traffic... on the reciprocal
track passing him by at 6500 or 6000... by some significant horizontal
error-distance, even if they didn't see each other (big sky theory :-) ).

GPS horizontal accuracy with WAAS is already in the order of magnitude of a
Cessna's wingspan, and some are talking about getting it down to mere
inches.

So the question is: If my Westbound Cessna at 6000 feet (with the autopilot
keeping it happily on the GPS-track centerline) meets the descending Bonanza
on the reciprocal track between the same two airports (using a similar
GPS/a-p combo), there is a distinct possibility that the horizontal
clearance may be zero...

...so is there anything in the current crop of GPS and/or Autopilot systems
that allow me to maintain a small cross-track error of my choosing, without
actually entering off-navaid-off-airport waypoints? ...or do we care; am I
overly concerned???

"Icebound" wrote in message ...
So the question is: If my Westbound Cessna at 6000 feet (with the autopilot
keeping it happily on the GPS-track centerline) meets the descending Bonanza
on the reciprocal track between the same two airports (using a similar
GPS/a-p combo), there is a distinct possibility that the horizontal
clearance may be zero...

...so is there anything in the current crop of GPS and/or Autopilot systems
that allow me to maintain a small cross-track error of my choosing, without
actually entering off-navaid-off-airport waypoints? ...or do we care; am I
overly concerned???

As others have pointed out, the GNS 480/CNX 80 does allow a
cross-track error. I did want to make one small philosophical
point...here in the deep south, we generally get direct routing. If
you have the capability, and you ask for the direct routing, you
generally are increasing your safety (by an admittedly tiny amount).
Further, anyone who is flying VFR and uses the airways is also
increasing his/her risk by that same small amount.

I do doubt that ATC will descend you through another aircraft on the
same airway, so the chances of meeting an IFR aircraft is
diminishingly small, but your concern is most likely regarding the VFR
aircraft sharing the airway.

Hence this discussion is of the "how many angels can dance on the head
of a pin" sort...making it perfect for USENET. And the take-home
points are(IMO) fly direct whenever allowed. Generate your courseline
from some NON-standard point. That is, don't go direct from some
intersection or from the runway...go from some offset point...avoiding
the other guy (most likely VFR) doing the same thing from your
destination. Don't use airways unless you must. And when you fly the
Atlantic (in a non-radar environment)...or the non-radar environments
of the USA...use SLOP!

Jim

Jim Harper wrote:

. And when you fly the
Atlantic (in a non-radar environment)...or the non-radar environments
of the USA...use SLOP!


SLOP is approved for oceanic. Do you know if it is approved for non-radar environments of the USA? If so, what
are the numbers to use? How do I determine when I am in a non-radar environment in the USA?

wrote:

How do I determine when I am in a non-radar environment in the USA?

When you hear "radar contact lost" or when you don't hear "radar contact".

wrote in message ...


SLOP is approved for oceanic. Do you know if it is approved for non-radar environments of the USA? If so, what
are the numbers to use? How do I determine when I am in a non-radar environment in the USA?

As a previous poster posted, you are in a non-radar environment when
you hear "radar contact lost". You are also in a non-radar environment
when you see mandatory reporting points. I believe that this mostly
applies to the great wide West.

No, SLOP does NOT apply, nor is it approved for the USA non-radar
environments. You are expected to fly right down the middle of the
airway.

Now, with respect to the ongoing argument about will/can you run into
someone on an IFR flightplan? Sure you can. I have always been
informed of traffic that the controllers see, so they _are_ trying to
tell us about VFR traffic.

Tell all your pilot friends...if you are going to fly VFR, especially
in IMC do NOT fly on airways. If you are flying IFR and you can do so,
file direct. It's a HUGE sky out there, and the airway and waypoint
system just funnels all of the traffic (which is using it) into a very
small portion of that huge sky. If I am not on an airway, I suspect
that the chances of hazarding upon some fool flying VFR in the clouds
are so small as to be non-existant. How often has it happened (hint,
try zero)? If I am flying IFR in VMC, I have my head out of the
cockpit and on a swivel. And in my (admittedly small) experince, the
controllers are in there pitching and telling me about traffic.

In any of these scenarios, does cross-track offset help? Nah, not
really.

Jim