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

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 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 are not overly concerned, it does present a greater chance of a
collision.

I haven't heard of a GPS unit that allows the user to set some sort of
"offset" from a course to follow, but it wouldn't surprise if such a feature
did exist somewhere.

Beyond that, the "big sky theory" still works reasonably well. Two
airplanes in level flight on opposite headings on the same airway stand a
decently improved chance of running into each other if they are using GPS.
But when at least one is climbing, they share their altitude for such a
short period of time, I would think that the *actual* risk is relatively
low, even if the GPS does significantly increase the risk when compared to a
VOR receiver.

In any case, even before GPS it was still reasonably important to be alert
for other traffic while traveling on airways (even beyond the general
importance of doing so at all times). GPS increases the risk, but the risk
was always there and I've certainly had my share of close encounters (under
1 mile) flying on airways with a VOR receiver.

Pete

"Peter Duniho" wrote:
I haven't heard of a GPS unit that allows the user to set some sort of
"offset" from a course to follow, but it wouldn't surprise if such a feature
did exist somewhere.

The CNX-80 / GNS-480 has it. It's called "Parallel Track". You tell it
if you want to fly left or right of course and by how much (in 1/10's of
a mile IIRC), and it invents a new course line for you to follow.

I believe the Apollo GX-60 had it too. The story I heard was the CAP
wanted to buy a bunch of GX-60's, but insisted Apollo add the Parallel
Track feature to facilitate flying grid search patterns.

"Icebound" wrote in message =
...
=20
So the question is: If my Westbound Cessna at 6000 feet (with the =
autopilot=20
keeping it happily on the GPS-track centerline) meets the descending =
Bonanza=20
on the reciprocal track between the same two airports (using a similar =

GPS/a-p combo), there is a distinct possibility that the horizontal=20
clearance may be zero...


The July 2004 issue of "International Procedures News" from
Flight Safety Inc., carried an item exactly about that,
in regard to the North Atlantic routes.
Here is one paragraph, which I quote from that article:

"Following a successful trial in the West Atlantic Route System (WATRS),
it has been determined that by allowing aircraft conducting oceanic =
flights
to fly lateral offsets not exceeding two NM right of centerline,
an additional safety margin will be provided and will mitigate the risk
of collision when non-normal events such as operational altitude =
deviation errors
and turbulence induced altitude deviations occur."

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---

"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...

"Peter Duniho" wrote in message
...
I haven't heard of a GPS unit that allows the user to set some sort of
"offset" from a course to follow, but it wouldn't surprise if such a
feature did exist somewhere.


My Northstar M3 has parallel track offset, as does the GX-60

"Peter Duniho" wrote:
I haven't heard of a GPS unit that allows the user to set some sort of
"offset" from a course to follow, but it wouldn't surprise if such a
feature
did exist somewhere.

The CNX-80 / GNS-480 has it. It's called "Parallel Track". You tell it
if you want to fly left or right of course and by how much (in 1/10's of
a mile IIRC), and it invents a new course line for you to follow.

I believe the Apollo GX-60 had it too. The story I heard was the CAP
wanted to buy a bunch of GX-60's, but insisted Apollo add the Parallel
Track feature to facilitate flying grid search patterns.

Parallel track and the grid search feature are not related to each other.
The GX series has the search grid lines on its display so CAP pilots can
stay oriented in their search grid. But even that wasn't as useful because
the grid lines they did show were not at the correct level. IIRC, you want
7.5 minute grid lines and the GX only showed down to 15 minute grid lines.

Roy Smith wrote in
:

"Peter Duniho" wrote:
I haven't heard of a GPS unit that allows the user to set some sort
of "offset" from a course to follow, but it wouldn't surprise if such
a feature did exist somewhere.

The CNX-80 / GNS-480 has it. It's called "Parallel Track". You tell
it if you want to fly left or right of course and by how much (in
1/10's of a mile IIRC), and it invents a new course line for you to
follow.

The Trimble 2102 has it, and so do most other units.

--
Regards,

Stan

"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

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.