Clarification about the term "GPS Shutdown"

[Mxsmanic]

Except when AWACS covers it, of course.

Which is pretty much never.

However, there's no NEED for radar coverage over oceans, flight
plans are calculated to make sure there's no spacing problem.

Provided that all aircraft have reliable navigation methods.

 

[jon]

Which is pretty much never.


Provided that all aircraft have reliable navigation methods.

that don't depend on just GPS.
Be sure to reach harder for the ring this time 'round the carousel.

 

[Iolaos]

that don't depend on just GPS.

It does if you're in RNP 10 airspace more than 6 hours from land.

 

[Alan Browne]

So far here he's merely providing light amusement. It'd be more fun if he
knew anything at all about navigation. Like, for instance, the points of
the compass, the shape of the world etc....

He does have a good sense of up and down... from falling on his ass a lot.

;-)

 

[Alan Browne]

We could answer his point of "Which hemisphere are you on" with the
classic "In the southern hemisphere everything is upside down" ;-)

Does that mean they really drive on the right in Aussie after all?

 

[Alan Browne]

Ed Seedhouse writes:




Crossing the Pacific, yes, but one tracks only a few nautical miles
wide? With other aircraft only a few miles away?

In trail dist is 10 - 15 minutes (60 NM or so)
Lateral sep is 1 deg of latitude (60 NM).

Pilots used to land with only a visual approach to the runway. Today
they still can, if the weather is clear. But for zero-visibility
approaches, special instruments are required, and if a pilot has to land
in zero visibility without them, his prior experience in visual
approaches is just as useless as no experience at all.

To be in low vis flight, you must be IFR qualified and the aircraft IFR
equipped... it's been like this since before WW II (with ever increasing
sophistication, of course.

0 vis approaches are the exclusive domain of CAT IIIc aircraft, runways and
aircrew. Most low vis approaches take place between much less stringent CAT II
and CAT I (or non-prec which is even less stringent).

What sort of backup will work in midocean, far from any ground
transmitter?

Not needed. INS is quite capable. eg: MD-11's run by KLM do not currently have
GPS. The costs of integration and certification would be horrendous ... to
enhance a system that is highly capable.

I'm tired of repeating all this Mx, so I hope it will sink in soon, GPS is not
the linchpin of aviation guidance. It is a preffered primary source for
accuracy. It is not currently on all transport aircraft and the older ones like
747-200's will, in the main, never be so equipped.

Cheers,
Alan.

 

[Alan Browne]

Ed Seedhouse writes:




Each increase in traffic density over the oceans makes older navigation
methods obsolete. Once this density requires GPS accuracy, there's no
turning back the clock.

See what I said about adjusting to emergencies. What's up stays up w/o change,
and what wants to up might be delayed to increase spacings during the emergency.

 

[Alan Browne]

Ed Seedhouse writes:




Other words are not necessary. If you put three balls on a pool table
and set them in motion, they aren't likely to hit each other very often;
if you do the same thing with 200 balls, they'll hit each other
regularly. Thus, precise navigation becomes more and more necessary as
the density of traffic rises. Simple math.

Fortunately for all concerned that is not how air navigation is set up. Many of
my replies to you yesterday refer.

 

[Alan Browne]

Alan Browne writes:




So if GPS is disabled in a way that produces inaccurate fixes, the INS
will be corrupted, and there won't be anything at all left.

You can't use one system to back up another if the back-up system
depends on the primary.

Oh dear. First of all, TSO-C129a compliant GPS' have an algorithm called RAIM
(Receiver Autonomous Integrity Monitor) which will quickly exclude sats that are
broadcasting screwy data. The GPS signals to the INS when its own solution is bad.

Secondly, the INS does automatically accept large changes in position, velocity
or fot that matter Range/Range-rate data from the GPS.

When the GPS goes down, the INS will be in safe, valid, accurate state.

Until they start to drift off course. Of course, they may collide at
"only" 30 knots or so.

Read what I've written about spacing and INS drift rates when unaided.

That's how they start out, but that's not how they stay.

There heading doesn't change (after a while the INS HDG error builds but it
takes a very long time). The airspeed of the aircraft reamians constant if the
power setting is unchanged (actually it slightly powers back as fuel is burned,
the aircraft is lighter and less lift is required and hence less induced drag
occurs, but that's fine detail for this discussion, in short, the auto throttle
maintains a constant Mach no.)

What ATC? This is thousands of miles from anywhere.

It's there. Atlantic Oceanic routes are controlled from Gander and Shannon.
Pilots make position reports at every 10° longitude crossing (as there is no
radar).

[As CNS/ATM develops, this reporting will be automated via SATCOM (Some routes
have this in limited test already). The enabler here is mainly SATCOM]

How does everyone else in the area know about this speed change?

Any speed change would be at ATC request via HF on SELCAL. But as I said
elsewhere, in such an emergency, the best course would be to moniotr, not to
begin changing things. Only new aircraft entering the system would be delayed
to transition to a lower density of traffic in the system.

What ATC?

See above. Most Oceanic routes are covered. There remain areas that hardly
need it as traffic density does not merit it.

Cheers,
Alam

 

[Alan Browne]

Alan Browne writes:




How do they do that for flights already out over the middle of the
Pacific, now deprived of the only navigation method that will keep them
on course with a safe degree of accuracy?

See other replies I've given you in this respect. GPS is not the sole means of
navigation.

 

[Alan Browne]

I'm not sure if a theta/rho solution (VOR/DME) is an available update in
the systems, that would do it too w/i about 0.5 NM at a range of about
28 NM from the station. (Assuming a 1° radial error which is probably
generous).

Coincidence: I ran into one of the engineers from my old company below and he
confirmed the theta/rho (VOR/DME) was not used as an RNAV solution for the
reason cited above.

Cheers,
Alan

 

[Fafnir]

A gyroscopic compass doesn't show you the magnetic north, it shows
true north. Airplanes tend to carry at least one of those

nowadays...

A gyroscopic compass has no concept of either true or magnetic
north.

It simply stays pointed in the direction to which you set it.

 

[Mxsmanic]

Not in my experience. I rarely find myself teleported into the middle of
the wilderness with no clue where I am.

It's a problem for the military, whence the motivation for GPS.

 

[Mxsmanic]

And if you send three pool balls off along a bowling alley in the same
direction at the same speed, they don't hit each other. Even if you put 200
poolballs there, they still don't collide. Simple Math.

Try it and see.

 

[Mxsmanic]

See other replies I've given you in this respect. GPS is not
the sole means of navigation.

It is the only means of navigation that will allow an aircraft to
navigate indefinintely without land radio contact and with precision
great enough to allow commercial traffic levels at modern densities
while doing so.

If GPS is so unnecessary in so many situations, why did the military
spend billions of dollars to develop it?

 

[Mxsmanic]

When the GPS goes down, the INS will be in safe, valid, accurate state.

Famous last words.

Read what I've written about spacing and INS drift rates when unaided.

That was the basis for my statement.

There heading doesn't change (after a while the INS HDG error builds but it
takes a very long time).

If the INS error is building, the heading is changing.

It's there. Atlantic Oceanic routes are controlled from Gander and Shannon.

Gander and Shannon don't reach the Pacific. Line of sight transmission
at 30,000 feet is about 300 nm, as I recall.

Any speed change would be at ATC request via HF on SELCAL.

What ATC?

See above. Most Oceanic routes are covered.

Ah ... most, therefore not all?

There remain areas that hardly need it as traffic density does not merit it.

Circular reasoning, no? It matters a lot to the aircraft in those
areas.

 

[Iolaos]

that don't depend on just GPS.

It does if you're in RNP 10 airspace more than 6 hours from land.

 

[Juergen Nieveler]

It does if you're in RNP 10 airspace more than 6 hours from land.

INS doesn't go bad THAT fast, unless you're feeding it wrong
information. This thread is about turning GPS off, however, not about
turning SA back on (with ludicrously huge errors, too)

Juergen Nieveler

 

[jon]

You need to go learn what "Integrity" means. Perhaps start with what it
meas in the aviation navigation realm, before tackling any other realms

Oh dear. First of all, TSO-C129a compliant GPS' have an algorithm called RAIM
(Receiver Autonomous Integrity Monitor) which will quickly exclude sats that are
broadcasting screwy data. The GPS signals to the INS when its own

solution is bad.

There's that word "Integrity" that keeps popping up. Alan, you seem to
have a thing for that word, don't you?

Secondly, the INS does automatically accept large changes in position, velocity
or fot that matter Range/Range-rate data from the GPS.

When the GPS goes down, the INS will be in safe, valid, accurate

state.

Oh, so if I understand you correctly, a GPS/INS hybrid operates with
INS serving as the primary and receving updates from GPS (when
available sufficient integrity). And this allows for periods of GPS
unavailability.

Thanks for clearing things up for me, Alan. I was about to fly (heh)
off the handle.

Read what I've written about spacing and INS drift rates when unaided.

There heading doesn't change (after a while the INS HDG error builds but it
takes a very long time).

I recall something like 1/100th of a degree per hour (gyro) which
translates to something like around 1 NM per hour?

The airspeed of the aircraft reamians constant if the
power setting is unchanged (actually it slightly powers back as fuel is burned,
the aircraft is lighter and less lift is required and hence less induced drag
occurs, but that's fine detail for this discussion, in short, the auto throttle
maintains a constant Mach no.)

Just a shot in the dark here Alan (most of the trans-oceanic flight
I've been on have been at night , but let me see if I grasp the
concept:

So, the lateral separation standards as well as the in-trail separation
standards are set sufficient high enough to account for the worst-case
error rates... and then some? I think I remember seeing an acronym TSE
(Total System Error).

So, assuming I'm flying Logan -> Frankfurt for a visit with the good
folks over at www.dfe.de , and one of my colleagues is returning home
on the 10:30AM (I bet it's still Flight #LH422 ), there's enough pad
built into the design of the routes and the air traffic requirements
and procedures such that if we were crossing the ocean at the same time
in 0 visibility AND we both lost GPS (Bush had a brain synapse and it
shut down, so backup receiver not an issue) for the entire time AND we
both lost every other navaid except INS....<takes a deep breath> AND we
both lost all of our voice comms after going out of radar contact so
both ATC was plotting increasingly less confident estimates of our
positions AND we couldn't boradcast any kind of "May Day" to any other
aircraft in the area who might be within range but far enough away to
engage evasive maneuvers AND we both lost our Collision Avoidance
goodies....... so, both of our INS' had been drifting the most it
possibly could before we get "near" each other (i'm assusming somewhere
around 3 or so hours each, given head/tail winds)..... that we STILL
wouldn't close enough to each other?

I'm not a pilot, but I want to be able to eat my little snacks on board
with some degree of comfort level, Alan. Please advise.



Ah, radio comms. Isn't technology great?

It's there. Atlantic Oceanic routes are controlled from Gander and Shannon.
Pilots make position reports at every 10° longitude crossing (as there is no
radar).

Ok, so what you're saying is the they make noises with their vocal
chords using a communications link to another human. And the controller
is able to know last reported position, heading, speed, etc. and
determine future position (to w/in a different set of tolerances than
GPS or INS, granted), yes?

So, could that whole link with human and another part of the RF
spectrum, taken togehter, be considered another navaid?

[As CNS/ATM develops, this reporting will be automated via SATCOM (Some routes
have this in limited test already). The enabler here is mainly SATCOM]

How does everyone else in the area know about this speed change?

Any speed change would be at ATC request via HF on SELCAL. But as I said
elsewhere, in such an emergency, the best course would be to moniotr, not to
begin changing things. Only new aircraft entering the system would be delayed
to transition to a lower density of traffic in the system.

What ATC?

See above. Most Oceanic routes are covered. There remain areas that hardly
need it as traffic density does not merit it.

I was under the impression that oceanic separation standards are
greater for some reason, Alan? Do you think it has anything to do with
contingency planning for outages (of any of one or more "navaids")?

Cheers,
Alam

Regards,
Jon

 

[Alan Browne]

Alan Browne writes:




It is the only means of navigation that will allow an aircraft to
navigate indefinintely without land radio contact and with precision
great enough to allow commercial traffic levels at modern densities
while doing so.

Again, while GPS functions, yes, higher densities will be attainable but the
seperation distance will not shrink in proportion to the nav accuracy
improvement. If this fantasy GPS outtage occurs, the drift rates in the INS'
will still keep everybody happily separated for 10's of hours, as I've
illustrated to you in other posts. New aircraft entering the system (during the
proposed emergency) may be delayed to increase separations for the duration of
the outtage.

This whole discussion is also missing the CNS/ATM evolution of which SATCOM is
another enabling component. In a nutshell, one part of this allows for the data
linking of position reports to oceanic controllers automatically and at a higher
rate than the HF voice relayed reports. This will allow oceanic controllers a
near real time view of the air traffic, and hence narrower (but by no means
'real close') separations between aircraft. The positions of aircraft are
reported by the FMS, which in turn is using the INS or GPS PPOS. And (again),
unbounded INS errors would take tens of hours to result in a lateral collision,
requiring perfectly opposing drift directions... not at all likely on many
counts. Regarding traffic fore and aft, well, they're traveling at the same
speed and direction and separated by 60NM or so, so impossible to colide.

When approaching land many variations on the theme can occur. One is flying a
radial towards a single VOR station using the VOR receiver in which case the
error is well within airways tolerances. VOR's can be tuned at 35,000 from
about 150-200 NM away. Another is that as soon as an acceptable geometry occurs
a DME-DME update to the position will reduce the error well within acceptable
accuracy.

If GPS is so unnecessary in so many situations, why did the military
spend billions of dollars to develop it?

They didn't develop it for commercial use, but they were politically astute
enough to allow the C/A to be non-encrypted for civil use in order to secure
funding. Prior to GPS commissioning as a full constellation, commercial
airlines were doing quite fine with INS and VLF/Omega for long range navigation,
the airways system (NDB, VOR) for short range over land, and the Microwave
Landing System (MLS) was in development to improve upon the post WW II ILS
system that was (and still is) the precision approach system.

Why did the military develop GPS? In the mid 70's, when the whole idea got
started, there were dozens of navigations systems from short range radio
navigation (VOR, DME, TACAN, NDB, RNAV, LOC, ILS, ...), INS, emerging VLF/Omega,
Doppler Radar, LORAN-A/C/D and probably a few others I can't remember. The Brits
had other systems such as ... hmmm, can't remember, but similar to LORAN.
Decca, I think.

The systems all had their own data formats and performace differences according
to their principle of operation. There were wonderfully complex integrations
(Doppler Radar to bound INS velocity drift; Omega/VLF to bound INS position
drift). Datalinks had been emerging sicne the 60's, but inter service system
integrationw as extremely difficult. (It was bad enough within groups within
the same service). This doesn't even begin to touch on the needs of the surface
Navy, grunts, artillery, cavalry and the emerging army aviation needs for
accuracy. Systems like DME, TACAN and Doppler radar are also emmisive,
requiring the aircraft to transmit to use the system, and this is taboo in
modern warfare: be passive. (TACAN, was a double jeopardy as ships at sea had
TACAN stations, as well as tanker aircraft. "I'm here, shoot me")

DOD recognized the problem and decided to get "above it" and go for one system
that could satisfy almost all needs for all services. GPS. It would provide
accurate navigation and timing and help all services evolve to a common
communications format for everything navigation related. (They're still not
there... a destination that won't be reached fully for another 10 years, I
expect). And all 'users' have passive receivers.

(ILS with only 40 channels was experiencing congestion in some high density
regions like LA and certain parts of Europe and Asia. Further, MLS was much
more precise than ILS and could cover a much larger volume of airspace... but,
the promise of what has become LAAS has put MLS to sleep... there are dozens of
MLS appraoches commissioned out there, and the US Mil has portable MLS ground
stations to setup approaches where needed. About 1100 airborne receivers that I
know direclty about were sold to the USAF. There are a few commercial operators
still using it as well, usually with privately owned ground stations).

GPS is of course the wunderkind of navigation for everyone. The expensive stuff
is done by the US taxpayer (Thanks!) in the space and ground segment. The
receiver end is relatively cheap. (Even when second generation 5 channel GPS
receivers cost over $20K, it was considered cheap). Today, for under $1000 a
grunt in the field can have his keyed receiver courtesy of the DOD.... and any
civilian can have a Wal*Mart special for $100 or so.

That's why GPS came about. Not as a grand design to help aviation. That was
secondary and a small part of justification to get funding. (Initial GPS
thought was that only the US+Allies would have access.)

Cheers,
Alan