Clarification about the term "GPS Shutdown"

[Alan Browne]

Alan Browne writes:




Famous last words.

No. Simple fact of design for the reasons mentioned. The actual details are
detailed and moderately complex and in any case I cannot reveal them for
confidentiality reasons regarding with my former employer and its customers.
You could, with some Googling, figure out half or more of them with a reasonable
chance of getting them right.

That was the basis for my statement.

Which is as wrong as it is misinformed (as almost all of your questions and
statements have shown).

BTW: I have no problem with your ignorance, I do have a problem with your
inability to accept the replies to your questions.

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

Go get a book on INS. Heading is one of the most stable, drift free issues of
INS. The heading source used by the pilot on INS equipped aircraft is usually
the INS, but can also be from a magnetic source (usually flux valves mounted in
the wingtips).

Face it Mx. The air navigation system, like all things aeronautical is the
result of slow, plodding, conservative evolution with safety being the first
gate. Integrating GPS into it (interating, not replacing things) was predicted
in 1990 (ish) to take upwards of 25 years. Well, it's 2004, and GPS is still
not on all long range commercial aircraft, such as I have mentioned elsewhere.

INS has done the job handilly since the 60's with ever increasing accuracy.
I've mentioned 2 NM/hr as being common today and that is the spec in most air
transport INS. The same companies such as Litton, Honeywell and Thales who make
these systems have military INS with accurcies of 1 NM/hr and better. The same
technology could be used for air transport at a higher cost, size and weight.
It is simply not required whether or not GPS is present.

Gander and Shannon don't reach the Pacific.

Sigh. The North Pacific has its own control centers which you can look up for
yourself. The 'middle' Pacific too.

Line of sight transmission
at 30,000 feet is about 300 nm, as I recall.

HF is not line of sight.

What ATC?

Stop asking the same question over and over. The major Oceanic route areas have
oceanic control centers.

Ah ... most, therefore not all?

Cheap shot, I completed the thought below.

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

No. They are so few and so far apart that there is no need for control. They
have HF to talk to their dept/arrival points (or many other places).

 

[Alan Browne]

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

For an air transport GPS receiver (TSO-C129a compliant), integrity comes from
several sources including RAIM (Receiver Autonomous Integrity Monitor). That,
in a simplistic nutshell, validates each satellite pseudo-range as fitting the
current PPOS within a certain tolerance. If it fails to meet that tolerance,
the satellite data is deemed unsafe and not admitted into the PVT solution.
(You would need a minimum of 5 sats to determine which particular one is unsafe;
4 sats would result in just saying the GPS nav solution is invalid if one of
them was invalid ... unless of course the altimeter was also being used as a
virtual ctr-of-the-earth satellite (and this is a common feature of most air
transport GPS'). Given all of the above, when you fall below 4 validated sats
(incl. altimeter) the solution is then invalid and flagged as such.

(I don't recall offhand how 2D solutions are flagged in a TSO-C129a receiver.
If integrating a GPS to an FMS or INS, I would reject such solutions and go INS,
RNAV or even DR (in that order) and wait for the number of available sats
solution to improve.)

SBAS (WAAS) provides further integrity warnings where available.

INS, before accepting the GPS data, verifies that it is close to the predicted
value that it should be. If it is outside of tolerance, it is not used for the
update. This can be quite simple or part of a Kalman filter, which in turn has
many checks before taking in data for the nav solution.

All of the above is occuring at at least 1 Hz.

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

Integrity in an Air Transport navigation solution is crown prince, if not king.

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.

Yes. Although "federated" would be the right word, as opposed to hybrid.
Hybrid INS/GPS is a deeper solution with more of the GPS data (including pseudo
range and psudo-range-rate being more directly integrated into the INS. I can't
describe it much further). For shorter range aircraft (non-Oceanic), this means
lower grade INS (lower cost) can be integrated.

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


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

Not sure. In any case you have a lot of things drifting in an INS. The
platform drift (HPR angles) and accelerometer integration (I mean literally
stepwise integration of the accelerometers to get velocity data) bias and random
walk, there is a thing called the Schuler oscillation which can be likened to a
pendulum with a lenght of 1/2 earth diameter and this drags the platform around
(I don't know the physics at all of this).

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:

(Most E-W are overnight, most W-E are during the day (N. Atlantic).

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

East and westbound a/c are at different altitudes. Usually the visibility is
quite good at 30 - 40,000'

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

Nope. Why would you lose voice comm. There are two means: the classic HF,
still in use and SATCOM.

There is no radar whasover over the ocean (for ATC, anyway).

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?

Yep. But more like 20 hours. Winds have nothing to do with it. You're all in
the same air and the fact that it is moving relative to the earth has nothing to
do with all the aircraft in flight in that same air. (Two boats on the river
will both drift with the river and not colide due to the river movement).

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.

Enjoy. Though do find business class standards have fallen in the past few
years, and now plastic cutlery!

there is no



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?

Yeah. They're main concern (ATC) is separation and 'feeding' the flights into
the European airspace (or NA coming back), and in feeding the continental
traffic into the Oceanic. Once there, everyone follows the protocol and
everyone is happy.

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

No. Since the ATC component doesn't even *know* where you are based on a voice
report, they can't 'guide' you. In the CNS/ATM env., the nav system of the
aircraft is reported to them by datalink, but you have the same data anyway.
ATC will just have a more up to date and accurate position for each aircraft.
This will permit tighter separations, but by no means close.

I'm a commercial pilot (light stuff) and flight instructor (not currently).
When I was 15 and learning to fly, my father (a pilot) said something like
'pilots need to do 3 things, and in this order: "Aviate, Navigate and
Communicate"'.

And that is really how it is all set up.

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")?

I'm not sure what you're referring to specifically. Over land and under enroute
facilities covered by radar, the separation can be closer. For instance, the
RNP in Europe is now 5 NM (nav system accuracy) and the altimeters are required
to be more accurate such that less vertical spacing can be used togehter with
less lateral sep.

Oceanic requires greater spacing because ATC does not 'see' the aircraft, and
because the accuracy of the nav systems is not as great. This is now being
tightened up with SATCOM allowing a closer to real time snapshot of where
everyone is, more accurate nav (GPS) and TCAS which alerts aircraft to other
aircraft in the vicinity along with collision prediction.

Ooops. I left TCAS out of the whole discussion didn't I? In the remotest
chance of two aircraft on a collision course, both crews will be informed, and
in fact told what to do by the TCAS system.

Cheers,
Alan

 

[Mxsmanic]

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)

We don't know exactly how GPS would be "turned off."

 

[Mxsmanic]

Again, while GPS functions, yes, higher densities will be attainable but the
seperation distance will not shrink in proportion to the nav accuracy
improvement.

Sure it will. That's the whole idea of improved accuracy in navigation.

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.

The P code was originally in the clear as well, as I recall. When
"anti-spoofing" was added it became the Y code, and was thereby
encrypted, which it has remained ever since.

 

[Alan Browne]

I haven't seen a LORAN in an airplane in decades. I'm not even
sure LORAN is available outside the USA.

In some places, but it is not that common. LORAN-C is at its origin a marine
nav system, not aviation. This is why there is good coverage on the coasts
(Pac, Atl, Gulf), the Great Lakes, the St-Lawrence and the Mississippi. There
are otherwise large gaps in US coverage between the coasts and the Mississippi.

Primary for oceanic flights is GPS.

Primary on aircraft equipped with it, but not sole means. One example I've
mentioned are KLM MD-11's. No GPS. Fly N. Atlantic daily AMS to many US/CDN
destinations. Most 747-200's don't have GPS, probably never will and N.
Atlantic is no problem as you're out of contact with a net of DME's for less
than 6 hours in any case. Older 747-400's don't have GPS, and I doubt very many
will be upgraded.

It's the only thing that provides the precision required for
flights on the new narrower tracks for flights over six hours.
The backup (primary on older airplanes) is INS, but that drifts
too much to be used after six hours.

The separations on Oceanic tracks are 1 degree of latitude, or 60NM and at least
10 minutes along track, which at 500 kts is about 80 NM. These separations are
beign reduced with higher RNP as you say above, but the the reduction in
separation is nowhere close to the RNP value (10 NM).

In the emergency being discussed here, even after reduced separation
requirements, the amount of drift in an INS will prevent collision for much more
than 6 hours.

In a nutshell, GPS is neccesary to achieve RNP for more than 6 hours or so, but
in an emergency, the lack of that performance is not going to cause any colisions.

In any case, as all air transport aircraft above 20 (?) seats are equipped with
TCAS, so even if two air transports got close over the ocean, there would be
more than sufficient warning. (Can't remember the exact number of seats, but
around there... some mutterings of this number being reduced further).

The majority of the airports in the world (and in the USA) don't
HAVE ILS.

They don't need to. OTOH the majority of airports used for air transport (and
many that just do bus jets or gen av) do have ILS, and they need it.

There's already talk of decomissioning the VOR/DME and possibly
the ILS system in favor of GPS.

There will be limited reductions, not elimination.

 

[Mxsmanic]

Which is as wrong as it is misinformed (as almost all of your questions and
statements have shown).

If your statement was wrong and misinformed, it's not my problem.

Sigh. The North Pacific has its own control centers which you can look up for
yourself. The 'middle' Pacific too.

And the South Pacific? And these are all in continuous communication
with aircraft?

HF is not line of sight.

HF meaning ATC bands? If so, how can these channels be reused?

 

[Alan Browne]

Alan Browne writes:




If your statement was wrong and misinformed, it's not my problem.

It wasn't. Your prev. declarartions and answers do show complete ignorance of
aviation navigation and I really can't help that. To the degree I've replied
here is merely the tip of the iceberg. So, go get educated.

And the South Pacific? And these are all in continuous communication
with aircraft?

I really don't know much about ops in the S.P. Aircraft there can communicate
by HF, if not with ATC, at least with their company. And most Air Transport
aircraft today on long routes have SATCOM, so communication with the company
base is easy, and passengers can phone home, too on the same system. (Airborne
high gain SATCOM has 6 voice/data channels, last I looked. Now they're looking
at higher BW and highspeed internet for your laptop via satellite).

These same SATCOM channels (and more) will be used for ATC as CNS/ATM evolves.

HF meaning ATC bands? If so, how can these channels be reused?

HF meaning High Frequency. Which despite its name is fairly low frequency by
today's standards and propagates quite far out of line of site. (Think of
"shortwave" and the great distance that covers).

Comms radios on aircraft are generally the aviation VHF (118.00 through 136.975
MHz) I might be off on the last bit by 50 Khz or so.

UHF, rarely used by air trasnport now, but often used by the military for ATC is
up in the mid-200 MHz or so. Never paid attention to that.

Oceanic generally requires HF which is at about 3 MHz through about 20 MHz for
aviation use. I don't recall the exact range of frequencies.

I have no idea what you mean by "reused".

 

[Alan Browne]

Alan Browne writes:




Sure it will. That's the whole idea of improved accuracy in navigation.

Read a little more carefully: "...will not shrink in proportion to the nav
accuracy improvement." and ponder "proportion".

The P code was originally in the clear as well, as I recall. When
"anti-spoofing" was added it became the Y code, and was thereby
encrypted, which it has remained ever since.

It was always designed to be encrypted (Y). P is (Y) decrypted. In the early
days of GPS, most sats were not operating encrypted. When the system reached a
certain maturity, they switched to full time (Y) code.

 

[Iolaos]

land.

INS doesn't go bad THAT fast, unless you're feeding it wrong
information.

The applicable regulation says that you need an update every 6.2
hours; it doesn't matter what you claim your INS will do.

This thread is about turning GPS off, however, not about
turning SA back on (with ludicrously huge errors, too)

Right now, that would not be a disaster.

In ten years, I expect that it would be.

And once again, it really doesn't matter what you or I think -
the possibility of the USA turning off GPS is what prompted the
EU to build Galileo. They wouldn't have decided to spend that
much money on a duplicative system if GPS were dependable.

 

[Mxsmanic]

And once again, it really doesn't matter what you or I think -
the possibility of the USA turning off GPS is what prompted the
EU to build Galileo. They wouldn't have decided to spend that
much money on a duplicative system if GPS were dependable.

Sure they would have. They're more interested in national pride than in
technical necessity. Besides, the US will be able to disable Galileo,
too, and even if it could not, with so many European hands in the system
there will be even more people who can disable Galileo than there are
who can disable GPS.

 

[Mxsmanic]

And most Air Transport aircraft today on long routes have
SATCOM, so communication with the company base is easy,
and passengers can phone home, too on the same system.

So all they have to do is call Detroit or Atlanta or wherever and ask
them to place a call to ATC and ask for instructions, eh?

These same SATCOM channels (and more) will be used for
ATC as CNS/ATM evolves.

Communications that depend on _satellites_? Why, that's just like GPS.
If GPS is untrustworthy as a means of navigation, why isn't SATCOM
untrustworthy as a means of radiocommunication?

It seems that some technologies are selectively favored, and others are
eschewed, but I don't see any technical reasons for this.

I have no idea what you mean by "reused".

The same frequencies are reused by transmitters at different locations.
If lines of sight were not limitations on transmission, this would be
much more difficult. The line of sight at 30,000 feet covers a circle
with a radius of 300 miles or so.

 

[Juergen Nieveler]

We don't know exactly how GPS would be "turned off."

Presumably they'll switch off the transmitter for the civilan
frequency while leaving the military one on?

On a related note:
http://news.bbc.co.uk/1/hi/sci/tech/4115761.stm

The first four Galileo sats have been ordered

Juergen Nieveler

 

[Juergen Nieveler]

And once again, it really doesn't matter what you or I think -
the possibility of the USA turning off GPS is what prompted the
EU to build Galileo. They wouldn't have decided to spend that
much money on a duplicative system if GPS were dependable.

Indeed, this is the main point. Whatever you, I or Mxsmaniac think
won't influence the decision of the President or the Pentagon wether
or not to shut down GPS. They do what they think is necessary, and
they won't give a smeg if we go complaining.

Juergen Nieveler

 

[Mxsmanic]

Indeed, this is the main point. Whatever you, I or Mxsmaniac think
won't influence the decision of the President or the Pentagon wether
or not to shut down GPS. They do what they think is necessary, and
they won't give a smeg if we go complaining.

In free countries with elected governments, the elected officials are in
fact very concerned with complaints they receive from the electorate. I
realize that this isn't the case in many of the more primitive societies
of the world.

 

[jon]

Nor do "we" because if "we" have a certified receiver, "we" have
Integrity. "We" don't care how the outage is manifested as it is the
receiver which is able to detect when it matters to providing a
navigation solution that can be trusted as often as the Integrity
requirements specifies.

If you run a mission critical app w/o using a certified receiver, you
assume the risk associated with the current constellation which does
not provide sufficient Integrity to support the mission critial app.

Again, you need to go read up on what Intergrity actually means. Until
you do, you will continue to be stuck in a mental corner of your own
choosing, while those with integrity continue on.

Want some more?

 

[Alan Browne]

Alan Browne writes:




So all they have to do is call Detroit or Atlanta or wherever and ask
them to place a call to ATC and ask for instructions, eh?

No. Any aircraft flying an uncontrolled route has no obligation or need to
coordinate with ATC until they are in range of a controlled area. Calling
"home" is more for logistical reasons.

Communications that depend on _satellites_? Why, that's just like GPS.
If GPS is untrustworthy as a means of navigation, why isn't SATCOM
untrustworthy as a means of radiocommunication?

It seems that some technologies are selectively favored, and others are
eschewed, but I don't see any technical reasons for this.

Different satellite network entirely. INMARSAT run SATCOM (originally for the
marine market). Integrity is demonstrably high. It is not likely to be used by
terrorists except to make expensive phone calls.

I never said GPS was untrustworthy, so stop bringing that up. Again, this whole
thread is about the unlikely event that GPS goes off for some short period.

The technical reason for SATCOM is gives near perfect coverage, HF does not. It
gives interference free communications (HF can be noisy). SATCOM can provide
for moderately high data rates. (HF wuld be low). The CNS/ATM plans for
oceanic take advantage of GPS, SATCOM and the sensor suite aboard the aircraft.

The same frequencies are reused by transmitters at different locations.
If lines of sight were not limitations on transmission, this would be
much more difficult. The line of sight at 30,000 feet covers a circle
with a radius of 300 miles or so.

VHF frequencies have fairly high voice traffic, HF do not. More and more
oceanic traffic will move to med-gain and high gain SATCOM. Further, as more of
the information provided to ATC (and received from them) will be data messages,
the amount of bandwidth will go way down (On SATCOM).

 

[Alan Browne]

they might


(East or



He has to do it within 10 nm of his assigned track to avoid
hitting other aircraft.

Not at all. RNP 10 means the position is w/i 10 NM. The separation from other
aircraft is much larger than the RNP. Current lateral separation is 60 NM.

I'm not sure where this is going, but one proposal calls for 30 NM lateral (with
RNP-10). Along track would be 30 NM too, but that doesn't matter as everyone
should maintain their Mach No.
http://www.faa.gov/programs/oep/v6/smart sheets/er/er-6 v6.htm

So even if, after GPS goes TU in this fantasy, and two INS' decided to drift
laterally towards each other at the worst drift rate, it would take 7.5 hours to
drift towards each other.

And then, of course, TCAS would alert both crews long before a colision occured.

Keep in mind, we're discussing an emergency, not normal conditions.

 

[Alan Browne]

have at least an NDB



All of which may be eliminated within the next decade in favor
of GPS.

"The phase-down of the VOR/DME and TACAN network is expected to begin in 2010.
The proposed phase-down will transition from today’s full coverage network
through an interim network and then to a minimum operational network. The
minimum operational network will support IFR operations for the busiest airports
in the NAS. A further 3-7 reduction may be possible to the level of a basic
backup network. Section 3.2 discusses the transition in more detail."

"As the GPS-based approach systems (WAAS/LAAS) are integrated into the NAS, and
user equipage and acceptance grows, the number of ILS systems will be reduced.
The phase-down will transition from today’s full coverage network through an
interim network to a minimum operational network and possibly to a basic backup
network. The minimum operational network will support IFR operations for the
busiest airports in the NAS. Section 3.2 discusses the transition in more detail."

"The phase-down of Category I ILS is expected to begin in 2010. The FAA does not
anticipate phasing out any Category II and III ILS systems. Until LAAS systems
are available, new and upgrade Category II and III precision approach
requirements will continue to be with ILS."

--US Federal Radionavigation Plan, 2001. (Next ed. 2006).

 

[Mxsmanic]

No. Any aircraft flying an uncontrolled route has no obligation or need to
coordinate with ATC until they are in range of a controlled area. Calling
"home" is more for logistical reasons.

So there's no ATC out there on the ocean.

Different satellite network entirely. INMARSAT run SATCOM (originally for the
marine market). Integrity is demonstrably high.

How do you demonstrate integrity?

It is not likely to be used by
terrorists except to make expensive phone calls.

What does that have to do with it?

The technical reason for SATCOM is gives near perfect coverage, HF does not. It
gives interference free communications (HF can be noisy). SATCOM can provide
for moderately high data rates.

How many channels does it provide?

 

[Alan Browne]

Alan Browne writes:




So there's no ATC out there on the ocean.

"controlled area". Where there is high traffic (N. Atlantic, N. Pacific) and
other areas, yes there is, as variously described to you. (I'd think it would
sink in by now). Where the number of aircraft crossing is low, there is little
need for control.

How do you demonstrate integrity?

Performance. It's there when you need it. (I don't know how INMARSAT
demonstrate integrity as a system. I do know that it is highly reliable. It
is, among other things, an emergency communications service for mariners.

How many channels does it provide?

Overall, I don't know. Do your own research. A single high gain antenna system
on an air transport provides 6 channels of voice or data. At a given time only
so many channels are available to the passengers for in flight calls, one or two
are reserved for the aircraft. Some airlines have integrated their aircraft
such that performance data is automatically relayed to the maintenance centre at
a specified interval, on certain conditions or on demand.