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Air France A330-200 lost over Atlantic ocean (Page 6)
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ghporter
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May 30, 2011, 06:55 AM
 
Originally Posted by chabig View Post
Air data is important so a lot of thought and redundancy goes into the design of the air data system.

<snip>

As for warm fuzzies about bad things happening on commercial aircraft, I remind you of the industry's unsurpassed safety record. When you step foot onto an airplane, you are safer than you were in the car on the way to the airport.
Got it. The sensor system is much more robust than I had anticipated, and with the press full of dumbed-down "information," it's easy to get an incorrect impression of the situation. I consider myself better informed in technical areas than the usual Joe News Consumer, and I was thoroughly misdirected by what I've read in the news. Your explanation helped a lot.

As for warm fuzzies, that is what is impacted by news like this, and it's what drives public opinion and consumer decisions. I don't think either Airbus or Air France are doing enough to make this less of a PR mess. Considering the other folks on those flights I took, and the kind of dreck they discussed (incessantly in a number of cases), knowing just a little about the AF crash could have had a very bad effect on "public concern" as expressed by the idiots typical travelers on my flights.

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May 30, 2011, 07:07 AM
 
Originally Posted by chabig View Post
Solid engineering goes into the design of these systems. Reference sensors, GPS, turbines(?) are all layman's ideas, with no solid engineering to back them up.
How is GPS a "layman's idea"? Correct me if I'm wrong, but wasn't it developed by the USAF? Don't the USAF do "solid engineering"?
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May 30, 2011, 07:31 AM
 
The idea "why not just use a GPS to determine speed" only makes sense to a layman, not an aeronautical engineer. That doesn't make the GPS any less valuable. What appears like common sense to laypeople, i.e. you and me, doesn't always hold up when you know all the details.
     
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May 31, 2011, 01:47 AM
 
Originally Posted by Doofy View Post
How is GPS a "layman's idea"? Correct me if I'm wrong, but wasn't it developed by the USAF? Don't the USAF do "solid engineering"?
Using GPS derived data as a proxy for airspeed is a layman's idea, much like the previously proposed RAT-in-a-duct for airspeed (which would likely be useless in severe icing conditions).

GPS was developed by USAF as a source of location data. From location, you can determine groundspeed. I don't see why the USAF's engineering of the GPS system is being called into question here.
     
ghporter
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May 31, 2011, 06:44 AM
 
Some of the most precise navigation's possible by using GPS, which can provide location data to within ONE meter in all three axes. Having a flight data computer combine GPS data with mechanically determined data for improved precision is something that is not a "layman's idea," but rather an advanced navigation technique. There is a rather interesting movement in hobby RC aircraft, building "drones," many of which include GPS modules to augment other location and navigation sensors. Military targeting systems use GPS to provide for exceptionally precise positioning of un-manned weapons, allowing not a "pretty close approximation" of an aircraft's location in airspace, but a precise location to within a meter or so. GPS is the planned "next generation" precision landing system, supplanting the relatively difficult to implement microwave-based ILS systems that have been developed, in part because MLS is subject to a number of potentially interfering signals.

GPS is, without using the encrypted "precision" data available to select military platforms, far more precise and reliable (with sufficiently advanced receivers, rather than "cheapest possible" units) than any other single system. It is used for long distance navigation for all types of vehicles; the Air Force's Hurricane Hunter WC-130 aircraft use a combination of inertial, LORAN, GPS and traditional sensors to determine their positions while IN hurricanes.

So, it's a "layman's solution?"

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May 31, 2011, 06:53 AM
 
Yes, because a GPS doesn't measure airspeed. When you're in a plane, groundspeed means nothing, airspeed everything.

All of the examples you gave above will help a plane to determine it's location, none will help it to measure it's speed relative to the air that surrounds it.
     
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May 31, 2011, 07:06 AM
 
GPS can measure location and speed over ground very precisely -- and it is used on commercial aircraft as well, but with no knowledge of the winds at a particular altitude, how are you supposed to measure the airspeed (which is the relevant velocity for the aerodynamics of the aircraft) with the help of GPS alone? I'm a layman, and it wouldn't even occur to me to suggest such a thing.
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May 31, 2011, 07:08 AM
 
Originally Posted by Phileas View Post
Yes, because a GPS doesn't measure airspeed. When you're in a plane, groundspeed means nothing, airspeed everything.
But if your airspeed instruments are measuring 60 kts and your ground speed GPS is measuring 450 kts, you'll have a better idea that your airspeed instruments are screwed. Or you're in a 390 kt tailwind. Surely that's some improvement over not knowing?
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May 31, 2011, 07:23 AM
 
Originally Posted by ghporter View Post
Military targeting systems use GPS to provide for exceptionally precise positioning of un-manned weapons, allowing not a "pretty close approximation" of an aircraft's location in airspace, but a precise location to within a meter or so. GPS is the planned "next generation" precision landing system, supplanting the relatively difficult to implement microwave-based ILS systems that have been developed, in part because MLS is subject to a number of potentially interfering signals.
Sorry to interrupt with non-aviation posts again, but that all scares me, because GPS is trivially easy to jam. (Also illegal to jam and easy to get caught doing it, as long as you don't have a business whose business model depends on GPS interference.

Also, LORAN's dead, Obama killed it.
     
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May 31, 2011, 07:43 AM
 
Originally Posted by Doofy View Post
But if your airspeed instruments are measuring 60 kts and your ground speed GPS is measuring 450 kts, you'll have a better idea that your airspeed instruments are screwed. Or you're in a 390 kt tailwind. Surely that's some improvement over not knowing?
That's not knowing, that just adds another level of unverifiable and confusing data. Also, remember that the nav system does read and analyze GPS data.
     
mduell
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May 31, 2011, 06:32 PM
 
Originally Posted by ghporter View Post
Some of the most precise navigation's possible by using GPS, which can provide location data to within ONE meter in all three axes.
Yes, GPS is a fantastic source of position information.

Originally Posted by ghporter View Post
Having a flight data computer combine GPS data with mechanically determined data for improved precision is something that is not a "layman's idea," but rather an advanced navigation technique.
I don't know what you're measuring mechanically (running some string out behind the airplane?), but you nailed the key word "navigation" when talking about GPS which leads us to your next paragraph:

Originally Posted by ghporter View Post
There is a rather interesting movement in hobby RC aircraft, building "drones," many of which include GPS modules to augment other location and navigation sensors. Military targeting systems use GPS to provide for exceptionally precise positioning of un-manned weapons, allowing not a "pretty close approximation" of an aircraft's location in airspace, but a precise location to within a meter or so. GPS is the planned "next generation" precision landing system, supplanting the relatively difficult to implement microwave-based ILS systems that have been developed, in part because MLS is subject to a number of potentially interfering signals.
... location ... navigation ... positioning ... location ... location ...

Nothing about airspeed... not even the RC guys using GPS as an airspeed proxy... wonder why that is.

Originally Posted by ghporter View Post
GPS is, without using the encrypted "precision" data available to select military platforms, far more precise and reliable (with sufficiently advanced receivers, rather than "cheapest possible" units) than any other single system. It is used for long distance navigation for all types of vehicles; the Air Force's Hurricane Hunter WC-130 aircraft use a combination of inertial, LORAN, GPS and traditional sensors to determine their positions while IN hurricanes.
... navigation ... position ...

Originally Posted by ghporter View Post
So, it's a "layman's solution?"
Using GPS data as a proxy for airspeed is a layman's solution. It "seems like" a good idea, but in no way addresses the real issues.
     
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May 31, 2011, 07:04 PM
 
The wreck was 2 years ago today.
     
mduell
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May 31, 2011, 07:27 PM
 
I think the display of information in the cockpit environment may be a significant gap between members familiar with the operation of transport category aircraft and those who are not.

Transport category aircraft generally have 4-8 displays in the cockpit, 2-3 for each pilot and 1-3 shared. One on each side will always be a Primary Flight Display (PFD) and the remainder will be Multi-Function Displays (MFD). On a MFD the pilot can display navigation information, engine information, systems (hydraulic, electric, etc) information, charts, etc.

The PFD contains an airspeed tape (on the left in this photo) that displays the current airspeed (IAS or TAS) derived from the pitot tubes:



Typically at least one MFD will display navigation information, including groundspeed (in the upper right corner in this flight sim screencap) which may be derived from GPS, ground radionavigation aids, or an IRU:



So the Air France 447 pilots always had or could have had (if they flipped a MFD to the navigation page) the groundspeed available to them if they felt it would be a useful reference. But it doesn't make any sense to put a GPS derived number on the PFD airspeed tape.
     
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May 31, 2011, 07:50 PM
 
Top left on an Airbus MFD.
(Pink if coming off the GPS, white if off the navaids (correct?))
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chabig
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May 31, 2011, 11:29 PM
 
Originally Posted by Doofy View Post
Top left on an Airbus MFD.
(Pink if coming off the GPS, white if off the navaids (correct?))
Close. Airbus displays groundspeed on the navigation display (ND), not the primary flight display (PFD). It's always displayed and it's always green. Here is an example:



In the top left, GS is groundspeed (190). TAS is true airspeed (198). The magnetic wind is displayed underneath (338/15) with a graphical arrow depiction.
     
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Jun 1, 2011, 02:23 AM
 
Just for comprehension (never having seen one of those displays):

190 kts is ground speed, plane is aimed slightly to the left of the target, as there is a 15-kt wind coming from 338°, which, since it's coming from an angle, works out to 8 kts of headwind?
     
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Jun 1, 2011, 06:37 AM
 
Yes. In that depiction, there is 8 knots of headwind component because the 15 knots wind is angled slightly--coming from the front left.
     
ghporter
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Jun 1, 2011, 07:05 AM
 
GPS provides instantaneous position information. The definition of velocity is rate of change in location, is it not?

At any flight altitude, it is "extremely difficult" to jam GPS signals... How do you get the jamming source anywhere near any random aircraft?

High end GPS systems can track position microsecond by microsecond, and can provide at least one meter precision. By simply using that data, extremely accurate velocity, direction and navigation information can be displayed.

Suggesting that GPS as a velocity and navigation aid is a "layman's solution" ignores the previously noted fact that the Space Shuttle program uses GPS for this very purpose.

Oh, and I should point out that my reference to LORAN was indeed dated. My one ride on a Hurricane Hunter aircraft was in 1981, though I did keep up with advances in the instrumentation on the WC-130 because it's a really amazing aircraft.

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Jun 1, 2011, 08:41 AM
 
Originally Posted by ghporter View Post
At any flight altitude, it is "extremely difficult" to jam GPS signals... How do you get the jamming source anywhere near any random aircraft?
An interference source on the plane itself would do it, but is probably impractical to get on a plane. Planes come close to the ground two times in their flight path, though, which could be enough opportunity to cause havoc. But you don't need to jam it: there's plenty of unintentional interference sources that can muck with the signal.

The transmission power of a single satellite's GPS signal is about the same as an incandescent lightbulb. By the time it gets to Earth, it's well below a femtowatt. There's all sorts of voodoo that the RF engineers do to extract that low signal and use it, but every system that uses GPS has to deal with the fact that unintentional interference happens. Mission-critical positioning systems must take input from multiple sources and "fall back" to them if GPS reception falters. They design it this way because they understand that there are some areas where you simply cannot receive a good enough signal to use GPS reliably (but the other methods are probably Good Enough until reception comes back.)

And quickly, regarding LORAN: Bush actually wanted to fund a LORAN modernization effort, because he understood that LORAN works well precisely in the areas where GPS can work poorly (or not at all), and thought it was good to fund a backup positioning system for the country. Obama saw the system as redundant, and killed it to save money (before the Tea Party made killing useful programs fashionable, thankyouverymuch). Here's hoping we don't need that backup anytime soon.


Sorry to keep derailing, I don't get a chance to put on my GPS Dork. hat very often....
     
chabig
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Jun 1, 2011, 09:25 AM
 
Ghporter, you are changing the subject. We had a an airspeed discussion going and you want to talk about GPS. That's fine, but let's not get the two mixed up.

Originally Posted by ghporter View Post
GPS provides instantaneous position information.
Sort of. GPS usually provides "instantaneous" position once per second. Between fixes, the receiver extrapolates.

Originally Posted by ghporter View Post
The definition of velocity is rate of change in location, is it not?
Yes, of course.

Originally Posted by ghporter View Post
At any flight altitude, it is "extremely difficult" to jam GPS signals... How do you get the jamming source anywhere near any random aircraft?
To jam a radio, the received jamming power must be great enough so that the receiver cannot discriminate the signal. You cannot make conclusions about the jammer's proximity to the target receiver.

Originally Posted by ghporter View Post
High end GPS systems can track position microsecond by microsecond, and can provide at least one meter precision. By simply using that data, extremely accurate velocity, direction and navigation information can be displayed.
As stated above, GPS usually provides position once per second, not microsecond by microsecond. And although 1 meter precision is obtainable for a stationary or very slow moving ground receiver, it's not achievable for an airplane in flight. Fifteen meter accuracy would be very very good.

Displaying velocity, direction, and navigation information is great, but it has nothing to do with airspeed.

Originally Posted by ghporter View Post
Suggesting that GPS as a velocity and navigation aid is a "layman's solution" ignores the previously noted fact that the Space Shuttle program uses GPS for this very purpose.
Either you're intentionally twisting the discussion or you didn't read the posts above. Nobody suggested that GPS was a "layman's" solution for velocity or navigation. What was said was that "the idea of using GPS to measure airspeed" was a layman's idea. GPS does not and cannot measure airspeed.

As for the shuttle, it first flew in 1972. The first operational GPS satellite was launched in 1989 and the GPS constellation first reached full operation capability in 1995. The shuttle flew for years without GPS. And although GPS was eventually integrated into the shuttle system, that is irrelevant to our airspeed discussion.

Do you know how the shuttles measure airspeed for the landing? They have two air data probes that extend from stowed positions into the slipstream to measure airspeed the conventional way.
     
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Jun 1, 2011, 09:28 AM
 
Glenn, from what I gather, the layman's error is the assumption that accurate navigation information has anything to do with keeping an aircraft aloft.

There is a connection, but it's rather incidental.

As i'm beginning to understand it, if you're dealing with strong winds, working with ground speed at all is probably more dangerous than beneficial.
     
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Jun 1, 2011, 09:29 AM
 
Originally Posted by Doofy View Post
But if your airspeed instruments are measuring 60 kts and your ground speed GPS is measuring 450 kts, you'll have a better idea that your airspeed instruments are screwed. Or you're in a 390 kt tailwind. Surely that's some improvement over not knowing?
Let's say I'm flying along in my Airbus and all of a sudden my airspeed indicates 60 knots. I already know my airspeed instruments are screwed!

By the way, if I'm flying 220 knots and I'm in a 390 knot tailwind, my airspeed indicator still reads 220 knots.
     
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Jun 1, 2011, 09:31 AM
 
Originally Posted by Spheric Harlot View Post
Glenn, from what I gather, the layman's error is the assumption that accurate navigation information has anything to do with keeping an aircraft aloft.
Bingo! Perfectly said.
     
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Jun 1, 2011, 09:56 AM
 
Originally Posted by Doofy View Post
Top left on an Airbus MFD.
Originally Posted by chabig View Post
Close. Airbus displays groundspeed on the navigation display (ND), not the primary flight display (PFD).
Chabig, if you're a pilot it's no wonder planes fall from the sky. You took my "MFD" and made like I'd said "PFD", and that's on a stationary Internet forum.

Just sayin'.
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Jun 1, 2011, 10:11 AM
 
Originally Posted by chabig View Post
Ghporter, you are changing the subject. We had a an airspeed discussion going and you want to talk about GPS. That's fine, but let's not get the two mixed up.
He's not changing the subject at all. We're talking about aircraft systems, not a specific system. You're talking about airspeed, we're talking about the whole kit and caboodle.

Originally Posted by chabig View Post
Either you're intentionally twisting the discussion or you didn't read the posts above. Nobody suggested that GPS was a "layman's" solution for velocity or navigation. What was said was that "the idea of using GPS to measure airspeed" was a layman's idea. GPS does not and cannot measure airspeed.
You didn't say that at all. You said:

Originally Posted by chabig View Post
Solid engineering goes into the design of these systems. Reference sensors, GPS, turbines(?) are all layman's ideas, with no solid engineering to back them up.
You suggested that GPS was a "layman's idea" with no qualification.

There's some serious bullshit going on somewhere here. Earlier in the thread, "GPS ground speed isn't displayed in the cockpit", then later it's "in green on the nav display".

And if ground speed is of no use, why's it on the display?
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Jun 1, 2011, 10:22 AM
 
@Doofy
You're mixing two things here: ground speed is displayed and I haven't seen anyone claim otherwise. But the ground speed as determined by the GPS is not displayed separately. So no, GPS ground speed isn't displayed, ground speed is. And the data from GPS are taken into account when calculating ground speed, but it's not the only source of data.
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Jun 1, 2011, 10:22 AM
 
Yes, I said that--in the context of measuring airspeed. Others have more distinctly restated what I meant.

As for groundspeed, GPS data is not displayed in the cockpit in the same way that it might be on a general aviation airplane. There is no "GPS receiver" or "GPS display" on the panel. GPS is not a data source by itself. It's just one of multiple inputs to the airplane's Flight Management and Guidance Computers (FMGC). The FMGCs take all of the inputs and generate an integrated nav solution, which is displayed on the cockpit Display Units (DUs). The DU in front of each pilot normally presents the Primary Flight Display (PFD) and the one next to it presents the Navigation Display (ND). The DU is hardware. The acronyms PFD and ND refer to the presentation of information on the hardware, like channels on a television.

Groundspeed is shown on the ND, but it's not GPS groundspeed. It's just groundspeed from the integrated FMGC solution. Groundspeed is displayed because it's useful for figuring out when you'll get there.
     
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Jun 1, 2011, 10:33 AM
 
Originally Posted by OreoCookie View Post
@Doofy
You're mixing two things here
No, I'm not.

Originally Posted by OreoCookie View Post
ground speed is displayed and I haven't seen anyone claim otherwise. But the ground speed as determined by the GPS is not displayed separately. So no, GPS ground speed isn't displayed, ground speed is. And the data from GPS are taken into account when calculating ground speed, but it's not the only source of data.
I know it's not the only source of data - I never said it was. However, I'd assume that while over the Atlantic there'd be less DME ground stations to measure from.
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Jun 1, 2011, 10:44 AM
 
Originally Posted by chabig View Post
As for groundspeed, GPS data is not displayed in the cockpit in the same way that it might be on a general aviation airplane. There is no "GPS receiver" or "GPS display" on the panel. GPS is not a data source by itself. It's just one of multiple inputs to the airplane's Flight Management and Guidance Computers (FMGC). The FMGCs take all of the inputs and generate an integrated nav solution, which is displayed on the cockpit Display Units (DUs). The DU in front of each pilot normally presents the Primary Flight Display (PFD) and the one next to it presents the Navigation Display (ND). The DU is hardware. The acronyms PFD and ND refer to the presentation of information on the hardware, like channels on a television.
Dude, I know my way around a G1000 and a few systems on Boeings (i.e. the FMC, MCP). No need to explain what a PFD is.
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Jun 1, 2011, 10:46 AM
 
Originally Posted by Doofy View Post
You suggested that GPS was a "layman's idea" with no qualification.
With rather considerable context, though - I mean, I haven't a clue about flying, but even I understood what he meant.
     
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Jun 1, 2011, 10:47 AM
 
Originally Posted by Spheric Harlot View Post
With rather considerable context, though - I mean, I haven't a clue about flying, but even I understood what he meant.
And Glenn's posts have rather considerable context too.
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Jun 1, 2011, 11:11 AM
 
Originally Posted by Doofy View Post
And Glenn's posts have rather considerable context too.
Just not relevant context, was the problem.
     
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Jun 1, 2011, 09:37 PM
 
A nice link for some basic understanding.


Airplane Upset Recovery Training Aid | Flight Safety Foundation
To know your Enemy, you must become your Enemy.”
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Jun 1, 2011, 10:09 PM
 
Originally Posted by chabig View Post
Ghporter, you are changing the subject. We had a an airspeed discussion going and you want to talk about GPS. That's fine, but let's not get the two mixed up.
You are right that my posts have become derailing. Mainly because my point seems to have not gotten across. Airspeed can't be directly measured by GPS, though absolute velocity can. Combine that with other data and you can augment other sensor systems' ability to provide more complete data to the aircrew.
Originally Posted by chabig View Post
Sort of. GPS usually provides "instantaneous" position once per second. Between fixes, the receiver extrapolates.
Not all GPS receivers are limited to one-second fixes. The absolute limit on frequency of position updates is based on the signals provided by the satellites. Those time base signals, which are basically "my clock says it is this time," are merely the inputs to a continuous matrix calculation (see this Penn State site for the actual algebra), which also takes into account "almanac data" that indicates the expected position of the satellite at that point in time, and with the more recent satellites, the known and expected errors in both almanac and clock data. Further, advanced receivers feature both multiple RF systems and multiple processors, providing numerous fix solutions with each processor and synthesizing a final fix from the multiple results. This all is to point out that the satellites themselves have nothing to do with position data. The receiver's processor calculates position based on time signals from as many satellites as possible. The "extrapolation" is more of a curve smoothing on the time scale of the processor's ability to compute each new fix, and the absolute data rates available from the satellites. Considering that GPS can easily provide sub-meter accuracy in 3 axes (given good signals from sufficient numbers of satellites), the data rate and processor calculation rate available on high end receiver devices seems to be more than adequate for continuous navigation information.
Originally Posted by chabig View Post
To jam a radio, the received jamming power must be great enough so that the receiver cannot discriminate the signal. You cannot make conclusions about the jammer's proximity to the target receiver.
The RF signal provided by the first two blocks of satellites was stated as around 50W. They orbit at approximately 11,000 miles in altitude, so the actual signal level at ground level is indeed miniscule. However, it is not a CW signal; it uses a spread spectrum system with sufficient redundancy of data to allow a receiver to pull solid data out of what appears to be background noise. The system uses direct-sequence spread spectrum codes, allowing the same channel frequency to be used by the fleet without mutual interference (each spacecraft has its own code sequence) while at the same time allowing such low RF power to be actually very effective. Civil systems depend on a roughly 1.2GHz frequency, which is close enough to half the resonant frequency of water to allow heavy clouds and foliage to degrade that signal, but the military system is not on that frequency, allowing it to evade such degradation.

Jamming an aircraft's external antennas with anything theoretically smuggled into the aircraft is a technical challenge that exceeds potential threats. The events of Sept. 11 demonstrated that multiple active cell phone conversations in the cabin while the aircraft is in flight do not noticeably impact flight systems-most likely because those systems are thoroughly enough shielded from RF interference to prevent that sort of problem. It is inconceivable that internal shielding would be less effective in more advanced aircraft featuring additional systems that depend on external signals. Jamming is not a valid concern in my opinion.
Originally Posted by chabig View Post
As stated above, GPS usually provides position once per second, not microsecond by microsecond. And although 1 meter precision is obtainable for a stationary or very slow moving ground receiver, it's not achievable for an airplane in flight. Fifteen meter accuracy would be very very good.
Processing speed and number of processors is the controlling factor here. The earliest portable military GPS devices could not provide heading information at speeds below about 5 MPH, but those devices were supplanted in the early 1990s. Multiple processors, each with multiple independent receivers can provide aggregate microsecond position data, though functionally anything faster than about .05 second updates are superfluous. For an inexpensive civilian GPS unit with a single processor, better than 15m accuracy is not only obtainable, but fairly easy to get with good conditions.
Originally Posted by chabig View Post
Displaying velocity, direction, and navigation information is great, but it has nothing to do with airspeed.
This information, as I stated several times, isn't expected to replace airspeed sensed from other sources. Rather, my suggestion was that a flight data computer that had GPS data and airspeed data could usefully indicate additional information, such as inconsistencies between physical data and expected data. I suggested that having additional information would be useful, not that the static/pitot system should be replaced.
Originally Posted by chabig View Post
Either you're intentionally twisting the discussion or you didn't read the posts above. Nobody suggested that GPS was a "layman's" solution for velocity or navigation. What was said was that "the idea of using GPS to measure airspeed" was a layman's idea. GPS does not and cannot measure airspeed.

As for the shuttle, it first flew in 1972. The first operational GPS satellite was launched in 1989 and the GPS constellation first reached full operation capability in 1995. The shuttle flew for years without GPS. And although GPS was eventually integrated into the shuttle system, that is irrelevant to our airspeed discussion.

Do you know how the shuttles measure airspeed for the landing? They have two air data probes that extend from stowed positions into the slipstream to measure airspeed the conventional way.
The statement that GPS was a "layman's solution" was not qualified, and from context it appeared that the intent was that it was not useful for anything. My posts were to that point. While the GPS system as we know it was not initiated until 1989, there were a number of experimental satellites that could be used for GPS as far before then. In fact, the original shuttle avionics package included a number of radio-based navigation systems, including TACAN, and FM altimiters (very little altered from the F-4D/E and RF-4C avionics suites). Until GPS was available, the early orbiters had to depend on inputs from a variety of systems which were integrated for an estimate of absolute velocity during hypersonic flight in reentry. And it is really only at the very end of the process, when the vehicle is subsonic, that airspeed (versus absolute velocity) is an issue in any case, and especially when managing the glide to final and landing.

Again, my point in all of this derailing rant has always been that GPS as an adjunct to traditional systems would be useful in a flight data system, providing the potential for more information about possible traditional sensor failures beyond error messages that boil down to "this sensor seems unreliable." Now that we're communicating and actually understanding each other, I'll put away my references and experience teaching the GPS system and GPS receiver fundamentals, and let people with actual aircrew experience discuss more concrete things. And probably learn quite a bit from that discussion, too.

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Jun 2, 2011, 07:22 AM
 
Originally Posted by ghporter View Post
Jamming an aircraft's external antennas with anything theoretically smuggled into the aircraft is a technical challenge that exceeds potential threats. The events of Sept. 11 demonstrated that multiple active cell phone conversations in the cabin while the aircraft is in flight do not noticeably impact flight systems-most likely because those systems are thoroughly enough shielded from RF interference to prevent that sort of problem. It is inconceivable that internal shielding would be less effective in more advanced aircraft featuring additional systems that depend on external signals. Jamming is not a valid concern in my opinion.
But those cell phones operate at frequencies other than the GPS L1 frequency, and their harmonics probably don't stomp on it either. No antenna can be shielded against interference on the frequencies that it is designed to detect. The best you can do is design the signal to be more robust so that you can receive it in the face of a given amount of interference. (Disclaimer: I am so, so not an RF guy...)
     
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Jun 2, 2011, 07:40 AM
 
Originally Posted by ghporter View Post
The statement that GPS was a "layman's solution" was not qualified, and from context it appeared that the intent was that it was not useful for anything. My posts were to that point.
Fair enough.

I am a layman, and I understood the remark entirely within the context of the remainder of his post, which was dedicated to airspeed.

At any rate, the comment has been clarified several times from various people who understood how it was meant, and I think it can be laid to rest.
     
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Jun 2, 2011, 02:11 PM
 
Recall the first post in this thread to suggest GPS:
Originally Posted by Doofy View Post
Looks to me like a total computer failure. All this pitot-failure-causing-the-problem is complete rubbish when you consider there's a GPS showing ground speed - they'd at least have had some idea of speed, altitude and an upcoming stall...
GPS provides no help with regard to a stall.

Originally Posted by ghporter View Post
Suggesting that GPS as a velocity and navigation aid is a "layman's solution" ignores the previously noted fact that the Space Shuttle program uses GPS for this very purpose.
Which is completely irrelevant to AFR447. AFR447 was not in orbit or reentry.
     
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Jun 2, 2011, 07:53 PM
 
Typical aircraft EM shielding is effective between 0Hz and several gigahertz; it is typically a combination of solid metal and very fine mesh at joins, and it has to prevent interference in systems that are sensitive to a very broad range of frequencies.

External antennas are shielded from the cabin primarily by the aircraft skin. The antennas are connected to their electronics via appropriate coaxial cables which block, as well as contain, the appropriate frequencies. It would definitely be difficult to produce a signal of any frequency within the cabin of an aircraft that could radiate to properly installed antennas. In many situations, GPS systems are built with monolithic receiver/antenna structures which are connected to data recovery components. In other words, a GPS receiver could be made with a remote unit that produces a data stream which is then processed remotely. Since the aircraft is basically a closed, electromagnetic container, and all the sensing antennas are outside this container, the whole concept of jamming a system from within the cabin is highly difficult from a technical standpoint.

While cell phones don't operate on anything near the L1 frequency, cell phones do operate on frequencies that could interfere with aircraft systems if they were able to radiate outside the cabin-which they cannot.

I never stated that absolute velocity could give any information about stall. But if aircrew had information that gave them reason to watch airspeed indications, it would definitely provide them with a alert that might hint that airspeed issues were in progress. Again, I never suggested replacement of traditional sensors, only that more information about the performance of those sensors would be helpful.

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Originally Posted by ghporter View Post
I never stated that absolute velocity could give any information about stall. But if aircrew had information that gave them reason to watch airspeed indications, it would definitely provide them with a alert that might hint that airspeed issues were in progress. Again, I never suggested replacement of traditional sensors, only that more information about the performance of those sensors would be helpful.
This amounts to a non-suggestion. The avionics already detect when the airspeed indications are unreliable and flag. The groundspeed is presented to the crew on the navigation screen. You're suggesting nothing.
     
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Dec 28, 2011, 05:54 AM
 
I hate to reopen this discussion, but here is a great summary of the event:

Air France 447 Flight-Data Recorder Transcript - What Really Happened Aboard Air France 447 - Popular Mechanics

Unfortunately, the truth is as I suspected--one pilot lost situational awareness and mishandled the airplane so badly that he flew them all into the ocean.
     
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Dec 29, 2011, 02:04 PM
 
Still hard to believe an experienced A340 pilot could have handled that situation as bad as a fresh-out-of-school commuter plane pilot. Boggles the mind really.

The part of the transcript where the one pilot says "I've been pulling up the whole time" is disturbing. (Insanity = doing the same thing an expecting a different result)

Also: Consider me less than impressed that:
- Airbus allows two pilots to give different control stick inputs without alerting EITHER that they are in conflict.
- Airbus averages the two conflicting inputs for the actual changes to the control surfaces. (I'd like to know in what use case this actually makes sense????)

Simply alerting the pilots that they weren't on the same page would have forced them to collaborate on a solution and possibly saved lives.
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Originally Posted by driven View Post
- Airbus averages the two conflicting inputs for the actual changes to the control surfaces. (I'd like to know in what use case this actually makes sense????)
As a layman that one confuses the hell out of me. Technologically, that seems awesome, but practically I don't see the situation where it would be beneficial.
     
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Jan 5, 2012, 06:23 PM
 
Originally Posted by driven View Post
Airbus allows two pilots to give different control stick inputs without alerting EITHER that they are in conflict.
This has long since been corrected in software. There is a loud audible warning and flashing lights when simultaneous inputs are detected from both sticks at the same time. However, the older Airbus airplanes didn't do this.

Originally Posted by driven View Post
Airbus averages the two conflicting inputs for the actual changes to the control surfaces. (I'd like to know in what use case this actually makes sense????)
This only makes sense because there is simply nothing better to do. As long as there are two sticks, there is no way that the machine can know which stick to obey and which one to ignore. So it's actually become pretty standard in dual-control fly by wire airplanes to just add the two sticks together (they aren't averaged).
     
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Jan 5, 2012, 06:45 PM
 
Originally Posted by chabig View Post
This has long since been corrected in software. There is a loud audible warning and flashing lights when simultaneous inputs are detected from both sticks at the same time. However, the older Airbus airplanes didn't do this.



This only makes sense because there is simply nothing better to do. As long as there are two sticks, there is no way that the machine can know which stick to obey and which one to ignore. So it's actually become pretty standard in dual-control fly by wire airplanes to just add the two sticks together (they aren't averaged).
As to the first point, this should be a software update for all existing aircraft. It certainly may have saved lives in this case.

As to the second point, wouldn't a better solution be to default to the left seat? (Since that's typically the command officer?) Or, if they resolve the first point, then the second point doesn't matter.

This is just a mess.
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Jan 5, 2012, 07:05 PM
 
Originally Posted by driven View Post
As to the first point, this should be a software update for all existing aircraft. It certainly may have saved lives in this case.
Software updates are expensive, complicated, and often require accompanying hardware updates. Yes it would save lives, but so would a lot of things.

Originally Posted by driven View Post
As to the second point, wouldn't a better solution be to default to the left seat? (Since that's typically the command officer?)
No. There is no reason to believe the pilot in the left seat is any more skilled or aware than the pilot in the right seat. There is a button on each stick that permits them to take sole control of the airplane by inhibiting inputs from the opposite stick. We are trained meticulously to use that button to take control. There is never a time when both pilots should be making inputs at the same time. If you want it, and time is critical, you push the button and fly. That didn't happen in this case, but it was a pilot training and performance problem, not a problem with the airplane.
     
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Jan 5, 2012, 07:10 PM
 
Originally Posted by chabig View Post
Software updates are expensive, complicated, and often require accompanying hardware updates. Yes it would save lives, but so would a lot of things.



No. There is no reason to believe the pilot in the left seat is any more skilled or aware than the pilot in the right seat. There is a button on each stick that permits them to take sole control of the airplane by inhibiting inputs from the opposite stick. We are trained meticulously to use that button to take control. There is never a time when both pilots should be making inputs at the same time. If you want it, and time is critical, you push the button and fly. That didn't happen in this case, but it was a pilot training and performance problem, not a problem with the airplane.
Well, thanks for the feedback. It's appreciated.

It does sound like this crew should have been flying cargo based on their performance in a crisis.
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Jan 5, 2012, 07:16 PM
 
I should have gone further to explain the left/right seat rationale. Remember, the reason the pilot in the left seat is in command is not because he's better, smarter, faster, or more experienced. It's just because he got hired first.
     
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Jan 5, 2012, 07:20 PM
 
Originally Posted by The Final Dakar View Post
As a layman that one confuses the hell out of me. Technologically, that seems awesome, but practically I don't see the situation where it would be beneficial.
Example:
You are approaching a light aircraft which you need to avoid.
One pilot choses to descend
One pilot choses to climb

The flight control system takes an average and everyone dies.
     
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Jan 5, 2012, 07:26 PM
 
Originally Posted by chabig View Post
I should have gone further to explain the left/right seat rationale. Remember, the reason the pilot in the left seat is in command is not because he's better, smarter, faster, or more experienced. It's just because he got hired first.
Not necessarily. Just more pilot in command time.
     
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Jan 5, 2012, 07:27 PM
 
Originally Posted by moonmonkey View Post
Example:
You are approaching a light aircraft which you need to avoid.
One pilot choses to descend
One pilot choses to climb

The flight control system takes an average and everyone dies.
Once again, it doesn't average. It adds the inputs together. And though we agree that you can make up situations where it might not make sense, I leave it to you to propose a better solution. I can make up a situation in a mechanical yoked airplane where everyone dies. Here you go:

You are approaching a light aircraft which you need to avoid.
One pilot choses to descend and pushes forward on the yoke
One pilot choses to climb and pulls back on the yoke with equal force

The yokes don't move and everyone dies.

Do you see? It's exactly the same! The forces were algebraically added together, exactly as they are in the fly-by-wire system.
     
 
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