4 ms·

I wonder why they keep using local & global air pressure-based altitude measurements instead of GPS. Is it just inertia, or does GPS have serious technical limi

by kijin 11y ago

I wonder why they keep using local & global air pressure-based altitude measurements instead of GPS. Is it just inertia, or does GPS have serious technical limitations when used in an airliner? I can understand relying on airspeed instead of GPS-measured groundspeed, because airspeed is important. But I can't think of a justification for not using true altitudes, especially during takeoff and landing. What happens if the pilots punch in a wrong number for the destination air pressure?

javert 11y ago
I wonder how robust the GPS system is. Could an unexpected space phenomenon bring it down? What would happen if a rogue nation started shooting down the satellites? This isn't a real answer. I hope someone with a real answer comes along!
- tfe 11y ago
  GPS satellites somewhat regularly go in and out of service for various reasons. There are more satellites in the constellation than strictly necessary in order to compensate for this. Information on this is distributed to pilots (in the US) via NOTAMs, and you can use this information to decide whether it's safe to use GPS navigation for your flight. Practically speaking though, aviation GPSs have a function called RAIM that continuously monitors the integrity of the position information and will inform you on a loss of precision. I believe most GPSs also allow predicting RAIM along your time and route of flight to make sure you'll have integrity from takeoff to landing.
- tanzam75 11y ago
  > I wonder how robust the GPS system is. Could an unexpected space phenomenon bring it down? What would happen if a rogue nation started shooting down the satellites? GPS is not the only system. We currently have about 2.7 global satellite navigation constellations available for use. The American GPS and the Russian GLONASS are both complete. The Chinese Beidou system is about 40% complete. The European Galileo system is about 30% complete. Each system also has about 20% redundancy in the form of on-orbit spares. Dual-system GPS/GLONASS receivers are now nearly everywhere. I was just looking at a phone that has a triple-system GPS/GLONASS/Beidou receiver. In a few years, quad-system receivers will be commonplace. A rogue nation would have to shoot down about 1.9 constellations, or about 60 satellites, before we drop below full coverage. The American, Chinese, and European systems have the same orbital inclination. If a gap opened, any satellite from any of these three systems could theoretically fill that gap. It would take some time to reposition the satellites, though. There is also additional redundancy in the eastern hemisphere. India and Japan have satellites that provide regional coverage. Part of the Chinese system also operates as a regional constellation. At this point, the rogue state is at war with the US, Russia, China, the EU, India, and Japan. They might as well just surrender.
  - spdustin 11y ago
    It would take only one nuclear device being "detonated" in low orbit, to wipe out nearly all of the satellites in a large area of that space. The resulting EMP may wreck quite a few ground-based systems too, IIRC from a book I read recently. Can't recall the name, though... In any case, disrupting satellite-based navigation is the simplest kind of rocket science: a missile that just has to go up and then detonate, no aiming required.
    - tanzam75 11y ago
      > It would take only one nuclear device being "detonated" in low orbit, to wipe out nearly all of the satellites in a large area of that space. The resulting EMP may wreck quite a few ground-based systems too, IIRC from a book I read recently. Can't recall the name, though... A nuclear weapon detonated in low earth orbit would not destroy a single global navigation satellite. They are not in low earth orbit. A nuclear weapon detonated in medium earth orbit would wipe out between 0 and 1 navigation satellite, depending on how close you get. The satellites just aren't that close together. An EMP would destroy electronics on the ground, but this will be a systemic effect. Satellite navigation would be collateral damage. When the ground-based electronics are replaced, though, the satellites will still be working. > In any case, disrupting satellite-based navigation is the simplest kind of rocket science: a missile that just has to go up and then detonate, no aiming required. Nuclear weapons are powerful, but they still have to obey the laws of physics. If you get far enough away, the inverse-squared law makes even a nuclear explosion look like a firecracker. A 1 megaton nuclear weapon would have to be detonated within half a kilometer to damage a satellite physically: http://www.projectrho.com/public_html/rocket/spacegunconvent.php http://www.projectrho.com/public_html/rocket/spacegunconvent... The shielding needed to protect a satellite against EMP only adds 5% to the cost of the satellite: http://fas.org/spp/starwars/congress/1997_h/h970716u.htm http://fas.org/spp/starwars/congress/1997_h/h970716u.htm So basically, it's going to take one nuclear warhead to destroy one navigation satellite. If a rogue nation has 60 nuclear warheads, it's hardly going to waste them to take out 60 navigation satellites.
bandwevil 11y ago
You'd need to be able to ensure that everyone is using GPS altitudes before requiring it, and there are some pretty ancient planes still flying out there. As far as I know, ATC radar doesn't actually measure altitude but rather relies on the plane's transponder to report it.If you have some mix of baro and GPS altitudes, you can no longer safely ensure separation. There should be enough checks for setting the proper baro setting - both pilots check the setting individually when changing it and it's reported to them a number of times throughout descent. Even if they do manage to get it wrong through all that, the GPWS should notify them if they get too close to any obstacles.
- Alupis 11y ago
  And not everyone uses GPS (GLONASS is a competing system used by some parts of the world, specifically developed by the USSR to not have to rely on a USA based system).
tfe 11y ago
I think the biggest hurdle is lack of precision. 200 feet isn't much error in the horizontal realm but is a big deal vertically. There are ways to supplement the precision of GPS altitude with ground-based stations (WAAS), and this is indeed used to construct GPS-based instrument approach procedures which are comparable in accuracy to traditional means (ILS). But these require ground-based transmitters; impractical to implement worldwide.
sokoloff 11y ago
The most unassailable answer is that the procedures were worked out before GPS equipped airplanes were in the fleet, and even today, GPS is not required equipment for flight in the flight levels (and many older jets are not equipped, especially older freighters). Other reasons are: the transponders that ATC (and other aircraft) interrogate report pressure altitude (uncorrected, meaning using 29.92 inHg as the standard). A sensitive altimeter can be had for a few hundred dollars and requires no electrical power (unless lighted) and basically no service throughout its life. To your last point, anywhere it matters (below the flight levels), pilots are using a local altimeter setting on the altimeters, often from the ATIS (automated terminal["airport"] information system) or from ATC. Of course, for takeoff, it doesn't matter. You're generally climbing at max available power or are doing a reduced power takeoff in aircraft so certified and on runways without obstacles to worry about. On approach, you're typically following a precision approach path provided by a radio signal (ILS includes lateral and vertical guidance beams) or that is GPS-based (where you have similar navigation performance levels provided by WAAS augmentation). There are non-precision approaches where the altimeter setting is more critical, but I can't recall the last time I had to fly one of those for other than training purposes.
- JHof 11y ago
  Even the newest airliners of today still triangulate their positions off of radio beacons (VORs) as a backup to the GPS system and also contain an inertial system. GPS being the most accurate, it's normally the primary navigation system, however, I've flown across the country, beacon to beacon, with the GPS inoperative, which some airliners still do from time to time. In the southwestern US, especially, GPS often becomes unusable due to military activity. It isn't uncommon for an airliner to experience a loss of GPS signal mid-flight when operating near military exercises. It's just the nature of our system. It was a good article. I enjoyed the allusions to ships on a sea, and moment before landing described as an unanswered question. I'm wondering if this will be passed along much in the pilot community. The most popular 'pilot life' creation in recent memory seems to be this Lego parody - https://www.youtube.com/watch?v=rNxz2hhSXuY https://www.youtube.com/watch?v=rNxz2hhSXuY.
csirac2 11y ago
The technical limitation is the basic geometry of GPS. Satellites are at ~20,000km, aircraft at ~10km max. And you want 0.0003km resolution in altitude. We barely even have 0.003km in the lat/long plane, and it's only that good because of fancy DSP algorithms which lean on special modeling of atmosphere and the satellites themselves - I.e. assumptions about a 3rd-party system, wholly inappropriate for safety critical stuff. In fact, cruising altitude (~"full span" indication) is roughly the magnitude of the specs for error in lat/lang back when GPS was first commissioned (IIRC).
omegant 11y ago
All the other replies are true, but the main reason is that airplanes fly a constant pressure altitude because it's important performance wise, while flying a constant geografical altitude is almost meaning less(except for obstacle avoidance. Pressure altitude is fundamental for the performance of airplanes because you fly in a known estable layer of the atmosphere that keeps your drag, lift and thrust almost constant (there are temperature changes but it takes docens of miles to change a single degree). Also if you select the standart atmosphere you know you are flying the same layer system of all the other airplanes in the air, with a fully autonomous, relatively cheap, and non electrical device. Also it's easy to calibrate on your own if you want to fly in a local airport and have no access to a weather report. It's hard to beat a conventional altimeter.
joshvm 11y ago
GPS altitude is considerably poorer than GPS lat/lon. There's a good chance that your GPS altitude could be 10-20 metres off what it should be, which is enough to be dangerous if you're using it as a landing aid. "True" altitude requires you to both have an excellent fix on the satellites and have a very, very good reference geodetic model (which you probably don't). Also depending on the quality of your fix, you'll have different altitudes to the rest of the people around you. It makes more sense to have two pilots independently check the pressure settings (and that's why you have a checklist).
WalterBright 11y ago
My dad was a navigator on a B-17. He'd tell me that he had to scrape the frost off of the dome with his fingernails so he could get a fix on a star while flying across the Atlantic. Crossing the Atlantic at the time was at the limit of the B-17's range, so messing up the navigation means you disappeared. Nobody dreamed of GPS back then :-)
- therockspush 11y ago
  Some operational C-130s still have a little sextant port for shooting a star but only a few of the old guys would still even know how it works. The GPS most of them use is almost as old, a $20 Garmin handheld could navigate circles around it.
  - ygra 11y ago
    > a $20 Garmin handheld could navigate circles around it Probably not, as it will refuse to work at both the altitude and speed of an airplane ;-) EDIT: I stand corrected. Neither the speed nor the altitude will be a problem.
    - votingprawn 11y ago
      > Probably not, as it will refuse to work at both the altitude and speed of an airplane ;-) Well sub in the garmin with one of the other low cost consumer units that work at those altitude and speeds then!
    - coob 11y ago
      The GPS altitude limit is 18km which is 60,000ft. Normal cruising altitude is 35,000ft. It's also trivial to obtain unrestricted GPS devices.
azernik 11y ago
Wikipedia [1] says something very different - that during takeoff and landing commercial aircraft generally rely on radar altimeters, which are extremely accurate and reliable, but generally don't work above about 750 meters. See: http://en.wikipedia.org/wiki/Radar_altimeter#Civil_aviation_applications http://en.wikipedia.org/wiki/Radar_altimeter#Civil_aviation_...

by kijin 11y ago

javert 11y ago

I wonder how robust the GPS system is. Could an unexpected space phenomenon bring it down? What would happen if a rogue nation started shooting down the satellites? This isn't a real answer. I hope someone with a real answer comes along!

tfe 11y ago
GPS satellites somewhat regularly go in and out of service for various reasons. There are more satellites in the constellation than strictly necessary in order to compensate for this. Information on this is distributed to pilots (in the US) via NOTAMs, and you can use this information to decide whether it's safe to use GPS navigation for your flight. Practically speaking though, aviation GPSs have a function called RAIM that continuously monitors the integrity of the position information and will inform you on a loss of precision. I believe most GPSs also allow predicting RAIM along your time and route of flight to make sure you'll have integrity from takeoff to landing.
tanzam75 11y ago
> I wonder how robust the GPS system is. Could an unexpected space phenomenon bring it down? What would happen if a rogue nation started shooting down the satellites? GPS is not the only system. We currently have about 2.7 global satellite navigation constellations available for use. The American GPS and the Russian GLONASS are both complete. The Chinese Beidou system is about 40% complete. The European Galileo system is about 30% complete. Each system also has about 20% redundancy in the form of on-orbit spares. Dual-system GPS/GLONASS receivers are now nearly everywhere. I was just looking at a phone that has a triple-system GPS/GLONASS/Beidou receiver. In a few years, quad-system receivers will be commonplace. A rogue nation would have to shoot down about 1.9 constellations, or about 60 satellites, before we drop below full coverage. The American, Chinese, and European systems have the same orbital inclination. If a gap opened, any satellite from any of these three systems could theoretically fill that gap. It would take some time to reposition the satellites, though. There is also additional redundancy in the eastern hemisphere. India and Japan have satellites that provide regional coverage. Part of the Chinese system also operates as a regional constellation. At this point, the rogue state is at war with the US, Russia, China, the EU, India, and Japan. They might as well just surrender.
- spdustin 11y ago
  It would take only one nuclear device being "detonated" in low orbit, to wipe out nearly all of the satellites in a large area of that space. The resulting EMP may wreck quite a few ground-based systems too, IIRC from a book I read recently. Can't recall the name, though... In any case, disrupting satellite-based navigation is the simplest kind of rocket science: a missile that just has to go up and then detonate, no aiming required.
  - tanzam75 11y ago
    > It would take only one nuclear device being "detonated" in low orbit, to wipe out nearly all of the satellites in a large area of that space. The resulting EMP may wreck quite a few ground-based systems too, IIRC from a book I read recently. Can't recall the name, though... A nuclear weapon detonated in low earth orbit would not destroy a single global navigation satellite. They are not in low earth orbit. A nuclear weapon detonated in medium earth orbit would wipe out between 0 and 1 navigation satellite, depending on how close you get. The satellites just aren't that close together. An EMP would destroy electronics on the ground, but this will be a systemic effect. Satellite navigation would be collateral damage. When the ground-based electronics are replaced, though, the satellites will still be working. > In any case, disrupting satellite-based navigation is the simplest kind of rocket science: a missile that just has to go up and then detonate, no aiming required. Nuclear weapons are powerful, but they still have to obey the laws of physics. If you get far enough away, the inverse-squared law makes even a nuclear explosion look like a firecracker. A 1 megaton nuclear weapon would have to be detonated within half a kilometer to damage a satellite physically: http://www.projectrho.com/public_html/rocket/spacegunconvent.php http://www.projectrho.com/public_html/rocket/spacegunconvent... The shielding needed to protect a satellite against EMP only adds 5% to the cost of the satellite: http://fas.org/spp/starwars/congress/1997_h/h970716u.htm http://fas.org/spp/starwars/congress/1997_h/h970716u.htm So basically, it's going to take one nuclear warhead to destroy one navigation satellite. If a rogue nation has 60 nuclear warheads, it's hardly going to waste them to take out 60 navigation satellites.

bandwevil 11y ago

You'd need to be able to ensure that everyone is using GPS altitudes before requiring it, and there are some pretty ancient planes still flying out there. As far as I know, ATC radar doesn't actually measure altitude but rather relies on the plane's transponder to report it.If you have some mix of baro and GPS altitudes, you can no longer safely ensure separation. There should be enough checks for setting the proper baro setting - both pilots check the setting individually when changing it and it's reported to them a number of times throughout descent. Even if they do manage to get it wrong through all that, the GPWS should notify them if they get too close to any obstacles.

Alupis 11y ago
And not everyone uses GPS (GLONASS is a competing system used by some parts of the world, specifically developed by the USSR to not have to rely on a USA based system).

tfe 11y ago

I think the biggest hurdle is lack of precision. 200 feet isn't much error in the horizontal realm but is a big deal vertically. There are ways to supplement the precision of GPS altitude with ground-based stations (WAAS), and this is indeed used to construct GPS-based instrument approach procedures which are comparable in accuracy to traditional means (ILS). But these require ground-based transmitters; impractical to implement worldwide.

sokoloff 11y ago

The most unassailable answer is that the procedures were worked out before GPS equipped airplanes were in the fleet, and even today, GPS is not required equipment for flight in the flight levels (and many older jets are not equipped, especially older freighters). Other reasons are: the transponders that ATC (and other aircraft) interrogate report pressure altitude (uncorrected, meaning using 29.92 inHg as the standard). A sensitive altimeter can be had for a few hundred dollars and requires no electrical power (unless lighted) and basically no service throughout its life. To your last point, anywhere it matters (below the flight levels), pilots are using a local altimeter setting on the altimeters, often from the ATIS (automated terminal["airport"] information system) or from ATC. Of course, for takeoff, it doesn't matter. You're generally climbing at max available power or are doing a reduced power takeoff in aircraft so certified and on runways without obstacles to worry about. On approach, you're typically following a precision approach path provided by a radio signal (ILS includes lateral and vertical guidance beams) or that is GPS-based (where you have similar navigation performance levels provided by WAAS augmentation). There are non-precision approaches where the altimeter setting is more critical, but I can't recall the last time I had to fly one of those for other than training purposes.

JHof 11y ago
Even the newest airliners of today still triangulate their positions off of radio beacons (VORs) as a backup to the GPS system and also contain an inertial system. GPS being the most accurate, it's normally the primary navigation system, however, I've flown across the country, beacon to beacon, with the GPS inoperative, which some airliners still do from time to time. In the southwestern US, especially, GPS often becomes unusable due to military activity. It isn't uncommon for an airliner to experience a loss of GPS signal mid-flight when operating near military exercises. It's just the nature of our system. It was a good article. I enjoyed the allusions to ships on a sea, and moment before landing described as an unanswered question. I'm wondering if this will be passed along much in the pilot community. The most popular 'pilot life' creation in recent memory seems to be this Lego parody - https://www.youtube.com/watch?v=rNxz2hhSXuY https://www.youtube.com/watch?v=rNxz2hhSXuY.

csirac2 11y ago

The technical limitation is the basic geometry of GPS. Satellites are at ~20,000km, aircraft at ~10km max. And you want 0.0003km resolution in altitude. We barely even have 0.003km in the lat/long plane, and it's only that good because of fancy DSP algorithms which lean on special modeling of atmosphere and the satellites themselves - I.e. assumptions about a 3rd-party system, wholly inappropriate for safety critical stuff. In fact, cruising altitude (~"full span" indication) is roughly the magnitude of the specs for error in lat/lang back when GPS was first commissioned (IIRC).

omegant 11y ago

All the other replies are true, but the main reason is that airplanes fly a constant pressure altitude because it's important performance wise, while flying a constant geografical altitude is almost meaning less(except for obstacle avoidance. Pressure altitude is fundamental for the performance of airplanes because you fly in a known estable layer of the atmosphere that keeps your drag, lift and thrust almost constant (there are temperature changes but it takes docens of miles to change a single degree). Also if you select the standart atmosphere you know you are flying the same layer system of all the other airplanes in the air, with a fully autonomous, relatively cheap, and non electrical device. Also it's easy to calibrate on your own if you want to fly in a local airport and have no access to a weather report. It's hard to beat a conventional altimeter.

joshvm 11y ago

GPS altitude is considerably poorer than GPS lat/lon. There's a good chance that your GPS altitude could be 10-20 metres off what it should be, which is enough to be dangerous if you're using it as a landing aid. "True" altitude requires you to both have an excellent fix on the satellites and have a very, very good reference geodetic model (which you probably don't). Also depending on the quality of your fix, you'll have different altitudes to the rest of the people around you. It makes more sense to have two pilots independently check the pressure settings (and that's why you have a checklist).

WalterBright 11y ago

My dad was a navigator on a B-17. He'd tell me that he had to scrape the frost off of the dome with his fingernails so he could get a fix on a star while flying across the Atlantic. Crossing the Atlantic at the time was at the limit of the B-17's range, so messing up the navigation means you disappeared. Nobody dreamed of GPS back then :-)

therockspush 11y ago
Some operational C-130s still have a little sextant port for shooting a star but only a few of the old guys would still even know how it works. The GPS most of them use is almost as old, a $20 Garmin handheld could navigate circles around it.
- ygra 11y ago
  > a $20 Garmin handheld could navigate circles around it Probably not, as it will refuse to work at both the altitude and speed of an airplane ;-) EDIT: I stand corrected. Neither the speed nor the altitude will be a problem.
  - votingprawn 11y ago
    > Probably not, as it will refuse to work at both the altitude and speed of an airplane ;-) Well sub in the garmin with one of the other low cost consumer units that work at those altitude and speeds then!
  - coob 11y ago
    The GPS altitude limit is 18km which is 60,000ft. Normal cruising altitude is 35,000ft. It's also trivial to obtain unrestricted GPS devices.

azernik 11y ago

Wikipedia [1] says something very different - that during takeoff and landing commercial aircraft generally rely on radar altimeters, which are extremely accurate and reliable, but generally don't work above about 750 meters. See: http://en.wikipedia.org/wiki/Radar_altimeter#Civil_aviation_applications http://en.wikipedia.org/wiki/Radar_altimeter#Civil_aviation_...