Westjet TV LiveMap: Data source?
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SierraPoppa
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Re: Westjet TV LiveMap: Data source?
Wow, talk about pickin' the flyshit out of the pepper.cpl_atc wrote:Just wondering where the LiveMap display on Westjet flights gets its data from? The altitude is always out of whack by several hundred feet vs. what the crew says. I.e. FL400 shows up as 39567 or something on the display.
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Cat Driver
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cpl_atc:
Sounds like that data is coming from a GPS source, altitude read outs from GPS uncorrected will vary depending on satellite positions...
Cat
Sounds like that data is coming from a GPS source, altitude read outs from GPS uncorrected will vary depending on satellite positions...
Cat
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After over a half a century of flying no one ever died because of my decision not to fly.
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Flying Nutcracker
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Look at the altitude next time you fly and enthusiastically find yourself looking at the screens and try to figure out if you are flying through low or high pressure systems.
GPS will give you actual altitude as opposed to a FL, unless ofcourse the GPS is fed data from the Air Data Computer or the Altimeters themselves...
GPS will give you actual altitude as opposed to a FL, unless ofcourse the GPS is fed data from the Air Data Computer or the Altimeters themselves...
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Flying Low
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If they get the 380 will the passengers on the upper deck get a higher altitude readout than those on the lower deck???? 
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Snow Monkey
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No, i think that would be compensated by miniature highly sensitive algorithmic barometers imbedded in the seat cushionsFlying Low wrote:If they get the 380 will the passengers on the upper deck get a higher altitude readout than those on the lower deck????
I'm givin er all she's got..
GPS altitude is calculated from mean geometric sea level. That is to say, it uses an imaginary sea level of a perfect circle who's diameter is more or less the average diameter of the earth.
Unfortunately the earth is an "oblate spheroid" (pear shaped) and not a perfect circle. Therefore the difference between GPS altitude (GSL) and actual MSL varies around the world.
Most satellite inflight entertainment systems receive air data and GPS position data to help optimize the reception. In many systems, the actual altitude shown on the displays are selectable. So the problem can be anything from someone selecting a mismatched altitude format to the fact they are actually displaying GSL.
Antother thing I have noticed, is that flight attendants sometimes use an imaginary cruise altitude in their announcments (Like "we'll be flying at an altitude of thirty thousand feet") or they use a generic announcement (like "we'll be cruising at altitudes UP TO forty one thousand feet")
So it might be just a miscommunication. It could even mean that they flight crew were assigned (or requested) a different altitude than the one they announced they will be flying at.
Unfortunately the earth is an "oblate spheroid" (pear shaped) and not a perfect circle. Therefore the difference between GPS altitude (GSL) and actual MSL varies around the world.
Most satellite inflight entertainment systems receive air data and GPS position data to help optimize the reception. In many systems, the actual altitude shown on the displays are selectable. So the problem can be anything from someone selecting a mismatched altitude format to the fact they are actually displaying GSL.
Antother thing I have noticed, is that flight attendants sometimes use an imaginary cruise altitude in their announcments (Like "we'll be flying at an altitude of thirty thousand feet") or they use a generic announcement (like "we'll be cruising at altitudes UP TO forty one thousand feet")
So it might be just a miscommunication. It could even mean that they flight crew were assigned (or requested) a different altitude than the one they announced they will be flying at.
GPS positions (Lat, Long, Height or X,Y,Z whatever you want) is all referenced to a ellipsoid. GPS is all references to the WGS84 Reference System, which uses the GRS1980 Reference Ellipsoid. Usually these reference ellipsoids are zeroed out at mean sea level, which is defined by tides and stuff and pretty much whatever they want. However, they are, as CID mentioned, mathematically designed so they are best fitting to a network of control points which will have their own elevations related to mean sea level.
For example, if you survey to a benchmark with GPS that has been given an elevation by conventional methods, you will most likely be off by tens of metres.
Heights measured relative to the geoid (an equipotental gravity surface) are called orthometric heights. These are the heights that you will see on legal survey plans or on the info cards for the benchmarks.
Thats why its very important to know what coordinate system you are using compared to the coordinates that you have. Some systems are very close, but some systems are very very different. And thats how the Oil companies get free wellsites, when the surveyors have to buy the land because they f'ed up the coordinate in the first place.
For example, if you survey to a benchmark with GPS that has been given an elevation by conventional methods, you will most likely be off by tens of metres.
Heights measured relative to the geoid (an equipotental gravity surface) are called orthometric heights. These are the heights that you will see on legal survey plans or on the info cards for the benchmarks.
Thats why its very important to know what coordinate system you are using compared to the coordinates that you have. Some systems are very close, but some systems are very very different. And thats how the Oil companies get free wellsites, when the surveyors have to buy the land because they f'ed up the coordinate in the first place.
