If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Its a 296. It seems the like the higher I go the farther off it is.
I am one to think the GPS would be more accurate than the altimeter. I was thinking that we get incorrect altimeter settings "Casey Wilson" N2310D @ gmail.com wrote in message news:x1Pqe.2896$9a1.185@trnddc01... "Aluckyguess" wrote in message ... I am getting over 500ft. difference and more. "Matt Whiting" wrote in message ... Aluckyguess wrote: How come the GPS reads a different altitude than the Altimeter? For many reasons. The main reason is that they derive altitude by two completely different methods and this inevitably introduces errors. Then each method has its own inherent errors. Even two altimeters side by side won't read the same altitude other than by accident. Matt The number of sattelites being received is one of the variables -- how many are you registering on the GPS? What brand/model is the GPS. Is the GPS certified for navigation? Does the GPS have a Kohlsman equivalent input? What are the altitude specifications of the GPS? Are you comparing it to an altimeter in an airplane? Is the calibration data on the altimeter valid? Have you compared the altimeter to the GPS while parked at a known reference (i.e., the benchmark of your local airport)? My Magellan handheld takes about five mintues of averaging to get a reasonably close altitude correlation -- how much time are you giving yours? Bottom line, forget the GPS altitude reading, dial in the local pressure data into the airplane's altimeter and trust it. |
#12
|
|||
|
|||
"Aluckyguess" wrote in message
How come the GPS reads a different altitude than the Altimeter? Altimeters are calibrated for local barometric pressure. GPS altitude based on a computed, simulated sphere that approximates sea level. If the Earth suddenly disappeared (as happened in Hitchhikers Guide to the Galaxy) and you happened to be flying at that moment, your GPS would continue to happily display an altitude even though there was no actual Earth underneath you. So, for safety reasons, I think we should all use GPS altitude so that we can all safely land if the Earth ever actually disappears. I've always been curious about something: The Earth is not a perfect sphere. It bulges at the equator due to forces of tidal gravitation and centrifugal force. My question is, do GPS altitude computations take this into effect when you're, for example, flying over the equator instead of over the north pole? -- Jim Fisher |
#13
|
|||
|
|||
Jim Fisher wrote:
"Aluckyguess" wrote in message How come the GPS reads a different altitude than the Altimeter? Altimeters are calibrated for local barometric pressure. GPS altitude based on a computed, simulated sphere that approximates sea level. If the Earth suddenly disappeared (as happened in Hitchhikers Guide to the Galaxy) and you happened to be flying at that moment, your GPS would continue to happily display an altitude even though there was no actual Earth underneath you. So, for safety reasons, I think we should all use GPS altitude so that we can all safely land if the Earth ever actually disappears. I've always been curious about something: The Earth is not a perfect sphere. It bulges at the equator due to forces of tidal gravitation and centrifugal force. My question is, do GPS altitude computations take this into effect when you're, for example, flying over the equator instead of over the north pole? -- Jim Fisher Yes - it's called the reference ellipsoid. There are actually several of them and some GPS receivers allow selecting your favourite one. Actually, if Earth would disppear with all its mass, GPS would get unusable, as the satellites would continue out of their tracks due to the lack of gravity pull. -- Tauno Voipio tauno voipio (at) iki fi |
#14
|
|||
|
|||
Aluckyguess wrote:
Its a 296. It seems the like the higher I go the farther off it is. I am one to think the GPS would be more accurate than the altimeter. I was thinking that we get incorrect altimeter settings The altimeter is only "accurate" when the temperature profile of the airmass is equal to the ISA standard atmosphere. The rest of the time, there will be some error which increases with altitude (wrt wherever the source for the altimeter setting is located). This isn't a concern for en-route operations since everybody's altimeter has the same "error". It can be a concern for terrain clearance in the mountains or in some cases instrument approaches in cold weather since the altimeter reads higher than the plane actually is. |
#15
|
|||
|
|||
Fred G. Black wrote:
This isn't a concern for en-route operations since everybody's altimeter has the same "error". It can be a concern for terrain clearance in the mountains or in some cases instrument approaches in cold weather since the altimeter reads higher than the plane actually is. No concern, either. When IMC, there is a safety margin built into the approaches, and when VMC, there are windows in the copckpit. Pneumatic altimeters are reliable, independant of any infrastructure and work without electricity. Stefan |
#16
|
|||
|
|||
AIM 1-1-19(a)(8) tells pilots not to use GPS altitude: "GPS altitude should
not be relied upon to determine aircraft altitude since the vertical error can be quite large." GPS altitude is measured above the GPS sphere, which is not sea level. Bob Gardner "Aluckyguess" wrote in message news How come the GPS reads a different altitude than the Altimeter? |
#17
|
|||
|
|||
"Thomas Borchert" wrote in message
... Main reason: Altimeter indication is corrected for barometric pressure. First order of business before comparing is to set the altimeter to 29.92. No. As Dan points out elsewhere, the main error in the barometer is not correcting for non-standard temperature. There are other errors as well, but none of them involve setting the altimeter to 29.92 in an attempt to make the altimeter more accurate. Unless the correct altimeter setting is actually 29.92, setting it to 29.92 will simply make the error worse. Pete |
#18
|
|||
|
|||
Stefan wrote:
Fred G. Black wrote: This isn't a concern for en-route operations since everybody's altimeter has the same "error". It can be a concern for terrain clearance in the mountains or in some cases instrument approaches in cold weather since the altimeter reads higher than the plane actually is. No concern, either. When IMC, there is a safety margin built into the approaches, and when VMC, there are windows in the copckpit. Pneumatic altimeters are reliable, independant of any infrastructure and work without electricity. Stefan There is a buffer in the procedure design however given that cold temperature corrections are published (at least in Canada, ref CAP GEN p19), it's not unreasonable to assume that there are situations where they should be used. Fred. |
#19
|
|||
|
|||
Peter,
Unless the correct altimeter setting is actually 29.92, setting it to 29.92 will simply make the error worse. Uh, yes. Brainfart on my side. Sorry -- Thomas Borchert (EDDH) |
#20
|
|||
|
|||
Fred G. Black wrote:
There is a buffer in the procedure design however given that cold temperature corrections are published (at least in Canada, ref CAP GEN p19), it's not unreasonable to assume that there are situations where they should be used. Of course. That's one more reason why you must pass a test to get the license. Stefan |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
GREAT AVIATION READS | Cribsheet | Piloting | 1 | September 12th 04 02:51 AM |
GREAT AVIATION READS | Cribsheet | Rotorcraft | 0 | September 10th 04 06:06 PM |
GREAT AVIATION READS | Cribsheet | Military Aviation | 0 | September 7th 04 06:37 PM |
When the ASI Reads Below Zero | BTIZ | Piloting | 11 | August 23rd 04 04:57 PM |
Newsweek Reads RAM! | Bob McKellar | Military Aviation | 0 | February 17th 04 02:10 AM |