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#1
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Say Altitude.
Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300 feet lower and gives me the altimer reading. I confirm. He says "turn off you mod C" I comply.Look at my GPS which is at 7060. I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at 7060. The controller comes back and says" I think this is due to a cold front in my flight area. 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod C. He concurs my altitude at 7000. Request lower for decend for landing. The gps works fine now, ATC is happy. HAS ANYONE ELSE EVER EXPERINCED THIS? |
#2
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Many times. Glitches can occur with their software. My most common
experience was having one controller tell me to turn off the altitude squawk, being handed off to another sector, asking the new controller if I should try Mode C again and having no further problems. Don't sweat it. If they don't like what they are seeing they will ask for verbal altitude reports. Bob Gardner "Hankal" wrote in message ... Long CX IFR at 7000 Controller comes on "say Altitude" My response "7000" He says you show 300 feet lower and gives me the altimer reading. I confirm. He says "turn off you mod C" I comply.Look at my GPS which is at 7060. I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at 7060. The controller comes back and says" I think this is due to a cold front in my flight area. 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod C. He concurs my altitude at 7000. Request lower for decend for landing. The gps works fine now, ATC is happy. HAS ANYONE ELSE EVER EXPERINCED THIS? |
#3
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In article , Hankal
wrote: I comply.Look at my GPS which is at 7060. I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at 7060. GPS altitude is only accurate to +/-150 feet. It is a "rough" reference, not to be considered "precise". |
#4
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You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude.
When WAAS is in common use this will probably change. Bob Gardner "EDR" wrote in message ... In article , Hankal wrote: I comply.Look at my GPS which is at 7060. I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at 7060. GPS altitude is only accurate to +/-150 feet. It is a "rough" reference, not to be considered "precise". |
#5
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"Bob Gardner" wrote in message news:R0hcc.78998$K91.171081@attbi_s02... You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude. When WAAS is in common use this will probably change. The altitude error is part of the WGS-84 model and is not a WAAS issue any longer. The CNX-80 provides VNAV capability through the use of pressure altitude from a transponder source and the pilot's baro correction input; married to a partial TAWS data base. GPS/WAAS is not likely to become an acceptable substitute for pressure altitude. Larger airplanes use air data derived pressure altitude in conjunction with GPS for improved RNP capability. |
#6
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I think the whole WAAS thing is in flux. The AIM gets wordier and more
difficult to interpret with each change to its discussion of GPS. IMHO, until there are a whole lot of WAAS capable boxes flying in the system we will be flailing about in the dark. Did you see the change to the AIM with regard to a reversed W on approach plates? Doesn't create much confidence in the system. Bob Gardner "Tarver Engineering" wrote in message ... "Bob Gardner" wrote in message news:R0hcc.78998$K91.171081@attbi_s02... You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude. When WAAS is in common use this will probably change. The altitude error is part of the WGS-84 model and is not a WAAS issue any longer. The CNX-80 provides VNAV capability through the use of pressure altitude from a transponder source and the pilot's baro correction input; married to a partial TAWS data base. GPS/WAAS is not likely to become an acceptable substitute for pressure altitude. Larger airplanes use air data derived pressure altitude in conjunction with GPS for improved RNP capability. |
#7
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"Bob Gardner" wrote in message news:%Glcc.191812$_w.1879080@attbi_s53... I think the whole WAAS thing is in flux. The AIM gets wordier and more difficult to interpret with each change to its discussion of GPS. IMHO, until there are a whole lot of WAAS capable boxes flying in the system we will be flailing about in the dark. I believe that VDL will be more of a factor moving forward than WAAS will be. FAA has spent a lot of money on space based WAAS while being in posession of a lot of underused VHF bandwidth. "Tarver Engineering" wrote in message ... "Bob Gardner" wrote in message news:R0hcc.78998$K91.171081@attbi_s02... You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude. When WAAS is in common use this will probably change. The altitude error is part of the WGS-84 model and is not a WAAS issue any longer. The CNX-80 provides VNAV capability through the use of pressure altitude from a transponder source and the pilot's baro correction input; married to a partial TAWS data base. GPS/WAAS is not likely to become an acceptable substitute for pressure altitude. Larger airplanes use air data derived pressure altitude in conjunction with GPS for improved RNP capability. |
#8
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Bob Gardner wrote: I think the whole WAAS thing is in flux. The AIM gets wordier and more difficult to interpret with each change to its discussion of GPS. IMHO, until there are a whole lot of WAAS capable boxes flying in the system we will be flailing about in the dark. You're right on. And, in an effort to sell LPV (WAAS) minimums, the friendlies "tweaked" the VNAV missed approach criteria so that VNAV minimums will increase, all to make WAAS look better. |
#9
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You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude.
When WAAS is in common use this will probably change. I was not using the GPS for altitude. Always use the altimeter The altimeter was showing 7000. ATC was showing 6700 Gps was 7060. However when I changed altitude the gps stayed frozen at 7060. Hank |
#10
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"Hankal" wrote in message ... You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude. When WAAS is in common use this will probably change. I was not using the GPS for altitude. Always use the altimeter The altimeter was showing 7000. ATC was showing 6700 Gps was 7060. However when I changed altitude the gps stayed frozen at 7060. Aren't controllers supposed to tell you to turn off your altimeter and go to VFR when an error that big is detected? |
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