GTX327 Mode-C Question

[647jc]

I have a GTX327 XPNDR and it receives pressure altitude information from my DYNON EFIS. While flight testing today I was at an indicated altitude of 5,000 ft. The pressure altitude displayed on the GTX327 was 4,900 which was probably correct since the barometric pressure today was not exactly 29.92. I contacted a local airport approach (approximately 40 miles away) and asked for a XPNDR check. They had me squawk a specified code and told me they saw me "bright" which I assume is good and they also said they saw me at 5,400 ft altitude. My XPNDR was displaying 4,900 ft so I don't understand why the tower saw me at an altitude 500 ft higher. Is there an adjustment in the GTX327 that can affect this? I tend to think in digital terms so I don't understand this descrepency. Perhaps the XPNDR converts the digital pressure altitude it receives from the EFIS to some analog equivalent before or during transmission and that is where the descrepency is. Any comments or expainations regarding this behavior, sure doesn't seem right to me?

 

[Mel]

The altitude they reported to you should have been your actual altitude, not pressure altitude. They correct for that. If you were 40 miles away there could have been a difference. Did they give you an altimeter setting?
Also the pressure altitude that your encoder reports changes at 50' above and below the numbers so if you were close to the edges, that will make some difference.
Still doesn't make the 400' difference you saw. You were at 5000' and they reported you at 5400'.

 

[Don Jones]

The way I understand it....

I have a GTX327 XPNDR and it receives pressure altitude information from my DYNON EFIS. While flight testing today I was at an indicated altitude of 5,000 ft. The pressure altitude displayed on the GTX327 was 4,900 which was probably correct since the barometric pressure today was not exactly 29.92. I contacted a local airport approach (approximately 40 miles away) and asked for a XPNDR check. They had me squawk a specified code and told me they saw me "bright" which I assume is good and they also said they saw me at 5,400 ft altitude. My XPNDR was displaying 4,900 ft so I don't understand why the tower saw me at an altitude 500 ft higher. Is there an adjustment in the GTX327 that can affect this? I tend to think in digital terms so I don't understand this descrepency. Perhaps the XPNDR converts the digital pressure altitude it receives from the EFIS to some analog equivalent before or during transmission and that is where the descrepency is. Any comments or expainations regarding this behavior, sure doesn't seem right to me?

The transponder transmits pressure altitude and ATC's equipment then converts it to a calculated actual altitude based on the barometric pressure of the day. So there would be a difference between what the GTX was showing and what ATC was showing. The only time they would be the same would be when the barometric pressure is 29.92.

 

[647jc]

Mel,

The tower never gave me an altimiter setting so I don't know what they were using to convert my pressure altitude to actual altitude. To account for the 400 ft descrepency there would have to have been approximately a .4 difference in barometric pressure between me and the tower which may have been possible since the tower was 40 miles away and a weather front was rolling through.

I think I'll try the test again later this week and next time ask for an altimeter setting also.


Thx.

 

[Alan Carroll]

To account for the 400 ft descrepency there would have to have been approximately a .4 difference in barometric pressure between me and the tower which may have been possible since the tower was 40 miles away and a weather front was rolling through.

Or maybe not. The 327 always shows pressure altitude, which is what the altimeter would read when set to 29.92. As mentioned above, ATC reports what it thinks is your true altitude, which is the pressure altitude you are squawking +/- any correction for non-standard pressure.

If the altimeter setting is 30.42 (pretty typical on a cold winter day), the correction is about +500'. A 4900 pressure altitude reading from the transponder would be corrected to 5400 true altitude by ATC. This would be true if even if you are flying directly over the airport.

Your altimeter should should agree with ATC (within ~100'), if you have the correct setting . If not then you might have a problem somewhere.

For more info:

http://flighttraining.aopa.org/ft_magazine/special/0903_weather.cfm

 

[647jc]

Alan,

My altimeter(s) (EFIS and steam gauge) were both set to my local AWOS barometric pressure therfore the 5000 ft msl readings from my altimeters should have been correct. The 4900 pressure altitude being sent by my XPNDR was also most likely correct since the local AWOS pressure was not exactely 29.92. So, I still think the only way the tower could be seeing me at 5400 ft was if the towers (40 miles away) local barometric pressure they used to convert my pressure altitude to actual altitude was approximatley .40 different from my local AWOS. My EFIS and steam altimeter are converting my pressure altitude of 4900 to 5000 based on my local AWOS barometric pressure setting. The tower does a similar conversion based on their local barometric pressure and the only way to account for a 400 ft descrepency (5400 - 5000 = 400) would be if the difference in the two barometric pressure values were .40 since a difference of 1.0 accounts for an altitude change of 1000 ft.

 

[Alan Carroll]

Alan,

My altimeter(s) (EFIS and steam gauge) were both set to my local AWOS barometric pressure therfore the 5000 ft msl readings from my altimeters should have been correct. The 4900 pressure altitude being sent by my XPNDR was also most likely correct since the local AWOS pressure was not exactely 29.92. So, I still think the only way the tower could be seeing me at 5400 ft was if the towers (40 miles away) local barometric pressure they used to convert my pressure altitude to actual altitude was approximatley .40 different from my local AWOS. My EFIS and steam altimeter are converting my pressure altitude of 4900 to 5000 based on my local AWOS barometric pressure setting. The tower does a similar conversion based on their local barometric pressure and the only way to account for a 400 ft descrepency (5400 - 5000 = 400) would be if the difference in the two barometric pressure values were .40 since a difference of 1.0 accounts for an altitude change of 1000 ft.

Joe - I glossed over the part in your original post where you mentioned 5000 indicated, sorry. Still, half an inch difference in pressure over 40 miles still sounds like a lot, even if there was a front.

More testing needed I guess?