WAAS GPS NONSTD TEMP's

[Sammy]

Per AIM para 7-2-3 ALTIMETER ERRORS, TBL 7-2-3 is published and recommeded for use when experiencing extreme cold temperatures. The error would then be added to the altitudes flown enroute and/or during an instrument approach procedure (IAP). Also, AIM para 1-1-21 WIDE AREA AUGMENTATION SYSTEM (WAAS), d. 1. states "One of the major improvements is the ability to generate an electronic glide path, independent of ground equipment or barometric aiding. This eliminates several problems such as cold temperature effects, incorrect altimeter setting or lack of a local alimeter source and allows approach procedures to be built without the cost of installing ground stations at each airport." Let us say I am shooting a cat I ILS. I reach the MAP when I reach the published DA while maintaining the glide slope. It is recommeded to increase the DA by the amount of the cold wx correction if extreme cold temperatures exist. If shooting a WAAS GPS down to the LNAV/VNAV minimums, I am still flying an electronic glide slope down to the published DA. Why don't the extreme temperatures effect this type of appraoch???

 

[JimNtexas]

In a WAAS system altitude is calculated entirely from the GPS signal. Barometric pressure is not used therefore there is no need for a cold weather correction.

I think.

 

[dgs]

Jim -- You are exactly correct. GPS altitudes are based on the absolute height above the WG-84 datum plane. Temperature has no effect because you are not using air pressure differentials to determine altitude. In fact, the issue of GPS vs. Baro altitudes is an issue for WAAS and LAAS approaches. The best procedures I've seen use baro until you begin the descent to prevent altitude variations in the terminal area between aircraft operating different equipment.