Originally Posted by
Hossharris
that’s not at all why vnav approaches have temperature limits.
and is also an incorrect understanding of how gps works.
Explained quickly, and poorly. A clarification. The FAA disagrees with you:
”
Effects of Cold Temperature on Baro-vertical Navigation (VNAV) Vertical Guidance Non-standard temperatures can result in a change to effective vertical paths and actual descent rates when using aircraft baro-VNAV equipment for vertical guidance on final approach segments. A lower than standard temperature will result in a shallower descent angle and reduced descent rate. Conversely, a higher than standard temperature will result in a steeper angle and increased descent rate. Pilots should consider potential consequences of these effects on approach minima, power settings, sight picture, visual cues, etc., especially for high-altitude or terrain-challenged locations and during low-visibility conditions.”
US AIP ENR 1.8
GPS provides the ability to calculate a precise position based on time delay of arrival and a very precise time signal from multiple satellites. You need at least four for a 3D position, five for fault detection, and six for fault detection and exclusion. This position is a geopotential height above an ellipsoid modal of the earth’s surface which is why a secondary system such as WAAS or Baro-Aiding is required to correct the altitude to a usable altitude.
When you use Baro-Aiding to correct the position, instead of WAAS, you introduce hot and cold temperature altimetry errors that change the slope of the glide path (which is slightly curved). Above the hot limit, the slope the aircraft will follow is too steep for approach design criteria in use by the FAA. Below the cold limit the slope the aircraft will follow is too shallow.