Confusing Your GPS

When Dorothy and her gang finally met the All-Powerful Oz, he turned out to be a snake-oil salesman from Kansas working busily out of sight behind the curtain. Your IFR-certified GPS is similar in a way, with the magic a result of engineers toiling under 1000+ pages of Minimum Operation Performance specs (although they probably did come from Kansas).You should never need to look behind the curtain and check up on your GPS. But it is possible to get the box to stumble if you try hard enough.

jwanwest

Tuesday, January 06, 2015 at 12:00 AM ET

Verified

Key Takeaways:

GPS units (non-WAAS and WAAS) continuously monitor integrity using systems like RAIM, FDE, or direct WAAS signals, with WAAS offering superior accuracy for approach minimums.
CDI sensitivity dynamically scales as you progress through flight phases, becoming progressively tighter from en route (5/2 miles) to terminal (1 mile) and ultimately to 0.3 miles by the Final Approach Fix (FAF).
Skipping segments or going direct to the FAF bypasses critical pre-FAF calculations, potentially causing sudden CDI deflections, autopilot disconnects, and a lack of displayed minimums just when they are most needed.
Understanding these "behind the curtain" operations and respecting the system's intended sequence is crucial for maintaining situational awareness and safely executing instrument approaches.

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When Dorothy and her gang finally met the All-Powerful Oz, he turned out to be a snake-oil salesman from Kansas working busily out of sight behind the curtain. Your IFR-certified GPS is similar in a way, with the magic a result of engineers toiling under 1000+ pages of Minimum Operation Performance specs (although they probably did come from Kansas).
You should never need to look behind the curtain and check up on your GPS. But it is possible to get the box to stumble if you try hard enough.

A Matter of Integrity
Most of what you could do that might cause problems happens in the terminal and approach environment. But to get a handle on why, let’s back up to en route navigation. Whether you’ve got a TSO-C129 (non-WAAS) or TSO-C145A/TSO-C146A (WAAS) GPS, it’s doing ongoing integrity checks as you fly.