LPV—Precision or not?

We all know an ILS is a precision approach. But there’s much confusion about whether an RNAV (GPS) approach to LPV minimums is. (Spoiler alert: Sometimes …)

BTeter

Friday, August 09, 2024 at 07:17 AM ET

Verified

This FAA diagram nicely shows the various antennas associated with an ILS.

Key Takeaways:

ILS approaches are traditional ground-based systems providing lateral and vertical guidance, while LPV approaches are satellite-based, relying on WAAS GPS corrections.
LPV approaches offer high accuracy, often exceeding that of a Category II ILS, providing a stable path free from bends or false glideslopes which can affect ground-based ILS.
Despite LPV's precision-level accuracy, the FAA classifies it as an "approach with vertical guidance" (APV) for operational purposes (e.g., alternate airport minimums), primarily due to the lack of mandatory approach lighting systems and specific time-to-alarm requirements for lower minimums, though it is considered precision for pilot currency.

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Perhaps it’d be best if we first take a broad look at the differences between an ILS and an LPV approach. A significant difference between the ILS and LPV is that the ILS is a ground-based system with several components that operate as a federated system to provide approach guidance. On the aircraft side, you must have a VHF localizer receiver, a UHF glideslope receiver, or a multimode receiver that incorporates both, and a method to determine noted distances from the runway threshold (marker beacon, compass locator, DME, radar, VOR cross radials).

The LPV approach is satellite-based; however, it also requires a ground-based station(s) to determine any errors in the “raw” GPS position and broadcast a correction. The detection of errors in the GPS position is the basis for the wide-area augmentation system (WAAS). GPS errors are also monitored via receiver autonomous integrity monitoring (RAIM), but WAAS makes it redundant. Let’s dig into this in more detail.