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RAIM, FDE, and WAAS Explained: GPS Integrity, Approach Types, and What to Do When You Lose RAIM

GPS feels infallible until you understand how much work goes into making it trustworthy enough to fly an instrument approach to low minimums. A GPS receiver computing your position from satellite signals has no inherent way to know if one of those satellites is transmitting bad data — and a bad satellite could put you a mile off course with no warning. That's the problem RAIM, FDE, and WAAS exist to solve. For instrument pilots, understanding GPS integrity monitoring isn't optional — it determines which approaches you can fly, what to do when integrity is lost, and how to plan flights that depend on satellite navigation.


This post covers GPS integrity in practical depth: RAIM and FDE, how WAAS changes the picture, the different GPS approach types and their requirements, the receiver certification categories, how to perform a RAIM prediction, and what to do when you lose RAIM in flight.



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The Core Problem: GPS Integrity

GPS provides position, velocity, and time information by measuring signals from multiple satellites. But there's a fundamental challenge: how does the receiver know the position it computes is correct?


The integrity problem:

  • A satellite could transmit erroneous data (clock error, orbit error)

  • The receiver computes position from the signals it receives

  • Without a check, a bad satellite produces a bad position

  • A bad position with no warning is dangerous, especially on approach


What "integrity" means:

  • Integrity is the ability to provide timely warnings when the system shouldn't be used

  • It's not just accuracy — it's knowing when the accuracy can't be trusted

  • For IFR, integrity is as important as accuracy

  • A system without integrity monitoring can't be trusted for approaches


The solutions:

  • RAIM: The receiver checks itself using redundant satellites

  • FDE: The receiver excludes a bad satellite and continues

  • WAAS: A ground-based augmentation system provides correction and integrity


These work together to make GPS trustworthy for aviation.


RAIM: Receiver Autonomous Integrity Monitoring

RAIM is the GPS receiver's self-checking function.


How RAIM works:

  • The receiver needs redundant satellites

  • With enough satellites, it can cross-check the position solution

  • If one satellite is inconsistent with the others, RAIM detects it

  • The receiver flags the integrity problem


The satellite requirement:

  • RAIM fault detection requires a minimum of 5 satellites

  • 4 satellites are needed for a basic 3D position

  • The 5th provides the redundancy for checking

  • More satellites improve RAIM availability


What RAIM does:

  • Detects when a satellite is faulty

  • Alerts the pilot that integrity can't be assured

  • Does NOT necessarily fix the problem (that's FDE)

  • Provides the warning that protects the pilot


The RAIM alert:

  • When RAIM is lost or a fault is detected, the receiver alerts

  • The pilot must respond appropriately

  • On approach, loss of RAIM may require a missed approach

  • Enroute, the pilot may need alternate navigation


FDE: Fault Detection and Exclusion

FDE builds on RAIM by not just detecting a fault, but excluding it.


How FDE works:

  • RAIM detects a faulty satellite

  • FDE identifies which satellite is bad

  • FDE excludes that satellite from the solution

  • Navigation continues with the remaining good satellites


The satellite requirement:

  • FDE requires a minimum of 6 satellites

  • 5 for RAIM detection

  • The 6th allows continuing after excluding the bad one

  • More satellites improve FDE capability


The benefit:

  • Navigation continues uninterrupted

  • The bad satellite is removed

  • No loss of navigation capability

  • Particularly valuable in remote areas (oceanic, etc.)


Why FDE matters for oceanic/remote:

  • Over oceans, there are no backup navaids

  • GPS is the primary (often only) navigation

  • FDE ensures continuity if a satellite fails

  • Required for some oceanic operations


WAAS: The Game Changer

WAAS (Wide Area Augmentation System) is a satellite-based augmentation system that dramatically improves GPS for aviation.


What WAAS does:

  • Ground stations monitor GPS signals

  • They calculate corrections for errors

  • Corrections are broadcast via geostationary satellites

  • The receiver applies corrections for better accuracy and integrity


WAAS benefits:

  • Improved accuracy: From ~15 meters to ~1-2 meters

  • Improved integrity: WAAS provides integrity information directly

  • Vertical guidance: Enables approaches with vertical guidance (LPV)

  • Reduced reliance on RAIM: WAAS provides integrity, reducing the need for RAIM prediction


The RAIM difference with WAAS:

  • Non-WAAS receivers rely on RAIM for integrity

  • WAAS receivers get integrity from WAAS itself

  • WAAS receivers generally don't require RAIM prediction checks

  • This is a major operational advantage


WAAS coverage:

  • Covers the continental U.S., most of Alaska, parts of Canada and Mexico

  • Geostationary satellites broadcast the corrections

  • Some areas (far north, etc.) have limited WAAS coverage


The bottom line:

WAAS receivers are more capable, more accurate, and don't typically require RAIM prediction. Non-WAAS receivers rely on RAIM and require prediction checks for approaches.


The GPS Approach Types

GPS enables several approach types with different capabilities and requirements. Understanding these is essential for instrument pilots.


LNAV (Lateral Navigation):

  • Lateral guidance only (no vertical guidance)

  • A non-precision approach

  • Minimum Descent Altitude (MDA)

  • Available to both WAAS and non-WAAS receivers (with RAIM)

  • The basic GPS approach


LNAV+V (Lateral Navigation with advisory vertical):

  • Lateral guidance with ADVISORY vertical guidance

  • The vertical guidance is advisory only (not for primary guidance)

  • Still flown to LNAV MDA

  • WAAS receivers provide this

  • The vertical guidance aids the descent but isn't certified


LNAV/VNAV (Lateral Navigation/Vertical Navigation):

  • Lateral AND vertical guidance

  • An approach with vertical guidance (APV)

  • Decision Altitude (DA) instead of MDA

  • Requires WAAS or baro-VNAV

  • More precise than LNAV


LPV (Localizer Performance with Vertical guidance):

  • The most precise GPS approach

  • Lateral and vertical guidance

  • Similar to an ILS in capability

  • Decision Altitude (DA), often as low as 200 feet

  • Requires WAAS

  • The "gold standard" GPS approach


LP (Localizer Performance):

  • Lateral guidance with localizer-like precision

  • No vertical guidance

  • MDA

  • Requires WAAS

  • Used where terrain prevents vertical guidance


The hierarchy:

  • LPV: Best (precise lateral + vertical, low DA)

  • LNAV/VNAV: Good (lateral + vertical, higher DA)

  • LP: Precise lateral, no vertical

  • LNAV+V: Lateral + advisory vertical

  • LNAV: Basic (lateral only, MDA)


The Receiver Certification Categories

GPS receivers are certified under different TSO (Technical Standard Order) categories that determine their capabilities.


TSO-C129 (legacy):

  • Older GPS receivers

  • Non-WAAS

  • Require RAIM for integrity

  • Require RAIM prediction for approaches

  • Can fly LNAV approaches

  • Being phased out


TSO-C145/C146 (WAAS):

  • WAAS-capable receivers

  • C145: WAAS sensor (used with a separate navigator)

  • C146: Standalone WAAS navigator

  • Can fly LPV, LNAV/VNAV, LP, LNAV+V

  • Don't typically require RAIM prediction

  • The modern standard


Why this matters:

  • Your receiver's certification determines which approaches you can fly

  • WAAS receivers (C145/146) have full capability

  • Non-WAAS receivers (C129) are limited to LNAV with RAIM

  • Check your aircraft's GPS certification


The IFR requirement:

  • The GPS must be IFR-certified (TSO'd) for IFR navigation

  • A handheld or non-certified GPS can't be used for IFR primary navigation

  • The certification must be appropriate for the operation


RAIM Prediction

For non-WAAS receivers, RAIM prediction is required before flying a GPS approach.


Why predict RAIM:

  • RAIM availability varies with satellite geometry

  • At certain times/locations, RAIM may not be available

  • An approach can't be flown without RAIM (non-WAAS)

  • Prediction confirms RAIM will be available


How to predict RAIM:

Method 1 - The receiver:

  • Many GPS units have a RAIM prediction function

  • Enter the destination and approach time

  • The unit predicts RAIM availability


Method 2 - Flight planning services:

  • 1800wxbrief.com and other services

  • RAIM prediction tools

  • Enter route and times


Method 3 - The FAA SAPT (Service Availability Prediction Tool):

  • Online RAIM/WAAS prediction

  • Detailed availability information


When RAIM prediction is required:

  • Non-WAAS receivers (TSO-C129) for approaches

  • Part of preflight planning for GPS approaches

  • WAAS receivers generally exempt (WAAS provides integrity)


If RAIM isn't predicted to be available:

  • Can't fly the GPS approach (non-WAAS)

  • Plan an alternate approach (ILS, VOR)

  • Or delay until RAIM is available


Baro-Aiding

Baro-aiding is a technique that improves RAIM availability.


What baro-aiding is:

  • Using barometric altitude as an additional input

  • The altimeter provides an independent altitude reference

  • This substitutes for one satellite in the RAIM calculation

  • Effectively reduces the satellite requirement


The benefit:

  • Improves RAIM availability

  • The baro altitude acts like an additional "satellite"

  • Allows RAIM with fewer actual satellites

  • Built into many IFR GPS units


How it works:

  • The receiver uses pressure altitude (corrected)

  • This provides geometric redundancy

  • RAIM can be computed with one fewer satellite

  • Enhances availability, especially with marginal satellite geometry



What to Do When You Lose RAIM in Flight

Losing RAIM during flight requires a response. This is a practical scenario instrument pilots must handle.


Enroute RAIM loss:

  • The GPS alerts that RAIM is unavailable

  • Position may still be displayed but isn't integrity-monitored

  • Revert to alternate navigation (VOR, etc.)

  • The MON (VOR network) is the backup

  • Advise ATC if navigation is affected


Approach RAIM loss:

  • If RAIM is lost before the FAF: don't begin the approach

  • If RAIM is lost during the approach (after FAF): execute a missed approach

  • The integrity can't be assured

  • Continuing without RAIM risks a bad position

  • Go missed and use an alternate


The general response:

  1. Recognize the RAIM alert

  2. Don't rely on GPS for the affected operation

  3. Revert to alternate navigation

  4. On approach, go missed if RAIM is lost

  5. Advise ATC as needed

  6. Use backup navigation (VOR/MON)


Why this matters:

  • RAIM loss means GPS integrity isn't assured

  • The position could be erroneous

  • On approach, this could put you off course near terrain

  • The conservative response (missed approach) protects safety


GPS Outages and Interference

A growing concern: GPS jamming and interference.


The threats:

  • GPS jamming (intentional interference)

  • GPS spoofing (false signals)

  • Solar activity affecting signals

  • Military testing causing outages


The reality:

  • GPS interference is increasingly common

  • Military testing creates planned outages (published in NOTAMs)

  • Jamming incidents are reported

  • Pilots must be prepared for GPS loss


Preparation:

  • Maintain VOR skills (the MON backup)

  • Check NOTAMs for GPS testing/outages

  • Have alternate navigation capability

  • Don't be solely dependent on GPS


GPS NOTAMs:

  • GPS interference testing is published

  • Check for GPS NOTAMs along your route

  • Plan alternates if GPS may be unavailable

  • Particularly important in military testing areas


Common Misconceptions

  • "GPS is always accurate and reliable.

    • "GPS is accurate, but integrity monitoring (RAIM/FDE/WAAS) is what makes it trustworthy. Without integrity, a bad satellite could cause errors.

  • "RAIM and FDE are the same.

    • "No — RAIM detects a fault (5 satellites); FDE excludes the faulty satellite and continues (6 satellites).

  • "WAAS receivers need RAIM prediction.

    • "Generally no — WAAS provides integrity, so WAAS receivers typically don't require RAIM prediction.

  • "Any GPS can be used for IFR.

    • "No — the GPS must be IFR-certified (appropriate TSO). Handhelds and non-certified units can't be used for IFR primary navigation.

  • "LPV is a precision approach.

    • "LPV is technically an APV (approach with vertical guidance), though it performs like a precision approach with DA as low as 200 feet. The distinction is technical.


On the Written Test and Checkride

GPS integrity appears on instrument tests. The most commonly tested topics:

  • RAIM (5 satellites for detection)

  • FDE (6 satellites for exclusion)

  • WAAS benefits and approach types

  • The GPS approach types (LNAV, LNAV/VNAV, LPV, LP)

  • RAIM prediction requirement

  • What to do when RAIM is lost


Quick Reference

RAIM:

  • Receiver Autonomous Integrity Monitoring

  • Detects a faulty satellite

  • Requires minimum 5 satellites

  • Alerts pilot to integrity issues


FDE:

  • Fault Detection and Exclusion

  • Detects AND excludes faulty satellite

  • Requires minimum 6 satellites

  • Continues navigation uninterrupted

  • Important for oceanic/remote


WAAS:

  • Wide Area Augmentation System

  • Improves accuracy (~1-2 meters)

  • Provides integrity (reduces RAIM need)

  • Enables LPV approaches

  • Covers CONUS, most of Alaska


Receiver Categories:

  • TSO-C129: Legacy, non-WAAS, RAIM required

  • TSO-C145: WAAS sensor

  • TSO-C146: WAAS standalone navigator


RAIM Prediction:

  • Required for non-WAAS approaches

  • Tools: receiver, 1800wxbrief, FAA SAPT

  • WAAS receivers generally exempt


Baro-Aiding:

  • Uses barometric altitude as input

  • Improves RAIM availability

  • Substitutes for one satellite


Losing RAIM in Flight:

  • Enroute: revert to VOR/MON

  • Before FAF: don't start approach

  • After FAF: execute missed approach

  • Advise ATC


Satellite Requirements:

  • 4: Basic 3D position

  • 5: RAIM (fault detection)

  • 6: FDE (fault exclusion)


GPS Threats:

  • Jamming, spoofing, interference

  • Check GPS NOTAMs

  • Maintain VOR skills (MON backup)


Key Principle:

GPS accuracy means little without integrity. RAIM (5 sats) detects faults, FDE (6 sats) excludes them, and WAAS provides integrity plus precise approaches. Know what to do when integrity is lost.



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Author: Nathan Hodell

CFI, CFII, MEI, ATP, Creator and CEO

Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.



 
 
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