4 ms·

GPS is not, and never will be, reliable enough for IFR on its own. Relying on a tower in instrument conditions because GPS failed is unacceptable, because ever

GPS is not, and never will be, reliable enough for IFR on its own. Relying on a tower in instrument conditions because GPS failed is unacceptable, because every other GPS user is in the same boat. If you’re flying IFR, you should have a working set of instruments and be using them. Why didn’t these pilots have their VORs tuned in for positioning?

toomuchtodo 2mo ago
> GPS is not, and never will be, reliable enough for IFR on its own. WAAS GPS (~7.6M accuracy) is reliable enough for IFR on its own that many approaches have removed legacy RF ILS equipment for instrument approaches (RNAV LPV with synthetic glidescope from GNSS). I would agree this does not apply to the vast majority of general aviation (although certified receivers supporting this are well within the reach of GA at ~$6k), but facts and specifics are important, so enumerating that GPS and these trimmings make it designed to replace traditional IFR equipment under certain circumstances. WAAS is not a paid service, the additional precision is free if your receiver and autopilot supports it. Further reading: https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/approaches https://www.faa.gov/about/office_org/headquarters_offices/at... https://www.aopa.org/news-and-media/all-news/2025/october/pilot/instrument-tip-gps-substitute-or-alternate https://www.aopa.org/news-and-media/all-news/2025/october/pi... ("Today most IFR pilots use GPS as a primary source of navigation, whether they’re flying a departure, airway, or arrival defined by VORs; or segments of approaches based on navaids or that require DME or ADF.") [My note: This is straight from the advocacy group for general aviation in the US, AOPA.] https://www.flyingmag.com/everything-you-need-to-know-about-rnav-gps-approaches/ https://www.flyingmag.com/everything-you-need-to-know-about-... https://boltflight.com/loc-vs-ils-vs-gps-approach-understanding-modern-instrument-landing-techniques/ https://boltflight.com/loc-vs-ils-vs-gps-approach-understand... https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html https://www.faa.gov/air_traffic/publications/atpubs/aim_html... https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System > "The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including approaches with vertical guidance to any airport within its coverage area. It may be further enhanced with the local-area augmentation system (LAAS) also known by the preferred ICAO term ground-based augmentation system (GBAS) in critical areas." > In 2007, WAAS vertical guidance was projected to be available nearly all the time (greater than 99%), and its coverage encompasses the full continental U.S., most of Alaska, northern Mexico, and southern Canada. At that time, the accuracy of WAAS would meet or exceed the requirements for Category 1 ILS approaches, namely, three-dimensional position information down to 200 feet (60 m) above touchdown zone elevation. Category II or III ILS GPS approaches are supported when the subject airport has a local reference station reporting corrections ("LAAS"). With enough precision, the aircraft can land itself with GPS and local corrections ("Autoland"). A King Air running Garmin's SafeReturn recently performed this autonomously with GPS alone (to the best of my knowledge). https://en.wikipedia.org/wiki/Local-area_augmentation_system https://en.wikipedia.org/wiki/Local-area_augmentation_system https://www.faa.gov/airports/planning_capacity/non_federal/gbas https://www.faa.gov/airports/planning_capacity/non_federal/g... Autoland saves King Air, everyone reported safe - https://news.ycombinator.com/item?id=46346214 https://news.ycombinator.com/item?id=46346214 - December 2025 (185 comments) Event ATC recording: https://www.youtube.com/watch?v=K3Nl3LOZNjc https://www.youtube.com/watch?v=K3Nl3LOZNjc Demo: https://www.youtube.com/watch?v=d-ruFmgTpqA https://www.youtube.com/watch?v=d-ruFmgTpqA
- trollbridge 2mo ago
  And if GPS fails, you can’t land at those airports under instruments conditions. Your flight plan has to take this into account and be ready to go to an alternate.
- nomel 2mo ago
  It's demonstrated fact that GPS is not 100% reliable, which would be required to meet the "on its own" requirement. See the article you're commenting under, and documentation about it from the FAA [1]. [1] https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs400/afs410/GNSS/GPS_GNSS_Interference_Resource_Guide.pdf https://www.faa.gov/about/office_org/headquarters_offices/av...
  - toomuchtodo 2mo ago
    100% reliable is not a requirement per FAA regs cited. Legacy navigation equipment fails too.
    - nomel 2mo ago
      Then I think you should re-read the original comment, with this revelation in mind.

toomuchtodo 2mo ago

> GPS is not, and never will be, reliable enough for IFR on its own. WAAS GPS (~7.6M accuracy) is reliable enough for IFR on its own that many approaches have removed legacy RF ILS equipment for instrument approaches (RNAV LPV with synthetic glidescope from GNSS). I would agree this does not apply to the vast majority of general aviation (although certified receivers supporting this are well within the reach of GA at ~$6k), but facts and specifics are important, so enumerating that GPS and these trimmings make it designed to replace traditional IFR equipment under certain circumstances. WAAS is not a paid service, the additional precision is free if your receiver and autopilot supports it. Further reading: https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/approaches https://www.faa.gov/about/office_org/headquarters_offices/at... https://www.aopa.org/news-and-media/all-news/2025/october/pilot/instrument-tip-gps-substitute-or-alternate https://www.aopa.org/news-and-media/all-news/2025/october/pi... ("Today most IFR pilots use GPS as a primary source of navigation, whether they’re flying a departure, airway, or arrival defined by VORs; or segments of approaches based on navaids or that require DME or ADF.") [My note: This is straight from the advocacy group for general aviation in the US, AOPA.] https://www.flyingmag.com/everything-you-need-to-know-about-rnav-gps-approaches/ https://www.flyingmag.com/everything-you-need-to-know-about-... https://boltflight.com/loc-vs-ils-vs-gps-approach-understanding-modern-instrument-landing-techniques/ https://boltflight.com/loc-vs-ils-vs-gps-approach-understand... https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html https://www.faa.gov/air_traffic/publications/atpubs/aim_html... https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System > "The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including approaches with vertical guidance to any airport within its coverage area. It may be further enhanced with the local-area augmentation system (LAAS) also known by the preferred ICAO term ground-based augmentation system (GBAS) in critical areas." > In 2007, WAAS vertical guidance was projected to be available nearly all the time (greater than 99%), and its coverage encompasses the full continental U.S., most of Alaska, northern Mexico, and southern Canada. At that time, the accuracy of WAAS would meet or exceed the requirements for Category 1 ILS approaches, namely, three-dimensional position information down to 200 feet (60 m) above touchdown zone elevation. Category II or III ILS GPS approaches are supported when the subject airport has a local reference station reporting corrections ("LAAS"). With enough precision, the aircraft can land itself with GPS and local corrections ("Autoland"). A King Air running Garmin's SafeReturn recently performed this autonomously with GPS alone (to the best of my knowledge). https://en.wikipedia.org/wiki/Local-area_augmentation_system https://en.wikipedia.org/wiki/Local-area_augmentation_system https://www.faa.gov/airports/planning_capacity/non_federal/gbas https://www.faa.gov/airports/planning_capacity/non_federal/g... Autoland saves King Air, everyone reported safe - https://news.ycombinator.com/item?id=46346214 https://news.ycombinator.com/item?id=46346214 - December 2025 (185 comments) Event ATC recording: https://www.youtube.com/watch?v=K3Nl3LOZNjc https://www.youtube.com/watch?v=K3Nl3LOZNjc Demo: https://www.youtube.com/watch?v=d-ruFmgTpqA https://www.youtube.com/watch?v=d-ruFmgTpqA

trollbridge 2mo ago
And if GPS fails, you can’t land at those airports under instruments conditions. Your flight plan has to take this into account and be ready to go to an alternate.
nomel 2mo ago
It's demonstrated fact that GPS is not 100% reliable, which would be required to meet the "on its own" requirement. See the article you're commenting under, and documentation about it from the FAA [1]. [1] https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs400/afs410/GNSS/GPS_GNSS_Interference_Resource_Guide.pdf https://www.faa.gov/about/office_org/headquarters_offices/av...
- toomuchtodo 2mo ago
  100% reliable is not a requirement per FAA regs cited. Legacy navigation equipment fails too.
  - nomel 2mo ago
    Then I think you should re-read the original comment, with this revelation in mind.