GPS approach downgrade

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
Yesterday I was flying into an airport in which the ceiling continued to lower until at approach time, it was at LPV minimums for ceiling (vis was 7).
That's cool I thought; might make it in.
Passing the IAF, had LPV illuminated then soon after got a warning about the approach being downgraded to LNAV which meant higher ceiling minimums and yes, a missed in actual.

I've been looking around to find out what can cause this downgrade to occur and have found nothing.
Nothing in AIM, using their search tool.
The unit displayed "Raim Available" prior to commencing approach.
There is no notam.
I have done this approach many times before using LPV all the way down.
Hoping it's not our equipment.
 
What FORANE said...we're in the middle of a massive geomagnetic storm courtesy of that little fireball that gives us life. The likes of which haven't been seen since 2005. Pretty much anything that uses a radio signal is degraded is some way. Hell, last night the UPS battery in my office started clicking on and off. There was enough interference in the power lines that the UPS was reading it as a power outage.
 
Most likely a side effect of the current G5 geomagnetic storm, but if it persists, it is possible that the vertical guidance board has failed. This happened to my 430W and it took a while for me to realize it was not environmental factors, but the unit itself that had partially failed. (It would happily fly LNAV approaches without any other flags or warnings.)
 
chemgeek said:
Most likely a side effect of the current G5 geomagnetic storm, but if it persists, it is possible that the vertical guidance board has failed.
Click to expand...
Is a G5 geomagnetic storm capable of damaging the vertical guidance board?
 
jeez, battling the weather down to mins with an autopilot acting up, passengers breathing down my neck and now I gotta worry about geomagnetic storms making me go missed and scrambling for an alternate!
Time to write that last chapter for E. Gann lol!
 
asicer said:
Is a G5 geomagnetic storm capable of damaging the vertical guidance board?
Click to expand...
Not likely. chemgeek is suggesting that there's a coincidental failure in the aircraft at the same time a major geomagnetic storm is causing problems to otherwise perfectly-functioning equipment.
 
asicer said:
Is a G5 geomagnetic storm capable of damaging the vertical guidance board?
Click to expand...
No. But a vertical guidance board failure will have the same symptoms. LPVs will automatically downgrade to LNAV without any other clues as to why.
 
I had an approach today act goofy too. LPV never downgraded but glide slope never popped up on the G600, tried reactivating the approach a couple of times but no joy. Loaded the visual approach and boom, glide slope popped up.
 
Skymac said:
I had an approach today act goofy too. LPV never downgraded but glide slope never popped up on the G600, tried reactivating the approach a couple of times but no joy. Loaded the visual approach and boom, glide slope popped up.
Click to expand...
I have seen that occur when VLOC is set on the GPS, but the CDI/PFD is set to GPS.
 
Let'sgoflying! said:
Yesterday I was flying into an airport in which the ceiling continued to lower until at approach time, it was at LPV minimums for ceiling (vis was 7).
That's cool I thought; might make it in.
Passing the IAF, had LPV illuminated then soon after got a warning about the approach being downgraded to LNAV which meant higher ceiling minimums and yes, a missed in actual.

I've been looking around to find out what can cause this downgrade to occur and have found nothing.
Nothing in AIM, using their search tool.
The unit displayed "Raim Available" prior to commencing approach.
There is no notam.
I have done this approach many times before using LPV all the way down.
Hoping it's not our equipment.
Click to expand...
Undoubtedly the Solar Storm. LPV, LP, and WAAS vertical guidance was wiped out for the entire US CONUS, Canada, and Alaska for about 8 hours on the date of your flight. If you look at the LPV annunciation, it would most likely have been amber rather than its normal green color on the GPS. Downgrades happen when you are 1 minute from the FAF. You can also check the satellite page and if the VFOM figure is above 40 feet, don't expect LPV200 (LPV200 is the service when the DH is below 250 feet) and don't expect LPV service (DH 250 feet or higher) when VFOM is 60+ feet.
 
Let'sgoflying! said:
Yesterday I was flying into an airport in which the ceiling continued to lower until at approach time, it was at LPV minimums for ceiling (vis was 7).
That's cool I thought; might make it in.
Passing the IAF, had LPV illuminated then soon after got a warning about the approach being downgraded to LNAV which meant higher ceiling minimums and yes, a missed in actual.

I've been looking around to find out what can cause this downgrade to occur and have found nothing.
Nothing in AIM, using their search tool.
The unit displayed "Raim Available" prior to commencing approach.
There is no notam.
I have done this approach many times before using LPV all the way down.
Hoping it's not our equipment.
Click to expand...
Had the same thing happen - I'm assuming it was the solar storm, that took out some of the GPS signals. I had the glide slope come and go twice on the same (normally LPV) approach. The same equipment worked fine the next day.
 
40YearDream said:
Had the same thing happen - I'm assuming it was the solar storm, that took out some of the GPS signals. I had the glide slope come and go twice on the same (normally LPV) approach. The same equipment worked fine the next day.
Click to expand...
I was surprised that it wasn’t till near the FAF that I learned there was a problem.
I always check “RAIM available”.
Maybe that’s not testing what I think it’s testing.
 
Let'sgoflying! said:
I was surprised that it wasn’t till near the FAF that I learned there was a problem.
I always check “RAIM available”.
Maybe that’s not testing what I think it’s testing.
Click to expand...
By design, downgrades occur when you are 1 minute out from the FAF. RAIM available will not have anything to do with WAAS service because WAAS does not use RAIM. Also RAIM available means that the algorithm can be performed in the GPS, that is different than the result of whether or not the RAIM determined integrity is acceptable or not for the operation being conducted. Assuming RAIM was available, meaning that the integrity can be determined, during the outage of LPV vertical and LP service that was recently experienced, since RAIM would only generate an alert if the LNAV approach integrity dropped below 0.3 NM (556 meters), no alert would have occurred. All during the outage, LNAV service was available. The criteria for LPV200 is vertical 35 meters and lateral 40 meters. Both of these limits were exceeded in the Solar storm, but the 556 meters required for LNAV was solid.

RAIM is not used for a WAAS GPS when inside a WAAS SV, so can usually be ignored unless there is a WAAS Outage NOTAM issued. Since 2007, there was only one such NOTAM issued for a 4 hour period when the WAAS system failed during a software update. As long as WAAS is available, RAIM is only relevant to non WAAS GPS systems. There is always a distinction between WAAS being available and the integrity values it provides either being suitable or not for a given PBN specification. WAAS integrity is broadcast from the three WAAS satellites in geostationary orbits with the SV covering the entire hemisphere over US CONUS, Canada, Alaska, Hawaii, Mexico, the Caribbean, and points south. See this for the limits of the WAAS footprint. https://www.nstb.tc.faa.gov/DisplayGEOStatus.htm
 
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Are we having another solar storm/LPV downgrading event?

Last Friday (Nov. 8), my son and I went and did some practice approaches (to our local airport, Wasilla, AK using a Garmin GNC355). The first LPV was fine. We went back out for another one. I noticed LPV was amber before the FAF, then went to green LNAV with a message saying “GPS approach downgraded. Use LNAV minimums.” We landed, fueled up, and tried it again with the same result.
I was talking to a friend who said he heard of trouble with GPS approaches in southeast Alaska. The northern lights were pretty stellar that night. I chalked it up to solar activity. I’m not familiar enough with the 355 yet to know what’s normal.

Today (Nov 10) my son went out again and got the same result.

I don’t know how to read the data on the website posted above…


This is just past the FAF after the amber LPV reverted to LNAV.
IMG_4276.jpeg
 
Alaska and most of Canada LPV service has been disrupted by solar activity over the past week. The sequence of first seeing amber LPV annunciation indicating that LPV service is not available ending in a downgrade to LNAV when one minute out from the FAF is the way this presents itself on WAAS GPS navigators. Here are the LPV200 availability for 11-8 and 11-10 showing most of Alaska and Canada without LPV200 service. All during the period, LNAV was solid.

lpv200 service on 11-8-2024.jpglpv200 service on 11-10-2024.jpg
 
Thanks John!

So what am I looking at? Red must indicate good coverage?
Edit: nevermind…. I just read the help tab in the chart website: darker red=better coverage.

Hopefully this will all blow over before my son’s checkride on Nov. 21st.
 
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The color graph is on the right and brown/magenta is good LPV service (like 100% availability or 1.0). As the availability drops, the color changes to orange around 95% availability, to a teal color around 90%, and to blue at 85%. White is off the chart. There are 1440 minutes in a day, so each 1% loss of availability is 14.4 minutes where LPV is not available. I use this site to view the current service levels, it is updated in real time every 3 minutes. https://www.nstb.tc.faa.gov/RT_VerticalProtectionLevel.htm The colors are reversed because it is not % availability, but Vertical Protection Level (VPL) which is measured in meters and Blue is good. The boundaries within the yellow line encompasses where LPV200 service is available. LPV200 requires a VPL <35 meters and LPV (DH >= 250 feet) requires 50 meters. LNAV only depends on the Horizontal Protection Level (HPL) to be above 556 Meters, whereas for LPV or LP, they require 40 meters. So you can use this site to get a good idea where things currently stand and whether or not you can expect downgrades. If the LPV service is below limits, your son and DPE may need to use an ILS to satisfy the precision approach task. LNAV has always been solid, the the NPA task should not present an issue. LNAV service level only needs HPL to be < 556 meters.

The sun is currently in its period of a solar maximum that may last for several years. During the maximum, solar events can cause disruptions in the ionosphere that messes with LPV. Also strong X ray bursts can have similar effect. When these events occur, the higher latitudes are more affected and you see this as stronger and more regular northern lights.

https://spaceweather.com/ and https://www.swpc.noaa.gov/ are the sites I use to understand solar weather. A Coronal Mass Ejection (CME) takes some time. like several days after the explosion to reach earth's ionosphere, so are more readily predicted in advance. An Xray event is less predictable and travels at the speed of light when it happens.
 
Good stuff! Thanks!

My son went out yesterday and got two successful LPV approaches!
 
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