GPS outage or system problem?

azure

azure

Final Approach
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azure
Yesterday at about 2330Z I was flying between 57D and KVLL at an altitude of between 2000 and 2500 MSL. Suddenly I noticed the following alerts on my CNX-80: "WARNING Loss of Navigation - Position Error" and "WARNING Loss of Navigation - Insufficient Sats" alternating with "Integrity restored normal ops". I had a WTF moment, and then by the time I switched to the SYS page, the alerts were gone, my course indicator had reappeared on my Sandel, and all was fine. I landed about 5 minutes later without any further trouble and haven't flown since, so I don't know if this was a transient problem or something recurring.

Anyone know of a good GPS satellite visibility predictor that can "predict" a past outage? I'm not sure whether I experienced a real outage or a transient receiver system failure.
 
azure said:
Yesterday at about 2330Z I was flying between 57D and KVLL at an altitude of between 2000 and 2500 MSL. Suddenly I noticed the following alerts on my CNX-80: "WARNING Loss of Navigation - Position Error" and "WARNING Loss of Navigation - Insufficient Sats" alternating with "Integrity restored normal ops". I had a WTF moment, and then by the time I switched to the SYS page, the alerts were gone, my course indicator had reappeared on my Sandel, and all was fine. I landed about 5 minutes later without any further trouble and haven't flown since, so I don't know if this was a transient problem or something recurring.

Anyone know of a good GPS satellite visibility predictor that can "predict" a past outage? I'm not sure whether I experienced a real outage or a transient receiver system failure.
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About the same time (perhaps 20 minutes earlier) I was flying with a friend in his Rocket and his 430W lost all satellites for a few minutes after which everything returned to normal. He said that's never happened to him before.
 
I didn't see any NOTAMs about GPS outages either. There have been multiple solar flares this week, it's possible one of them was the cause. Apparently this is ongoing, so if that's the case it could very well happen again.

http://www.space.com/15643-monster-sunspot-unleashes-solar-flares.html

In any case, it seems it was some kind of unforecast outage and likely not an equipment failure. That's somewhat comforting... not sure I would like to be flying RNAV approaches in IMC this week though. :eek:

But yes, this is the first time I've seen anything like that since I started flying with a GPS.
 
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There is no way this was a satellite visibility issue though. When I finally got to the satellite page, there were about 8 of them showing, all green. Visibility of ~5 satellites doesn't change that quickly.

Something else was going on, though we'll probably never know what it was for sure.
 
I had a similar problem on Sunday, in this same area. Betw the 3 pilots aboard, we had 4 GPSs aboard. All 4 lost position. The G396 showed all the sats with nice tall signal strength bars, but could not resolve a position for 30-60 secs, but did not rpt losing lock.
 
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rainsux said:
I had a similar problem on Sunday, in this same area. Betw the 3 pilots aboard, we had 4 GPSs aboard. All 4 lost position. The G396 showed all the sats with nice tall signal strength bars, but could not resolve a position for 30-60 secs, but did not rpt losing lock.
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Just because there was a signal doesn't mean it contained useful data.
 
rainsux said:
I had a similar problem on Sunday, in this same area. Betw the 3 pilots aboard, we had 4 GPSs aboard. All 4 lost position. The G396 showed all the sats with nice tall signal strength bars, but could not resolve a position for 30-60 secs, but did not rpt losing lock.
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Interesting!
 
The US Military could have been testing a scramble unannounced. That would basically mean you get full signal but the data is useless.
 
Apache123 said:
The US Military could have been testing a scramble unannounced. That would basically mean you get full signal but the data is useless.
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The "signal strength" bars on a GPS receiver are not actually an indication of the received signal amplitude. That would be somewhat useless anyway since the nominal signal level is well below the noise floor in most places. Instead they show something like what percentage of the "chips" (short bursts of data that ID the particular satellite) were received without error. Therefore it would seem that "scrambled" data would still show low or no bars. In the past when the military wanted to make the GPS accuracy less than optimal they would dither the least significant portion of the time code. That wouldn't make the signal "useless" but it did degrade the position solutions. I'm not aware that they can do that to a greater extent than they have in the past but I wouldn't be totally surprised if that was possible. Don't know what a GPS navigator would show in that case though. And FWIW, the outage I witnessed did involve the "signal strength" bars dropping off for a couple minutes but they did come back fairly rapidly so unless one switched to the GPS signal page as soon as the GPS indicated a problem they might not have seen the bars gone.
 
Apache123 said:
The US Military could have been testing a scramble unannounced. That would basically mean you get full signal but the data is useless.
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The US Air Force is compelled by Executive Order to ensure that the GPS signal is available for all peaceful uses. Therefore they will never scramble to signal coming from the satellites. They do plenty of testing on the ground in localized areas.
 
They also issue NOTAMs. Been a ton of them in Southern Colorado and Alamagordo, NM in the last few years.
 
It happened again today while I was shooting an RNAV approach under the hood with my CFII. We were on our way to the IAF so it was trivial, we didn't have to miss, and asked for vectors to the ILS instead. It took a good 3 minutes to recover full WAAS capability. Then it happened twice again later on, though it was much shorter both times, almost fleeting (an alert, then the message saying normal ops restored).

This was in the vicinity of KDET around 2200Z.
 
Has to be the solar flares. The Sun is quite active right now. Does take much to spoof that signal to noise ratio.
 
comanchepilot said:
Has to be the solar flares. The Sun is quite active right now. Does take much to spoof that signal to noise ratio.
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I'd check into that, instead of speculation. So far, the flares haven't done squat.

Could be, but I'd sure like to know for sure...

L.Adamson

edit: BTW, I have a friend who fly's Boeing 737 800's. GPS is the principal navigation tool in this airplane. Whenever I see him, I always ask about GPS outages (being the GPS fanatic that I am). The last.........he mentioned, was a few seconds, close to a military installation in the mountain west. That was about six years ago. He does fly from coast to coast. It was last Thursday, that I again inquired about the GPS reliability.
 
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There's been more NOTAMs out for the usual jamming near Alamagordo, NM that cover all the way into Wyoming in the Flight Levels. It's virtually a standing NOTAM at this point around here.
 
I called up my avionics tech today and described the problem. He suspects cellphone interference as the likeliest cause (due to overflying one or more cell towers at fairly low altitude), followed by something else in the plane putting out a harmonic that's clobbering the GPS. I went for a fuel run today to 57D, and just as I put the gear down in the pattern back at VLL, I got the familiar LOI alert again, followed quickly by the message saying "normal ops restored". I was setting up for a landing and really didn't want to run the gear motor again so soon to put the gear back up and exit the area to troubleshoot the problem, so I just clumsily took out my phone and shut it off. I could swear there was one more LOI alert immediately after I turned off my phone, but I'm not 100% sure (could have been from an outage that began before the phone was completely shut off) and then it cleared up on final. So I tried one more run around the pattern with my cellphone off. It didn't act up. Of course, this means not very much. Tomorrow I have another checkride prep session with my CFII. This time I'll make sure we have both of our cellphones off for the whole flight and see what happens...
 
Interesting possibility. Will be interested in how it behaves.

You can start giving those "all personal electonic devices off" flight attendant speeches now. ;)
 
azure said:
I called up my avionics tech today and described the problem. He suspects cellphone interference as the likeliest cause (due to overflying one or more cell towers at fairly low altitude), followed by something else in the plane putting out a harmonic that's clobbering the GPS.
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I've only had my Garmin 430 lose signal once while flying fairly near a very powerful radar center. Several other people lost signal in the same area.

I'm think I routinely fly significantly closer to cell phone towers on a regular basis than most people and have not had problems. :dunno: :D

I'm sure you've had the antennae connections checked. Is there a way to make sure the antennae itself is OK?

Barb
 
One Short said:
I've only had my Garmin 430 lose signal once while flying fairly near a very powerful radar center. Several other people lost signal in the same area.

I'm think I routinely fly significantly closer to cell phone towers on a regular basis than most people and have not had problems. :dunno: :D

I'm sure you've had the antennae connections checked. Is there a way to make sure the antennae itself is OK?

Barb
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Actually I haven't had them checked yet -- Bill sounds pretty sure that this kind of behavior is due to interference, but I'm not so sure either and will ask him about that possibility if it continues.

More this evening, since I'll be doing another round of checkride prep in a couple of hours.
 
One Short said:
I've only had my Garmin 430 lose signal once while flying fairly near a very powerful radar center. Several other people lost signal in the same area.

I'm think I routinely fly significantly closer to cell phone towers on a regular basis than most people and have not had problems. :dunno: :D

I'm sure you've had the antennae connections checked. Is there a way to make sure the antennae itself is OK?

Barb
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Guess those moose antlers don't put up much interference! :lol:


Liz - FWIW, I've lost GPS near Grayling several times; but never "close to home" (although, I haven't flown into Troy in awhile). :idea:
 
It happened again this afternoon, just once, about 6 nm north of VLL, so same general area as the first time. We did a bunch of approaches at PTK, and once west of VLL it was rock solid. Still, that's very flimsy evidence for external interference. Out of sheer paranoia I had the 480 on the satellites page whenever I was using it as a primary nav source, so that hopefully I'd get some advance warning if I was losing sats. All the displayed sats were green and showed strong signal.

We had our cellphones off the whole time so obviously it isn't that. I'm going to call the avionics shop tomorrow and see what the next step is. He'll probably tell me to switch off my transponder (a SL-70), which means pulling the CB since it isn't panel mounted. Luckily I have a #2 xpdr I can switch to and still be legal within the Mode C veil. I've also heard of people getting GPS interference from the ELT (without it necessarily transmitting on 121.5).
 
rainsux said:
Next time it happens, drop a waypoint. Send me the lat:lon.
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I can do that, but if it's a specific location I'd have to know it with a lot greater precision than a GPS-generated lat:long. It happened yesterday in the pattern at VLL, midfield downwind for rwy 9. First time in that spot, though I've been in that general location dozens of times before.

I think it's most likely generated by something in the plane, or else a bad antenna connection. (Or maybe sunspots.)
 
There was a massive Coronal Mass Ejection today in the Northern Hemisphere. Mostly the U.S. side.

Probably not related, but it flattened 20 meters.
 
denverpilot said:
There was a massive Coronal Mass Ejection today in the Northern Hemisphere. Mostly the U.S. side.

Probably not related, but it flattened 20 meters.
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And GPS reception is in the... well, roughly 0.2 meter band IIRC.

As I understand it, the main way that solar activity affects GPS reception is by altering the ionosphere's index of refraction and thus the signal propagation time, resulting in range errors and loss of accuracy. If the event was major enough to flat-out interfere with signal reception, I would think it would affect a lot more users than just my little 480. ;)
 
azure said:
And GPS reception is in the... well, roughly 0.2 meter band IIRC.

As I understand it, the main way that solar activity affects GPS reception is by altering the ionosphere's index of refraction and thus the signal propagation time, resulting in range errors and loss of accuracy. If the event was major enough to flat-out interfere with signal reception, I would think it would affect a lot more users than just my little 480. ;)
Click to expand...

Oh, very true... was just a comment that a big flare was going on.

Another one headed this way, too...

Space Weather News for July 12, 2012
http://spaceweather.com

EARTH-DIRECTED X-FLARE: Big sunspot AR1520 erupted on July 12th around 16:53 UT, producing an X-class solar flare and hurling a CME directly toward Earth. Forecasters expect the cloud to arrive on July 14th. Its impact could spark moderate to severe geomagnetic storms, allowing auroras to be seen at lower latitudes than usual.
 
azure said:
And GPS reception is in the... well, roughly 0.2 meter band IIRC.

As I understand it, the main way that solar activity affects GPS reception is by altering the ionosphere's index of refraction and thus the signal propagation time, resulting in range errors and loss of accuracy. If the event was major enough to flat-out interfere with signal reception, I would think it would affect a lot more users than just my little 480. ;)
Click to expand...
The Garmin GPS antennas have been found prone to breaking into oscillation under certain conditions. Symptoms are an abrupt loss of all satellites (no bars) on the attached GPS and weak or no GPS reception on other units in the vicinity (within a meter or two). Recycling power to the affected radio usually restores proper functionality, at least for a while. If yours recovered on it's own this was probably not your problem.
 
I haven't seen a complete loss, but I've seen RAIM failures in areas that were not in the RAIM prediction. I also saw a LPV fall back to LNAV the other day and the guy familiar with the area says it happens everytime he flies that particular approach. Something in the area must be whacking the signal.
 
I had the same problem a few years ago. it began to get worse and worse. I would loose gps signal on the 430w and I would also lose signal on my handheld GPS. I finally started troubleshooting it on the ground when it got worse. I finally discovered it was the gps antenna of the 430w. Most gps antennas are powered and can go into oscillation and turn into a transmitter jamming all gps receivers in the area. If you have a handheld gps, this is easy to test. when it happens turn to the gps status page on the handheld gps. watch the signal strength of sats. the turn off the 430w and see if the signal comes back. it should almost be instant.

here is a link to a video I made of the problem

http://www.youtube.com/watch?v=sUX73dWuSsY&feature=youtube_gdata_player

This seems to be a problem garmin has had that they are not talking much about. I talked to garmin and they said " oh yea....a bad antenna". the avionics shop I work with was troubleshooting a similar problem when I discovered mine. they have had several since but garmin just said it was an isolated problem. mi am not so sure. I know several other people that were chancing a similar problem for months until they tested and discovered it was the antenna. I would go there first.

also to quickly kill the power you could simply pull the breaker. my bad antenna jammed any gps receiver within 20 feet.
 
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