Archive of GPS Interference NOTAMs?

Palmpilot

Palmpilot

Touchdown! Greaser!
Joined
Apr 1, 2007
Messages
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Location
PUDBY
Display Name
Richard Palm
Home Airfield
Palo Alto (SFO area)
Does anyone know of a Web archive of past NOTAMs about GPS interference testing?

The reason I ask is that the CAP plane I'm responsible for gets occasional squawks for intermittent loss of position, and I would like to see if these correlate to the military's interference testing.
 
They tried to compile those in one digital document but it was too big, overwhelmed the byte container.
 
I don't get the same squawks for the CAP plane I'm responsible for, and since it's a neighboring squadron and the two airplanes fly to the same places, I don't think it's outside your airplane.
 
Could the newer GPS be more tolerant of interference testing?

Troubleshooting intermittents is the WORST!
 
Palmpilot said:
Could the newer GPS be more tolerant of interference testing?

Troubleshooting intermittents is the WORST!
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You have two GPSs in that plane! If they aren't both reporting LOI, the problem is not interference. I know no one ever looks at that GX65, but it's a perfectly valid VFR GPS.

I may need to use that airplane for IFR flight Saturday....I guess it's not gonna happen except in VMC.
 
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MAKG1 said:
You have two GPSs in that plane! If they aren't both reporting LOI, the problem is not interference. I know no one ever looks at that GX65, but it's a perfectly valid VFR GPS.
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The pilot who wrote up the squawk didn't mention the GX55, so I don't know whether he looked at it or not. However, the site linked above shows the current interference NOTAM when I search on "ZOA," but it doesn't show anything on the dates of his flights, so I guess that possibility is ruled out.

I may need to use that airplane for IFR flight Saturday....I guess it's not gonna happen except in VMC.
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Especially since the #1 CDI does not work when it's switched to Nav 1. Since the IFR GPS's reliability been called into question, you could be down to just Nav 2 and the DME, so not only can the IFR GPS not be counted on, there's no ILS capability. (If you feel that a single VOR plus DME would be enough for the flight you have in mind, I could be persuaded to remove the IFR prohibition.)

I called the local avionics shop yesterday, but the person who answered the phone said she would have to have someone call me back about scheduling the work. So far, no call back.)

At least the oil has been changed and the beacon has been fixed!
 
With DME, a lot of stuff is possible, as all of the local ILSs are also LOC/DME. But it won't get me into SQL without the GPS. I forgot that plane had a real DME. It has ADF, too, but that won't help.
 
MAKG1 said:
With DME, a lot of stuff is possible, as all of the local ILSs are also LOC/DME. But it won't get me into SQL without the GPS. I forgot that plane had a real DME. It has ADF, too, but that won't help.
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SFO is forecasting clearing by 9:00 AM tomorrow. I hope that turns out to be correct!
 
Palmpilot said:
SFO is forecasting clearing by 9:00 AM tomorrow. I hope that turns out to be correct!
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Ya think NorCal would let me track the SFO LOC 28L until under the clag, then break off to SQL? :D

I suppose that would go along with the monkeys flying out of my butt.
 
MAKG1 said:
Ya think NorCal would let me track the SFO LOC 28L until under the clag, then break off to SQL? :D

I suppose that would go along with the monkeys flying out of my butt.
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Light aircraft do fly approaches into SFO sometimes, but I wouldn't be surprised if there was a delay in getting the approach. Once on the approach, I don't think they could stop you from cancelling IFR.

Before Palo Alto and San Carlos got instrument approaches, the usual method was to fly an approach into Hayward, cancel, and cross the bay VFR. (This was also before Moffett got an ILS.)
 
MAKG1 said:
I suppose that would go along with the monkeys flying out of my butt.
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Had no idea the monkey had made it to SFO. But you really shouldn't have let him up in there in the first place, knowing his behavior and history. ;)
 
Just a hunch, but I'd say it _might_ be a defective Garmin antenna. We had similar issues, and I know many others have as well. Problem is solved by replacing the antenna.
 
dans2992 said:
Just a hunch, but I'd say it _might_ be a defective Garmin antenna. We had similar issues, and I know many others have as well. Problem is solved by replacing the antenna.
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Hard to imagine defective. That installation is many years old. Maybe broken.

I'll fly it tomorrow (VFR) and see how bad it is. I'll even ask for a practice RNAV approach into SQL if possible.
 
dans2992 said:
Just a hunch, but I'd say it _might_ be a defective Garmin antenna. We had similar issues, and I know many others have as well. Problem is solved by replacing the antenna.
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Thanks for the suggestion. Bad or loose antenna cable connections is another possibility.

I'm just hoping we can find a way to repeatably reproduce the problem. Otherwise, we won't be able to positively determine whether anything we try works or not.
 
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MAKG1 said:
Hard to imagine defective. That installation is many years old. Maybe broken.

I'll fly it tomorrow (VFR) and see how bad it is. I'll even ask for a practice RNAV approach into SQL if possible.
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Yeah, I'm definitely interested in getting pilot feedback on whether the problem is reproducible.

By the way, I finally got around to gluing the rubber cushion back onto the iron maiden. I kept forgetting to bring adhesive with me, and I forgot again this week, so I got fed up and went over to the Walgreens on Story Road to get some rubber cement. It was FAA-certified rubber cement, so it cost a hundred dollars. ;)
 
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Palmpilot said:
Thanks for the suggestion. Bad or loose antenna cable connections is another possibility.

I'm just hoping we can find a way to repeatably reproduce teh problem. Otherwise, we won't be able to positively determine whether anything we try works or not.
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You'd be better off looking at the satellite status page to see how the "signal strength" bars (they actually measure the time weighted ratio of valid vs invalid data "chunks") look. If there's an intermittent antenna or antenna cable issue you should see a rapid drop on all the bars.
A common failure with Garmin antennas involves the internal pre-amp breaking into oscillation which not only kills all reception on the radio it feeds, it can also interfere with other nearby GPS antennas. Other local problems to consider:

ELT re-radiated harmonics. Many 121.5/243MHz ELTs have very cheap RF front ends which can generate harmonics of any strong RF and radiate that energy from the ELT antenna. The strong RF can be transmissions from comm radios in the airplane or even a nearby broadcast radio or TV transmitter. Since this doesn't require the ELT to be on or transmitting the best way to eliminate this a your cause is to disconnect the external antenna from the ELT or remove the ELT if it has a built in antenna.

Comm radio transmitter harmonics. The GPS frequency is close to the 12th harmonic of 131.275MHz and the 13th harmonic of 121.175MHz. Some amount of those harmonics exists in the transmit output of any comm radio but some are worse than others. Check the GPS signal bars while transmitting a verbal count on each of those frequencies for at least 30 seconds to see if this is a problem with your comm radios. Because of this issue it's important to get pilots to record all comm frequencies in use in addition to their position and time when they experience an inflight GPS outage. Keep in mind that if the comm does interfere with the GPS the problem could be the comm radio or something else re-radiating the harmonic such as the ELT.
 
Great suggestions - thanks!
 
FWIW, I flew that airplane 3.5 hours yesterday with no complaints from either GPS.

I would question whether one of those DIY ADSB gadgets the Oakland guys like so much was operating. Yes I do know who was flying and that he has one. Those are the cheapest antennas you can imagine, and they like to put them right on top of the radio stack.

I'd also question where he was, for the possibility of interference from the ground.
 
MAKG1 said:
FWIW, I flew that airplane 3.5 hours yesterday with no complaints from either GPS.

I would question whether one of those DIY ADSB gadgets the Oakland guys like so much was operating. Yes I do know who was flying and that he has one. Those are the cheapest antennas you can imagine, and they like to put them right on top of the radio stack.

I'd also question where he was, for the possibility of interference from the ground.
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Thanks for the report, and I'll add those to the list of possibilities to be investigated.
 
An update: The IFR GPS has now failed completely, as in, no light on the screen.
 
MAKG1 said:
Good to know. So, when it relocates for a tow, it had better be VMC, 'cause both airports have only RNAV approaches.

Will it be replaced with another Garmin 400 or similar? That GPS was never standard issue.

When are you guys getting that 206? I'm itching to fly it.
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Answered via email.
 
Palmpilot said:
An update: The IFR GPS has now failed completely, as in, no light on the screen.
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It has been discovered that the GPS was not fully plugged into the tray. Once that was corrected, it started working again. It's possible that it has been working its way out for some time. If so, that could also be the cause of the intermittent loss of position. Time will tell.
 
Palmpilot said:
It has been discovered that the GPS was not fully plugged into the tray. Once that was corrected, it started working again. It's possible that it has been working its way out for some time. If so, that could also be the cause of the intermittent loss of position. Time will tell.
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Excellent!

And that explains why it seemed to work fine when I flew it last weekend....
 
The Nav 1 CDI also was working on the ground the last couple of times I checked it. I'm wondering if the button that switches the CDI between Nav 1 and the GPS requires the Nav 1 output to go through the GPS. If so, then the looseness in the tray could explain the problem we had with that as well. Did you happen to try Nav 1 on the CDI last weekend?
 
Palmpilot said:
The Nav 1 CDI also was working on the ground the last couple of times I checked it. I'm wondering if the button that switches the CDI between Nav 1 and the GPS requires the Nav 1 output to go through the GPS. If so, then the looseness in the tray could explain the problem we had with that as well. Did you happen to try Nav 1 on the CDI last weekend?
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No, I was flying O rides VFR. I use the GPS to manage time to return, and navigate by pilotage.
 
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