GTX 345 Idle too Long?

MBDiagMan

MBDiagMan

Final Approach
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May 8, 2011
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Mount Pleasant,Texas
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Doc
I was sick and the Mooney was on the ground since February 13th. I flew it about 45 minutes this morning, landed then flew about 45 minutes home. In ForeFlight More/Devices it showed connected to the transponder, but it didn’t show anything at all in the upper left corner of the chart to indicate ADS-B Good or anything like I have seen before.

After seeing the messages above I looked them up in the GTX 345 Pilot Guide. In the troubleshooting chart common between all three messages is a possible problem worded as follows: “Extended periods between use or GPS position source disconnection from the aircraft battery could cause a longer than normal GPS acquisition time

Anyone ever seen this?

 

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GPS units have five modes of satellite acquisition: Cold start, warm start, hot start, assisted start and restart. The problem with long periods of inactivity is that the satellite ephemeris data (catalog of visible satellites) becomes outdated and the receiver takes a while to lock onto one satellite to determine what other satellites should be visible from its location (each GPS satellite broadcasts the ephemeris for all satellites). A "warm start" means the ephemeris is not (at least totally) outdated, hence more rapid acquisition. Typically, a "cold start" may take 30 seconds or a bit longer depending on the sky visibility at the location. "Warm start" may acquire within a few seconds (depending, again, on sky visibility and receiver design). "Hot start" is virtually instantaneous (like you just fueled the plane and cranked up again) since the satellites probably haven't moved too far in the local sky. The remaining two modes are not really applicable to the kind of GPS units typically in our airplanes.
 
I’m not sure, but I don’t think GPS satellites are geosynchronous like the satellites we receive TV on.
 
I think I found the problem. Kind of embarrassing. Remember I’m just beginning to learn all 5his stuff. I think the Traffic selection in Foreflight was off, causing me not to see traffic. I discovered this when in the Cessna with th3 Stratus ESG this afternoon. After flying the Mooney and seeing no traffic this morning, I saw the messages after the flight. I believe they set AFTER the flight. That’s when I took the picture.

Cockpit error. “Guilty your honor.”
 
Your -345 has no idea what the settings to your IPad are...it only transmits the data, it doesnt receive a reply. That picture you showed only relates to your transponder. So that may explain the lack of traffic on your tablet, but it doesn’t explain the messages you received. FWIW, I’ve never seen those messages (I have a -345 too). I’ve also gone longer without flying than what you’ve reported.
 
Timbeck2 said:
I thought satellites were geosynchronous.
Click to expand...


Depends on the satellite. Weather and many communication satellites are, but most are lower.

1024px-Comparison_satellite_navigation_orbits.svg.png
 
Arrow76R said:
GPS units have five modes of satellite acquisition: Cold start, warm start, hot start, assisted start and restart. The problem with long periods of inactivity is that the satellite ephemeris data (catalog of visible satellites) becomes outdated and the receiver takes a while to lock onto one satellite to determine what other satellites should be visible from its location (each GPS satellite broadcasts the ephemeris for all satellites). A "warm start" means the ephemeris is not (at least totally) outdated, hence more rapid acquisition. Typically, a "cold start" may take 30 seconds or a bit longer depending on the sky visibility at the location. "Warm start" may acquire within a few seconds (depending, again, on sky visibility and receiver design). "Hot start" is virtually instantaneous (like you just fueled the plane and cranked up again) since the satellites probably haven't moved too far in the local sky. The remaining two modes are not really applicable to the kind of GPS units typically in our airplanes.
Click to expand...

It's longer than a few minutes on a truly cold start. The full ephemeris information takes 12.5 minutes to transmit, then there's some time for the box to process. 345 is pretty recent, so I imagine that's pretty minimal. Older boxes could take 15-20 minutes total. I've had my KLN-94 take 15 minutes. I've never had the GTN-750 that replaced it ever take more than 10 minutes, so I wonder if the boxes are now smart enough to start working with partial info.

Either way, 45 minutes should have been plenty.
 
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