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GTN650 excessive time to get initial 3D position

pecanflyboy

Well Known Member
Howdy,
I purchased a new GTN650 in January, 2019 and swapped out my GNS480. The unit worked well until the fall when I began experiencing excessive time to acquire 3D position. It seemed the colder the temperature and the longer the airplane had been unpowered, the longer the acquisition time. I spent a lot of time troubleshooting with Garmin. I finally received and "Internal battery low" message and Garmin swapped the GTN650 with a reconditioned unit. Initially, I had the same battery warning with the new unit, but it eventually went away (left over from the previous unit, or the increase in temperatures gave the battery new life?). I'm still having excessive initial 3D position acquisition. Here is a typical scenario (I'm still working with Garmin).

Before taking the aircraft out of the hangar, I checked the GTN650 position and UTC time in the configuration mode. It appeared accurate, having flown the day before.

Placed the aircraft with a clear view of the sky. G3X system found 3D position within 30 seconds. GTN650 had no satellites and no position. Turned everything off, pulled circuit breakers on everything except the GTN 650, powered on and waited over 5 minutes with no satellites on the GPS system page. Powered down, pushed in all CB?s, powered on and waited.

After 5 minutes I finally received 3D position on the GTN650. Powered down, powered on???GTN 650 3D position in one minute. I repeated with same result. So, it seems that once the GTN650 finds an initial position it reacquires it quickly with a re-power. This tells me the integrity of the installation is good (antenna position, wiring, interference, etc).

The longer the airplane sits in the hangar, and the colder the temperature, the longer it takes to acquire its initial 3D position. It seems the unit either looses its current position and has to do a full ?initialization? position, or it is something related to a cold unit?

Yesterday, after the aircraft sat for over a week, with the temperature at 35F, I flew for 30 minutes with no 3D position on the GTN650. Took off less than an hour later, and finally got the 3D position after another 5 minutes.

Here is me:
? RV-6
? G3X (non-touch)
o GDU 370 (PFD)
o GDU 375 (MFD)
? GSU 73
? GTN 650 (Replaced GNS480)
? G5 (SPD) (CANBUS to G3X, Not connected to SL30)
? SL30
? GMA 340
? GTX 23ES
? GDL 39
? TruTrak GXPilot autopilot
GMC305

Any ideas?

Jimmy
 
It's a big sky and the gps needs to know where to look for a satellite. It maintains an almanac style table that it uses to find sat's (similar to a tide table). It needs the current date and time to use that table and the battery is supposed to keep the date/time current. Without the date/time, it is systematically sweeping and looking and it finds one via chance. Time to acquire will be variable, just as you would be poking around a dark room looking for a light switch. Once it gets one sat, it downloads the date/time from it and the rest are found rapidly.

My old KLN let me put in the current date time and that would allow for a rapid acquisition, even if the battery was dead. Not sure if garmin suppoirts that though.

Having a known position, in a addition to the table makes it more rapid. I suspect that last position is stored in EPROM and doesn't require a battery.

It is possible that the temp is messing with the battery or opening and closing a poor connection to the battery. Also likely that the battey is holding a short time and that is why the 5 minute restart works well. Often a battery will hold for a bit after the unit it on to recharge it.

Larry
 
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You may want to look at the satellite page signal strengths as you isolate everything in the plane other than the GTN-650, then shut breakers one at at time.

You may find something that is adding noise to the GTN-650 satellite receiver.

Carl
 
You may want to look at the satellite page signal strengths as you isolate everything in the plane other than the GTN-650, then shut breakers one at at time.

You may find something that is adding noise to the GTN-650 satellite receiver.

Carl

Thanks, I've done that with no issues.

It sounds like an internal battery issue to me. But, the chances are low that two units would have the same problem? I don't have a battery message on the current unit.
 
Did you replace the GPS Antenna when you swapped to the GTN? Have you tried another antenna? Where on the aircraft is the antenna and G3X antennas?

Jay
 
Did you replace the GPS Antenna when you swapped to the GTN? Have you tried another antenna? Where on the aircraft is the antenna and G3X antennas?

Jay

I used a new GA-35 with the GTN650 installation. Have not been offered another to use from Garmin. The antenna is behind the canopy on top of the fuselage. Unshared. Away from any other antenna or device. Unpainted.
 
GA-35 Antenna

A few years ago, Gamin had a batch of bad GA-35 antennas. So it could be bad. Since you already swapped out the radio, I would swap the antenna out next.

Another thing to check, since it might be a temperature problem, check the antenna connectors on both ends. If the center pin was soldered, it could have a cold sold joint, and temperatures can effect that. Also check the cable.

If you have a spare cable around, just plug one end into the radio and the other into the antenna. No need to run it, until you see if that works. If it fixes it, then run a new cable with connectors.

It could still be the radio, but check out the antenna and cable too.

Good luck.

Brian
 
Keep in mind when the GTN-650 is working perfectly, it will still frequently take an amazingly long time to lock into a position. It does for mine.

In other words, you may not have a problem.

Carl
 
I've noticed a delay in getting position since the last GTN software update. I used to load satellites really quick but it now takes about 5 minutes, sometimes longer. Never had a problem NOT getting position.
 
I've noticed a delay in getting position since the last GTN software update. I used to load satellites really quick but it now takes about 5 minutes, sometimes longer. Never had a problem NOT getting position.

That's a good observation. My initial thought was that a recent software upload may have been the culprit. Of course, troubleshooting that issue is difficult.

The manual states that the "initial" position acquisition can take as long as 20 minutes, but subsequent acquisition should be shorter. It's taken more the 30 minutes to get a position if the aircraft has been unpowered for more than 2 weeks.

In my view 5-30 minutes is excessive. It's not inertial nav. I should not have to wait at the end of the runway for my ADSB WAAS to find itself. Especially when the G3X does it in 30 seconds.
 
A few years ago, Gamin had a batch of bad GA-35 antennas. So it could be bad. Since you already swapped out the radio, I would swap the antenna out next.

Another thing to check, since it might be a temperature problem, check the antenna connectors on both ends. If the center pin was soldered, it could have a cold sold joint, and temperatures can effect that. Also check the cable.

Brian

I've not heard about the bad batch of GA-35's. Garmin is sending me a new antenna, so that is next. I'm not sure how that affects the issue. I still think it's the unit loosing its position and time and going into and "initial" acquisition mode. The GPS System screen shows no satellites until it finds the first one, and then it unzips them in 30 seconds and I have a position.
 
I see you are and RV-6, but you don't note which canopy. My RV-7 Slider frame causes errors with my GNS-480 antenna, mounted on the upper fuselage behind the baggage bulkhead. It finds satellites but gives me integrity errors on the ground with the sliding canopy open. The antenna doesn't like the canopy steel framework with my Kroger sunshade wires.
 
That's a good observation. My initial thought was that a recent software upload may have been the culprit. Of course, troubleshooting that issue is difficult.

The manual states that the "initial" position acquisition can take as long as 20 minutes, but subsequent acquisition should be shorter. It's taken more the 30 minutes to get a position if the aircraft has been unpowered for more than 2 weeks.

In my view 5-30 minutes is excessive. It's not inertial nav. I should not have to wait at the end of the runway for my ADSB WAAS to find itself. Especially when the G3X does it in 30 seconds.


I just looked at my logs and I can absolutely say the delays started happening after installation of v6.7 software in the GTN.
 
I just looked at my logs and I can absolutely say the delays started happening after installation of v6.7 software in the GTN.

That update came out in February, and my problems started last fall? I'll bring it up to Garmin. I remember when I first started troubleshooting that I had asked about the update. But, you know how that works. "Not in the software," and then everything starts working again with a new update.
 
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I see you are and RV-6, but you don't note which canopy. My RV-7 Slider frame causes errors with my GNS-480 antenna, mounted on the upper fuselage behind the baggage bulkhead. It finds satellites but gives me integrity errors on the ground with the sliding canopy open. The antenna doesn't like the canopy steel framework with my Kroger sunshade wires.

I love you guys. You make me think of everything. I do have a slider that covers the antenna when the canopy is aft. However, I still don't get antenna reception 20-30 minutes after the canopy is closed and flying. I see no satellites.
 
Try this: pull the aircraft to where it has a good view of the sky. Close the canopy, and only then power up the gps. Does it acquire a fix within a few minutes?
 
Garmin sent me a replacement GA-35 antenna. I cleaned the ground plane and connections. Checked for 4.7V at the antenna. Coax continuity was good. Moved aircraft away from the hangar and close the canopy. Turned off all other electronics.

INITIAL acquisition took over 10 minutes. I noticed that the GTN was looking for the same satellites as the G3X (which took less than 30 seconds to acquire 3D position). Another interesting observation was that I did not have the semi-transparent "ghost" bars above the GTN satellites. Once I did see the "ghost" bars I had 3D position within a minute.

SUBSEQUENT start ups consistently gave me a GTN 3D position within 90 seconds. I also had the "ghost" bars immediately.

The only thing left to replace is the RG-400 coax which will be a major ordeal. I can't see that it's the coax or the installation as I get consistent rapid acquisition after the initial.

Garmin tells me that I should have a 3D position within 2 minutes.

Has anyone else had this issue after the 6.7 update?
 
Another interesting observation was that I did not have the semi-transparent "ghost" bars above the GTN satellites. Once I did see the "ghost" bars I had 3D position within a minute.

SUBSEQUENT start ups consistently gave me a GTN 3D position within 90 seconds. I also had the "ghost" bars immediately.

Still sounds like a position / date time issue. I believe the unit looks for one satellite first to positively identify it's location in the sky and get it's broadcast date/time. It then uses the internal table to map where all of the other satellites should be. That is when the ghost bars show up (unit now knows where they are and just need to confirm reception from those postions. Unit needs to fine tune each sat's location based upon signal strength. GPS antennae must be told where in the sky to look for each signal. They are not omni directional like VHF antennae.

That fact that a shut down and restart will give you a quick synch up points to the fact that the unit is not maintaining the aircraft's position or current time over the long haul (or is somehow lost or corrupted) and spends a lot of time searching. Having a good aircraft locatoin and time will allow the GPS to quickly find the first sat. If that doesn't work, it must spend a lot of time searching for it.

Larry
 
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I just re-read your first post. This should be under warrantee. Contact the Garmin exp rep (here on VAF) and get him on-board.
 
I just re-read your first post. This should be under warrantee. Contact the Garmin exp rep (here on VAF) and get him on-board.

It?s all been under warranty. Garmin has replaced the new unit with a refurbished unit. Garmin has replaced the antenna. Fortunately, I?ve been able to do all the labor. They?ve been very supportive and patient. However, the problem still exists. I suspect there are others that just tolerate it. My issue is that I?m based at a field very near a Mode C vail which requires ADSB. I have to wait for the GTN to find itself before I takeoff.
 
When you pay that kind of $$ for avionics you have a right to get a working unit. I admire your patience, not sure I'd be so civil.... Of course double check that the issue isn't on your end, but having had a working unit that now doesn't work...
 
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