GTN650 excessive time to get initial 3D position

pecanflyboy · #1 04-16-2020, 10:22 AM

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

lr172 · #2 04-16-2020, 10:44 AM

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

Carl Froehlich · #3 04-16-2020, 10:49 AM

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

pecanflyboy · #4 04-16-2020, 11:25 AM

Quote:

Originally Posted by Carl Froehlich

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.

pilot2512 · #5 04-16-2020, 11:33 AM

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

pecanflyboy · #6 04-16-2020, 12:26 PM

Quote:

Originally Posted by pilot2512

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.

Pdtofly · #7 04-16-2020, 01:24 PM

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

Carl Froehlich · #8 04-16-2020, 01:46 PM

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

Planecrazy232 · #9 04-16-2020, 02:15 PM

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.

pecanflyboy · #10 04-17-2020, 08:13 AM

Quote:

Originally Posted by Planecrazy232

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.