GPS Questions

[Marauder]

As some of you know, I recently upgraded my panel to include a GTN 650 and an Aspen PFD/MFD combo. I have been flying with it to gain familiarity with the new technology and to work out any gremlins. Moving up from VOR/LOC/GS has been an interesting journey! And I can't believe how much I was missing out on!

 

For those of you with GPS WAAS systems, does the GPS signal ever drop out unexpectedly? I had one occurrence of it about a week ago while flying a practice approach. It was on a transition from GPS to an ILS. Shortly after moving over to the localizer, the GPS announced a GPS signal loss and a LOI message showed up. Is this normal to see these? Are there websites that can show GPS outages? I know about the RAIM prediction tool in the GTN, are there other tools? It only happened once, but I wasn't happy to see that on an approach.

 

Can someone explain to me how exactly the "suspend" mode is supposed to work? More importantly the timing of it. On one ILS approach I was asked to fly the published procedure turn (with a hold). The GPS showed the entry to the PT and I proceeded to fly it. During this entry, I was told to expect a 10 minute hold and I hit the suspend button. The hold pattern was depicted in magenta and I was able to fly it. When I was cleared for the approach, I wasn't certain when I could unsuspend it. I waited until I was on inbound leg and it seemed to be fine. Could I have hit it sooner? I was on the outbound leg of the hold when I got the clearance.

 

Speaking of transition from GPS to localizer, the GTN has a feature that allows the unit to automatically switch from GPS to localizer. Even though I had this turned on, I still had to switch manually (the CDI button on the GTN) from GPS to the ILS (was under the GS and had a valid localizer signal) because it never switched. Anyone familiar how this is supposed to work and if it can be programmed to be turned off by the avionics shop? I am wondering if some installations (like those with Aspens) require it to be turned off.

 

As for the Aspens; I'm impressed. Like any big purchase, I had some buyer's remorse. After flying with them and seeing firsthand the integration and additional functionalities you gain (like winds aloft, obstacles, etc.), I am glad I made the decision. Just having all of your navigation and situational awareness information in front of you while in flying is a big plus. Add in the integration with the older equipment and it makes for a common sense approach to upgrading an older panel.

[carusoam]

About four or five years ago there were several incidences on this site involving lost signals on Garmin 430/530 equipment.

I believe it to be antenna placement related.

I had a similar problem with a portable Garmin going off line because of a Narco nav radio, when tuned to a certain VOR frequency, would bleed too much.

Search for Garmin Antenna issues and pick the oldest thread you can find....

Overall, in modern GPS days, I have never lost a GPS signal....

Best regards,

-a-

[Marauder]

About four or five years ago there were several incidences on this site involving lost signals on Garmin 430/530 equipment.

I believe it to be antenna placement related.

I had a similar problem with a portable Garmin going off line because of a Narco nav radio, when tuned to a certain VOR frequency, would bleed too much.

Search for Garmin Antenna issues and pick the oldest thread you can find....

Overall, in modern GPS days, I have never lost a GPS signal....

Best regards,

-a-

Thanks! A good starting point. I mentioned it to the avionics shop today and they indicated that bleed over can cause it. I know what freqs I was on, so I will try to duplicate it.

[PTK]

Marauder, as long as you have the approach activated on the GTN, it will automatically switch over to vloc when you intercept the localizer, about 99% of the time! The other 1% when it won't, is when you make some kind of funny turn when you really should be executing a PT. In other words if it thinks you should be making a pt but you don't, it will stay on GPS.

The unit enters suspend mode when you pass the holding fix entering a hold. You can unsuspend anytime as long as you are passed the fix and outbound in the hold. Say for example, you cross the fix and are outbound on your last circuit. As long as you are passed the fix you can unsuspend anytime. It will still show you around your final circuit and prepare you for the next leg. When you are inbound and cross the fix it will switch to the next leg.

Hope this helps.

[Marauder]

Marauder, as long as you have activated the approach on the GTN, it will automatically switch over to vloc when you intercept the localizer, about 99% of the time! The other 1% when it won't, is when you make some kind of funny turn when you really should be executing a PT. In other words if it thinks you should be making a pt but you don't, it will stay on GPS.

The unit enters suspend mode when you pass the holding fix entering a hold. You can unsuspend anytime as long as you are passed the fix and outbound in the hold. Say you cross the fix and you begin your last circuit. As long as you are passed the fix you can unsuspend anytime. It will still show you around your final circuit and prepare you for the next leg. When you are inbound and cross the fix it will switch to the next leg.

Hope this helps.

 

Yes, that helps. It looks like (at least I am hoping) that what I saw was the 1%. I was inbound on the localizer for an ILS and unsuspended the unit about a mile out on the inbound leg. When I crossed the FAF, it still had not switched and I switched it manually. I think I waited too long on the "suspend" mode. I read in the manual that it automatically switches from 2.0 to 15 NM from the FAF. Since I was closer than that, the manual says it won't switch.

 

As for the suspend mode; if I understand you correctly, I don't need to do anything when entering the PT. It will automatically enter suspend mode when I cross the holding fix. Let me know if my thinking is correct... If I am flying an ILS with a holding pattern at the FAF and I am doing a parallel entry, once I cross the fix, the unit goes into suspend mode and will stay there until I take it out of the mode. So, if ATC tells me to expect to hold there for a period of time, I just continue to fly the hold and won't need to unsuspend until cleared for the approach. Once cleared, it should automatically switch to VLOC for the ILS unless, I am inside of the 2 NM window. I got this right?

[PTK]

Yes to both. Weather at a published hold or a hold at a faf is same. If you are doing parallel entry at the faf presumably you are doing it for course reversal and to lose altitude and get lined up for approach. So the system helps you by automatically suspending way point sequencing to allow you to do at least one circuit around to accomplish all that and get lined up. And it's what ATC expects also. When you are ready, you simply unsuspend before the faf inbound. If you don't need to fly any circuit you can simply fly the course reversal and unsuspend when inbound. Same idea except no holding circuit. This is intented to help us by lowering workload at a high workload phase. It is easier to simply press unsuspend rather than worrying about having to suspend when you are trying to get set up for the approach or hold and then unsuspend. Also, as you know, you manually force suspend and then un-suspend for an unpublished hold.

Also the nm distances are GPS derived values. Depending on the autopilot you have it will automatically go to vloc as long as you are roughly at between 2 and 15 nm outside the faf.

[Txbyker]

I have struggled with the GPS to VLOC switching too in my GNS650.  When you select the ILS approach it should load the freq.  Mine loads in standby which is probably correct.  In section 6.11 of the GNS 650 pilot guide it says within 1.2 miles of final approach course it will switch from GPS to VLOC as long as the ILS frequency is loaded and you are configured to switch automatically.  I have to ensure the ILS frequency is primary because sometimes it does not switch from standby.

 

I got a new Aspen too and need to go fly some ILS under the hood just to get the flow down.  The built-in GPSS is fantastic.

 

Have never lost the GPS signal once acquired.  Might want to have them check the connections to make sure vibration is not causing something to disconnect.

[Marauder]

I have struggled with the GPS to VLOC switching too in my GNS650. When you select the ILS approach it should load the freq. Mine loads in standby which is probably correct. In section 6.11 of the GNS 650 pilot guide it says within 1.2 miles of final approach course it will switch from GPS to VLOC as long as the ILS frequency is loaded and you are configured to switch automatically. I have to ensure the ILS frequency is primary because sometimes it does not switch from standby. I got a new Aspen too and need to go fly some ILS under the hood just to get the flow down. The built-in GPSS is fantastic. Have never lost the GPS signal once acquired. Might want to have them check the connections to make sure vibration is not causing something to disconnect.

I took the GTN trainer for a ride on my iPad. Loaded the ILS I was having problems with and did duplicate what I thought I was seeing in the airplane. Like your situation, when I activated the ILS approach, the ILS freq was loaded in the standby slot. I had it fly the course reversal (tear drop entry). It did not put the approach into suspend mode. It flew the tear drop, turned inbound and did not move the ILS freq into the active position. It continued on in GPS mode until just outside the FAF and a message popped up to switch the ILS freq to active and to switch the CDI. I will keep plugging away with my testing...

[PTK]

Auto switching the CDI output is autopilot dependent. The GTN 750 that I have (and I'm sure the 650 should be the same) switches to vloc automatically anywhere between 2 and 15 nm from the faf. I have the KFC150. You must switch mannually with the KAP140 or KFC225.

Which ap do you guys have?

[Txbyker]

Allsmiles, you are new school.  I have a KAP100.  Is there something newer than that?   I have 60 PSS pitch stablizer too but with the Aspen built in GPSS I can't see upgrading at least until they STC the Avidyne DFC90 for Mooney.  I guess that explains having to manually configure the VLOC.

[Marauder]

Auto switching the CDI output is autopilot dependent. The GTN 750 that I have (and I'm sure the 650 should be the same) switches to vloc automatically anywhere between 2 and 15 nm from the faf. I have the KFC150. You must switch mannually with the KAP140 or KFC225. Which ap do you guys have?

Did not know that. I have the STEC 60-2.

[Oscar Avalle]

Auto switching the CDI output is autopilot dependent. The GTN 750 that I have (and I'm sure the 650 should be the same) switches to vloc automatically anywhere between 2 and 15 nm from the faf. I have the KFC150. You must switch mannually with the KAP140 or KFC225.

Which ap do you guys have?

Interesting.... I did not know that. I will try it out with my STEC

[PTK]

I'm not familiar with S-Tec! But the reason it will not auto switch with the KAP 140 and 225 is that those ap's have both, GPS mode and NAV mode. They are not smart enough to discern that they are getting signal from GTN. The KFC 150 has one mode and so it recognizes the signal from GTN.

[Marauder]

I'm not familiar with S-Tec! But the reason it will not auto switch with the KAP 140 and 225 is that those ap's have both, GPS mode and NAV mode. They are not smart enough to discern that they are getting signal from GTN. The KFC 150 has one mode and so it recognizes the signal from GTN.

I have an email out to my avionics shop. It definitely is not switching and I am trying to find out why.

[PTK]

Marauder and Oscar, you might also want to call Garmin directly and ask them. (866) 739-5687 and then option 2 for panel mounted products.

[Marauder]

Marauder and Oscar, you might also want to call Garmin directly and ask them. (866) 739-5687 and then option 2 for panel mounted products.

Well, I called. No help. I did something a lot of us don't do, I read the manual. I have the GTN setup correctly. There is only one place to turn that feature on and off, and I have it set correctly (Garmin confirmed this but suggested a possible software problem with the Aspen). The Aspen manual only talks about the 430 features, but uses the same terminology. I can't imagine the software logic for the CDI function to have changed. I think I have an excuse to go fly and try some more The only difference between hand flying an ILS and using the AP coupled, is switching the AP to "approach" mode before the FAF. I should be getting the same indications on the Aspen and the GTN should be doing an automatic switchover from GPS to VLOC.

[Txbyker]

Just for grins when you load the approach press the large knob and make sure the NAV frequency is loaded primary and not standby. 

[Marauder]

Just for grins when you load the approach press the large knob and make sure the NAV frequency is loaded primary and not standby. 

I saw that that was a problem when I loaded my first ILS approach. I'm pretty sure that I had the ILS frequency loaded the next few times I flew. Hopefully the weather will cooperate this weekend and I'll be able to do more testing. Sent from my Nexus 7 using Tapatalk HD

[co2bruce]

My understanding is you need to put the ILS / LOC freq in active. When you load approach it is put in standby. The GTN will switch from GPS to VLOC on its own but you must push the freq to active manually.

[Bryan]

Sorry to bring up an old topic, @Marauder did you ever find anything on your GTN losing GPS?  I have a new (2 month old) GTN 750 install that is starting to lose GPS more and more. Lost it today on both flights while working on my IFR training, once on an RNav Approach.

I am thinking it could be equipment b/c my 430 (non-WAAS) did not lose signal.

[carusoam]

Any idea how close your GPS antennae are to each other?  

This was a usual challenge for new Garmin GPS installations, a decade ago.  

Some people have got this again when trying to use the same holes from previous antennae.

There may be some hints of what worked in terms of grounding planes...

Best regards,

-a-

[Marauder]

1. 7 minutes ago, Bryan said:

   Sorry to bring up an old topic, @Marauder did you ever find anything on your GTN losing GPS?  I have a new (2 month old) GTN 750 install that is starting to lose GPS more and more. Lost it today on both flights while working on my IFR training, once on an RNav Approach.

   I am thinking it could be equipment b/c my 430 (non-WAAS) did not lose signal.

   Bryan -- probably going to curse myself by saying everything has been fine since I reported this issue. I was able to determine that it was happening at a specific location and no other. It only happened a couple of times and I racked it up to some sort of interference in the area. I have heard antenna placement and quality of the coax to the antenna could have a significant impact on LOI being triggered. Wish I could be more help.

[Marauder]

7 minutes ago, carusoam said:

Any idea how close your GPS antennae are to each other?  

This was a usual challenge for new Garmin GPS installations, a decade ago.  

Some people have got this again when trying to use the same holes from previous antennae.

There may be some hints of what worked in terms of grounding planes...

Best regards,

-a-

Someone say Antenna Farm?

[carusoam]

And I used a Boston accent when I did it... 

Antenna Faaahm.

A hint that it is an interference issue, turn the other radios off and see if the GTN comes back on line.  It would help if you can display the GPS sattelites being used page.

best when done in IMC...

Best regards,

-a-

[Bryan]

Was the GPS to VLOC issue to do with the ILS being in the standby vs. the active?

FAA notes a GPS Flight Advisory today and tomorrow near my area so hopefully that is my issue but my suspicion is an antenna issue as the 430 had signal.