GDL88 Diversity or not

[DesertNomad]

I am adding a GDL88 to my Dakota. Should I get a Diversity option?

I fly in Northern Nevada were we are out of ATC radar coverage quite a lot.

Thanks.

 

[Unit74]

Nope. No need unless you are flying into big iron parking where they are using ground based adsb. In the air it won't do anything for you. All the stations are on the ground and the transmitter is placed on the belly.


Or in other words, don't waste your money.

 

[GLMS_NC]

Nope. No need unless you are flying into big iron parking where they are using ground based adsb. In the air it won't do anything for you. All the stations are on the ground and the transmitter is placed on the belly.


Or in other words, don't waste your money.

+1. Not needed.

 

[petrolero]

Not needed. I asked my avionics shop the same question. Not enough metal in our planes to be a problem.

The idea on big iron is that when they are in a bank, a belly or dorsal antenna alone could be shielded - either from the ground or from another aircraft's receiver. Remember the 2020 concept calls for air-to-air reception too. For their operations that was deemed to be a problem worthy of an extra antenna.

 

[dans2992]

So I wonder if small GA planes with just a belly antenna can be seen from space-based ADSB i.e. Aireon...?

 

[petrolero]

So I wonder if small GA planes with just a belly antenna can be seen from space-based ADSB i.e. Aireon...?

Does aireon receive data from ground stations or other uplink of data?

My imagination without knowing much about aireon: In that case their sats might not receive the signal directly from the small aircraft but may receive processed data from either the ground stations themselves or some other source (proprietary uplink?) of aggregated data that would include small airplanes?

Reading this now: http://aireon.com/resources/technical-overview/

ETA:

The Antenna
In order to ensure reliable satellite reception, an A1 class transmitter and top mount aircraft antenna (commonly found on most commercial aircraft and private jets), is required due to the space-based nature of Aireon’s receivers. Aircraft with a Traffic Alert and Collision Avoidance System (TCAS) to help prevent midair collisions are typically equipped with both top and bottom mount antennas.

Plus it says it is only receiving 1090 transponders:

Space-based Automatic Dependent Surveillance-Broadcast (ADS-B) will provide unparalleled 100 percent global surveillance coverage to receive and process ADS-B signals broadcast from aircraft equipped with 1090 MHz ADS-B transponders, which operate on the same frequency as traditional Mode A/C/S transponders, including DO-260, DO-260A and DO-260B (Link Versions 0, 1 and 2, respectively), as well as DO-260B/ED-102A, the current standard.

Interesting. So no 978 UAT and it's not going to see us (necessarily) with belly antennas. But I would think it could receive blasts from the ground stations, no?

 

[bnt83]

Satellites can see wimpy 406 ELT transmitters but not a top mounted antenna on an ADS-B transceiver?


To diversity or not is simply going to equate to "where am I flying". It may not be required in the US, but Canadian or Mexican neighbors may require it.

Future requirements of said top mounted antennas are likely already being talked about somewhere.

 

[petrolero]

Satellites can see wimpy 406 ELT transmitters but not a top mounted antenna on an ADS-B transceiver?

My ELT antenna is top-mounted but ADS-B out is on the belly.

Future requirements of said top mounted antennas are likely already being talked about somewhere.

No doubt.

 

[bnt83]

It should be fairly painless to add diversity later if needed. Pressurized aircraft requiring just a bit more.

Depending on the airplane, another antenna doubler = another airworthiness limitation / eddy current inspection to track.

 

[Jim Rosenow]

It should be fairly painless to add diversity later if needed.

Not being argumentative, but it it's painless why not do it with the install?

I know nothing other than my personal experience... When we did the 182, our avionics shop owner, who has been in the business forever, recommended doing dual antennae. I frankly don't remember why.

I do know that it wasn't to make more $$ on the install, as giving demonstrably bad advice could have an adverse affect on his continuing to work on several turbine aircraft he does avionics for.

FWIW...YMMV

Jim

 

[John Collins]

ETA:

Plus it says it is only receiving 1090 transponders:



Interesting. So no 978 UAT and it's not going to see us (necessarily) with belly antennas. But I would think it could receive blasts from the ground stations, no?

Client's don't receive their own position as a TISB broadcast from a ground station, if the ground station were to repeat the client's data as a TISB, it would show as a ghost to the client.

 

[John Collins]

It should be fairly painless to add diversity later if needed. Pressurized aircraft requiring just a bit more.

Depending on the airplane, another antenna doubler = another airworthiness limitation / eddy current inspection to track.

I checked into this with Garmin and they claimed it would require a different PN for the GDL88 box, so it is much more than just adding the antenna, at least for the GDL88. That said, I don't see the point unless one frequents the Class B airports where surface ADS-B is available and a top antenna makes sense. Even then it only makes sense if the display system supports the surface functions. My GNS530W does not, so I did not waste the money, but I would have considered it if I had installed a GTN system which does provide the surface ADS-B functions.

 

[Jim Rosenow]

There's the answer, John! When we did the GDL88 we also put in a GTN, and we do go to Class B on occasion.

Jim

 

[bnt83]

I checked into this with Garmin and they claimed it would require a different PN for the GDL88 box, so it is much more than just adding the antenna, at least for the GDL88. That said, I don't see the point unless one frequents the Class B airports where surface ADS-B is available and a top antenna makes sense. Even then it only makes sense if the display system supports the surface functions. My GNS530W does not, so I did not waste the money, but I would have considered it if I had installed a GTN system which does provide the surface ADS-B functions.

I'd say swapping a box that uses the same tray and adding one coax and one antenna is pretty painless. Power, ground, interfaces and mounting of the tray is already there.

Most backplates I've seen have the optional equipment holes already in them.

Easily done in less than a day with paperwork finished on a typical unpressurized GA aircraft.

 

[John Collins]

I'd say swapping a box that uses the same tray and adding one coax and one antenna is pretty painless. Power, ground, interfaces and mounting of the tray is already there.

Most backplates I've seen have the optional equipment holes already in them.

Easily done in less than a day with paperwork finished on a typical unpressurized GA aircraft.

The part that is not painless is buying a new GDL88 with diversity and selling or trading your old one. This is not something that can be done for free.

 

[bnt83]

The part that is not painless is buying a new GDL88 with diversity and selling or trading your old one. This is not something that can be done for free.

There will probably be an upgrade program for them. Just like all the other LRUs that get sent out one part number and come back a different part number. No need to buy a new one and sell the other.

I never implied it was free, but certainly painless compared to the initial installation of the GDL-88. The difference between non-diversity and diversity is only about $1200 which includes the whole installation kit package which an owner shouldn't need to pay for again.

Antenna spacing on the top of small airplane could prove problematic tho.

 

[John Collins]

There will probably be an upgrade program for them. Just like all the other LRUs that get sent out one part number and come back a different part number. No need to buy a new one and sell the other.

I never implied it was free, but certainly painless compared to the initial installation of the GDL-88. The difference between non-diversity and diversity is only about $1200 which includes the whole installation kit package which an owner shouldn't need to pay for again.

Antenna spacing on the top of small airplane could prove problematic tho.

So has Garmin agreed to this type of swap?

 

[MooneyDriver78]

I thought the 88 didn't sit in a tray, it's standalone in the tail, at least mine is.

 

[bnt83]

I thought the 88 didn't sit in a tray, it's standalone in the tail, at least mine is.

Its still has a rack tho. Tray, rack, its all the same If you already have this installed, no reason to buy the entire install kit again.

 

[bnt83]

So has Garmin agreed to this type of swap?

Even if Garmin doesn't you should be able to find a dealer willing to take the used one on trade.

Avionics is similar to car dealers you know?

Customer A:
Removed 430w for GTN 650 or aAidyne IFD . Avionics shop takes 430w as a $X credit.

Avionics shop sells 430w to Customer B for $X + markup...

 

[bnt83]

Not being argumentative, but it it's painless why not do it with the install?

I know nothing other than my personal experience... When we did the 182, our avionics shop owner, who has been in the business forever, recommended doing dual antennae. I frankly don't remember why.

I do know that it wasn't to make more $$ on the install, as giving demonstrably bad advice could have an adverse affect on his continuing to work on several turbine aircraft he does avionics for.

FWIW...YMMV

Jim

Costs?

Depending on the airplane there could be significant challenges with antenna spacing. Especially aircraft that have all com and GPS antennas on the top. Then they may have an RSM (Aspen) up there too.

Turbine - I'm not a specialist in oceanic flight but how are they tracked if there are few ground based radar stations? Diversity IMHO is about global tracking.

There are many differences in equipment requirements around the world. We don't have to have a 406 ELT here. We don't have to have TCAS software version 7.1. We don't have to have 8.33 kHz VHF com radio spacing. The list goes on. But if you fly international, it CAN be required.

 

[TangoWhiskey]

My avionics' guy's opinion:

In my opinion (everyone has one) diversity is not necessary. The stations are on the ground. Airborne traffic must approach you from afar. Your bottom antenna will pick them up. An aircraft would need to ‘appear’ right above you in order to be shadowed by your wings (in reality, you will still pick up a guy flying right overhead). If he did ‘appear’ overhead, the ground station will still pick him up, and relay his position to your box.

The quote I received this week for GDL-88, FS210, L-band antenna and a new altitude encoder (serial data, not gray code) was 2 weeks and ~ $9500. For those that have had the work done, does that sound "in the ballpark"? Garmin offers a GDL-88/FlightStream 210 bundle.

That would make me 2020 compliant except for the flight levels or outside the US. My TR182 is cerfied to FL200, so if I want to fly it above FL180 I'd also need to do a 330ES transponder (now, or later).

 

[John Collins]

Sounds like it is on the pricy side. I would get competitive quotes.

 

[John Collins]

In my opinion (everyone has one) diversity is not necessary. The stations are on the ground. Airborne traffic must approach you from afar. Your bottom antenna will pick them up. An aircraft would need to ‘appear’ right above you in order to be shadowed by your wings (in reality, you will still pick up a guy flying right overhead). If he did ‘appear’ overhead, the ground station will still pick him up, and relay his position to your box.

I agree with only needing one antenna, except for the last sentence. The ground station doesn't relay any positions of ADS-B Out equipped aircraft to your aircraft with a GDL88. The ground station only broadcasts the positions of aircraft that do not have ADS-B Out, have a mode A/C transponder, and are detected by ground radar. In theory some ADS-B Out equipped aircraft might have a relay broadcast, but this doesn't apply to a GDL88 because it receives both frequencies via air to air.

I would select a CI-105 type antenna for around $150 verses the Garmin one that is way too expensive. The only difference is the more expensive antenna has better protection from lightning strikes.

 

[petrolero]

My avionics' guy's opinion:



The quote I received this week for GDL-88, FS210, L-band antenna and a new altitude encoder (serial data, not gray code) was 2 weeks and ~ $9500. For those that have had the work done, does that sound "in the ballpark"? Garmin offers a GDL-88/FlightStream 210 bundle.

That would make me 2020 compliant except for the flight levels or outside the US. My TR182 is cerfied to FL200, so if I want to fly it above FL180 I'd also need to do a 330ES transponder (now, or later).

I'm heading down the same path as you but I started with the 1090ES transponder just by pure chance. A company used my plane to test/STC the Trig TT-21/GTN 750 combination so I got to keep the xpdr and that satisfies my ADS-B out requirement.

So I'm 2020/Class A compliant but I still intend to get the GDL-88 and FS210 so that I can have traffic alerts and weather on the GTN 750. I've heard the presentation of both is excellent. Plus I want traffic alerts with resolution recommendations in my headset. FF/Stratus traffic alerts simply aren't reliable enough IMHO and I have to make sure it's connected to my headset's Bluetooth anyway. Meh.

As far as price, yeah $9500 sounds about right to me. The GDL-88 installation is a fair amount of work. In fact, they installed the wiring for the GDL-88 (and antenna) during my last annual in Oct. while the interior was out. I'll put in the actual unit and actual antenna later.

Thanks for the info on the GDL-88/FlightStream bundle - I'll have to ask about that.

 

[bnt83]

I'm heading down the same path as you but I started with the 1090ES transponder just by pure chance. A company used my plane to test/STC the Trig TT-21/GTN 750 combination so I got to keep the xpdr and that satisfies my ADS-B out requirement.

So I'm 2020/Class A compliant but I still intend to get the GDL-88 and FS210 so that I can have traffic alerts and weather on the GTN 750. I've heard the presentation of both is excellent. Plus I want traffic alerts with resolution recommendations in my headset. FF/Stratus traffic alerts simply aren't reliable enough IMHO and I have to make sure it's connected to my headset's Bluetooth anyway. Meh.

As far as price, yeah $9500 sounds about right to me. The GDL-88 installation is a fair amount of work. In fact, they installed the wiring for the GDL-88 (and antenna) during my last annual in Oct. while the interior was out. I'll put in the actual unit and actual antenna later.

Thanks for the info on the GDL-88/FlightStream bundle - I'll have to ask about that.

Any TIS display from the Trig on the GTN? Are you remote controlling the transponder via GTN?

I'm getting a GTN 650 and wiring several options just in case I change my mind on path to ADS-B compliance...

 

[petrolero]

Any TIS display from the Trig on the GTN? Are you remote controlling the transponder via GTN?

I'm getting a GTN 650 and wiring several options just in case I change my mind on path to ADS-B compliance...

No. I need to ask about the TIS input. I don't know if that wasn't part of thhe STC or if it's just not hooked up or what.

The GTN does not control the TT21.

I like the transponder a lot. It fits into an unused CDI slot so I now have a metric ass-ton of panel space. The main feature I like about the xpdr is that it switches to/from FLT/GND upon takeoff and landing using either speed or gear switch or both, not sure.

 

[bnt83]

No. I need to ask about the TIS input. I don't know if that wasn't part of thhe STC or if it's just not hooked up or what.

The GTN does not control the TT21.

I like the transponder a lot. It fits into an unused CDI slot so I now have a metric ass-ton of panel space. The main feature I like about the xpdr is that it switches to/from FLT/GND upon takeoff and landing using either speed or gear switch or both, not sure.

Ok thanks. Sounds like all its probably getting from the GTN is GPS positional and cannot send any data to the GTN.

(I noticed there is an installation manual online that says it accepts RS-232 Garmin ADS-B. I'm wondering if there is some licensing agreement between Garmin and Trig going on there because I haven't seen that with other 3 party products)

 

[N53KL]

My avionics' guy's opinion:



The quote I received this week for GDL-88, FS210, L-band antenna and a new altitude encoder (serial data, not gray code) was 2 weeks and ~ $9500. For those that have had the work done, does that sound "in the ballpark"? Garmin offers a GDL-88/FlightStream 210 bundle.

That would make me 2020 compliant except for the flight levels or outside the US. My TR182 is cerfied to FL200, so if I want to fly it above FL180 I'd also need to do a 330ES transponder (now, or later).

Troy,

My GDL88/FS210 ran $8200 (non-diversity). This included a -$200 rebate and a software upgrade for my G530W and my GTX327 (+$284). If the encoder cost $1000 you are in the ballpark.

Kevin

 

[TangoWhiskey]

As far as price, yeah $9500 sounds about right to me. The GDL-88 installation is a fair amount of work. In fact, they installed the wiring for the GDL-88 (and antenna) during my last annual in Oct. while the interior was out. I'll put in the actual unit and actual antenna later.

Thanks for the info on the GDL-88/FlightStream bundle - I'll have to ask about that.

Troy,

My GDL88/FS210 ran $8200 (non-diversity). This included a -$200 rebate and a software upgrade for my G530W and my GTX327 (+$284). If the encoder cost $1000 you are in the ballpark.

Thanks guys!

 

[petrolero]

Ok thanks. Sounds like all its probably getting from the GTN is GPS positional and cannot send any data to the GTN.

(I noticed there is an installation manual online that says it accepts RS-232 Garmin ADS-B. I'm wondering if there is some licensing agreement between Garmin and Trig going on there because I haven't seen that with other 3 party products)

Correct. There is a licensing agreement. The STC shop first contacted me not long after I installed my GTN 750 in ~June 2013. There was a long wait for the licensing piece to get done. I believe Bendix/King also has one but then that is because Trig manufactures the Bendix/King transponders (or at least the one that was to be tested - the KT-74 IIRC). They initially contemplated testing the KT-74 in my plane but we ended up testing the TT-21 instead.

I have asked in passing whether this STC combo of TT21/GTN is only applicable to the current software version. I think the answer to that is yes based on my avionics shop's first guess without reviewing the actual STC language.

The explanation for the restriction, if true, would be that, in theory, Garmin could change its proprietary data format with any new SW version and that would render the transponder useless until it received an update. So the STC *may* be limited to SW Version 5.13 with further testing required upon upgrading to a new SW version. Would be a good question for Trig if you don't want to try and find the STC itself. It should be in my records... which aren't here.

 

[bnt83]

Correct. There is a licensing agreement. The STC shop first contacted me not long after I installed my GTN 750 in ~June 2013. There was a long wait for the licensing piece to get done. I believe Bendix/King also has one but then that is because Trig manufactures the Bendix/King transponders.

I have asked in passing whether this STC combo of TT21/GTN is only applicable to the current software version. I think the answer to that is yes based on my avionics shop's first guess without reviewing the actual STC language.

The explanation for the restriction, if true, would be that, in theory, Garmin could change its proprietary data format with any new SW version and that would render the transponder useless until it received an update. So the STC *may* be limited to SW Version 5.13 with further testing required upon upgrading to a new SW version. Would be a good question for Trig if you don't want to try and find the STC itself. It should be in my records... which aren't here.

I doubt they would change any parameters in that data format simply because it would affect any other radios that use it, including the garmin transponders themselves. For instance, I can buy a GTX330ES off ebay and wire it to the GTN, but they both have to have a software version higher than _______ to make ADS-B (out) work. Same thing goes with 430w/530w units.

 

[petrolero]

I doubt they would change any parameters in that data format simply because it would affect any other radios that use it, including the garmin transponders themselves. For instance, I can buy a GTX330ES off ebay and wire it to the GTN, but they both have to have a software version higher than _______ to make ADS-B (out) work. Same thing goes with 430w/530w units.

I agree. But that was the question related to the STC - what happens when I upgrade my SW version on the GTN? The STC is new enough that this hasn't come up in practice yet.

 

[bnt83]

I agree. But that was the question related to the STC - what happens when I upgrade my SW version on the GTN? The STC is new enough that this hasn't come up in practice yet.

STCs --- often times the software requirements are on the wiring diagrams themselves. Usually the Instructions for Continued Airworthiness says to reference the wiring diagrams.

If functions are added or removed, Trig could technically issue a service bulletin against the STC. If there is any changes needed to a Flight Manual Supplement or ICA then it seems more likely the STC would be revised.

 

[bnt83]

If all that fails, then you probably could just add a blind GPS receiver to get a position source to the Trig via field approval. What's one more antenna ?

(Never have seen a GPS antenna splitter)