Who owns/maintains the definition of this last one?
Noone. And there exist multiple slightly incompatible versions.
I think (but am not sure) King originally invented this.
I should think a converter wouldn’t be extremely hard to build, around some microcontroller?
Yes and no. No for the basic functionality. Yes because of the slightly incompatible versions. And “aviation data format” (as Garmin calls it) is extremely thin on some information commonly expected from NMEA equipment, such as the satellite status.
Here is my parser
After all aviation data is not that bad. Usually it is specified in the installation manual which can not be said of most other protocols used by the panel mount GPS boxes. In the GNS530 manual which can be found on the internet it is in Appendix C.
There are some differences between different manufacturers but it is possible to write a parser which works for most of them. The big difference between boxes is if they support the full type 2 protocol which contains the whole flight plan or only the reduced type 1.
To give an example here this is what it look like in ASCII. Some parts are human readable others (which do not show in this post well) are binary:
z00126
AN 52 1250
BE 013 0913
C357
D000
E00158
GL0011
I0507
KEDDB
L0511
QE030
S——-
T————-
l000158
w01EDAZ
w02bEDDB
There are some differences between different manufacturers
Well, the differences start with obscure features like the altitude 
The “aviation format” isn’t really relevant to feeding a device which is bluetooth attached. It normally contains the programmed route (also called the “flight plan”) but a bluetooth GPS receiver obviously isn’t going to know your programmed route. And there are loads of incompatibilities in the aviation format, for example Garmin and King are mutually incompatible in the OBS mode. I was unable to get any answer from Avidyne on the compatibility of their MFDs in this mode, which to me suggests their programmers have left.
I think most certified GPS boxes have a separate GPS module which transmits NMEA to the rest of the box. The King boxes certainly have – they bought it in and it is mentioned in say the KLN94 IM. The NMEA data is even available on a connector pin on the back. I think the GNS boxes have the function incorporated on the main board, maybe even implemented on the main processor, but I have never seen the inside of a GNS and even the GNS430 MM gives no clue.
Yes one could build a box which connects to a standard active GPS antenna on the roof and can run two concurrent bluetooth sessions, one with an NMEA device and one with an IOS device. But it would be a lot of work.
And the IOS one is likely impossible because Apple have locked down their bluetooth interface so it works only with Apple-approved devices. AFAIK it is impossible to build a bluetooth GPS which works with an IOS device, without purchasing a license from Apple…
There are IOS GPSs which can do NMEA also but not concurrently; you have to flip a switch. This strikes me as bizzare because that switch must be more hassle than implementing both protocols concurrently, so maybe there is a reason for it. Maybe Apple force them to do it so one cannot have a non-Apple client running concurrently.
You tested it with both concurrently?
I mean two concurrent BT sessions. Very very few NMEA GPSs can run more than one session. Emtac brought out one which did 4 sessions, shortly before they went bust.
It doesn’t have external antenna / external power though, does it? I guess one could power it via USB though, with +5V.
I have not tested it with both but the product description explicitly highlights this feature so why should it not work? Yes, it has a USB connector for power. I never saw the need for an external antenna with it and my airplane more shielded than yours. You will probably find it at AERO.
The Gamin Glo supports simultaneous connections (with the latest firmware update), certainly on IOS. I’ve had three simultaneous IOS connections and it’s worked great. I haven’t tried it with any non-IOS devices (I don’t have any!). What device would I need to test the NMEA stuff for you, I can see if I can borrow something.
There’s no external antennae connection, but I’ve never had any problems with reception. I usually leave the GLO in my flight bag on the back seat and it has no problems. The battery supposedly lasts for 12 hours, but you can use it and charge it at the same time if you like. I have an Anker USB battery charger that I keep in my bag, just in case.
What device would I need to test the NMEA stuff for you
I think anything that isn’t IOS 
I think any mobile phone which doesn’t have internal GPS, enable location services (or whatever it is called), take a photo, copy the photo to a PC, and look under Properties to see if the EXIF data contains lat/long. Or just run google maps or whatever and see if it finds you.
Or any laptop/tablet running say windows, which has bluetooth. Just find the GPS in the bluetooth connections, connect to it, you should see a COM port under Control Panel / Systems / Hardware / Device Manager / Ports (COM & LPT) and if you get that far, it has probably worked. But to test it fully, run Hyperterminal (or better still download Teraterm) and see if there is NMEA stuff coming out of the COM port (4800 baud 8 bits no parity). The data is really obvious.
I don’t know if there is an easier way.
I appreciate you being able to do this.
Not having an external antenna is OK, but it needs an accessible and reliable on/off switch. Some of the IOS compatible units used to turn on by themselves.
Hi Peter.
I’ve just tested this out for you by:
1. connecting my ipad mini (no built-in gps) with the garmin glo and running SkyDemon
2. connecting my Mac to the GLO via bluetooth and opening a serial connection to the GLO
I can confirm both devices were running concurrently and successfully.
The serial output I see is this sort of thing:
$GPRMC,151021.5,A,5142.55843,N,00253.72590,W,000.03,000.2,310314,002.4,W,A*35
$GPGGA,151021.5,5142.55843,N,00253.72590,W,1,04,1.4,41.4,M,48.8,M,,79
$GPVTG,000.2,T,002.6,M,000.03,N,0000.06,K,A10
$GPRMC,151021.6,A,5142.55843,N,00253.72590,W,000.03,000.2,310314,002.4,W,A*36
$GPGGA,151021.6,5142.55843,N,00253.72590,W,1,04,1.4,41.4,M,48.8,M,,7A
$GPVTG,000.2,T,002.6,M,000.03,N,0000.06,K,A10
$GPRMC,151021.7,A,5142.55843,N,00253.72590,W,000.03,000.2,310314,002.4,W,A*37
$GPGGA,151021.7,5142.55843,N,00253.72590,W,1,04,1.4,41.4,M,48.8,M,,7B
$GPVTG,000.2,T,002.6,M,000.03,N,0000.05,K,A13
With regards to the power-on button, it is fairly easy to accidentally press the on-button whilst the GLO is in your bag. However the recent firmware update included a new feature that automatically powers off the GLO if it doesn’t pair with a device after 2 minutes, to minimise the risk of inadvertently draining the battery.
