NMEA out to transponder.

[Paul_Sengupta]

John, have a look at this thread:

http://forums.flyer.co.uk/viewtopic.php?f=1&t=97653

Rob P has tried wiring up a KT74 and appears to have got nothing out. I don't know what firmware he has.

[simonjarman]

You might have a look at the Garrecht VT-2000 transponder, maybe a bit more expensive, but it is easy to use and mine accepted NMEA serial data from a Garmin GPS90. It fits in a KT76 rack and only needs a small solder bridge to connect the external GPS.
Simon

[The Westmorland Flyer]

John, have a look at this thread

Thanks Paul. I'm there now. This trial with its unusual parameter requirements has certainly forced some issues to the surface!

[AlanB]

This trial with its unusual parameter requirements has certainly forced some issues to the surface!

It certainly has. It would appear that the NEMA message set output from the GPS devices does include an indication of accuracy.

The NEMA GGA message in particular defines if the position fix is:

0 Fix not available or invalid

1 GPS SPS Mode, fix valid

2 Differential GPS, SPS Mode, fix valid

3 GPS PPS Mode, fix valid

I've analysed the outputs from a Garmin 196 and the GPS/GLONOS UBS device used on the PilotAware.

GPS/GLONASS USB Dongle
$GPGGA,113156.00,5126.31393,N,00103.39777,W,1,08,1.01,59.2,M,46.7,M,,*78

Garmin 196 WAAS not available
$GPGGA,120720,5126.3133,N,00103.4034,W,1,06,2.4,55.9,M,48.2,M,,*62

Garmin 196 WAAS available
$GPGGA,133754,5126.3067,N,00103.4207,W,2,07,1.1,51.6,M,48.2,M,,*6E

The humble Garmin 196 updates the output of the appropriate NEMA message to indicate increased accuracy when WAAS is available and used. The GPS/GLONASS USB Stick does not a the device description say is supports, note not uses, DGPS (WAAS).

In my opinion therefore the combination of Transponder and GPS Data source needs to be carefully thought through. For example;

On the Flyer Thread a SkyMap IIIC is DGPS Capable but in the manual indicates it requires a separate receiver as it is not built into the device. it was connected to a KT74 but was not providing the GPS data to allow ADS-B out.

If, and I don't know the inner workings of the device, the KT74 requires the appropriate flag in the NEMA GGA message to indicate DGPS then these two combinations are not going to function. That might change if the firmware in the KT754 is upgraded but again I have no knowledge of the inner workings of the KT74.

It would appear that the Garmin 196, and the AVMap V, have these built in so provide the GGA message to indicate increased accuracy from a DGPS (inbuilt WAAS) source.

From an earlier thread I believe that FASVIG posted something from Trig that indicated that the TT21 devices used the NEMA RMC message. This message only has a flag data valid or not valid but no indication of DGPS therefore more flexible in the GPS source.

As an aside I have just completed a standalone GPS project using off-the-shelf components which provides the NEMA message set on an RS232 out cable. It does require some soldering knowledge. Powered by 12volts it can drive up to two devices but is not DGPS (WAAS) enabled.

Hope this helps someone.

[Paul_Sengupta]

That's very interesting. Lee could always set the parameter on the output of the PAW rather than sending through what the GPS sends, but that would be cheating! 

[The Westmorland Flyer]

Indeed, very interesting analysis, Alan. Especially noteworthy is the distinction between being "WAAS capable" or "WAAS enabled" and actually setting the $GPGGA sentence correctly. That's a whole new Pandora's box of potential issues! This is a level of detail that we probably can't expect suppliers, even those as good as Mendelssohn to be able to fully resolve.

Is anyone dealing with Trig, Garmin et al to find out more about this? I am happy to have a go if no-one else is on the case.

[Admin]

There was a question regarding which sentences are sent from PilotAware to the Transponder
in fact if you want to 'sniff' the sentences , as somebody said, simply tick the 'trp' checkbox
on the config page of the web browser, and you will see everything that goes to the transponder

so I am sending  (if available)
$GPRMC
$GPGGA
$GPVTG

There may need to be some configuration of these at some point, either by transponder type,
or a set of checkboxes to say which sentences you want passed to the transponder

Thx
Lee

[The Westmorland Flyer]

I've just checked the PAW->transponder serial output and can confirm that PAW is outputting the three sentences exactly as Less says. The position fix field in the $GPGGA sentence is 1 for my UBlox7 GPS suggesting that it's not seeing EGNOS WAAS (or at least is not setting the field correctly). The GPS antenna is indoors at the moment so it might just be not seeing the necessary satellites. I'll extend the USB outdoors tomorrow to see if things change.

BTW, Lee, I don't see a "trp" check box in the config on my 20151114 PAW. I did it the hard way via a serial port into PuTTYtel!

[Admin]

BTW, Lee, I don't see a "trp" check box in the config on my 20151114 PAW. I did it the hard way via a serial port into PuTTYtel!

Doh ?!?
You are right, sorry, this is not in the released software yet, it will be in the next release.
Putty is another great way, I use it all the time.

Regarding position fix, you will get a better fix when you have a decent satellite view.
Also remember you can get the fix info from the web page, rather than decoding the sentences
http://192.168.1.1

Thx
Lee

[Paul_Sengupta]

EGNOS satellites are geostationary, so you need a clear view of the sky to the south, like with your Sky dish. Unless you're in the southern hemisphere of course.

[SteveHutt]

Is anyone dealing with Trig, Garmin et al to find out more about this?

As you know, FASVIG is keen to encourage voluntary adoption of ADS-B, and we see support for uncertified GPS ADS-B Out as the most likely means of maximising adoption in GA.

So, I am hoping to do the following:
1) Encourage the CAA to more heavily support uncert GPS ADS-B and press others to do so to.
2) Make available info on which transponder makes/models/firmware version support uncert GPS ADS-B.
3) Persuade LAA and other associations to simplify MOD approval using new PAW Traffic Display feature.
4) Make available better info on exactly how (including precise data input reqs) to get uncert GPS ADS-B working.
5) Use the above to 'encourage' those manufacturers that do not support uncert GPS ADS-B to reconsider.

I am already doing 1) via the CAA Conspicuity Working Group.

On 2) I have info from Trig (already on www.fasvig.org) and a reply from Air Avionics (Garrecht) saying they will help. I am awaiting responses from Becker, Bendix King, Dittel and Funke. I did not email Garmin as I already know they are not interested in supporting uncert GPS ADS-B Out with their transponders. They declined invitations to assist in the NATS uncertified GPS trial so there seemed no point. If there are any other manufacturers you think I should contact please let me know.

On 3) I was at LAA Turweston yesterday and had further discussion with the CEO but unfortunately LAA Engineering staff were not available. They have my proposal. If I succeed with the LAA I will turn my attention to other associations.

I already knew there was a need for 4) but the KT74 discussion here and elsewhere reinforce this.

On 5) the more manufacturer support for uncertified GPS ADS-B that can be achieved, the greater the commercial pressure on non-supporting manufacturers to review their position. Activity on 1) helps too.

Any other suggestions on approach gratefully received.

Steve

[The Westmorland Flyer]

The NEMA GGA message in particular defines if the position fix is:

0 Fix not available or invalid
1 GPS SPS Mode, fix valid
2 Differential GPS, SPS Mode, fix valid
3 GPS PPS Mode, fix valid

I've analysed the outputs from a Garmin 196 and the GPS/GLONOS UBS device used on the PilotAware.

GPS/GLONASS USB Dongle
$GPGGA,113156.00,5126.31393,N,00103.39777,W,1,08,1.01,59.2,M,46.7,M,,*78

I did some tests today with the GPS/GLONASS USB Dongle outside with a clear view of the southern sky and found that it does see EGNOS and it sets the DGPS flag accordingly:

$GPGGA,122133.00,5434.29993,N,00237.87591,W,2,09,1.10,210.7,M,49.1,M,,0000*46

The NMEA sentence above is a copy and paste of PAW's output to the transponder, so PAW is correctly sending the DGPS flag to the transponder.

It must be said though that I had to get the GPS well out into the clear before it picked up the EGNOS satellite. I'm not sure one could rely on it with the GPS/GLONASS USB Dongle on the coaming - unless you only fly ever south that is

[The Westmorland Flyer]

Any other suggestions on approach gratefully received.

It sounds like you're covering all the bases there Steve. I'm the Senior Air Traffic Engineer at Carlisle Airport and as such have various useful contacts in CAA SARG. Generally when I engage with them on GA issues they are very happy to put me in contact with the relevant key personnel. For example I've been doing quite a lot of work on the 8.33kHz saga. If you think this might provide you with another route in to CAA or if I can help with your workload then I am happy to assist.

[AlanB]

It must be said though that I had to get the GPS well out into the clear before it picked up the EGNOS satellite. I'm not sure one could rely on it with the GPS/GLONASS USB Dongle on the coaming - unless you only fly ever south that is

Given mine a better view of the sky and likewise now showing the same.

Location, Location, Location 

[SteveHutt]

Thanks John,

Appreciate that. I want to think you did, but just in case.....
Did you do this: http://fasvig.org/volunteer
Maybe you did already. If so, apologies.
We have 97 Champions now and I cannot remember all their names!

See here: http://fasvig.org/fasvip/poc/vee/vee-5

This is the basis underpinning what I am doing. It is just me at the moment as the Champion project teams have not yet started up, but will be very soon. I could include you on this project team.

Regards,
Steve