Erratic height indication on FR24

[davep]

We transmit ADSB using a Garrecht transponder with PAW as the GPS source. Just recently we have noticed that the height displayed on FR24 occasionally drops to 0 feet and other times rises to 40,000 feet for varying periods of time. What height source is used by the transponder for ADSB?

[Ian Melville]

Dave, have you a second PAW available to log your ADSB transmissions? Eliminate any FR24 errors.

Your mode S transponder will have had height info prior to connecting the PAW, you need to find out from the transponder people if the GPS data overrides the original barometric data. I assume there was no issue before the link was done?

[Admin]

I am pretty sure that the height reported is barometric height, not GNSS.
In which case, this unrelated to PAW, which should only be referenced for the
Lat/Lng positions.

I could be wrong, sounds like you need to confirm with Garrecht, which NMEA sentences are used, and in particular which fields.

Thx
Lee

[davep]

We acquired PAW and the transponder at much the same time so there has been no significant period when the transponder has operated in isolation. We are still using an old external altitude encoder for the transponder but it also has an internal encoder which can be used in place of the existing external one. This change requires a fair amount of work as the mounting tray needs drilling to install a static adapter. If someone can confirm that the altitude encoder is the cause of the erratic height rather than PAW then the extra work to change the encoder would be justified.
I must say that for the majority of the time the transponder/PAW system works perfectly and an increasing number of people around the airfield are becoming very interested.

[Keithvinning]

Hi Dave

What airfield is that perhaps we should come up and give a presentation

Keith

[Ian Melville]

Dave, I have had a look at the documentation for Garrecht transponders in general(not knowing which transponder model you have), and there is nothing to indicate that they use anything other than the internal Alticoder, or an external Alticoder for the VT-02 and VT-2000. For these two there are just four modes:
Possible values:
int-25ft, uses the internal pressure sensor for alticoding with 25ft resolution
int-100ft, uses the internal pressure sensor for alticoding with 100ft resolution (not recommended anymore),
ext-25ft, uses the data provided by an external alticoder with 25ft resolution. Never use this setting, if you don't know exactly, which resolution the alticoder connected to the VT-02 provides.
ext-100ft, uses the data provided by an external alticoding with 100ft resolution. This is the default setting for parallel alticoders as used in former KT-76 / KT-76A/ KT-76C installations.

The ADSB settings make no mention of the altitude.

I would switch to the internal Alticoder?

[davep]

I have sent a query to Garrecht asking what height source is used for ADSB. Unfortunately they are on company holiday so it will be next week before I get a reply.
In the meantime we may as well do the extra work to transfer to the internal alticoder anyway as it will be more accurate at 25 feet increments instead of the 100 feet external encoder.
The transponder is a type VT-2000 and it was chosen partly because it transmits ADSB as soon as an uncertified GPS is connected.