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How does Mode S Enhanced Surveillance (GPS position) fit in with ADS-B OUT (and checking your ADS-B OUT using Foreflight)

Go back some years to say the early days of Mode S. If you connected a GNS430 to a GTX330 (for e.g. GPS ground speed based auto AIR/GND switching) the GTX330 would radiate the whole lot including the GPS position and probably altitude on the Mode S data. It would do this only when pinged by SSR radar.

The EU had set up the Elementary Mode S versus Enhanced Mode S distinction (something the US makers of all the equipment knew nothing about ) and tried to prohibit the radiation of this extra data, for aircraft which did not qualify for the Enhanced level (5700kg+ 250kt TAS+ etc). So a TBM was ok but a TB20 would have been illegal to have that wire. Details here and that regulation has probably been abandoned by now. Anyway, N-regs etc arriving to Europe from outside would routinely have that connection already. This is just a bit of background…

Now, this GPS position radiation is just like ADS-B OUT, except that it doesn’t happen continuously.

Aircraft types like a TBM, PC12, perhaps most bizjets, and many others as described above, would be radiating this GPS position data whenever they were pinged by SSR, which in Europe is most of the time at any altitude.

So what happens when you have say a GTX330, GNS750, have that connection in place, and then set up ADS-B OUT. Do you get both lots coming out?

And are there any TAS products which make use of the older GPS-derived radiated data? I think there is a lot of it about and it is a waste to not use it.

The people I know about here who might know are @garryiae, @wigglyamp, @ncyankee and a few others I don’t remember.

Garmin later produced firmware for the GTX330 which made it possible to use GPS data for auto AIR/GND switching but did not radiate the “illegal” stuff.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

If you connected a GNS430 to a GTX330 (for e.g. GPS ground speed based auto AIR/GND switching) the GTX330 would radiate the whole lot including the GPS position and probably altitude on the Mode S data. It would do this only when pinged by SSR radar.

This isn’t correct. The standard link between the GNS and GTX is an RS232 (Garmin Aviation format) which is used for air-ground switching using GPS groundspeed. Whilst GPS position is included in the data stream, the transponder does not transmit it.

With Enhanced surveillance, the following DAPs are transmitted:

(1) MCP/FCU Selected Altitude;
(2) Roll Angle;
(3) True Track Angle;
(4) Ground Speed;
(5) Magnetic Heading;
(6) Indicated Airspeed or Mach No ;
(7) Vertical rate: Barometric Altitude rate or Inertial vertical Velocity. When barometric altitude rate field is provided, it is derived solely from barometric measurement;
(8) Barometric Pressure Setting in use minus 80 000 Pascal; and
(9) Track Angle Rate or True Airspeed.

The link between the GNS and GTX is ARINC 429 and if connected in most GA aircraft, this will only provide the GPS ground speed and True Track Angle DAPs. EASA didn’t make this connection illegal at all, but required that any such connection was certified via an STC, just as they also require for ADS-B. Again, there is no GPS position transmitted. The transmission of the DAPs can be switched on or off in the set-up configuration pages (ES ENABLED).

Once you get into ADS-B, then the data source from the GPS for a certified SIL=1 is a different data bus, even though it can be RS232 (Garmin ADS-B+) or ARINC429.

Rumour has it that Garmin will be releasing new GTX software to allow a GTX330-ES to transmit NMEA GPS position data with SIL=0 from KLN90/94, GPS150/155 etc to satisfy the many GA users of portable ADS-B receivers with tablet displays.

Avionics geek.
Somewhere remote in Devon, UK.

Garmin added EHS capability to its GTX330 transponder. EHS adds 8 parameters that get included in the secondary reply. Most installations don’t provide the data to the transponder, for example selected altitude. So a transponder may be EHS capable, but if the data required for the EHS parameters are not available, the installation is not EHS, so you would not code the transponder as H or L. Although some of the EHS information may be derived from a GPS, position is not one of them. The data used by EHS and the source of that data is:

Selected Altitude – the pilot entered value on an FMS or an aircraft equipped with altitude-preselect. Most GA autopilots do not provide this data in an electronic form that can be ingested by the transponder.
Roll angle – comes from an AHRS
Track angle – can come from FMS or a GPS
Track angle rate – aka rate of turn, comes from an AHRS
Ground speed – can come from an FMS or a GPS
Magnetic Heading – can come from an AHRS or a bootstrap on an HSI
IAS – comes from an air data unit
Vertical rate – comes from an air data unit

Also most piston aircraft don’t have an AHRS or an air data unit and if they do, the data is not always in a digital form that can be used by a transponder. A G1000 installation is likely to have all of this data in a form that can be sent to a mode S transponder. A GNS530 equipped aircraft with a legacy autopilot is highly unlikely to have this data.

Just because some of the data may be derived from a GPS, it is an invalid assumption that latitude-longitude position is amongst the data elements in a mode S reply.

KUZA, United States

This isn’t correct. The standard link between the GNS and GTX is an RS232 (Garmin Aviation format) which is used for air-ground switching using GPS groundspeed. Whilst GPS position is included in the data stream, the transponder does not transmit it.

This subject is firmware version dependent.

Although some of the EHS information may be derived from a GPS, position is not one of them.

That answers my question then; the position was never transmitted by a GTX330. Very interesting, because I had read in so many places that it was…

However this suggests that a GTX330ES does transmit the GPS position.

Rumour has it that Garmin will be releasing new GTX software to allow a GTX330-ES to transmit NMEA GPS position data with SIL=0 from KLN90/94, GPS150/155 etc to satisfy the many GA users of portable ADS-B receivers with tablet displays.

That seems to confirm that a GTX330ES can transmit the position, so my Q stands as to how this is handled if you are also using it for ADS-B OUT? Does the unit transmit the GPS position regularly in its ADS-B OUT mode, as well as in the response to SSR?

But this is very interesting anyway. Is the objective to make a KLN94+GTX330ES aircraft generate “uncertified” ADS-B OUT, which would presumably be visible on tablet ADS-B IN devices but not visible on certified ADS-B IN devices such as a TAS605 (once upgraded to ADS-B IN, of course).

Administrator
Shoreham EGKA, United Kingdom

The point is that ADS-B and ATCRBS radar are two independent surveillance systems with two separate theories of operation.. The ATCRBS system is based on a ground radar interrogating the transponder for information on one frequency (1030 MHz) and responding on another (1090 MHz) where Altitude uses DF 4, and identity uses DF 5. The location is determined by physics, the speed of light and the scan azimuth of the radar antenna. ADS-B uses a separate format (DF 17) with longer messages that can include a latitude and longitude and that are periodically broadcast as unsolicited Extended Squitter messages (1090 MHz) and are received and processed by the ADS-B ground station. The transponder is used as a transmission medium and the messages are tagged as to their download format (DF) and function. The two capabilities coexist with each other. ATCRBS radar does not process ADS-B ES squitter messages and ADS-B ground stations do not process transponder replies. In the US, there is a fusion server that merges the two sources of position into a single displayed target on the controllers scope.

The ADS-B Out broadcast via the Extended squitter includes much more information than just position, which is just one of 19 items. Many of the items are simply not provided by a non approved position source and are easy to detect that they are missing. So when you ask about if you attach a position source to an ES transponder that is enabled to broadcast the DF 17 Extended Squitter messages, it will undoubtedly broadcast the latitude and longitude if it is supplied in one of the transponder supported formats. The transponders I am familiar support multiple types of RS232 and ARINC 429 interfaces that include latitude and longitude, but only one specific format carries all the required data. So yes, they will broadcast position and be able to be seen on non certified receivers if latitude and longitude is conveyed to the transponder. But currently in the US, these folks are getting the polite letters from the FAA to fix it or turn it off. After Jan 1, 2020, the letters won’t be nice. I am guessing that European governments won’t be as tolerant as the FAA.

KUZA, United States

NCYankee wrote:

The transponders I am familiar support multiple types of RS232 and ARINC 429 interfaces that include latitude and longitude, but only one specific format carries all the required data

What precisely is “all the required data” beside the lat/long?

The installation manual of the popular Trig TT31 transponder (also sold as the KT74) says that “The ADS-B Out capability is enabled automatically when the transponder is configured to use a GPS position source over an RS232 interface. With a GPS source connected the transponder supports all required features of ADS-B Out.”

This suggests that the other required data can be generated by the transponder itself and indeed the TT31 has numerous data field settings for ADS-B.

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

This suggests that the other required data can be generated by the transponder itself and indeed the TT31 has numerous data field settings for ADS-B.

Here is the US regulation for the data required, I added the source of the data:

(d) Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c. The pilot must enter information for message elements listed in paragraphs (d)(7) through (d)(10) of this section during the appropriate phase of flight.

(1) The length and width of the aircraft; (This can come from settings in the transponder)

(2) An indication of the aircraft’s latitude and longitude; (This comes from the position source, any supported format for the transponder could be used)

(3) An indication of the aircraft’s barometric pressure altitude; (this usually comes from a altimeter encoder or from a air data unit)

(4) An indication of the aircraft’s velocity; (This comes from the position source)

(5) An indication if TCAS II or ACAS is installed and operating in a mode that can generate resolution advisory alerts; (This comes from the transponder configuration)

(6) If an operable TCAS II or ACAS is installed, an indication if a resolution advisory is in effect; (This comes from the TCAS system if installed)

(7) An indication of the Mode 3/A transponder code specified by ATC; (this is from the transponder and set by the pilot)

(8) An indication of the aircraft’s call sign that is submitted on the flight plan, or the aircraft’s registration number, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address; (This can come from the transponder and in some cases is entered by the pilot.)

(9) An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference; (This comes from the transponder based on the code value entered).

(10) An indication of the aircraft’s “IDENT” to ATC;(this comes from the transponder based on the pilot using ident)

(11) An indication of the aircraft assigned ICAO 24-bit address, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address; (This value is configured into the transponder).

(12) An indication of the aircraft’s emitter category; (this value is a configuration in the transponder)

(13) An indication of whether an ADS-B In capability is installed; (this is a configuration value entered into the transponder)

(14) An indication of the aircraft’s geometric altitude; (this is an altitude from the position source with reference to the WGS84 and is not corrected to the geoid. Not all position sources provide this in a usable format or at all.

(15) An indication of the Navigation Accuracy Category for Position (NACP); (This is from the position source and non compliant position sources don’t provide this data)

(16) An indication of the Navigation Accuracy Category for Velocity (NACV); (This is from the position source and non compliant position sources don’t provide this data)

(17) An indication of the Navigation Integrity Category (NIC); (This is from the position source and non compliant position sources don’t provide this data)

(18) An indication of the System Design Assurance (SDA); and (This is a value usually assigned to a particular transponder thru the certification process)

(19) An indication of the Source Integrity Level (SIL).(This is a value dependent on the position source dynamic data and a setting in the transponder)

For example, many of the GPS systems provide a position source data stream via RS232 that is named Aviation Format. It does not include items 15, 16, 17 and items 4 and 14 may not be the required data.

Also a key element of getting the STC is the overall latency of the position source and the ADS-B Out transponder, measured as a system. Looking at transponders using unapproved position sources, I do not see velocity data,accuracy data, altitude data, or integrity data and the SDA and SIL values are not compliant. The FAA designates Non Performing Emitters that don’t have valid or missing data, even if the latitude-longitude data is correct format. After 2020, any transponders detected as NPE will be ordered to cease and desist use of the units or face possible enforcement action.

KUZA, United States

Thanks. So the question is if the position source (properly) generates items 4 and 14-17.

The IM of the Trig TT31 claims that it will comply with EASA ADS-B requirements (CS-ACNS) when connected to a SBAS Garmin GPS. Interestingly, for FAA compliance, the GNSxxxW units are listed, but not the GTN units!

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

The IM of the Trig TT31 claims that it will comply with EASA ADS-B requirements (CS-ACNS) when connected to a SBAS Garmin GPS. Interestingly, for FAA compliance, the GNSxxxW units are listed, but not the GTN units!

I believe the STC now includes the GTN. There are several formats of RS232 from the Garmin units that will get the Trig to generate a position, but only one format is compliant and must be used if the Trig software is at the version that supports RTCA DO 260B. It must be configured for ADS-B Out + format. There is an older version that is compliant with RTCA DO 260A and the early software versions of the Trig unit, it is named ADS-B Out. Aviation Data or ADS-B Out will make the combination with the Trig NPE. So it is a specific pairing with a specific version of the GPS and transponder. The GPS software version and transponder version must match that on the STC, as must the format of the RS232 data. The GTN also supports the format ADS-B Out +, but the pair had to be demonstrated with the Trig to add it to the STC, even though the same protocol was in use with the GTX330ES. So Garmin held an STC that included a GTN or GNS430W/530W or GNS480 as approved positions sources for the GTX330ES, and Trig had an STC for the combo of the GNS430W/530W using ADS-B Out +, but the extra pairing had to be tested and receive an STC.

KUZA, United States

This is so complex… Can you imagine getting this stuff installed, here in Europe? Most installers don’t even know about this, let alone have test kit enabling them to check what exactly is being emitted. You could so easily pay x k for an installation whose ADS-B OUT is invisible to certified ADS-B IN.

Has anyone heard of Europe checking for NPE installations and going after the aircraft? Maybe 30 years from now

Getting back to my original Q, I guess that

The point is that ADS-B and ATCRBS radar are two independent surveillance systems with two separate theories of operation..

means that the two will work concurrently; one doesn’t disable the other. Presumably the software must be written so that the periodic ADS-B OUT emission is slightly delayed if the transponder is responding to SSR.

Can a GTX330 (year 2005) be upgraded to ADS-B OUT?

Administrator
Shoreham EGKA, United Kingdom
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