Menu Sign In Contact FAQ
Banner
Welcome to our forums

Mysteries of ARINC407 and ARINC429

Google maps?

EGTR

That will plot textual data.

What I’ve been wondering is whether some old Avidyne MFD might accept external GPS data via this route. The IFD boxes don’t appear to, unless it is undocumented.

I have the IM for an EX500 which says it has ARINC429 but no details. Well, there is this

which shows it getting GPS data via 429. The IM for the KLN90B shown above shows it transmits position via 310+311, which is the standard “coarse” format. Same for the GNS/W boxes.

I wonder if anyone has an IM for anything which might accept ARINC429 labels 110, 120, 111, 121?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I wonder if anyone has an IM for anything which might accept ARINC429 labels 110, 120, 111, 121?

The Sandel SN3500 uses label 121 input from GPS/FMS to provide the roll-steering output to autopilots.

SN3500_82005_im_t_installation_manual_2_pdf

Avionics geek.
Somewhere remote in Devon, UK.

I think this is one of the cases of multiple definitions – 121 is Longitude Fine (the last 11 bits of the 32-bit longitude; 0.4" resolution) in the GPS group

But the Sandel uses another one:

I get the impression that 110, 120, 111, 121 are very old. They are the only way in which a TAS6xxA (the ADS-B IN version) can receive WAAS GPS position via 429 (because of Avidyne design choice, not because labels 310,311 would not have provided sufficient resolution) but no GPS I can find outputs these.

Perhaps this conflict between GPS label 121 and the roll steering label 121 is why the “secret” Garmin RS232 data stream is used for all WAAS GPS → ADS-B IN connections in GA? All the recent nav boxes are outputting roll steering label 121, which precludes ARINC429 being used for GPS because that would need all four of 110, 120, 111, 121.

But would it? Is there some bizzare certification issue which says that the GPS position used for ADS-B IN feed needs a higher resolution that 310,311 could provide? These deliver 180 degrees spread over 20 bits which translates to approximately 20m.

I’ve now coded a GPS position converter from NMEA to ARINC429 labels 110, 120, 111, 121 and also the more usual 310, 311. Might be useful to somebody You can get a $20 WAAS/EGNOS GPS module to feed it. Or just any old NMEA GPS like a KLN94…

I had a dig around for the description for the Roll Steering Label 121 and eventually found this in the DAC roll steering converter IM (normally used together with label 312 ground speed, presumably to provide a better estimation of the turn rate)

As usual, we have been here before

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Perhaps this conflict with 429 roll steering label 121 is why the “secret” Garmin RS232 data stream is used for all WAAS GPS position data in GA? Because all the recent nav boxes are outputting label 121 roll steering, which precludes the ARINC429 channel being used for GPS because that would need all four of 110, 120, 111, 121.

Garmin went to the serial port because there were 4 or 5 ports on the GNS430W and GNS530W respectively and only a single general purpose ARINC 429 output port, so it was strictly a matter of how many devices could be connected to the GPS. The single ARINC output port could be configured to use one of 8 selections of labels, non GAMA 429, GAMA 429, GAMA 429 for BK, GAMA 429 Graphics Honeywell, GAMA 429 Graphics Sandel, GAMA 429 Pro Line 21, or GAMA 429 Sextant. It did not include ARINC 743A as an option. Regardless, with only a single port available and too many conflicts with both speed and labels, it was just not feasible to use 429. Serial offered up to 5 different output data streams and a similar number of input data streams. There were over a hundred fifty thousand GNS systems already in the field and using RS232 was just software.

KUZA, United States

Yes indeed; cost of an RS232 port is about $0.30 (MAX232A) whereas the cost of an ARINC429 output is in the area of $10, or more if you are using one of the “easy” chips, and this is relevant when building say a 430 or a 530 whose total parts cost is c. $300.

Unfortunately it has resulted in the industry having to use a “secret” Garmin RS232 data stream, which presumably they license from Garmin. I can see why Garmin did it, but Avidyne didn’t have to do that.

Another thing is that while RS232 is supposed to have just one load on it (in reality a “few” will work) a single ARINC429 transmitter can drive 20 loads, and a single transmitter can transmit every label required. At at high speed mode, it takes about 300us to transmit one 32-bit packet, so even if you are trying to do 50/second that is only 15ms per second i.e. a 1.5% duty cycle. Even at the slow speed, it is only a 13% duty cycle. So one port can transmit a load of data to all the various devices.

Incidentally, is there a spec somewhere on the position accuracy for ADS-B IN? I know it has to be a WAAS (SBAS) GPS source, but IIRC it does not have to be receiving SBAS all the time.

Administrator
Shoreham EGKA, United Kingdom

There is a specification for ADS-B Out when flying inside the rule airspace, but that does not apply outside of rule airspace or to ADS-B in, In the US regulations, it is specified as:

(c) ADS-B Out Performance Requirements for NACP, NACV, NIC, SDA, and SIL—

(1) For aircraft broadcasting ADS-B Out as required under Sec. 91.225 (a) and (b)—
(i) The aircraft’s NACP must be less than 0.05 nautical miles;
(ii) The aircraft’s NACV must be less than 10 meters per second;
(iii) The aircraft’s NIC must be less than 0.2 nautical miles;
(iv) The aircraft’s SDA must be 2; and
(v) The aircraft’s SIL must be 3.
(2) Changes in NACP, NACV, SDA, and SIL must be broadcast within 10 seconds.
(3) Changes in NIC must be broadcast within 12 seconds.

These requirements can be met more than 95% of the time by a non WAAS position source. There is no requirement for WAAS per se, but WAAS does meet the requirement a very high percent of the time. NACP and NACV are the 95% accuracy values. NIC is the 99.99999% integrity containment radius. Any ADS-B Out system is expected to broadcast whatever these values are at the time in the form of 16 category codes such as NACp of 8 is an accuracy between 30 meters and 92.6 meters and meets the minimum accuracy requirement of 0.05 NM in rule airspace.

KUZA, United States

I’d say the requirements can be met 100% of the time by a non WAAS position source

The SIL=3 requirement differs from the SIL=1 mentioned elsewhere, and renders SIL=1 emitters invisible, but we’ve done that one to death in other threads

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I’d say the requirements can be met 100% of the time by a non WAAS position source

They can as long as all the required data is provided.

KUZA, United States

Peter wrote:

The SIL=3 requirement differs from the SIL=1 mentioned elsewhere, and renders SIL=1 emitters invisible, but we’ve done that one to death in other threads

It’s SIL=0 that’s invisible. SIL=1 from something like a Trig TN72 TABS GPS will be visible on certified receivers.

Avionics geek.
Somewhere remote in Devon, UK.
Sign in to add your message

Back to Top