ADS-B IN/OUT TECHNICAL QUESTIONS

[Bennett]

1. Original attempt to move this post here seemingly disappeared

 

2.  I think this forum may be the better place for this thread

 

I have been extolling the virtues of traffic on ADS-B in/out, but I do have some technical questions that I am sure some of gurus on this forum can answer.

One: I assume that all participating "out" aircraft report their altitude via the blind encoders (29.92") as opposed to their own barometric altitude, and that ATC corrects the encoder altitude by their local barometric models. If this is the case, then all participating aircraft can believe the relative altitude differences. Is this true? If true, then aircraft that have set their barometric altitude incorrectly (below 18,000') are not a problem as to relative altitude.

Two: The GTN 650/750 units have the ability to set their map orientation to Heading, Track, or North. I prefer Heading. Suppose I am showing a Heading of 300 degrees (magnetic), and the strong winds give me a track of 320 degrees. The traffic display on my GTN 750 Traffic page or overlay shows an aircraft 4 miles away, at the same altitude, and about 20 degrees off the nose to the left. I assume that the ADS-B in/out reports the aircraft's track in magnetic degrees, and my magnetic track to him. Where should I be looking to spot that other aircraft? Would I be better off using Track up for the GTNs for traffic? Does it make a difference if I select Relative to Absolute on the Traffic page as to his azimuth relative to me? 

Three: Is it true that NOAA and National Weather report wind directions in True, and not magnetic? What about the wind barbs in NextGen via ADS-B? 

Thank you for the replies. 

 

[chrisk]

From aviationweather.gov

 

All levels through 12,000 feet are true altitude (MSL). The levels 18,000 feet and above are pressure altitude. Wind direction is from true north. No winds are forecast within 1,500 feet of station elevation. Also, no temperatures are forecast for the 3,000 foot level or for any level within 2,500 feet of station elevation. Temperature is in whole degrees Celsius and assumed to be negative above 24,000 feet.

[Bennett]

Thanks Chrisk, but how is True Altitude determined for traffic purposes?  And what happens between 12,000' and 18,000'? 

Obviously I am concerned that the + and - altitude differences shown on ABS-B in/out traffic displays are consistent for all participating (and non-participating aircraft). Please see my assumption about the use of blind encoders using 29.92", corrected by ATC through their local models.  

[carusoam]

Bennett,

I am thinking track up will generate a better warning for traffic, no matter which direction or accuracy things have.

The danger zone will always be displayed in the same place on the screen.

Less interpretation required. Less opportunity for a mistake to creep in.

Ipad1 users: check if you get track up as an option. I believe the I1 may be too slow to handle this feature....

Looking forward to the discussion on accuracy and calibrations...

Best regards,

-a-

[Bennett]

I've always used heading up, where available, so that my "flight prediction line", or whatever it is called (I use 10 minutes) shows a crude wind correction, but track-up would also work.  Easy to change on the GTN units, and Foreflight, I believe, now allows track up display. 

 

I think my questions are valid as few of us had had much experience with ADS-B in/out yet, but I believe that many more will participate well before the 2020 mandate. 

[N601RX]

I'm not sure about the ADS-B, but the Monroy system picks up the other planes transponder altitude, which is uncorrected and the picks up your own transponder signal which is also uncorrected and compares the two.

[Bennett]

I sure don't want to nit pick here, but I do think it important that we all understand what altitude data is being transmitted to/from participating aircraft.  I think that your "uncorrected transponder" altitude is what blind encoders transmit (Baro at 29.92"), irrespective of what your aircraft's altimeter shows, using your barometric settings. At least I hope that this is what is happening as relative altitude is a vital component of traffic alerting.  

[N601RX]

Yes, by "uncorrected transponder" I mean what blind encoder is sending to the transponder. The Baro correction takes place on the ATC side of things.

[Piloto]

As per ICAO and the FAA all altitude reported by an aircraft transponder (Mode S, Mode C, ADS-B out) is uncorrected pressure altitude reference to 29.92" or just pressure altitude. This assures that all aircraft relative to each other will be on the same reference irrelevant of altitude or altimeter setting. This assure proper separation by TCAS and ATC radar. For traffic below 18,000 over land the controller reads the corrected altitude on his screen but the radar algorithm and TCAS use uncorrected pressure altitude to determine vertical separation and conflict. Over water beyond 100nm of the nearest FSS pressure altitude (29.92") is used for all altitudes. The onboard source for pressure altitude is the altitude encoder or an air data computer. GPS can not provide this altitude. There could  be a difference of 1,000 feet between pressure altitude and GPS altitude due to the variability of the atmosphere (pressure and temperature).

 

If you are using a conventional TCAS (ARINC 735A) all the traffic indicated on the TCAS window is relative to the nose of the aircraft, weather you are standing still (hovering or on the ground) or moving. Conventional TCAS systems employs directional antennas that determine the actual angular position of the traffic in relation to the nose of the aircraft. On the other hand most ADS-B traffic sensors do not employ directional antennas so they rely on the traffic reported Lat/Long. The ADS-B sensor needs to know your (GPS) location and heading to locate the traffic relative to the nose. In abscense of a heading source most of this portable ADS-B sensor use ground track or only show traffic on a north oriented map. Ground track only will not show traffic proper orientation on the ground or when hovering or going backward on helicopters.

 

José      

[Bennett]

Thank you Jose. That validates my understanding of ADS-B altitude, and thus I can trust the + - altitude differences on the Traffic screens. I still have some concerns about relative azimuth for ADS-B, since we do not have directional antennas, even in the permanent Gamin GDL 88 installations. As mentioned in my initial questions I am assuming that GPS track is used for reported directions for all aircraft, which brings me to how traffic is presented in Heading Up moving map displays. I would hate to have all this data and still be looking (eyeballs) in the wrong place. Finally, does ABS-B NexGen winds aloft display true or magnetic directions. Here in the Bay Area we have a magnetic variation of approximately 14 degrees East, which is significant in determining crosswind and heading components. Hard for these ancient fingers to type on the iPhone. Don't know how the teenagers use their thumbs to type.

[Piloto]

Thank you Jose. That validates my understanding of ADS-B altitude, and thus I can trust the + - altitude differences on the Traffic screens. I still have some concerns about relative azimuth for ADS-B, since we do not have directional antennas, even in the permanent Gamin GDL 88 installations. As mentioned in my initial questions I am assuming that GPS track is used for reported directions for all aircraft, which brings me to how traffic is presented in Heading Up moving map displays. I would hate to have all this data and still be looking (eyeballs) in the wrong place. Finally, does ABS-B NexGen winds aloft display true or magnetic directions. Here in the Bay Area we have a magnetic variation of approximately 14 degrees East, which is significant in determining crosswind and heading components. Hard for these ancient fingers to type on the iPhone. Don't know how the teenagers use their thumbs to type.

On the G530 mag heading input is required to display traffic on the moving maps. I assume the same is for the GTN-750. The winds maps that I have seen all are referenced to the true north like this one http://aviationweather.gov/iffdp/fdwndh But this may not be the case for winds on the ground at a local airport since the runway alignment is referenced to the magnetic north. Either way look for the wind sock.

 

José

[Bennett]

Jose, I agree that NOAA and National Weather, etc. report True North for winds aloft, and that local airports report magnetic directions in their ATIS, or equivalent.  I am just curious if WM weather and/or NextGen wind barbs show magnetic direction or true, and if the winds shown while the display is set to either Heading or Track shows relative or absolute.  I've combed through various Pilot's Guides (GDL 88 etc.) and while I am probably missing the right section, I cannot find information related to these questions. 

 

I believe that the GTN series uses underlying magnetic headings for any of the three display modes: Heading, Track, or North. The early software versions had a problem with their internal magnetic/true tables for the US. Furthermore, Garmin had a bulletin for the GTN series requiring the disablement of the magnetic headings sent from the Bendix/King/Allied/ remote flux compass. This latter problem was addressed in one of the newer software releases, but I, and some other users, still have a "yawing" airplane icon related, I believe, to Garmin's still faulty internal magnetic/true tables across the US.  I show less yaw in Texas than California, roughly proportional to the difference in magnetic variation. 

 

I think you can see why one would like to have answers to the questions posed. I will try again to talk to knowledgeable Garmin engineers (if I can find any) at Oshkosh, although the last time I tried this, all I got were shrugs and sales pitches. Very frustrating as I think that these are simple, but important, questions.

[Piloto]

Bennett, common sense tells me that anytime the wind barbs are overlayed over a map they have to be true north referenced, just like  the longitude lines and the topography. Otherwise the winds barbs will not be in correct alignment with the map topography. Even the windsock shows the winds reference to the topography thus the true north.

 

There is no real advantage on getting mag heading into the GTN-750 unless you are flying a helicopter. Even for TCAS the typical map range setting is to big to discern nearby traffic location. This is why traffic is also displayed on the small inset window at 6nm range.

 

José 

[Bennett]

Thanks. That is in line with my thinking as well. I just couldn't find any documentation to that effect. I am always fascinated how much information is not available from Garmin - large .pdf User and Pilot's guides that leave out vital information. Probably written by engineers, or perhaps worse, tech writers.