Peter wrote:
Does ADS-B radiate baro altitude and/or GPS altitude, and how does this vary between certified and uncertified emitters?
ADS-B broadcasts both Baro pressure Altitude (flight level) in the position message and the difference between Baro Altitude and Geometric altitude in the Velocity message. That way, both are available. If the Baro Altitude fails, the Geometric altitude can be broadcast in the position message. The source of the altitude is indicated in the message. With ADS-B, the source for the Baro altitude must be the same as the one used in the transponder. With a portable device, there is not necessarily a baro altitude and if their is, it is likely to be a cabin altitude which will not match the transponder altitude. Altitude is provided in increments of 25 feet, from -1000 to just over 103,000 feet. In the US, we can have a transponder that is mode A/C and a UAT device that is ADS-B Out. Some systems receive the transponder reply to get a matching value for pressure altitude. Geometric altitude can vary from pressure altitude by quite a bit, at higher altitudes this can be in the order of thousands of feet.
Dimme wrote:
ADS-B provides altitude, aircraft flight ID and vertical air speed. Will it also furnish horizontal air speed?
These are the items that ADS-B provides:
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;
(2) An indication of the aircraft’s latitude and longitude;
(3) An indication of the aircraft’s barometric pressure altitude;
(4) An indication of the aircraft’s velocity;
(5) An indication if TCAS II or ACAS is installed and operating in a mode that can generate resolution advisory alerts;
(6) If an operable TCAS II or ACAS is installed, an indication if a resolution advisory is in effect;
(7) An indication of the Mode 3/A transponder code specified by ATC;
(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;
(9) An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference;
(10) An indication of the aircraft’s “IDENT” to ATC;
(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;
(12) An indication of the aircraft’s emitter category;
(13) An indication of whether an ADS-B In capability is installed;
(14) An indication of the aircraft’s geometric altitude;
(15) An indication of the Navigation Accuracy Category for Position (NACP);
(16) An indication of the Navigation Accuracy Category for Velocity (NACV);
(17) An indication of the Navigation Integrity Category (NIC);
(18) An indication of the System Design Assurance (SDA); and
(19) An indication of the Source Integrity Level (SIL).
Silvaire wrote:
The traffic display on a portable IN device in the cockpit provides relative altitude for other traffic, so it wouldn’t seem to matter what altitude baseline is used for OUT as long as it’s the same between aircraft. I think it’s GPS data, all ADS-B OUT transmitters have a GPS receiver, no? It works pretty well in my experience, better than relying on ATC. When I make visual contact it always seems to be at the expected relative altitude.
Relative altitude is based on baro pressure altitude of own ship verses the target baro altitude. This will match the relative altitude of a TAS or TCAS system. One can fall back to comparing Geometric Altitudes if Baro altitude is not available, but this can result in small errors, like 100 feet or so. If one ship has baro pressure altitude and the other has geometric altitude, there can be large differences and the two are equivalent to comparing apples with oranges. Differences can easily be over 500 feet and at higher altitudes can be in the thousands of feet. To convert Geo altitude to an equivalent MSL, you need to adjust for the local gravity. The yields a GPS based true MSL altitude sort of equivalent. To use baro pressure altitude, you need to first convert to MSL by adjusting for the local barometric setting and then adjust for the temperature at the local barometric setting location and its altitude, finally you have to determine an estimated true MSL altitude based on your height above the baro setting measurement point. Now you can compare the two true MSL altitudes for relative altitude differences. Lots more error.
Dimme wrote:
These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.
It really only allows for a test of reasonableness of the two altitude sources, since the accumulated differences can be on the order of 500 to thousands of feet.
NCYankee wrote:
Relative altitude is based on baro pressure altitude of own ship verses the target baro altitude.
OK, thanks for that.
I think you explained this above, but do I properly understand that for example a non-connected UAT Uavionix tailBeacon is receiving barometric altitude data transmitted by a plane’s KT76 Mode C transponder and retransmitting it as ADS-B OUT altitude data to be received directly by another nearby plane (no ground station in play)?
This basically means that anyone running a “portable” ADS-B OUT device in a plane without a transponder is going to be radiating a baro altitude which is useless for traffic avoidance.
And the receiving device cannot possibly know that it is useless. If it did, it could use the GPS altitude instead (if the above plane was radiating that also) but does that happen?
Do all portable ADS-B OUT products flag their radiated baro altitude as invalid whenever they are not receiving own-ship transponder Mode C altitude?
The “useless” bit works in both directions. If you are running a box which gets “baro altitude” from the cockpit air, and it is telling you there is a Mode C/S contact at the same altitude (and you won’t get any other data on any portable device relative to a Mode C/S target, because none of them do azimuth or distance) that warning is nearly useless, because while the target is radiating a hopefully accurate pressure altitude, the one at your end is uncertain. And there won’t be a GPS altitude to work with because Mode C/S doesn’t radiate that.
Silvaire wrote:
I think you explained this above, but do I properly understand that for example a non-connected UAT Uavionix tailBeacon is receiving barometric altitude data transmitted by a plane’s KT76 Mode C transponder and retransmitting it as ADS-B OUT altitude data to be received directly by another nearby plane (no ground station in play)?
As I understand it, they sense the voltage variations during the transponder transmissions. There was a Garmin patent that claims to receive the 1090 messages to determine this and there is or was a law suit regarding infringement. That lawsuit was settled, so regardless of the technique used, the uAvionix solution determines the altitude from the Mode A/C transponder, so the pressure altitude broadcast by the tailBeacon and the KT76 transponder remain congruent.
Thanks again for the explanation… clever stuff.
The Uavionix tailbeacon is UAT, and US only.
I wonder how this works in Europe, and particularly where there is no transponder. A lot of people are being pushed into getting one of the “portable” boxes, and are hoping that this will save them installing a transponder. So there can’t be any useful baro altitude data being emitted. The Q, I guess, is whether GPS altitude is being emitted instead, and the other end is able to select that.
A related Q is whether the transmitting device has a config option to not transmit what is known to be an inaccurate baro altitude.
I am surprised that Garmin sued, because the practice of picking stuff off the transponder transmission is quite old. The Avidyne TAS6xx boxes have always been doing that. Not extracting pressure altitude though; the TAS gets it via gray code from the aircraft wiring.
The SkyEcho 2 has an onboard barometer as I understand it – which is why it works with SkyDemon when most others do not
