ADS-B question

[Pilot DAR]

I'm an ADS-B newbie. Can someone tell me: Is the "Groundspeed" reported on ADS-B output the speed across the ground? or, the indicated airspeed? If the airplane is in a very steep dive (like near vertical) the "airspeed" and "groundspeed" will be very different values, which is reported in the output? (is it ASI based? or GPS based?)

[The Ancient Geek]

Reported groundspeed is GPS derived speed.

[Jan Olieslagers]

[[ excuse me for linguistic nitpicking ]]
It is not from the ASI, it is satellite derived. However GPS is only one satellite constellation among a handful, better to use the generic term GNSS.

Myself wonder about the reported altitude - I suspect that this will be satellite derived, too, but probably with correction for QNH?

[Pilot DAR]

Reported groundspeed is GPS derived speed

Okay, I should ask my question a little differently then... Is the GPS (or GNSS) derived [ground] speed considered in two dimensions - parallel to the earth's surface? Or in three dimensions, considering an altitude change as an element in the speed? If the plane is diving at a severe angle, is the ADS-B reported speed considering the vertical speed as an element in the "groundspeed" in the third dimension? IN the extreme, if a plane dived vertically, would it indicate any "groundspeed" on the ADS-B output?

[First_Principal]

As with a lot of these things it seems somewhat more difficult to get the information than it should be ... however, from CS-ACNS (issue 4):

The ADS-B Out system provides the following minimum set of data parameters:
(1) Aircraft Identification;
(2) Mode A Code;
(3) ICAO 24-bit aircraft address;
(4a) Airborne Horizontal Position — Latitude and Longitude;
(4b) Airborne Navigation Integrity Category: NIC;
(4c) Airborne/Surface Navigation Accuracy Category for Position: NACp;
(4d) Airborne/Surface Source Integrity Level: SIL;
(4e) Airborne/Surface System Design Assurance: SDA;
(5) Pressure Altitude (incl. NICbaro);
(6) Special Position Identification (SPI);
(7a) Emergency Status;
(7b) Emergency Indication;
(8) 1090 ES Version Number;
(9a) Airborne velocity over Ground — (East/West and North/South);
(9b) Airborne/Surface Navigation Accuracy Category for Velocity: NACv;
(10) Emitter Category;
(11) Vertical Rate;
(12a) Surface Horizontal Position — Latitude and Longitude;
(12b) Surface Navigation Integrity Category: NIC;
(13) Surface Ground Track;
(14) Movement (surface ground speed);
(15) Length/width of Aircraft;
(16) GPS Antenna Longitudinal Offset;
(17a) Geometric Altitude; and
(17b) Geometric Altitude Quality: GVA.

Where available in a suitable format, the ADS-B Out system provides the following data
parameters:
(1) Selected Altitude;
(2) Barometric Pressure Setting; an

Thus, I think 9(a) strictly answers your question? However along with some of the other transmitted information (+ historical data where needed) this should allow a receiver to derive (and display if required) the aircraft actual vector+velocity fairly simply.

That's the way I see it from the information I've read to date, but I say that from first principles - someone with experience in the field may be able to provide a more useful real-world answer.

FP.

[TheOddOne]

Our ADS-B 'out' derived from our Garmin transponder gives GNSS-derived speed over the ground and GNSS-derived altitude referenced to World Geophysical Survey 1984 (WGS84). This is somewhat different from baro-derived altitude which is also different from 'real' altitude.This is in the UK with no enhancements the raw ADS-B as other places benefit from. I guess velocity can be derived from the track shown on various web sites that track ADS-B.
Other more sophisticated ABS--B units can give more information, but nowhere near as much as Mode Select (known as Mode 'S').
When you look at some of the accident analysis both from Youtube people such as Juan Browne and official sources, they sometimes show a timeline of horizontal and vertical speeds derived from ADS-B, but as separate traces. The recent tragedy involving test flying a C208 is a case in point.
TOO

[India Four Two]

Minor point: WGS is World Geodetic System.

[wrench1]

Can someone tell me: Is the "Groundspeed" reported on ADS-B output the speed across the ground? or, the indicated airspeed?
ADS-B can report up to 4 speeds. Here is the simple explanation:
https://www.adsbexchange.com/ads-b-data-field-explanations/
https://blog.flightaware.com/speed-values-in-ads-b-data-gs-ias-tas-and-mach'

Here is a more technical explanation:
https://mode-s.org/decode/content/ads-b/5-airborne-velocity.html

[Pilot DAR]

Thanks Wrench,

From the link you provided:

Subtypes 1 and 2 are used to report ground speeds of aircraft. Subtypes 3 and 4 are used to report aircraft true airspeed or indicated airspeed. Reporting of airspeed in ADS-B only occurs when aircraft position cannot be determined based on the GNSS system. In the real world, subtype 3 messages are very rare.

Sub-type 2 and 4 are designed for supersonic aircraft. Their message structures are identical to subtypes 1 and 3, but with the speed resolution of 4 kt instead of 1 kt. However, since there are no operational supersonic airliners currently, there is no ADS-B airborne velocity message with sub-type 2 and 4 at this moment.1 (https://mode-s.org/decode/content/ads-b/5-airborne-velocity.html#fn1)

These messages contain more information than just horizontal and vertical velocity. Two other significant types of information are the navigation uncertainty category for velocity, and the difference between the GNSS height and barometric altitude.


GNSS height and baro altitude were reported, and the aircraft position based upon GNSS system was known and reported, and it was not a supersonic aircraft. So, from my inference of the information above, sub-type 2 & 4 speed information would not have been reported, as it was not supersonic, and sub-type 3 speed information would not have been reported, as the GNSS position was known and being reported. So we're left with sub-type 1 information, which is a GNSS derived "groundspeed" (IAS/TAS not being reported at all). Do I have it right so far? And, vertical speed (down) was reported, I presume from the GNSS position.

What remains unclear to me, is the "groundspeed" being reported, two dimensional, parallel to the surface of the earth? Or three dimensional over the earth? If the aircraft is in a very steep dive, the two dimensional groundspeed value would be artificially slow, and no longer useful information in the context of "did the plane exceed Vne?" If the ADS-B GNSS based "groundspeed" is actually a speed based upon a three dimensional calculation, it probably equates to IAS, at least roughly, and now has value in understanding how the airplane saw the speed relative to it's limitations.

The last sentence of the linked article says:

It is possible that with strong tail wind, the ground speed of an aircraft may appear to be “supersonic” during the flight.

Which makes me think that in two dimensions, ADS-B reported "groundspeed" and IAS could be quite different, but I don't know how to infer a vertical speed component in that.

[wrench1]

What remains unclear to me, is the "groundspeed" being reported, two dimensional, parallel to the surface of the earth?
The link is more than an article, its a complete book. The link below has an index of all chapters which may possibly contain your answers. If not, perhaps reach out to the author for those answers? I follow the theory side mainly to assist with troubleshooting plus had a unique opportunity to learn the workings of ADSB when the company I worked for participated in the FAA's 2nd ADSB critical service test back in 2009 or 10. Beyond that I don't have much to offer except point you in the different directions for your possible answers.
https://mode-s.org/decode/index.html

[First_Principal]

It's worth remembering that velocity and speed are different things, sometimes these can be confused or used interchangeably where they probably shouldn't.

I'd tend to defer to the standard I posted above which, to me, suggests that ADS-B out reports horizontal velocity E-W and N-S separately to vertical rate and altitude. Given it specifically states 'horizontal' then I'd take that as meaning the two-dimensional velocity over ground. To me this makes sense (that you'd transmit the raw data as much as possible with minimal or no processing), and if it's solely GNSS based then that's what you'd expect.

Given the data stream is quite rich (also reporting Z / positional components) it's then trivial for a receiving system to determine and supply the 3-dimensional aircraft velocity in space. IOW yes you could determine the speed + direction (and therefore velocity) in a dive.

However this is the derived velocity in space, which is not necessarily the airspeed of the 'craft, and thus the VNE question might remain unanswered if it weren't for other general data that you could also incorporate. Such data might come from area met reports, or aircraft local to the one in question that may also report IAS/TAS (if the subject craft didn't), but as I see it 'simple' ADS-B out by itself won't actually transmit everything you need to determine if the aircraft exceeded limits.

Once again this is largely from first principles and a quick review of the standard detail I've found, so I'd very much defer to people more experienced in the field than me should an authoritative source arrive.

FP.

[TheOddOne]

Minor point: WGS is World Geodetic System.

Quite right, apologies for my laziness in not double-checking half-remembered things.

​​​​​​​TOO

[Abrahn]

What remains unclear to me, is the "groundspeed" being reported, two dimensional, parallel to the surface of the earth? Or three dimensional over the earth?

I know very little about ADS-B, but I do have some training in geodesy, so may be able to add something useful.

There are multiple ways of measuring speed from a GNSS. One is by comparing two positions and another is by looking at the shift in the GNSS signal itself, which will give a velocity relative to the satellite (and therefore will include speed in a vertical dive).

Assuming the first, the difference in position can be calculated using spherical geometry to give a line along the spheroid (i.e. WGS84 for GPS) and converted to a speed either by using the spherical distance or the straight line, depending on which approximation you consider appropriate.

Or you can convert the positions to Cartesian coordinates and work out the distance using 3D Pythagoras.

Only if you decide to ignore height and use 2D Pythagoras will get the "horizontal" component only.

It is unlikely that you'll get speed parallel to the ground (as opposed to a mathematical approximation of the ground) just because that requires a surface elevation model.

I don't know how real world ADS-B transponders do it.

[Pilot DAR]

In my use of the term "groundspeed", I'm thinking of that as movement parallel to the earth's surface, terrain disregarded.

The answer is going to come out in a report later, I was just trying to get my knowledge one step further along...

[Jan Olieslagers]

In my use of the term "groundspeed", I'm thinking of that as movement parallel to the earth's surface, terrain disregarded. That sounds very much like my own interpretation ;) Perhaps a bit more elaborated: "ground speed" might well be defined as the report between the time spent to travel between two coordinate pairs, and the "surface" distance between them, at an agreed "surface" standard like (for one example, but a quite likely candidate) WGS84. Such would be close to the thinking of the marine world, where non-military useage of GNSS orginated. As so often in history, aviators are only following in the track of mariners.

I don't know how real world ADS-B transponders do it. Perhaps there is no general answer - it seems not impossible that one ADS-B set does it one way, and the other does differently.

Or, if they have to conform to certain standards to be certified - as I assume they should - then we should consult those certification standards. Perhaps a DAR has access to them (which I certainly don't)?

[First_Principal]

"Groundspeed" is derived from a horizontal vector with reference to the WGS84 ellipsoid (as Abrahn mentioned), but yes Pilot DAR I should think your understanding is, for practical purposes, correct.

Jan Olieslagers There is very definitely a standard! The reference I used earlier to research this was ICAO Aeronautical Telecommunications Annex 10 Vol 4, and CS-ACNS (as reported above).

wrench1 posted some useful links that give good explanations around ADSB velocity data, these are definitely worth a read. Further useful information can also be found in papers such as Modeling ADS-B Position and Velocity Errors for Airborne Merging and Spacing in Interval Management Application (Satish C. Mohleji and Ganghuai Wang), another description of ADS-B airborne velocity here (https://1library.net/article/mode-s-extended-squitter-airborne-velocity-reg.zljnwggy), GNSS time reference info here (https://gssc.esa.int/navipedia/index.php/Time_References_in_GNSS), and geodetic detail here (https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/).

While Pilot DAR hasn't said exactly what this is about I've assumed a light a/c that doesn't transmit extended information?

FP.

[Pilot DAR]

While Pilot DAR (https://www.pprune.org/members/150561-pilot-dar) hasn't said exactly what this is about I've assumed a light a/c that doesn't transmit extended information?

Have a look at this thread, and the dots will join up:

https://www.pprune.org/accidents-close-calls/649928-cessna-c208b-down-near-seattle-4-deceased.html

From the ADS-B information presented in the link, either the Caravan was flying one knot faster than its 175 KIAS Vmo, at a hell of a rate down, or, if the descent rate is not factored into the ADS-B "groundspeed", it was flying at an IAS significantly faster than Vmo. In either case, the plane could not stand what happened to it.

[First_Principal]

Ah, thanks, I'd seen that thread but hadn't noted your particular interest in it - I can understand why you'd want to know more.

Initially it appears there's some further (useful) detail given on the ADS-B Exchange page, but a quick manual calculation just on the displayed altitude and time intervals shows a possibly different rate of descent than reported, which suggests to me it may be interpreting the RoD from the full set of data (and/or there's further data inputted, but which may be discarded for clarity of display).

As I've (locally) done a bit with the tar1090 code that ADS-B Exchange runs on, I'll see if I can find some time to have a quick look around what calculations it may be doing with the raw ADSB-out data, and from that what it displays...

FP.

[Pilot DAR]

The airplane appears to accelerate from 50 some knots to more than 170 in less than ten seconds. The RoD shown is probably a worst case snapshot during the dive, but it does correspond to the peak RoD I experienced during Caravan spin testing. I won't say that the Caravan has a weakness, 'cause I don't believe it to be so - but, they broke it up in flight, I opine, either over G, overspeed, or a combination. I hope to learn which, or what proportion. I'm hoping that the investigation, and final report are able to extract G information from some [hopefully] retained data somewhere...

I can say from experience (good, in my case) that a G meter is required equipment if you think you might spin a Caravan. Not knowing how much G you can pull during the dive recovery will likely result in an accidental overspeed, it accelerates really fast when you point it nearly straight down!

[First_Principal]

So I've had a look at the tar1090 and associated code (readsb for example), which has served to remind me a little of my last interaction with the software.

I hasten to say that my past experience and current appreciation of the work put into this open-source software is entirely positive, however I don't think I'd use it for the purposes of accurately determining what happened in the last few moments of this sad event.

For example; while my perusal of the code was necessarily cursory I saw several places where there is some apparent massaging of the speed component, depending upon certain parameters. Thus without detailed examination (along with the raw unfiltered ads-b out data) I'd treat the presentation you've seen as of 'interest' rather than something of serious investigatory value.

Furthermore, while it seems that raw data may be available upon request from ADS-B Exchange I assume at this point that it's still that derived from readsb or similar, and thus may contain some interpretive component(s). Although that's probably perfectly reasonable for most purposes it could muddy the water in extreme events such as this.

That said if someone with good experience of the code came along and showed me an error in my assumptions, and/or how to account for the filtering/extract the most accurate detail from the resultant datastream, I'd be happy to acknowledge that.

Coming to your comments about having a G-meter; do you utilise a purpose-built device (or maybe these days your telephone with some suitable application)? I have a personal interest in asking because I once developed an inexpensive unit to record, plot, and transmit specific seismic events - it could be easily adapted for aviation use if that was at all useful...

Otherwise thanks for the suggestion, but I can assure you I've not thought about spinning a Caravan! My last experience was in a 172, and a 150, and for me that was enough ;-)

FP.