Is it possible to get the reported ADSB-GPS-height?

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aitorC
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Is it possible to get the reported ADSB-GPS-height?

Postby aitorC » December 8th, 2010, 10:03 am

Hello all,

I have access to one SBS-1e, I have some questions however about if there is any GPS Height reported by the ADSB in the aircrafts that the SBS-1e is able to report. I mean, the BaseStation software reports ONLY the Altitude (Barometic Altitude) with a resolution of 25f/100f but it does not report the GPS Altitude/Height (Geometric Height/ Altitude GNSS), I will call it GPS height from now on ;). I read a couple of post in the forums mentioning this but no clear answers.

I read here (http://www.homepages.mcb.net/bones/SBS/ ... t_Data.htm) that the software (BaseStation) does not display the GPS Height but in reallity it can be extracted from the ethernet socket in the packet "MSG 6" (Surveillance ID message) (Is this the socket from the SBS or the socket from the BaseSation software?). There is a message not very well explain there that I quote here:

"BaseStation only displays Barometer altitude but in the data are HAE (height above ellipsoid), which is sent as the difference between GPS altitude and barometric altitude."

I guess that what it means is:

"BaseStation only displays Barometer altitude but in the data there is HAE (height above ellipsoid) (GPS height) available, which is sent in the field 12 instead of the barometric altitude at MSG5,6."

Can anyone that has been playing around with the sockets confirm that there is actually GPS height coming from the SBS-1e but the software never displays it? Does the BaseStation socket provide this data?

PD: Probably you are wondering why I would be interested in this if the accuracy in height is quite bad and probably useless as ATC, ok, I am planning to create a meteorological model based in the data retrieved from the aircrafts with the assumtion of GPS having between 50 and 100m error in height and the aircrafts performing alwways with an accuracy of 200f from the reported flight level (assumed for RVSM levels in Europe), some statistics, some filters and the met could be studied without the use of weather balloons??? Will se ;)

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Re: Is it possible to get the reported ADSB-GPS-height?

Postby DaveReid » December 8th, 2010, 10:58 am

If baro altitude was as inaccurate as you suggest, then we would see lots of mid-air collisions and level busts :)

You are correct in that the ADS-B MOPS provide for Format Types 20-22, which are Airborne Position transmissions using GNSS HAE instead of baro altitude. However I have never seen any of those, so I have no idea whether BaseStation decodes them, and even if it did the socket messages have no provision for differentiating between the two types of altitude report.

aitorC
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Re: Is it possible to get the reported ADSB-GPS-height?

Postby aitorC » December 8th, 2010, 11:44 am

DaveReid wrote:If baro altitude was as inaccurate as you suggest, then we would see lots of mid-air collisions and level busts :)


I said 200feet for barometric altitude error reported by the aircraft (with the GPS I guess it will be between 100 and 50m). That 200feet is actually accurate "enough": 60m. Aircrafts move in levels 300m appart (1000feet) in RVSM. I would expect most of them reporting the correct altitude, but dont forget that Eurocontrol has a RVSM monitoring cell (using multilateration systems) to detect aircrafts flying with altimeter system error of around 200feet or more. And if they have it, probably means that it is because there is a small amount of aircrafts arriving to these limits. If someone has access to ICAO annex 6 I think there you can see some of the probabilities of finding an aircraft flying with an altimeter system error of 300feet!.

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Re: Is it possible to get the reported ADSB-GPS-height?

Postby DaveReid » December 8th, 2010, 12:23 pm

Incidentally your weather modelling idea is certainly feasible.

Most airliners report TAS as well as groundspeed, and track of course, so it's not difficult to derive a reasonable estimate of winds aloft if you have enough aircraft flying in different directions.

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Re: Is it possible to get the reported ADSB-GPS-height?

Postby threemiles » December 17th, 2010, 10:46 pm

Eurocontrol has stopped using the height monitoring last year.

Baro altitude is pressure related. HAE is a correction to baro alitutde, the result is height over the ellipsoid WGS-84. Corrections can be significant, tonight I can see -2475 ft offset at 31000 feet, whichs means the actual GPS height is 28525 ft. Local QNH is 997. But what is the use for?

From other Mode-S data I derive winds aloft with amazing precision for every single airplane (crosschecked when the controller asks for the wind at altitude). It needs Hdg, True Track, TAS and GS. All available from Mode-S. Outside temp can also be derived from TAS, altitude and Mach, I would guess.

These data are not available from Basestation, it needs a different decoder.
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aitorC
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Re: Is it possible to get the reported ADSB-GPS-height?

Postby aitorC » January 12th, 2011, 2:49 pm

threemiles wrote:Eurocontrol has stopped using the height monitoring last year.

Baro altitude is pressure related. HAE is a correction to baro alitutde, the result is height over the ellipsoid WGS-84. Corrections can be significant, tonight I can see -2475 ft offset at 31000 feet, whichs means the actual GPS height is 28525 ft. Local QNH is 997. But what is the use for?

From other Mode-S data I derive winds aloft with amazing precision for every single airplane (crosschecked when the controller asks for the wind at altitude). It needs Hdg, True Track, TAS and GS. All available from Mode-S. Outside temp can also be derived from TAS, altitude and Mach, I would guess.

These data are not available from Basestation, it needs a different decoder.


Hi threemiles, I am sorry for this delay, I went back to my country of origin for xmas and I have been out of everything, now finally I am back to my desk with my computer :)

About Eurocontrol stopping height monitoring last year? where did you get that information from? They have 3 monitoring units (HMU): http://www.ecacnav.com/content.asp?PageID=62
And as I said I was in september in their workshop and I have good friends working in the HMUs systems, their systems are up and running today and also I just heard they are planning to upgrade them: http://www.asdnews.com/news/32377/Indra ... tems__.htm

Going back to the subject. You said "Baro altitude is pressure related", I think that is clear.
About HAE, you said is "correction to baro altitude, the result over the ellipsoid WGS-84". And that is definitely not clear to me now. I have read somewhere else that it is actually "GNSS altitude difference from barometric altitude" so knowing the Baro altitude as well you could calculate the GNSS altitude. I am losing a litle bit of hope now because you might be right and basestation could not be the solution for me :(

I am not interested in winds for the moment, I just wanted to have the GNSS altitude from where all the pressure levels can be calculated and from there the temperatures. Next step would be about winds and as you said with GS and AS you can "easily" get the info if you have enough flights around.

Anyone knows anything about this?

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Re: Is it possible to get the reported ADSB-GPS-height?

Postby DaveReid » January 12th, 2011, 4:56 pm

aitorC wrote:About Eurocontrol stopping height monitoring last year? where did you get that information from? They have 3 monitoring units (HMU): http://www.ecacnav.com/content.asp?PageID=62
And as I said I was in september in their workshop and I have good friends working in the HMUs systems, their systems are up and running today and also I just heard they are planning to upgrade them: http://www.asdnews.com/news/32377/Indra ... tems__.htm

The Height Monitoring Results pages on the ECACNAV website are certainly starting to show results for monitoring flights in 2011.

Going back to the subject. You said "Baro altitude is pressure related", I think that is clear.
About HAE, you said is "correction to baro altitude, the result over the ellipsoid WGS-84". And that is definitely not clear to me now. I have read somewhere else that it is actually "GNSS altitude difference from barometric altitude" so knowing the Baro altitude as well you could calculate the GNSS altitude. I am losing a litle bit of hope now because you might be right and basestation could not be the solution for me :(

In the good old days you could ask a technical question like that on the forum here and get an answer from Kinetic :-)

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KineticChris
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Re: Is it possible to get the reported ADSB-GPS-height?

Postby KineticChris » January 13th, 2011, 11:45 am

I have had a chat with our programmers and here is a summary of the reply!

Yes, some aircraft report GPS/GNSS altitude aka Geometric altitude or HAE, as well as the obligatory baro altitude. Basestation does not currently report this, but could do as an enhancement.

It is actually reported as the difference between GNSS and baro altitudes, which allows the GNSS altitude to be calculated from the baro altitude, which is always sent.

HAE is height above the WGS-84 ellipsoid, not the same as height above mean sea level.

It is not used for air traffic control as it is less reliable. Aircraft altimeters are generally pretty accurate and have always been used for height separation. An airliner will have multiple altimeters which can be set for different atmospheric pressures as required. The encoding altimeters used by transponders are always referenced to 1013.25 millibars and allow the controller to check independently that the pilot is flying at their assigned altitude. Hence the altitude reported by Basestation will not in general be the exact height above sea level.

Basestation could correct for this by knowing the current atmospheric pressure (QNH) but the problem is this can vary quite a bit across the reception range of an SBS-1, as well as changing during the day.


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Re: Is it possible to get the reported ADSB-GPS-height?

Postby DaveReid » January 13th, 2011, 12:31 pm

KineticChris wrote:Yes, some aircraft report GPS/GNSS altitude aka Geometric altitude or HAE, as well as the obligatory baro altitude. Basestation does not currently report this, but could do as an enhancement.

That's interesting. Notwithstanding my previous post, where I said I'd never seen any Format Types 20-22 (GNSS HAE instead of baro altitude), it's just dawned on me that many (probably most) ADS-B aircraft send the difference between baro and GNSS altitude as part of the normal Format Type 19 Airborne Velocity squitter (except for non-GNSS types like 757, some 767s, many turboprops, etc). It's sent in 25' increments as the last 8 bits of the packet.

I haven't paid much attention to decoding it to date, but it's certainly there for the asking if you have a utility that captures raw packet data - for example yesterday evening RYR B738 4CA6FE was at FL350 and reporting 325' to 375' difference.

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Re: Is it possible to get the reported ADSB-GPS-height?

Postby bones » January 13th, 2011, 1:48 pm

Basestation could correct for this by knowing the current atmospheric pressure (QNH) but the problem is this can vary quite a bit across the reception range of an SBS-1, as well as changing during the day.


This isn't really an issue. Most aircraft operate on 1013mb once above the Transition Altitude so anything higher than 6000ft can be discounted. The TA varies across UK TMA's with the default being 3000ft but the busier TMA's use 6000ft. Aircraft operating on QNH are therefore reasonably low level and would be within a limited range of our SBS's.

The facility to add local QNH would enhance the display considerably. It is somethng we do on actual ATC radar systems and I consider it to be one of the major omissions from the BaseStation software. The Transition Altitude is already built into the software so it doesn't take a big step to add QNH calculations.

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Re: Is it possible to get the reported ADSB-GPS-height?

Postby Supel » January 22nd, 2011, 2:10 pm

bones wrote:
Basestation could correct for this by knowing the current atmospheric pressure (QNH) but the problem is this can vary quite a bit across the reception range of an SBS-1, as well as changing during the day.


This isn't really an issue. Most aircraft operate on 1013mb once above the Transition Altitude so anything higher than 6000ft can be discounted. The TA varies across UK TMA's with the default being 3000ft but the busier TMA's use 6000ft. Aircraft operating on QNH are therefore reasonably low level and would be within a limited range of our SBS's.

The facility to add local QNH would enhance the display considerably. It is somethng we do on actual ATC radar systems and I consider it to be one of the major omissions from the BaseStation software. The Transition Altitude is already built into the software so it doesn't take a big step to add QNH calculations.


I agree with Bones 100% it's not very nice to see and ac landing at EGSH with a height display of -250ft. I have my screen set to 60 miles I do not wish see any further. The QNH does not change over such a short distance.
BFN John c4.


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