The XML data output on port 7879

[feraudyh]

Hello All,
  I've got a few questions about the data that appears as XML data on port 7879. I dont come from the aviation business and I have been asked to write software so my questions may appear naive.
Is the MODES field a string representing a positive integer using hexadecimal coding?
Is the CALLSIGN field a string of with a length that is bounded by a well known value like 7?
Is the ALTITUDE a decimal  string representing a positive integer. Is it measured in feet?
Are longitude and latitude expressed as decimal real values so that 60.5 represents 60 degrees and 30 minutes?
Are ground speed and airspeed always in knots? These are integers?
It seems that airspeed can be missing, but perhaps groundspeed is always reported.
Is Vrate in feet/seconds? Can it be negative for a plane that is descending? Is that represented by a "-" prefix?
Oh and one last thing: who is responsible for assigning the TRACK number?

thankyou
Henri

[feraudyh]

I should add that the reply to the FAQ
here
http://www.airnavsystems.com/forum/index.php?topic=9.0
does not correspond precisely with what I am receiving on my Terminal that is receiving data from the 7879 port. For instance the Squawk field I see on my terminal is is not mentionned on that page.
Actually I dont know what it means, if anyone can help me.

But worse, much worse is the fact that Bearing is not displayed on my terminal.
This means that we shall have to dump this product.

[AirNav Support]

feraudyh,

If your thinking of creating an app from the data best to try and read up on some aviation knoweldege otherwise some of the terms will be confusing.

To answer your questions:

Mode S -  Its not representing a integer, its a unique identifier for aircraft
Altitude - This can be negative in a few cases (Amsterdam is an example). It is in feet. It won't have decimal values.
Lat/Lon - Its the decimal values, not as your suggest above though.
Grounspeed/Airspeed - In kts
Vrate - This is in feet per minute. And yes it can be negative and has the minus sign.
Track - This is the bearing I think your refering to i.e (the heading of the aircraft)
Squawk - This is a number assigned by ATC for identification in a controllers area.

[feraudyh]

Thanks for your helpful answer,
  What a relief to know that Track means bearing/heading (or route?). I suppose it means the
real direction (tangent to the actual path of the plane) so that wind on the plane is taken into account.
  I  have just worked on AIS, NMEA data for the navy. Longitude and Latitude are not coded with simple a decimal representation but with a coding that makes the minutes appear explicitly. Is this the case here?
Do you have any suggested reading material? The English Wikipedia article is not terribly helpful.
Henri

[feraudyh]

A chap from the office next to mine suddenly came to me and said he was a former air-traffic controller,
so I wont need you to provide me with the appropriate reading list.

[feraudyh]

When I look at http://adsb.tc.faa.gov/WG5_Meetings/Changes/UAT-App_C.pdf
it seems to me that the ModeS call  is a 6 character hex-coded integer.
Of course I can leave it as a string, but I can also code it as an int.

[feraudyh]

Lat/Lon - Its the decimal values, not as your suggest above though.

I suppose that the English of this sentence should be
They are decimal values but not as you suggest above.

So please tell me, is it minutes divided by a thousand (as in AIS)
or is it as in the following link:
http://notinthemanual.blogspot.fr/2008/07/convert-nmea-latitude-longitude-to.html
Thankyou.

[feraudyh]

Another possibility of encoding that comes to mind is that say "67.30" means
67 degrees and 30 minutes.

Is this what you use?
thankyou
Henri