Wow~I'm speechless to see your tremendous knowledge. Many thanks.
Too many information to take in, so firstly let me summarize it.
1. ADS-B data can be either picked up by ATC on the ground or by Satellite which are fed to a contracted subscribers(ATC, or Flight Radar24 etc.). The first case requires antennas and processors etc. to display targets while the second case only requires processors to assemble data obtained from data provider such as Aireon.
2. ATC only care about what's written on the filed flight plan. If it is a LAT/LONG coordinate, it is LAT/LONG not ARINC424. However ATC do receive ARINC424 standard notation in reality which they are well aware of.
Okay!
To clarify my previous question, let's take an example in the second last page of NAT OPS Bulletin2018_003(Rev01).
Filed route contains a waypoint 5230N02000W.
FMS can only display this waypoint as N52W020 due to its capability(Max 7 digit)
1. In this case, when ATC picks up FMS route, Half degree is missing. So my understanding is that to avoid this confusion it's better to use a special format that ARINC424 use which includes letter 'H' to represent a half coordinate. I stand here to be corrected.
2. Ultimate question (Reason why I started this thread)
If a filed route contains a waypoint 5200N02030W, FMS can only display this waypoint as N52W020. Again, this is not representing a real coordinate. Unfortunately there's no special format to represent Longitude half degree coordinat in the ARINC424 standard. Pilot always crosscheck the waypoint coordinate to see if the filed coordinate is correctly inserted into this waypoint. However there's no way that ATC can check whether this waypoint is correctly representing a filed coordinate by only looking at the name of the waypoint.
I hope my question is better described this time. Again, any comments are highly welcomed.