[Orekit Developers] Extracting TAI-UTC from IERS bulletin A

[MAISONOBE Luc <luc.maisonobe@c-s.fr>]

Hi all,

I had recently a request by an operational project to extract TAI-UTC  
from bulletin A
files which are already managed by the project rather than extracting  
them from
UTC-TAI.history.

I have done it and it is available as a new implementation of the  
existing UTCTAILoader
interface. However, I am clearly unhappy with this.

The problem is that there are known issues with TAI-UTC values in some  
bulletin A. One
issue was in bulletina-xix-001.txt from 2006-01-05 has a wrong year  
for last leap second
and bulletina-xxi-053.tx from 2008-12-31 has an off by one value for  
TAI-UTC on MJD 54832).
The Earth Orientation Department at USNO told us this TAI-UTC data was  
only provided as a
convenience and this data should rather be sourced from other official  
files. They
refered me to bulletin C, but there are also UTC-TAI.history at Paris  
observatory or
tai-utc.dat at USNO.

As the bulletin A files are a record of past publications, they cannot  
modify archived
bulletins, hence the errors above will remain forever.

The issue from 2008-12-31 seemed to be due to the limit between the  
rapid data and the
prediction section being exactly on the day of a leap second  
introduction. According to
bulletin A publication schedule, this may well happen again in the  
case a leap second
were introduced on 2016-07-01 (which we don't know yet).

Another problem is that bulletin A are not available far in the past  
(I could only find back
to 2005) and I am not sure projects that use them keep the full  
history, whereas UTC-TAI.history
and tai-utc.dat both preserve all data in one single file.

So I would prefer to remove this feature I just added, and only give  
this UTCTAILoader
implementation to this project (the project specifications mandate  
this use) and not keep
it within Orekit as it could be misused and is not reliable.

On the other hand, I will probably add support for tai-utc.dat, it is  
pretty straightforward.

What to you think?

best regards,
Luc