Satgen487 Keps2000 Questions etc  by GM4IHJ  (BID SGEN487)  1998-07-24

Keplerian orbital elements are a familiar part of amateur satellite
tracking . They are the things you have to change in your tracking
software every 2 months, or so , in order to follow your favourite
satellite. But there is a lot more to them than this . Indeed they can
tell us a great deal, as with the recent Zenith launch of six
satellites by the same rocket.

You can distinguish this sextet one from another , because while they
all carry the same International Designator made up of the year number
and launch of that year number Eg 98043. This number is followed by a
letter A to F. Where A is the RESURS ;B is FASAT ;C is TMSAT ; D is
Techsat ; E is Westpac and F is Safir. Then if you look at the
individual mean motions you can see how fast they are likely to
separate . While their right ascensions shows the alignment of their
individual orbit planes and, their Epoch times and mean anomalies give
a rough guide as to how they are arranged in orbit ie not always the
same as follow my leader A to F.
So that is the simple arrangement for now. But will it carry through
to year 2000 ? 

Year 2000 millenium problem. The first thing that must change is the
way NASA/NORAD date the elements . Simple 98043 Epoch 98194.202 will
do for now , but most computer software will be stuck, if automatioc
input of Keps after 1 Jan 2000 , is entered as 98043 00001.202.
Clearly something has to change, and the sooner we know how Keps will
be published post day 1 2000, the sooner we can prepare our software
to cope with this. It is all very well having a computer that is 2000
proof , but the present method of using Keplerian Elements is not
going to survive the change, and it looks as if both 2 line elements
and our software will have to change.

Astronomers are of course much more sensible about this . They do not
use this 2 line element format. They quote orbital Ephemeris dated
from either noon 1 Jan 1950 or noon 1 Jan 2000. So all you have to do
is work out the number of Julian days since or up to this Ephemeris
date. Despite this, one regularly sees pleas from radio amateurs
asking for Lunar or Solar Keps, because they do not realise that these
distant heavenly bodies move so slowly that you can predict where they
will be 25 years from now using the published Ephemeris data.

Unfortunately however the Ephemeris data that works for slow changing
Sun , Moon, Planets and Stars, is no good for near earth artificial
satellites . So it is clear that radio amateurs need to start
discussing their particular version of the "Millenium Bug" . We must
find out what NASA/NORAD intend to do or we will have to input all
Keps by hand after 1999-12-31. Equally we must check all tracking
software to ensure that it to will not fall apart , just as we try to
cope, with our post Hogmanay hangovers.