Re: Year 2000 Problem for TLEs??

Bruno Tilgner (
Fri, 22 Aug 1997 16:37:10 -0400

Bjoern Gimle wrote:

>Without looking into the code, I would speculate that NORAD models could=

>suffer from (7), at least for the SDP4 model used for nodal periods =

>over 225 minutes. I can accept Bruno's statement that there is no
>intrinsic fault in the model - it is up to the programmer to feed it
>the correct time difference for the propagation. But I think SDP4
>computes Moon and Sun effects, and thus need an absolute date.
>Although the Sun's position is approximately the same in both centuries,=

>the Moon's (maybe) quite different.
>If this (four-digit) year (or Julian Date?) is the programmers' =

>responsibility, we are back to situation (5).

I have looked into the code in the meantime. Bjoern is quite right. The
SDP4 model needs the absolute date to calculate solar and lunar effects.

To this end, a variable DS50 (Julian days since 1950.0) is calculated
in the routine ThetaG. The way this is done is such that the result will
DEFINITELY BE WRONG for years beyond 1999. Every programmer using this
code is well advised to change it in time. This can be done by a simple
range check of the year. For instance, if  year < 57 (before the first
Sputnik) then it must be in the next century.

Therefore, as far as the deep space model is concerned, I must revise my
opinion expressed yesterday that everything will work fine.

This does not solve the problem of the transition from 2099 to 2100,
but perhaps we can leave this up to our grand-grandchildren to deal with.=

Bruno Tilgner
48.85N 2.02E