Re: Possible further elset developments

Jim Varney (
Thu, 1 Apr 1999 21:07:44 -0800

On 1 Apr 99, Phil Rogers wrote:

> NORAD's Special Perturbations model is nothing new and is indeed the
> model which has for years been used to generate all NORAD elsets.
> As far as I know, NORAD has never used the General Perturbations
> model in their data generation, but rather has distributed this
> algorithm to all of their sensors to be used for their own orbital
> calculations since it is more compact, faster and easier to use on the
> small to medium scale mainframes commonly used for that purpose.
> The primary difference between the two models is that while the
> GP model is suited to generating elsets which remain valid only for a
> few days, the SP model elsets can still be valid as long as two weeks or
> more after generation, barring any external intervention with a given
> sat's orbit. For this reason, NORAD's elsets are always to be preferred
> over elsets of any other source because of their longevity of life.

The difference between the "special" elsets and the two-lines we all know 
and love is much greater than just their longevity.

I have an unclassified SPACECOM document that talks about this.  It says 
the special perturbations element set is a "numerically derived 'XYZ' 
vector."  It's a state vector.  It also says that the form of the elements 
differ depending on whether the object is LEO or deep space.

If this is correct, then there's no need for SPACECOM to distribute their 
model.  Any numerical integration model can be used with varying success 
depending on the sophistication of the atmospheric and gravitational 
models.  The "increased longevity" does not come from the elsets 
themselves; it comes from the numerical integration method, which can be 
built to consider many more perturbation factors than SGP4 does.

If SPACECOM started issuing state vectors accurate to a meter instead of 
mean TLE's accurate to a kilometer, I would say hooray for them!  Despite 
the short-term pain of some broken software, we should welcome this change 
to higher accuracy in the long term with open arms.

 -- Jim

Jim Varney
Member, Sacramento Valley Astronomical Society
Sacramento Iridiums