For the reasons discussed in Section 3, it is not always possible to use the linear identification theory based upon all 6 orbital elements. The question is how to use the same algorithm on a subset of the orbital elements? The answer is implicit in the arguments presented in [Paper I], Section 2.3, Case 2, which we are going to use without repeating the formal proofs.
Let us suppose that the vector of estimated parameters is split into two
components, along linear subspaces of the parameter space:
Then the uncertainty of E for arbitrary L can be described by
the penalty, with respect to the minimum point E*
Note that the marginal normal matrix CE is not CE and that
to obtain the penalty of the above formula as a function of E, the
value of L has to be changed with respect to the nominal solution L* of
the unrestricted problem, by an amount which is a function of E:
Let us apply this restricted penalty formula to the restricted
identification problem. Let (L1,E1) and (L2,E2) be the nominal
solutions for the two arcs considered separately, and CE1 and
CE2 the corresponding marginal normal matrices. The variables L
are given as function of E by:
By the same formalism of
the previous subsection:
Note that KE is not the same as the complete minimum penalty K of the previous section. The estimate KE of the minimum penalty is obtained by assuming that L=L1(E) in the computation of while L=L2(E) in the computation of . Thus there is, in general, no complete solution with a single X=(L,E) to be fit to the observations of both arcs with penalty KE, but such value is obtained by using (L1(E0), E0) in the first arc, (L2(E0), E0) in the second arc, E0 being the proposed restricted identification.
We claim that : KE is the minimum of the penalty over the space of variables (E,L1, L2), while K is the minimum of the same penalty over the same space but with the additional constraint L1=L2, and the minimum of a function can only increase when constraints are added.
In conclusion, the proposed restricted identification E0 is not a complete identification, and the corresponding minimum penalty KEis not the full penalty to be paid to achieve a full identification. This procedure is, however, a good way to filter the possible identifications because : if a couple can be discarded as a possible identification with the restricted computation, because KEis too large, then it does not need to be tested with the complete algorithm.
Note that it is also possible to define a constrained identification algorithm, based upon the conditional covariance matrices and the algorithm for constrained optimization on linear subspaces, as outlined in [Paper I], Section 2.3.