[ldd-geom] Vectors_Cartesian_Specific allows multiple instances of the same vector

[benjhirsch]

Describe the issue identified (if applicable)
The Vectors_Cartesian_Specific class allows multiple instances of the same vector with no mandate to disambiguate between them. For example, the class could contain two instances of Vector_Cartesian_Velocity_Target_Relative_To_Spacecraft with different component values, and users/programs would have no way to know which values are correct for what should be a uniquely defined vector.

Describe the solution you'd like
The dictionary should either prohibit multiple instances of the same vector within the class or require that the instances be disambiguated in some way (i.e. different target, different spacecraft).

LDD Dictionary Version
1.9.7.0

PDS4 IM Version
1.19.0.0 and 1.21.0.0 at least

[edguinness]

The dictionary team is working on this request. The fix is probably complicated.

[cgobat]

Does the fix need to be any more complicated than simply changing it from

<DD_Association>
    <identifier_reference>XSChoice#</identifier_reference>
    <identifier_reference>Vector_Cartesian_Position_Central_Body_To_Spacecraft</identifier_reference>
    <identifier_reference>Vector_Cartesian_Position_Central_Body_To_Target</identifier_reference>
    ...
    <identifier_reference>Vector_Cartesian_Velocity_Target_Relative_To_Sun</identifier_reference>
    <reference_type>component_of</reference_type>
    <minimum_occurrences>1</minimum_occurrences>
    <maximum_occurrences>*</maximum_occurrences>
</DD_Association>

to something like

<DD_Association>
    <identifier_reference>Vector_Cartesian_Position_Central_Body_To_Spacecraft</identifier_reference>
    <reference_type>component_of</reference_type>
    <minimum_occurrences>0</minimum_occurrences>
    <maximum_occurrences>1</maximum_occurrences>
</DD_Association>
<DD_Association>
    <identifier_reference>Vector_Cartesian_Position_Central_Body_To_Target</identifier_reference>
    <reference_type>component_of</reference_type>
    <minimum_occurrences>0</minimum_occurrences>
    <maximum_occurrences>1</maximum_occurrences>
</DD_Association>
...
<DD_Association>
    <identifier_reference>Vector_Cartesian_Velocity_Target_Relative_To_Sun</identifier_reference>
    <reference_type>component_of</reference_type>
    <minimum_occurrences>0</minimum_occurrences>
    <maximum_occurrences>1</maximum_occurrences>
</DD_Association>

?

The only possible regression here that I can see is that it would be possible to have an empty <geom:Vectors_Cartesian_Specific/>, whereas currently at least 1 child is required. If that's undesirable, it would be pretty easily fixed with a schematron rule though.