# Satellite submodels

A satellite submodel is one that has a single role arrow that connects to it from another submodel. An example is illustrated in the Role arrow topic. Put simply, the effect of the role arrow is to allow the satellite model to behave in some ways as if it were a submodel of the base model, while still also being a submodel of the model actually containing it. Each instance of the satellite model is associated with an instance of the base model, and influences running between the two will only reference data between base model instances and their own associated satellite model instances. By default there is one satellite model instance per base model instance, but this can be made more by giving the satellite model its own dimensions, or nesting it in another submodel which has dimensions. Also satellite models can be conditional, so some base model instances have different numbers of satellites, or none at all.

Satellite submodels are not used very often, because most modelling problems are expressed more clearly by association submodels. But they are worth discussing because they are the simplest use case of the role arrow, so they are a good way of understanding its semantics, which remain the same when it is used in more complex cases. Also, some problems are best modelled in terms of satellite submodels.

Let's say that you have a multiple-instance submodel containing information on the species and volume of a set of individual trees: each instance is one tree. You would like to find the total volume of all trees belong to species 1.

This is easy to do if you have model the trees using a fixed-membership submodel (i.e. assuming that you have a fixed number of trees). You simply take influence arrows from the species and volume variables inside the submodel to a variable outside (say total), and give total the equation:

total = sum(if [species]==1 then [volume] else 0)

[species] and [volume] are both arrays with the same number of elements, and Simile's array language matches them up. If you use a population submodel to model the trees, then you do not have a problem: you can use more than one list in the arguments of a Simile function or operator, provided they always have the same length, and the result will be a list of the same length, which is fine so long as it is not assigned to the result without being summed or otherwise converted into a single value. The equatuion would be:

total = sum(if {species}==1 then {volume} else 0)

If you want to get lots of summary information about trees of one particular species, then rather than summing the results of a conditional expression like this for each one, you could make the equations simpler by creating a satellite submodel with variables for the same values that are in the base model. In the above case, it would involve creating a new submodel for the species 1 trees, using a single role arrow from the tree submodel to this satellite submodel, and entering the condition "species==1". An instance of this submodel will be created for each tree of species 1, and not for the others. If you then take the "volume" value into the submodel, then you can extract the volumes just for species 1.