First, it should be noted that, strictly speaking, ground services are usually provided by ground handling companies, which might or might not always be from an actual airline that operates there.
Those big US airlines like Southwest, United or American, are basically a default fallback, meaning they have a chance to appear anywhere in the US, unless we have more specific information about an airport, which might really have ground handling provided by those same airlines, but it's not always the case.
I made a quick check on KPSP website, and it seems right now, they have two handlers, which are Atlantic and Signature, and they are in fact missing from GSX, so we'll add them both in the next update.
However, if you want to change an operator for any airport ( in this case you can only choose from the available ones ), you have two ways to do so:
- Just select the operator for Handling or Catering explicitly in the airport customization parking, for all parking spots you want to assign a specific single operator.
- Change the rules for one or more operators by creating a custom rules files, as explained in the "Rules Customization chapter at Page 86 of the Manual, although I suggest start reading from Page 84, to get a better understanding how rules works.
