According to Planetstorm rules
Unspotted Indirect Fire has a modifier of +2
Spotted Indirect Fire has a modifier of 0 (zero)
What I am going to suggest makes things a bit more detailed (complicated?)
Unspotted will remain a mod of +2
Spotted unaided will be a mod of +1
Spotted using a Target Designator will be a mod of 0 (zero)
The reason I am suggesting this, is as in RL there are call for fires without a spotter, with a spotter and call for fires with actual Target Designators (TD) equipment.
Rarely would a I see the use of the Spotted unaided call for fire but by having it in the rules it would allow for it to take place.
I say rarely because if you are planning on having and using Indirect fire weapons, you most likely will have individuals in your force who have a TD.
Also (me attempting to plan ahead), when there is a RPG and conversion charts (for other figures) for making your own figures, it might actually come in handy.
Dave Chase