It's says there's a way to increase a drones sensor in both the core book and Rigger 3, but no matter where I look I can't find the requirements or cost. Can anybody help a guy out?

Per the books, I don't believe you can increase sensors after production. IIRC, the sensor packages are all wrapped in during design and have design point costs, but not upgrade costs. However, as a GM, I just fool with the numbers until they seem right and permit that.

You can also change sensor ratings with the flux rules. I believe flux rules apply to all vehicles, so drones would be included. Using that, you can boost sensor ratings up to 50% (again, IIRC). The rules for this are in the main book.

The final thing I'd probably allow is to make a 'sensors only vehicle' or find some sensor equipment and just mount that on the drone. It's probably the least space-efficient route, but as a GM, I'd be cool with it.

Rigger 3 modifications section --> sensors page. 147

From the chart I believe anything rating 5+ requires specialized fixers.

In addition to all that, R3 p30 gives rules for upgrading sensors with additional capabilities without increasing rating. SR3 p135 lists the capabilities included at each rating.

~J