You only need one GPS system to be run by a regime friendly to your cause and use that. Given in that in that part of the world there is going to be at least one player who will be symathetic to your cause with either a GPS system or will get you access to the key codes even if the system owner would perfer you didn't have them. (it's complex out there)
Basically GPS is now so widley used by so many countries, organisations etc etc that turning it off is not an option. This is why there is now more than one system. Because one side is worried the other side will exclude them.
It would not take the "key player" to be a state actor. Most reasonably skilled embedded/electronics developers can knock up a sophisticated system. Actually most hobbyists and a lot of drone racers could do this quite easily with standad cheap commercially available modules.
GPS reciver/decoders are about $30 each from amazon, ebay, marshals, RS, Farnel, Element 14. etc.
TECNOIOT SIM808 Module GSM GPRS GPS Breakout Board SIM808 for Arduino Raspberry: Amazon.co.uk: Computers & Accessories
How about one with a compass?
WFHhsxfh OCDAY NEO-M8N GPS Module with Compass Sensitive for DJI NAZA Lite V1 V2 Flight Controller RC783. Electronic Accessories: Amazon.co.uk: Kitchen & Home
about the same for GSM modems.
OSTENT GSM Modem with Wavecom Q2303A Module USB Interface AT Commands SMS: Amazon.co.uk: Computers & Accessories
That gives you three forms of passive navigation with one two way comms.
Also all you need is a radio compass module. The modules are of a similar price these days. This will give you the bearing of a transmitter. For example any of the VOR, VORTAC, TACAN transmitters or indeed any civil radio station (if you know the exact location) You can then navigate to anywhere within a few feet. Most of the information is on civil air maps and the ONC & TPC maps. With a burner phone you could have some near the target area to (ie with in a few KM with eyes on) to do any last minute changes via SMS.
Add in a rasberry Pi (or any one of several 100 similar systems that are very widely available ) and you have an extremely sophisticated multi channel navigation system for less than $150 + a bit of time knocking the code together. There is no reliable defence against this sort of attack other than things like phalanx. .
The problem is with a range of 1500 miles you could lauch anywhere in side one hell of an area, probalby spanning several countries.
OTOH given the size of these drones you could smuggle them in to a local launch site in a pickup truck (with in a few miles) and lunch them balistically given the size of the target, ie forward x yards/metres and go bang.
There is no reliable defence against this sort of attack either.
