I'd like to allow for larger Sensor arrays giving greater detection ranges, so for passive sensors you'd need 4* the array to get 2* the range.
Strategic Operations p247 (the Battleforce: Advanced Rules chapter, the story on Detection) mentions detecting the emergence wave at 15 AU with specialized military sensors. Page 118 (Advanced Aerospace Combat -> Advanced Sensors - Emergence Wave) says that you make a detection check with a modifier based on half the AU between the detector and the arriving unit.
From the rules (IIRC) the emergence wave duration is proportional to distance (in ly) traveled times the number of Dropships that can be carried (so a
Potemkin making a long jump will be long at 375 seconds, or just over 6 minutes).
But if you want amazing sensors, treat it as 4* the current sensor mass giving you 2* the detection range. Figure it will be used for Large Naval Comm-scanner suites only. So if you want the existing ranges (i.e. the 5 million kilometer drive plume range modifier, tripled by LNCSS to 15 million km), you use one of the Large Naval Comm-Scanner suites at 500 tons.
But you want more range. Doubling the range means you need 4 of the LNCSS, and you now have the range modifier for equal to 30 million km, to use when trying to detect drive plumes.
But the range to the Zenith/Nadir point (assuming Terra) is 10 AU, or about 1,500,000,000 km. So trying to detect anything at that range is still at +50 penalty.
So you double the range again, to 60 million km. You now need 16 LNCSS, and your divisor is 60 million km. Only a +25 penalty now.
Double the range again, to 120 million km. You now need 64 LNCSS, and your divisor is 120 million km. Only a +13 penalty now.
At this point you would also be dividing the range to the Jump point in AU by 16, for only a 1 pt penalty to detect anything arriving via KF drive. You are pretty much guaranteed to spot anything arriving at the Z/N points.
But you are really paranoid and want to be able to watch their exhaust plumes as well to track Dropships.
Double the range again, to 240 million km. You now need 256 LNCSS, and your divisor is 240 million km. Only a +7 penalty now.
Double the range again, to 480 million km. You now need 1024 LNCSS, and your divisor is 480 million km. Only a +4 penalty now.
Double the range again, to 960 million km. You now need 4096 LNCSS, and your divisor is 960 million km. Only a +2 penalty now.
I suppose you could stop now. Your Sensor array masses 2,048,000 tons, costs 1024 Billion C-Bills (before the multiplier for whatever platform it is mounted upon), needs almost 50,000 crew, can track exhaust plumes of Dropships at the Jump Points reliably, and only needs one point of damage to be knocked out.
It can also easily spot the mobs coming to kick you out of power for even proposing this monstrosity.
Edit - I got the math wrong and recaculated the price