What's the cheapest possible way to monitor a stellar system for intruders?
As per
this thread, satellites and airships count as aerospace vessels and large craft are aerospace vessels larger than 200 tons with legal satellites and airships going up to 300 tons. As such, a military satellite or airship that is 201 tons or more can perform system monitoring watching for jump signatures (to AU distances), drive plumes (to 10s of gigameters), radio comms (to 10 gigameters), enemy radar (to a gigameter), radar reflection (to 10s of megameters), IR jump signatures (to 10s of megameters), and optical sighting (to megameters) as per Strat Ops pages 117-119.
The Skywatch satellite is a cheap transportable satellite capable of automated monitoring for vessels and relaying results to an in-system base station.
Skywatch satellite
201 ton satellite support vessel
1,628,952 C-bills
Structural Integrity: 1
Tech Level D
Chassis: 32.5 tons
Fusion Engine: 20.5 tons
Armor: 4 tons (88 armor points BAR 7)
Communications Equipment: 13 tons (+1 ton built in implies detection check base role 0)
Fuel: 130 tons (71 burn-years)
Cargo: 1 ton
Just having one of these in-system isn't enough to reliably detect in-system jumps, but one at the Zenith, one at the Nadir, and a couple at about 2 AU monitoring the inner system provides semi-reliable discovery of nearby jumps at a price of <6M c-bills. Since the satellites are fully automated and have the fuel to last for 100 years (!), they are easily deployed to even relatively poor systems. Richer systems feature one satellite in an in orbit perpendicular to the orbital plane around every major body capable of generating an L1 jump point.
The Skywatch satellite is a good order of magnitude cheaper than a minimal automated space station capable of performing similar duties.
Is it possible to go even cheaper? It turns out yes, using an Airship.
Skywatch airship
201 ton airship support vessel
601,313 C-bills
Structural Integrity: 4
Safe Thrust 1
Tech Level D
Chassis: 50.5 tons
Fusion Engine: 8.5 tons
Armor: 3 tons (66 armor points BAR 7)
Communications Equipment: 14 tons (detection check base roll 0)
Smart Robotic Control System: 10.5 tons
Cargo: 114.5 tons
The fully automated Skywatch Airship operates on worlds with an atmosphere. The low-pressure atmosphere above it plausibly interferes with some detectors, particularly the shorter range ones. Nevertheless, at 1/3 price, this is incredibly affordable.
A good reference point here is the
Skybus smallcraft at 6.7M C-bills, about the cheapest of any surface to orbit system. For the cost of a Skybus, it's reasonable to deploy sensors capable of detecting jumps within Sol-like systems.
Improvements:
An active probe would make these systems perform better and plausibly be worth the price.
I'm skeptical that a Naval Comm Scanner is worth the price in most circumstances since it doubles ranges and increase the price by more than a factor of 8---you might as well just deploy more satellites.
Tricky things:
Deploying a 201 ton satellite legally seems to require the use of a repair bay. Plausibly you can shift one from cargo to a repair bay and then drop them out of the repair bay as discussed
here. Alternatively, it may be possible to deploy it in pieces from a smallcraft bay and assemble in space.
Deploying a 201 ton airship on a world is relatively easy---land a dropship, unload the airship, prep it, and launch. What appears impossible is getting an airship into the upper atmosphere of a gas giant. Fortunately, gas giants often have moons and some of those moons (like Titan) may have an adequate atmosphere. This does make you wonder: if a skwatch airship is parked on an airless moon, can it function?
