What the different sensor types do.
The gravitational sensor was a major issue in 2300ad. The question was is the grav scanner an "all seeing eye" (ASE)? If it was then stealth is pointless. The way the grav scanner is used in "Three Blind Mice" does not require an ASE. The grav scanner can detect the gravity waves coming from an operational stutterwarp, but there is no directional component and indeed no range component. It is just a meter. There may be a couple of hacks to get extra information. Firstly your own baffles block detection, and so you can get some directional sense by turning until a signal disappears. Secondly, if you had multiple scanners very far apart and a lot of time you can triangulate. Neither is tactically that useful.
We should note that an operating stutterwarp emits many orders of magnitude more gravity waves than a planet. Earth radiates 200 watts of G-waves, and Jupiter around a hundred times that.
DSS is just the "civvie spec" passive sensor.
Passive sensors are spherical scanners that pick up the emissions of enemy craft. They make "black globes" on maps. Detection ranges with the most advanced sensors against a typical large warship is 40 light seconds (20 hexes). Beyond this the sensors can't see the contact and they don't appear on the board. The information that passives will reveal is simply the board position, the current radiated signature and any movement. All these are naturally disclosed in the sensor process.
Active sensors are highly directional, and can interrogate black globes and place the counter on the table. Unlike passives, actives give range information and can be used for weapons fire. The only additional information the active array gives is the effective reflected signature and enough range information to make a target lock. It will give no more identity information, and so a model is really inappropriate. The resolution is such than even when "revealed" and placed on a table
there is likely to be no identification data beyond an estimate of size
and pseudovelocity. At actually work out what a ship or object is the
scanning ship needs to get to "melee" range, as we will discuss below.
On that note, when you irradiate a target and it has it's own passive sensors (ESM), then the irradiated passive will give directional but not range information. It will turn a vessel truly "hidden" into a black globe.
To get identity information requires literally doing a "VID" (visual identification) which requires entering "melee" range.
How Sensors Should Work in Star Cruiser
Essentially for SC to really work there should be a degree of hidden movement. Hidden movement is when an object on the board isn't even a black globe. Given the SC rules, almost every object within the passive range of any enemy ship is a black globe, and those a bit further out (passive+ radiated signature) are also. This means that at long range (or with weak sensors) it should be possible to spot an enemy ship as a black globe and launch missiles in advance. For the actual attack the shooter will have to light up an active sensor and illuminate the target, which will reveal the shooter as a black globe, or a drone which is revealed.
Tactically this actually fixes the game back to how it was envisioned in the notes. Stealthiness matters to an extent, and surprise long range missile strikes become possible. Having a sensor drone with a powerful active array makes sense as they the attacker doesn't reveal their position for free when attacking. Of course said illuminating sensor could be on a fighter, frigate etc.
This adds an additional wrinkle. An active sensor is committed against "one target". What if said target is a swarm of missiles? Do we only illuminate one missile out of multiple missiles stacked in a hex? The answer is yes. To get any usable return the emitted radar beam needs to be incredibly tight. One hex in SC is 4,000 times the 150 km that a modern fighter aircraft might be able to lock onto a target. The return at one hex is 10^-14 that of said fighter for the same beam width and power. The same factors that prevent scanning with a radar also prevent locking a radar onto more than a very narrow target. Said radar likely is more like a MASER than what we think of as a radar in operation given the need for insanely tight angular divergence to get any return at a distance of many light seconds.
What of turret weapon fire, and TTA's and UTES? Well, the TTA and UTES is a very weak radar, as it consumes less than 10 kW (i.e. it consumes no power in the design as consumption below 10 kW is housekeeping) vs the 1-8 MW for a long range targeting radar. Their range is at best a few percent of a hex. That means a turrets integral UTES/TTA can't even target an enemy in the same hex. What it can do is target an object approaching to within a few ten thousands of km. That is approaching to particle beam range and entering nuclear explosive range (bomb pumped "lasers" have realistic ranges measured in 10's of km, not ls). Thus we need to disambiguate two ranges.
Within the same hex is "range = 0", which essentially means within about a light second of the target. A starship in orbit of Earth's moon and another orbiting Earth as technically within the range =0 range (although you might set up Earth in the centre of a hex, in which case the Moon is just into the next hex...) but the passive sensors certainly could not resolve any shape details, and the little TTA radars could not get an effective return to target it. By the same token, a bomb-pumped laser or a particle beam's attack would have been spread to nothing and be ineffective at such a range. Only the focused high frequency X-Ray lasers of turrets with 3+ m focal arrays will be effective, and only dedicated and fairly powerful radars could target it.
However, if you move said ship to Earth orbit and both ships are now separated by only a few hundred km then then can resolve details of each other, and target each other effectively using UTES. Bomb-pumped lasers and particles beams will be effective.
Thus there should be two situations; range = 0 and range = close. The latter is the starship equivalent of crossing bayonets.
TTA/UTES is effective only at "close range". This allows the guns to target incoming missiles in their terminal phase, and any starships closing to use particle beams or submunitions. It removes the sensor situation here. This is also the range where finer detail than a blob can be seen, sense full disclosure of the vessel type etc. should be made.
Sensors and Scenario Staring Range
Two ships, or groups of ships, will not see each other simultaneously. The one that see the other group first has the sensor advantage. Since only passive sensors can scan, it is the best passive sensor value and the highest effective radiated signature that matters.
The board is set up with the ships being a distance apart equal to the initial detection range of the disadvantaged side. If the advantaged side has a significant advantage then they can have deployed missiles etc. already. The advantaged side only reveals the objects in passive sensor range (such as the launched missiles) and any objects who've lit up an active targeting radar against a vessel with passive sensors.
This presents a problem, as the objects out of passive range are completely invisible until they close to passive range or light up a targeting radar. It requires a GM to utilise true "hidden movement".
Imagine a convoy of French frigates under attack by a Kennedy. All they'd see was an active sensor lighting them up (i.e. the drone) and some waves of incoming missiles. However, all the Kennedy would see at long range is a bunch of passive contacts all with similar signatures; the attacking missiles are essentially being targeted randomly because the Kennedy can't tell escorts and merchants apart.
Welcome in information-centric warfare....
