FIRST OF ALL I THINK IT SHOULD BE REMOVED BECAUSE IT RUINED THE GAME RABBLE RABBLE this'll probably be a megathread but it needs to be to cover the complexities.
i probably don't need to outline how the current sensor system works. i think it's a good baseline.
first a note on philosophy: this game isn't a simulation. what does that mean? it means that you shouldn't add extra depth and confusion to it for realism's sake. that does not mean that you cannot add realism, just that it must serve a gameplay purpose. my suggestions are taken from how some sensor systems behave in real life as a baseline, however in the end they don't really need to compare that closely; as long as they add depth and interesting gameplay options, they can behave however.
now: what are the problems with the current sensor system?
1. in the end-game it pretty much stops mattering. once fleets blow up to a reasonable size they can see each other from halfway across the sector even if they are perfectly stationary.
2. sensors are not very granular. there's a huge mishmash of sensor types that are all rolled into one "sensor strength" which allows you to see, and a "sensor signature" which is really simple and allows you to be seen.
3. the omnidirectional and "invincible" nature of the sensors of all fleets mean that there are very few strategies that can be employed in trying to lose or gain a sensor advantage over an enemy fleet. this is mostly a result of problem #2
4. sensors are too binary. it's almost impossible to gain a sensor advantage with the current mechanics. you are nearly always detected at almost exactly the same range they detect you. this means that it's almost impossible to sneak up on fleets, or to hide effectively while surveying an area.
so, my suggestion is diversify sensors a little bit into distinct types. there are a few types of sensors we can reasonably extrapolate will be used in pretty much any space setting, as they are relatively simple, robust, and can be produced by pretty much anyone. all sensors should have tradeoffs, strengths and weaknesses and i'll do my best to point out what these are and why.
first, sensor types: for our purposes we will divide sensors into active and passive.
active sensors emit detectable radiation or 'noise' that means that anyone who is listening for an active system and is in it's area of effect will 'hear' it with whatever system they are using. an example of this is active sonar. you emit a pulse. the pulse hits the enemy. the enemy hears the pulse hit them. the pulse comes back. you can detect the enemy. active systems necessarily reveal at least your rough position. that means they should allow you to be detected better by the enemy (if they have some means of detecting you)
passive sensors don't emit anything. the eyeball is a passive sensor; the light hits the object, the light enters your eye, you can see it. you can of course look at someone who isn't looking in your direction and they won't notice. infrared systems are also passive, you can see the heat coming off something without emitting anything. passive systems are necessarily more stealthy because there isn't a way to detect that something is looking at you, however they usually have other downsides such as lower detection range, uncertainty about detected object's bearing, speed, range etc. or possibility of false-positives in detection.
now, onto specific sensors.
infrared: simple, lowtech and in use in pretty much every combat vehicle since the late 40s, infrared is a sensor that sees heat, or more accurately heat radiated off of surfaces. infrared leakage is pretty much impossible to stop (in fact, totally impossible) and because of the extremely low temperature in space it is easy to detect over vast distances. infrared sensors should detect larger heat sources better, and anything that has it's engines engaged much better. e-burn should light up like a christmas tree in infrared. infrared should probably have a 360 degree detection ability, however it would not detect as well on your tail because of your own drive plume blinding the sensors. crazy ivans may be required. infrared is a passive sensor, meaning you do not have to emit anything to see hot objects. because of this, it is far stealthier than an active system and thus should not increase your sensor signature. since infrared is technologically fairly simple, nearly every ship should have an infrared detection system.
RADAR: basically a radio echolocation system. radar is an active system (should increase your signature if you turn it on) but provides a superior detection against targets at long range, especially if they are large. detection arcs can be anything from a forward fixed beam to a 360 degree system, but a smaller frontal arc might be better for gameplay and simplicity's sake. note that unlike infrared, RADAR only cares about how big and how far away you are. it doesn't matter if you are standing still or what your engines are doing, it can detect you anyway. the downside is that anyone caught in the radar beam knows they are being looked at. radar systems should be standard on most ships, perhaps barring civilian vessels. I'd expect pretty much any warship to have one.
RW (radar warning) not so much a detection system as a defensive measure, the RW system tells you if a radar is looking at you. useful to know if someone is trying to find you, it's range should be almost unlimited. it is a passive sensor as it does not emit anything. most military-grade ships should have a comparable system.
exotic sensors: these should be rarer, and the amount of them is almost unlimited. but for starsector i'd suggest two.
gravimetric: gravimetric sensors detect massive objects, and the more massive the easier and the further away it can be detected. for our purposes we can assume these are relatively coarse, however they should be very effective at detecting large concentrations of cruiser+ ships as the entire point should be hunter-killer type actions. passive, as you are not really emitting anything, only reading the gravity of an object from range. should be standard on most phase-ships, as they are the u-boats of space. dedicated scout or surveying ships may also have them.
phase detection (cool name needed): detects phase fields at long range. effectively cuts through phase ships' sensor signature reduction. active(?) should be present on most expansion epoch+ or tri-tachyon ships. domain prototypes or very expensive scout ships may also have them.
other things:
sensor fuzz
Q: what's the difference between a hound and a cerberus at millions of kilometers?
A: uh...
some 'fuzziness' to sensors at long ranges might not be amiss. you should nearly always be able to tell with any sensor what class the ship is, but the difference between the aforementioned hound and cerberus might be impossible to tell. some sensors should be better at determining the exact ship, while others (infrared, probably) just tells you the rough direction it's moving and it's approximate size.
likewise, not being able to detect the whole fleet, but only the less stealthy parts of it would be pretty cool. for example, you turn on your IR system and hide in a belt. you're watching the jump point and you see an onslaught. it gets closer. then you see it's medium escort of destroyers, then it gets closer, you start to see it's less stealthy frigates. it gets closer. then you see it's phase frigates. the sky's the limit with this sort of thing.
more granularity to move speed: would be nice to have a few more buttons that limit your fleet's speed but at which you gain bonuses to your signature, but which aren't as crazy as go dark is.
deployables, more specifically sensor buoys: these are of course attendant on a system that isn't there (yet?) but i mean come on that's the rest of the post too so whatever.
deployable sensor buoys would make a ton of sense in-setting (in the tactical map already!) and add a far larger non-personal aspect to sensors. these should be anchorable to objects or able to be put into orbit around planets, asteroids, or the sun; depending on the sensor type they are using they should have their own signature. they should of course be able to be destroyed, perhaps more interesting though is the opportunities to tie these to factions and allow the AI to use them as more limited objectives in their constant wars. not only that, but they would be of great help to the player in allowing them to scope out jump points or ring systems or what-have-you in order to get a much better picture of local areas without directly approaching and risk being blown up.
the uses for these are of course endless, but they have downsides; they would be vulnerable to destruction by your's or your factions enemies, perhaps only have a limited duration, etc. if or when these are added the sector factions should likely have them deployed around their space already, as that would make sense. a deliberate campaign to blind a specific faction seems like it would be great fun.
so, if we implement any of this what will we have to modify about the sensor system as it is right now?
well, a bunch of new hotkeys for the bar will need to be added to turn off and on various systems. several more 'layers' will have to be added to the detection mechanic; each fleet will need a signature for each type of sensor in the game, and each fleet will need as many sensors as that fleet will have. how difficult is that? i'm honestly not sure. it could be anything from easy to impossible. the hardest part seems to me like it would be teaching the AI to use new systems, though maybe the backbone of that is already there. it seems pretty smart about the current system, for one.
so what's the payoff?
the payoff i believe will be relatively awesome. the sensor mechanics of cat & mouse will get more interesting. in particular, ambushes should be easier to pull off as a stationary target will be comparably much harder to see (due to infrared having trouble seeing stationary ships) breaking up the omnidirectional nature of current sensors means that how you approach the enemy will matter. ships can be easily diversified within the system via sensor types and strengths. factions, too.
this of course skips the effects entirely of electronic warfare, jamming etc. perhaps i will go on about that later.
Spoiler
nobody's going to read this are they?
anyway those are my thoughts. thoughts?