How would you make "Sensors" for a starship?

[Junghalli]

Actually trying to defend the observation posts seems like it'd probably be a waste of resources for the most part. It seems a better idea to invest resources in:

1) Trying to hide their exact orbits from the enemy, so actually dismantling your observation grid would require a prolonged, tedious search.

2) Trying to find a way to build them cheaply so you can hopefully throw up new ones faster than the enemy can find and blow up all the old ones. They'd be expendable assets, like ammunition.

[erik_t]

That depends on just how far away these observation posts are. It is entirely possible that the defender would need more delta-v than the attacker in order to scramble a spacecraft to the observation post in time, depending on the distance to the nearest staging area.

For example, just for the sake of argument, let's say each post is capable of identifying attackers at a range of ½AU, but you have several layers of posts out to a range of 4AU, and you only have a network of staging areas out to 1 AU.

Oh, no, I am unclear. They don't need delta-v to protect the outpost; as SS says, the outpost is boned anyway. They need delta-v to intercept at the final target of the bogeys.

Beowulf raised an interesting point on AIM that trying to mix active and passive long-range systems could be troublesome. At Pluto's orbit (5.5 light-hours from Sol), your passive kinematic fix must be extrapolated five hours into the future to use active systems. However, I'm not sure that this is a huge problem. You just extrapolate into the future and look where the object should be based on Newtonian motion. If the object is not there, it is thrusting and ergo is an unidentified spacecraft, at least neutral if not hostile. If the object is there, then you can look it over more closely to determine what it is, be it spacecraft or natural object.

You still reduce the uncertainty from (neutral/hostile/natural) to (neutral/hostile) or you active-query the thing directly. The latter is preferable, but the former is a serious improvement.

[Ariphaos]

That depends on just how far away these observation posts are. It is entirely possible that the defender would need more delta-v than the attacker in order to scramble a spacecraft to the observation post in time, depending on the distance to the nearest staging area.

For example, just for the sake of argument, let's say each post is capable of identifying attackers at a range of ½AU, but you have several layers of posts out to a range of 4AU, and you only have a network of staging areas out to 1 AU.

Why would you?

Point your star at the idiot and be done with them. No delta-v required, just their location.

[Darth Wong]

Oh, no, I am unclear. They don't need delta-v to protect the outpost; as SS says, the outpost is boned anyway. They need delta-v to intercept at the final target of the bogeys.

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

There's always the idea that they won't know where your forward posts are, but that's one of those offense/defense things, and depends on how vigilantly they are watching your activity at the edges of the territory under your control. And they can potentially tell whether you have any undiscovered observation posts along their line of attack by launching probing attacks and seeing if you respond. I suppose you could conceal your knowledge of their approach by delaying your response, but that's a gamble (and it would also make for an interesting sci-fi plot).

Beowulf raised an interesting point on AIM that trying to mix active and passive long-range systems could be troublesome. At Pluto's orbit (5.5 light-hours from Sol), your passive kinematic fix must be extrapolated five hours into the future to use active systems. However, I'm not sure that this is a huge problem. You just extrapolate into the future and look where the object should be based on Newtonian motion. If the object is not there, it is thrusting and ergo is an unidentified spacecraft, at least neutral if not hostile. If the object is there, then you can look it over more closely to determine what it is, be it spacecraft or natural object.

You still reduce the uncertainty from (neutral/hostile/natural) to (neutral/hostile) or you active-query the thing directly. The latter is preferable, but the former is a serious improvement.

There's still the question of the accuracy of the passive fix, especially if you need to get it from a very distant observer platform because your forward outposts were destroyed and it will take you many months to replace them.

[Drooling Iguana]

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

You'd know that you're being attacked, and possibly who's doing to attacking so you can send your own counterattack force before the enemy gets to you. Sure, you're still boned, but if the enemy knew of this policy of yours beforehand then they'd know that any attack would be a mutually assured destruction scenario and, if they're smart, they wouldn't launch the attack to begin with. If you don't have that network of sensor stations then the enemy might think they could wipe you out before you can get your own fleet away, which would make them more likely to attack.

[Darth Wong]

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

You'd know that you're being attacked, and possibly who's doing to attacking so you can send your own counterattack force before the enemy gets to you. Sure, you're still boned, but if the enemy knew of this policy of yours beforehand then they'd know that any attack would be a mutually assured destruction scenario and, if they're smart, they wouldn't launch the attack to begin with. If you don't have that network of sensor stations then the enemy might think they could wipe you out before you can get your own fleet away, which would make them more likely to attack.

People wrote stories about wars even in the Cold War era of real-life despite a MAD situation existing. Hell, I'm sure Tom Clancy was able to buy a shitload of his navy-looking baseball caps and nondescript windbreakers with the money he made from Red Storm Rising. There are ways to make it work (eg- traitors aiding the attackers etc) even before we look at other options such as simply making one side or the other inherently stronger (eg- superior technology or numbers) or some destabilizing event (eg- a disaster).

[Drooling Iguana]

People wrote stories about wars even in the Cold War era of real-life despite a MAD situation existing. Hell, I'm sure Tom Clancy was able to buy a shitload of his navy-looking baseball caps and nondescript windbreakers with the money he made from Red Storm Rising. There are ways to make it work (eg- traitors aiding the attackers etc) even before we look at other options such as simply making one side or the other inherently stronger (eg- superior technology or numbers) or some destabilizing event (eg- a disaster).

Still, it would make the sensors useful to have even if they weren't capable of actually stopping an attack.

[TheLostVikings]

Oh, no, I am unclear. They don't need delta-v to protect the outpost; as SS says, the outpost is boned anyway. They need delta-v to intercept at the final target of the bogeys.

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

But how exactly is this a problem? The entire point of having forward sensor platforms is to give you advance warning of an impending attack. If the last signal you receive is that "I'm under atta..." then it already managed to make fulfill its purpose.

You know know:
1. That someone is intending to attack you.
2. The proximate position of one of their units (even if it was just a missile)
3. exactly where they decided to attack. (even if it might have been a diversion)
4. a general direction to point massive amounts of sensors at (but not all of them, for obvious reasons)

Unless all they did was burn out some optical sensors from afar with a laser (with incidentally gives you an approximate attack vector based on where the sensor was pointing at the time) it makes more sense to design the entire thing to explode (as "loudly" as possible) instead of worrying about repairing it.

Because even if the enemy could get some sort of stealthy projectile close enough to lase/nuke the sensor platform, hard enough to render it unable to transmit an actual message back home (if not outright destroying it), it can still blow itself up to give the people back home pretty obvious hint that something is wrong.

Sure it's essentially a high-tech version of burning beacons on hilltops, but hey, if it isn't broken... etc. etc.

Edit:
Also the attacker is alien, vaporizing your sensors before they can get their "hands" on them means they won't be able to analyze our tech. Which prevent them from getting an tactical advantage if our technologies are completely different. (which they are pretty damn likely to be for any even mildly realistic depictions of aliens)

[erik_t]

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

It goes both ways; your outposts are going to be crazy far apart and likely possessing enough station-keeping thrusters that they could dodge a rock casually tossed. Beyond that, we start needing a more solid definition of the scenario. Where are your? Where is the enemy? What drive systems (STL and probably FTL if we're talking multisystem combat) are in use?

Regardless, you're going to have some defensive perimeter, and it's going to be some distance out that is much cheaper for you to reach than your opponent. The fuel for him to come and harass your outposts might well make the whole thing not worth it, never mind the ship-days he's spending, the wear and tear on equipment, and whatever capabilities he might be exposing (since outposts can presumably see each other).

I dunno. I return to my previous thought - we're getting into needing a more precise definition of our universe.

[Wyrm]

On the question of detecting decoys, what about radar? The decoy is a lamp, not a ship, so it should look rather different if you got a good look at it.

Power issues aside, radar waves are much longer wavelength. If you're only seeing a dot on visual, radar's going to reveal even less detail for the same (or even grossly expanded) receiver dish. Also, the individual photons are longer wavelength; you'd have to catch more of the little buggers to trip the threshold. You would need a simply gargantuan dish to gather enough energy to ping your sensors.

You may not be able to tell the path initially. But a light that just appears out of nowhere is exactly the sort of anomaly that a competently programmed sensor grid should pay attention to (as it's exactly what you'd expect of a rocket burn in deep space). If you have more than one sensor (and you should) you should be able to get the distance via parallax. Your sensor grid would note the existence, sky position, and distance of this weird light that winked on and off, keep a record of it, and be on the lookout for any more.

Thanks, you just made the invader's job much easier. Do you realize how much junk there is in interplanetary space, each of which have uneven reflectivity, which will rotate in just a way to cause a visible flash? How much noise there is in not only stray photons, but also in your own detector? That's why you need to collect lots of photons to be sure you've seen anything of interest. Otherwise, your bots are going to be logging thousands of such events each second.

Let's assume your ship is accelerating by 20 km/s and has an acceleration of 1 m/s^2. It'll need 20,000 seconds or 5.5 hours to get up to change its velocity by 20 km/s. That's lots and lots of short bursts. The next short burst the sensor net should, again, note the existence, sky position, and distance. Now assuming each burst is 5 seconds you need 4000 bursts. That's plenty of time to work out that these bursts are happening in a straight line of decreasing distance, and you should be able to use the parallax measurements to calculate velocity and acceleration.

During those same five seconds, there are going to be ~5000 similar events. You'll need to comb through 5000^4000 possible tracks to find the one track that looks like a deceleration curve. Good luck.

In order to identify paralax, you have to correlate the speck you saw at one observation post to be the same speck you saw the same instant at another. But unless those two observation posts are exactly the same distance from the ship, you'll need to throw in a trilateration as well as a paralax calculation. With a lot of junk and random noise, those calculations are going to have large error bars. Also, you have to know where your own observation posts are to within awesome accuracy, or that will be a significant source of error, too.

[TheLostVikings]

Ops, ran out of editing time on my previous post. Consider this an addendum to that:

Also: bonus point of you have enough resources to attach a bomb-pumped laser to your sensor platforms. After all any sensors in the proximity of the first have a decent idea where to look for an attacker, and if they are going to be destroyed by the enemy anyway they might as well give them a dose of hot laser while they are at it. (it probably wouldn't be able to make a dent in the defenses of an actual warship, but even burning out an optical sensor or two would still be an advantage, even if a rather minuscule one)

Thus the sensor array is now upgraded to a mine field.

[Darth Wong]

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

It goes both ways; your outposts are going to be crazy far apart and likely possessing enough station-keeping thrusters that they could dodge a rock casually tossed. Beyond that, we start needing a more solid definition of the scenario. Where are your? Where is the enemy? What drive systems (STL and probably FTL if we're talking multisystem combat) are in use?

Regardless, you're going to have some defensive perimeter, and it's going to be some distance out that is much cheaper for you to reach than your opponent. The fuel for him to come and harass your outposts might well make the whole thing not worth it, never mind the ship-days he's spending, the wear and tear on equipment, and whatever capabilities he might be exposing (since outposts can presumably see each other).

I dunno. I return to my previous thought - we're getting into needing a more precise definition of our universe.

Oh, I absolutely agree that it goes both ways. I was just trying to show that the enemy has some options available to him. The way some people were talking, they were building up an idea that it was simply impossible to hide your precise movements from a well-prepared enemy. It seems to me that it would be more of a classic offense vs defense struggle, with both sides working hard to overcome the other.

[Darth Wong]

But how exactly is this a problem? The entire point of having forward sensor platforms is to give you advance warning of an impending attack. If the last signal you receive is that "I'm under atta..." then it already managed to make fulfill its purpose.

You're making unwarranted assumptions.

You know know:
1. That someone is intending to attack you.
2. The proximate position of one of their units (even if it was just a missile)
3. exactly where they decided to attack. (even if it might have been a diversion)
4. a general direction to point massive amounts of sensors at (but not all of them, for obvious reasons)

"know" and "now" and not the same word. As I said, you are making unwarranted assumptions. The destruction of a forward observation post might be a precursor to an attack, but it might not. He might simply be testing your defenses. He might be trying to force you to waste time, money, and resources rushing your forces about in anticipation of an attack, and replacing the lost observation post, which I assume to be considerably more expensive than a guided missile.

Unless all they did was burn out some optical sensors from afar with a laser (with incidentally gives you an approximate attack vector based on where the sensor was pointing at the time) it makes more sense to design the entire thing to explode (as "loudly" as possible) instead of worrying about repairing it.

That doesn't give you an approximate attack vector of jack shit. You're assuming that the attack on the observation platform is the attack on the system. As if the two are necessarily synonymous.

[Ziggy Stardust]

Excuse my ignorance on the subject, and not to pollute this incredibly interesting discussion, but could someone maybe give a brief explanation of how relativistic speeds would affect all of this? A ship travelling FTL would have a difficult time employing any sort of long-range detection, due to the "distortion" (I don't know the exact term to use here). But how would it be different for a stationary outpost to detect something moving FTL versus SL? Would it be easier, harder, or what?

[Beowulf]

There's no basis whatsoever to come to any conclusions once you bring FTL into the picture. We have no clue what would happen, because we don't have any clue how to FTL would work.

[Ziggy Stardust]

There's no basis whatsoever to come to any conclusions once you bring FTL into the picture. We have no clue what would happen, because we don't have any clue how to FTL would work.

But aren't there basic assumptions about how an object moving faster than c would interact with the universe?

[erik_t]

That's really the subject of another thread. We'll get bogged down in causality and other such foulness. For now, I refer you to Wiki's tachyon page.

If it doesn't hurt your brain, then you don't understand it.

[Junghalli]

And they can potentially tell whether you have any undiscovered observation posts along their line of attack by launching probing attacks and seeing if you respond. I suppose you could conceal your knowledge of their approach by delaying your response, but that's a gamble (and it would also make for an interesting sci-fi plot).

How much would that really help? Yeah, they'd know there was an observation platform in sensor range somewhere along this straight line between their base and yours, but assuming the sensor is dark and quiet that leaves a lot of space to comb over to find it. If effective sensor range is, say, 10 million km and the distance between bases is 2 AU, and you launch your response when they're halfway there, the observation platform could be anywhere in a cylendrical volume an AU long and 10 million km in radius. And it gets better still when you consider that the observation platforms will be in orbit and therefore in motion relative to your and their base, so there's probably a whole constellation of platforms moving in and out of that volume, not just one.

This isn't even getting into the possibility of the observation platforms having ranges long enough that they could spot you from "behind" (from a position beyond your own base).

Power issues aside, radar waves are much longer wavelength. If you're only seeing a dot on visual, radar's going to reveal even less detail for the same (or even grossly expanded) receiver dish. Also, the individual photons are longer wavelength; you'd have to catch more of the little buggers to trip the threshold. You would need a simply gargantuan dish to gather enough energy to ping your sensors.

A radar beam would be directed though, not omnidirectional. Although I guess the return scatter would be so widely spread out it might as well be?

Thanks, you just made the invader's job much easier. Do you realize how much junk there is in interplanetary space, each of which have uneven reflectivity, which will rotate in just a way to cause a visible flash? How much noise there is in not only stray photons, but also in your own detector? That's why you need to collect lots of photons to be sure you've seen anything of interest. Otherwise, your bots are going to be logging thousands of such events each second.

True. You'd have to have enough sensors that they could be close enough that they could get an "interesting" image, i.e. more than a handful of photons. The observation platform has to be close enough that you can look at the flash of a ship drive and go "hmm, that's interesting, there wasn't a large asteroid there a moment ago" or "hmm, that's interesting that flash of light had the spectroscopic lines of helium plasma". You'd want to be close enough to be able to get such an "interesting" image of a light source in the high-megawatt to low-gigawatt range.

Question is how far apart you can put the observation platforms and still get this. I have no idea how to figure out how far away the observation platforms must be though, I must confess. How far away can we get that kind of resolution with real life telescopes?

Either that, or you need an analyses computer capable of handling math on the order of keeping track of 5000^4000 possible tracks.

Excuse my ignorance on the subject, and not to pollute this incredibly interesting discussion, but could someone maybe give a brief explanation of how relativistic speeds would affect all of this? A ship travelling FTL would have a difficult time employing any sort of long-range detection, due to the "distortion" (I don't know the exact term to use here). But how would it be different for a stationary outpost to detect something moving FTL versus SL? Would it be easier, harder, or what?

I imagine detecting anything "moving" at FTL would be impossible without further magitech, because it'd be outracing its own light and so would be invisible.

[Darth Wong]

This isn't even getting into the possibility of the observation platforms having ranges long enough that they could spot you from "behind" (from a position beyond your own base).

Why would you build your base so close to their observation posts? Or if your base is pre-existing, why would you have allowed them to build observation posts close enough to monitor your base in great detail? And if your base is far away, how many outposts must they have built, in order to have such deep observation capabilities? And why would you not move your base or attack the forward outposts if these conditions exist?

It seems to me that many of your arguments rely upon the defender having constructed all manner of infrastructure to aid in detection while the attacker has passively taken no real measures of his own, and then clumsily blunders about.

[Junghalli]

Why would you build your base so close to their observation posts? Or if your base is pre-existing, why would you have allowed them to build observation posts close enough to monitor your base in great detail? And if your base is far away, how many outposts must they have built, in order to have such deep observation capabilities? And why would you not move your base or attack the forward outposts if these conditions exist?

I was thinking more in the nature of a scenario where, say, the attacking ship is based at Mars and the observation platform is somewhere in the asteroid belt. I'm not sure how well your ship could be resolved at that distance though, which is why I only suggested it as a possibility.

[Junghalli]

Something I didn't think of in my reply to Wyrm:

Another point to consider is that, as I said, it's fairly likely that you'd know the locations of enemy staging areas. If so, your sensors wouldn't have to watch the entire sky for anomalies. They could watch the space immediately around those known staging areas, and the tracks of any ships leaving them. This enormously cuts down the amount of sky that must be watched and lets you eliminate a huge number of anomalies because they just aren't on any track that leads back an enemy base. This would make the entire job of detection much easier.

[erik_t]

These observation posts would take a long time to replace; it's not as if you can just drive a couple of transport trucks out and repair it. The enemy would presumably take out a few posts in order to effectively blind a sector before launching an attack, so there's no reason to assume that you would have gotten a fix on their attack force before you lost the outposts. Sure, you'd see the drone they launched at the outpost, for all the good that would do you.

Further thoughts:

Hubble was originally budgeted to cost about $400 million in 80s-ish dollars (although it ended up much more than that). The Voyagers were around $800 million each in 70s-ish dollars. Hubble probably has a larger and higher-quality mirror than we'd be using in the observation-post role; further, note that these are for essentially one-off vehicles. We could expect that quasi-mass-produced stations would be much cheaper, probably trending towards but not approaching mass-produced GPS at $40 million each.

In the meantime, I think a space missile is unlikely to be smaller or less complex than the ground-based ABM interceptors now deployed by the US. CDI claims these go for $37 million each, although this is by no means an unbiased source. Falling back on the old axiom that aviation cost scales directly with weight, we could extrapolate from SM-3 and arrive at a unit cost of $100 million. The truth is probably somewhere in between. Note these are quasi-mass-produced.

I freely admit to playing fast and loose with the facts (aieeee wiki!) here. However I think it's reasonable to say that an observation post would not be more than, say, an order of magnitude more expensive than a space missile. The ratio would, if anything, probably be lower.

[Wyrm]

A radar beam would be directed though, not omnidirectional. Although I guess the return scatter would be so widely spread out it might as well be?

Once the radar waves bounce off the target, all bets are off. Even if you manage this trick, the radar is going to have to be more powerful than the engine just to achieve the same detectability, for reasons I hope are obvious.

True. You'd have to have enough sensors that they could be close enough that they could get an "interesting" image, i.e. more than a handful of photons. The observation platform has to be close enough that you can look at the flash of a ship drive and go "hmm, that's interesting, there wasn't a large asteroid there a moment ago"

Why do you assume it was a large asteroid? Why can't it be a fleck of dust or ice close up? Do you know where all the dust is in a system?

or "hmm, that's interesting that flash of light had the spectroscopic lines of helium plasma".

You're not going to get spectroscopic lines from a flash. Unless you already have a grating over your detector, in which case you're getting the spectroscopic lines of everything. Good luck teasing out the flash.

You'd want to be close enough to be able to get such an "interesting" image of a light source in the high-megawatt to low-gigawatt range.

Question is how far apart you can put the observation platforms and still get this. I have no idea how to figure out how far away the observation platforms must be though, I must confess. How far away can we get that kind of resolution with real life telescopes?

Calculate it out. As a ballpark, something barely detectible is 1 pJ, and 100 pJ to make sure you've detected something, and 1 mJ to take its spectrograph. With the inverse square law, it should be easy.

Either that, or you need an analyses computer capable of handling math on the order of keeping track of 5000^4000 possible tracks.

I doubt it. 5000^4000 >>> googol >> number of atoms in the universe.

Another point to consider is that, as I said, it's fairly likely that you'd know the locations of enemy staging areas. If so, your sensors wouldn't have to watch the entire sky for anomalies. They could watch the space immediately around those known staging areas, and the tracks of any ships leaving them. This enormously cuts down the amount of sky that must be watched and lets you eliminate a huge number of anomalies because they just aren't on any track that leads back an enemy base. This would make the entire job of detection much easier.

You're having enough trouble finding a track in the first place, even when you're staring right at it, let alone figuring out it came from an enemy staging area.

[aimless]

You're having enough trouble finding a track in the first place, even when you're staring right at it, let alone figuring out it came from an enemy staging area.

I hope this is assuming the enemy is trying to avoid detection? Going to the other extreme, you're not telling me a ship scrambling from a staging area doing a 3g burn to turnaround so they can get to something in time is going to be hard to track in any way if you're looking closely at that staging area.

[Junghalli]

Once the radar waves bounce off the target, all bets are off. Even if you manage this trick, the radar is going to have to be more powerful than the engine just to achieve the same detectability, for reasons I hope are obvious.

Ah, so radar probably won't work.

To turn my earlier question on its head, how would you go about sorting lamp decoys from actual ships, assuming this is possible?

Why do you assume it was a large asteroid? Why can't it be a fleck of dust or ice close up? Do you know where all the dust is in a system?

Good point. Having a dual detector and using parallax on the light source should sort that out. Of course, I imagine doing parallax on a light source that only lasts a few seconds to a fraction of a second might be quite technically challenging (again, this isn't my field).

Calculate it out. As a ballpark, something barely detectible is 1 pJ, and 100 pJ to make sure you've detected something, and 1 mJ to take its spectrograph. With the inverse square law, it should be easy.

Hmm, using GMT's numbers from earlier, I got a 4 gigawatt energy source being detectable from around 1 AU for a 1 second exposure by the New Horizons telescope, and a 60 megawatt source being detectable from around 17.5 million km. Of course this is "we're sure we've got something but it's a dot of light". Spectroscopy would require something on the order of thousands of kilometers, which ... would probably require a grid density that doesn't seem realistically feasible.

You're having enough trouble finding a track in the first place, even when you're staring right at it, let alone figuring out it came from an enemy staging area.

What I mean is this. You know where the enemy bases are. To get away from the immediate vicinity of these bases on a practical timescale the enemy ships will have to do burns, and these burns if detected will allow the courses to be tracked. This lets you eliminate everything but tiny slices of sky around the enemy bases and narrow corridors that mark the pathways of enemy shipping. You can look for anomalies in those tiny pieces of the sky and ignore the ones that happen in the rest of the sky. This should make your job a lot easier.

Of course, the enemy can try creating secret bases, but they'll have to launch some sort of constructor ships, which gets you back to the original problem. Of course, this does assume you've been watching them the whole time. If they constructed these secret bases before you built your sensor grid, then obviously you won't know about them. Although said bases have to recieve no shipping (personnel rotations etc.), or else the ships' burns would lead you right back to them.

You might also try finding a way to launch ships under external power while dumping the heat into a large planetoid or something, though I'm not sure how this would work. Lightsails and such invariably involve energy hitting and scattering off the spacecraft, which would be visible; the only thing I can think of is a spring-like system, and I doubt you could impart much velocity with that.