First off, there are two broad classes of sensors: passive and active. Passive sensors just detect any emissions from the target, i.e., they passively look for the target. Passive sensors include telescopes and heat sensors. Active sensors emit various frequencies and detect their reflection off the target, i.e., they actively "shine a light" on the target. Active sensors include radar and lidar/ladar.
Active sensors are much better at detection, but have the annoying side effect of virtually placing a huge flashing neon sign on your ship that says: "LOOK AT ME! I'M HERE! SHOOT ME, SHOOT ME!!" . This not only lets all hostiles (detected and undetected) know where you are, but also gives their deadly radar-homing missiles some radar to home in on.
Passive sensors, on the other hand, are more blind but are undetectable. Much better if you are trying to hide. Passive sensors also generally can vaguely detect the presence of objects at a much greater range than active sensors. But active sensors can determine the precise location of an object with much greater precision.
Why? An active sensor emits "pings" of electromagnetic radiation in order to illuminate the target, the sensor "sees" the target if the energy returned by reflecting off the target is high enough to be detected. If the target has a small dimension compared to the angular and range resolution of the active sensor, the strength of the return signal is proportional to the inverse fourth-power of the distance to the target (i.e., signal fall-off is 1/r4).
Why this fall off is 1/r4 instead of the 1/r2 you'd expect from the inverse square law is explained here and here. Basically only a fraction of the initial pulse energy is reflected back. So the target acts as if it was an active sensor emitting pings with a strength of 1/r2 of the original pulse. These pseudo-pings travel back to the original ship, suffering a further loss of 1/r2. This combines to make an effective loss of 1/r4.
But on the third hand an active sensor uses tightly focused pings while a passive sensor has to make do with whatever unfocused radiation flux the target emits.
There is one cute real-world trick. If your active radar pulses mimic radio static, enemy radar detectors will filter the pulses out as random noise and fail to see them. This will make your active radar invisible. Until the enemy catches on to the trick and redesigns their detectors.
In some SF novels, passive sensors are called "sensors" while active sensors are called "scanners."
Detection is something that's been in my family for decades. My dear departed grandfather Charles Haney Davis was a civilian contractor for the US Navy, with a retired rank of Admiral. He was on the USS Semmes (DD-189) in the 1940's, working on something that would eventually become Sonar.
It would be a jolly science fictional idea to postulate a break-through that could detect passive sensors, keeping in mind that there doesn't seem to be any basis for this in reality. Wave your hands real hard, and vaguely mutter about "psionics", or something based on a Schrödinger's cat-like collapse of wave function (Captain, the wave function collapsed, it means somebody is peeking at us!) or specially trained experts who feel itchy sensations between their shoulders when somebody is looking at them. But to reiterate, this is strictly science fiction.
Late breaking news: maybe it isn't strictly science fiction after all. I stumbled over a scientific paper with the provocative title Information Transmission Without Energy Exchange. This seems related to detecting passive sensors. Or maybe not, the math is over my head like a cirrus cloud.
Strategic combat sensors detect hostile spacecraft at long range, giving advanced warning of enemy attack.
First off, as Ken Burnside explains, there is one major way that detection in space is different from detection on Terra's surface: There Is No Horizon. Since Terra is a sphere, the curvature means if you are of average height, the fact your eyes are about 1.7 meters off the ground means anything much further away than 4.7 kilometers will be invisible. That is the distance to the horizon, anything further (that is not outrageously tall) will be hidden below the horizon.
Space don't have no horizon, nohow. The range is pretty much to infinity (or 13.798 ± 0.037 billion light years if you want to be picky).
Yes, there will be a bit of a horizon effect if you and the target are in close orbit around a planet. The target will be hidden for about one-eighth of an orbital period. For something in LEO around Terra, this means it will be hidden for about 15 minutes, max. Which is not really a militarily significant amount of time.
Secondly, there are three different ranges:
- Detection Range: You become aware there is something out there, at that position in the celestial sphere. You may or may not know how far away it is (e.g., there is a bogey, a blip on the radar screen).
- Identification Range: You know there is an object of a certain type at range x (e.g., there is a Blortch CL-23 "FenderBender" light cruiser at x 135.2, y 17.3, z 325.1 ).
- Targeting Range: Your sensors have enough data for a firing solution (Your casaba howitzers have a target lock on the enemy FenderBender, designated Target Tango 13. You may fire at will. ).
For a given sensor, these range are arranged above in order of decreasing distance.
In space, Detection (as opposed to Identification and Targeting) is basically a matter of time. You can purchase off-the-shelf software fully capable of processing a full spherical sky search and flag any bogeys. The processing power of an average PC graphics card is more than up to the task. Since it takes about three days to travel from Luna to Terra with current technology, it is not like there is any rush.
If the enemy is using torchships, then you can probably spot them with your naked eyes. At least if they are closer than a few astronomical units (1 AU = distance between Terra and Sol).
And thirdly, refer to the next section.
Wargames like GDW's STAR CRUISER describe interplanetary combat as being like hide and go seek with bazookas. Stealthy ships are tiny needles hidden in the huge haystack of deep space. The first ship that detects its opponent wins by vaporizing said opponent with a nuclear warhead. Turning on active sensors is tantamount to suicide. It is like one of the bazooka-packing seekers clicking on a flashlight: all your enemies instantly see and shoot you before you get a good look. You'd best have all your sensors and weapons far from your ship on expendable remote drones.
Well, that turns out not to be the case.
The "bazooka" part is accurate, but not the "hiding" part. If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor.
The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit.
As of 2013, the Voyager 1 space probe is about 18 billion kilometers away from Terra and its radio signal is a pathetic 20 watts (or about as dim as the light bulb in your refrigerator). But as faint as it is, the Green Bank telescope can pick it out from the background noise in one second flat.
This is with current off-the-shelf technology. Presumably future technology would be better.
Read the essay in the Rocketpunk Manifesto entitled Stealth Reconsidered.
Now I know you do not want to accept the fact that stealth in space is all but impossible. This I know from experience (Every day I have new email from somebody who thinks they've figured out a way to do it. So far all of them have had fatal flaws.). The only thing that upsets budding SF writers more is Albert Einstein denying them their faster than light starships. But don't shoot me, I'm just the messenger. The good folk on the usenet newsgroup rec.arts.sf.science went through all the arguments but it all came to naught.
Offhand I can think of a couple limited cases that provide something stealth-like in space:
- If the battling ships are in close orbit around a planet, obviously ship A will not be able to detect ship B if it is on the far side of the planet. Nor detect course changes or launches of missiles. This becomes impossible if there are more than two ships involved and/or scouting satellites.
- If the battling ships are deep inside a gas giant's atmosphere, detection range will be drastically lowered. You will have a more cinematically interesting "Battle of Midway" situation (naval fleets sending fights of scouting aircraft all over the place, desperately trying to locate an enemy fleet before they locate you).
If you are bound and determined to have stealth in space, you will have to postulate some sort of hand-waving technology. Popular in science fiction are "cloaking devices" and stealth as a side effect of the faster-than-light propulsion used by starships ("We can't detect the Zorg ship until it comes out of warp, sir!"). Much more rare is something like a heat radiator, where the radiator sticks into hyperspace to make the heat invisibly go away into the fifth dimension.
It is not like the absence of stealth in space takes all the fun out of things. Sometimes things are more interesting this way. For example, John Reiher shows how to incorporate this in to the tabletop role playing game Diaspora (incidentally, Diaspora has been awarded the Atomic Rocket Seal of Approval).
If you want to really argue on this topic, I'd advise you to cut out the middle man and go directly to rec.arts.sf.science and lay your case out before the experts. You might also want to review the section on Respecting Science.
This is true. Take my word for it, I know from bitter experience.
First off, the answer is NO, you cannot solve the problem by using a thermocouple to convert the heat into electricity.
Ken Burnside said:
For reference purposes, here follows some brief summaries of the more common arguments and their rebuttals.
If you are hoping to lose your tiny heat signature in the vastness of the sky, I've got some bad news for you. Current astronomical instruments can do a complete sky survey in about four hours, or less. Presumably future technology can do it even faster.
Ken Burnside said:
Rd = ( 17.8E6 * sqrt( Ms*As*Isp*(1-Nd)*(1-Ns) ) )
- Rd = maximum detection range (kilometers)
- Ms = bogey spacecraft mass (tons)
- As = bogey spacecraft acceleration (G)
- Isp = bogey drive specific impulse (seconds)
- Nd = bogey drive efficiency (0.0 to 1.0)
- Ns = bogey "stealth efficiency", i.e. fraction of waste energy which can be magically shielded from enemy detectors. (0.0 to 1.0)
Current chemical rockets have Nd of roughly 0.95. Ion drives get about 0.50, and steady-state plasma thrusters 0.65 or so — both can in principle be pushed to 0.90 with some difficulty, but not much beyond that. For realistic rockets, Ns = 0.0. There really isn't any way to hide your waste energy from your opponents, short of science fiction.
Here they note that the assumption was a telescope with a Field Of View (FoV) of 0.8° and 0.7 seconds to scan that FoV. At 0.8° the entire sky has about 64,000 FoVs. At 0.7 seconds per FoV scan, that would take about 12.54 hours.
Dr. Schilling says the total sky scan time can be reduced to one hour at the cost of reducing the range by a factor of 3.54. Alternatively the telescope can be fitted with nine detectors instead of one (a 3x3 macro array) which would increase the FoV by three. The entire sky would then be about 7,000 FoVs. At 0.7 FoV scan, a total sky scan would take 1.3 hours.
And of course this was assuming astronomical equipment that was top-of-the-line in 1998. The state of the art has advanced quite a bit since then.
"Well FINE!!", you say, "I'll turn off the engines and run silent like a submarine in a World War II movie. I'll be invisible." Unfortunately that won't work either. The life support for your crew emits enough heat to be detected at an exceedingly long range. The 285 Kelvin habitat module will stand out like a search-light against the three Kelvin background of outer space.
The maximum range a ship running silent with engines shut down can be detected with current technology is:
Rd = 13.4 * sqrt(A) * T2
- Rd = detection range (km)
- A = spacecraft projected area (m2 )
- T = surface temperature (Kelvin, room temperature is about 285-290 K)
If the ship is a convex shape, its projected area will be roughly one quarter of its surface area.
To keep the lifesystem in the spacecraft at levels where the crew can live, you probably want it above 273 K (where water freezes), and preferably at 285-290 K (room temperature).
Glancing at the above equation it is evident that the lower the spacecraft's temperature, the harder it is to detect. "Aha!" you say, "why not refrigerate the ship and radiate the heat from the side facing away from the enemy?"
Ken Burnside explains why not. To actively refrigerate, you need power. So you have to fire up the nuclear reactor. Suddenly you have a hot spot on your ship that is about 800 K, minimum, so you now have even more waste heat to dump.
This means a larger radiator surface to dump all the heat, which means more mass. Much more mass. It will be either a whopping two to three times the mass of your reactor or it will be so flimsy it will snap the moment you engage the thrusters. It is a bigger target, and now you have to start worrying about a hostile ship noticing that you occluded a star.
Dr. John Schilling had some more bad news for would be stealthers trying to radiate the heat from the side facing away from the enemy.
If you are actually trying to apply thrust, the upper equation comes into play, and they can see you all over the solar system. What's worse, they can measure the spectrum of your drive to estimate the thrust and use a telescope to observe your acceleration. Simple division will reveal the mass of your ship.
"Well fine!", you say, "I'll just burn once and drift silently"
But now you will be months in getting to your target. The extra time increases the chance that the enemy will spot you. It will be harder to keep your directional radiator aimed away from any enemy observers. And if you are spotted, so much of your ship mass will be radiators instead of weapons, so that the enemy ships will out-gun you by an obscene margin.
Not to mention the fact that once your initial burn is spotted, the enemy will be able to calculate your future position anytime in the future. They can set a computer controlled telescope to track your current calculated position, and will quickly spot any future course correction burns.
So much for being ambushed by a space pirate appearing out of nowhere. And everybody on a cruiser would know that the hostile bogey would be within combat range in two months, three days, five hours, and thirty-three minutes. You might as well take it easy and get your rest before the battle. You know the cliché: long stretches of boredom punctuated by brief moments of stark terror.
And to forestall your next question, decoys do not work particularly well either. More specifically, a decoy capable of fooling the enemy would wind up costing almost as much as a full ship.
Just to make sure that we are both on the same page here, I am talking about time frames of weeks to months. Such as found when a task force weeks or months away from their target, attempting to fool the enemey observers into thinking that your are a force of twenty warships, when you are actually a force of one warship and nineteen decoys.
I am not talking about time frames of a few seconds. Such as found when a combat spacecraft, with a hostile heat-seaking missile attempting to fly up its rear, dumps off a couple of decoy thermal flares hoping the missile will be confused.
First off, a decoy needs to emit a similar amount of radiation and heat as the ship it is pretending to be. This means each decoy needs a power source comparable in size to a full ship, the same goes for radiator area.
If the decoy and the real ship thrusts, it becomes worse. The exhaust plume has to be the same, which means both the decoy and the real ship has to have the same thrust. This means the decoy has to have the same mass as a real ship, or it will accelerate faster, thus giving itself away. If you down-rate the decoy's thrust, the dimness of the exhaust plume will give it away.
So if each decoy needs a spaceship sized engine in a spaceship sized hull with a spaceship sized mass isn't much of a decoy. Why not add weapons an make it an actual spaceship?
And you'd better add defenses as well. Otherwise the decoy is nothing more than an unusually expensive, unusually easy to destroy missile.
Isaac Kuo points out that all of this assumes that the decoy and the warship are using rocket propulsion. It does not apply if they are using solar sails, laser light sails, magnsails, or other non-rocket propulsion.
But I repeat: while it is more or less impossible to use decoys to fool distant observers, it may be possible to use something like decoys in a dog-fight to protect your ship from enemy short-range antiship missiles. In the latter case, you are not trying to make a fake image of your ship so much as you are trying to break the target lock the hostile missiles have on your ship's vulnerable posterior.
Dr. John Schilling discusses why the exhaust plume of a decoy will have to have the same thrust as a real ship:
Actually, I too agree it is possible under certain circumstances, any disagreement is over where one draws the line. Matterbeam is not talking about a Romulan cloaking device that will let that dastardly Romulan Warbird from unexpectedly appearing a couple of meters behind the Starship Enterprise and shooting a plasma torpedo up her tailpipe. He states that a spacecraft is eventually going to become visible to its enemies, but there are strategies that can put that off as long as possible.
It appears that Matterbeam and I mostly differ on our assumptions about sensor platforms. My opinion is that a full-sky scanning sensor capable of detecting a hostile stealthy spacecraft at absurd distances will be so inexpensive that any astromilitary will fill their entire solar system with the little darlings, while Matterbeam says there are plenty of valid reasons that ain't necessarily so. Such reasons can be used by any science fiction author or game designer who wants more stealth. The number of sensor platforms is important because the prime stealth technique is jettisoning waste heat in a direction not seen by any sensor platform. The more platforms, the fewer the safe directions.
He had run a four article series on the topic on his blog, but asked permission to write a couple of specific article for inclusion here. Which I instantly granted. I am a strong upholder of the scientific method, especially the part about it being self-correcting by peer review and data from new experiments. His two articles are below:
A Cloaking Device is handwaving technology that violates physics and gives you your ardently desired stealth in space. In fact, it goes a step further and makes the freaking ship utterly invisible. Be warned that if you use this in your novel RocketCat will hunt you down and give you an atomic wedgie.
The tension of the submarine movie is because the captain of the US surface destroy escort ship knows there is a deadly German submarine lurking somewhere like Jaws, but you can't see where the blasted thing is hiding. Since there is no ocean in space to hide in, Schneider postulated incredibly advanced stealth technology (but probably thought of it as a high-tech cloak of invisibility). Another Star Trek screenwriter, D.C. Fontana, coined the term cloaking device for the 1968 episode "The Enterprise Incident".
If you have faster-than-light starships in your science fictional universe, and also want to have starship combat (and instersetllar empires), you have to have a carefully crafted set of limitations.
The Alderson Drive or "jump point" drive has been used in many SF starship combat games, for the same reason Niven and Pournelle used it: unlike most other FTL, it allows the possibility of interstellar battles.
Most other FTL is a "fly anywhere" kind of propulsion, which generally does not allow battles to occur except by mutual consent. Often a planet cannot even detect an enemy invasion fleet until it suddenly pops out of hyperspace. Interstellar wars only last long enough for your hyperspace bombers to fly to the enemy's planets, then a brief emergence to spit out a hellburner, a planet-wrecker nuclear bomb, a planet-sterilizing torch warhead, a planet-cracker antimatter warhead, or a planet-buster neutronium-antimatter warhead. Then they fly home, only to discover that the enemy's bombers were on a similar mission. Go to The Tough Guide to the Known Galaxy and read the entry "SLAG"
Please note that there is a second FTL situation that can allow interstellar combat. You need two things.
[A] Ships travel faster-than-light taking some time to travel the distance (i.e, travel is NOT instantaneous).
[B] There must exist some kind of faster-than-light radar that can detect the invading ships far enough in advance that the defenders have time to do something about it.
In other words you have to postulate Strategic FTL Sensors.
This will create something like wet navy combat in the Pacific ocean in the period after the time the navy was equipped with radar, but before the advent of orbital spy satellites that can see every ship on the ocean. This is more or less the situation in the Star Trek TV show(s).
On Terra Air-Sea Rescue has survivors of aircraft that have ditched in the ocean or of seagoing ships that sank being searched for by rescue vehicles.
In the hard-SF world, the "search" part is easy since there ain't no stealth in space. Offhand I'd say the only reason for searching is if the lost object contains no living beings or active energy sources, and is at more or less the ambient temperature of deep space.
In non-hard SF, especially with faster-than-light starships, searching can become difficult again. Or even impossible.
In the two-dimensional ocean surface environment, the standard search pattern is the "expanding square". You start at the last known location of the people in peril. You then travel along calculated route legs in a pattern that expands every 3rd leg so you systematically cover the the search area in an expanding pattern. The expansion is limited to ensure that your detection range covers the entire search area. If the expansion is too wide then you might leave detection gaps and fail to detect the stricken people.
Figuring the legs and turns is generally a task for the computer or smart-phone electronic-charting app, but sailor tests require doing it manually. Or by using the Weems & Plath slide rule.
Note that the expanding square pattern makes a few assumptions. Primarily it assumes that the people in peril are in a raft or something that moves very slowly relative to the search vessel (or at least slow relative to the time it takes for the search vessel to traverse the search pattern). In the space environment most objects are at least moving in an orbit around the sun or planet, and at most could have a sizeable velocity. Secondarily it assumes that you have a good value for the last known location.
Now what would the three-dimensional outer-space version of an expanding square search pattern be? Casey Handmer, Conrad Teves, and Paul Drye all pointed out to me that the logical 3D analog would be the 3D version of the Hilbert space-filling curve.
If I am doing my math correctly, if the ship's detectors have a range of 1 unit, the length of each leg of the Hilbert curve will have to be 1.4 units long. This is to ensure there are no detection gaps.
Theoretically a spacecraft traversing a 3D Hilbert curve could coast during the legs and only burn its engines at the turns.
Scuba divers and submarines searching for objects on the ocean floor use what is called a circular search. In the diagram above note how once a circular section is traversed, the next circular path has a larger radius.
M Harold Page, Astrographer, Nick Husher, and dziban303 suggested "ball of yarn" search patterns. That is, a circular path continually expanding and precessing (i.e., changing its latitude). Much like starting with a skein of yarn and hand winding it into a ball. The center axis of the yarn is the path of the searching spacecraft, and the radius of the yarn thickness is the detection radius. As long as the ball contains no voids, there are no detection gaps.
However, unless there is a planet or other object with lots of gravity in the center, the searching spacecraft will have to be constantly burning its engines. Without a convenient source of gravity to bend the ship's trajectory into a circle, ship's thrust will have to be expended to force the curve.
Please do not confuse ball-o'-yarn search patterns with Heinlein's "ball of twine" orbits. Those have a circular path that is constant, it does not expand. They are polar orbits beloved by military spy satellites, since they eventually pass over every square centimeter of Terra's surface.
Gabriel Fonseca had a different idea. He envisioned the spacecraft following a path similar to GAIA's sky watching pattern, but constantly expanding away from the point-of-origin as it does its sweeps. He was assuming that the lost object was more or less stationary. He wrote a parametric equation for this in spherical coordinates.
Prez Cannady suggested not quite a cone, but more like a system of concentric horns bounded by the target's known max acceleration.
Somebody asked about what if the object had a last known location and a last known vector. Mr. Fonseca said that would basically be a cone, depicted above with the Z axis the last known vector and the origin being the last known location.
Then Mr. Fonseca thought about it some more and realized that there woudl be a problem once the radius of the spiral search path exceeded the detection radius. There would be a growing detection gap near the center. So he altered the parametric equation to make the search path dip in closer to center, and avoid the detection gaps. He made the radius vary as t * sin(t), instead of just t.
Combat scoutships are spacecraft whose primary function is military reconnaissance and military surveillance. They may also be used as military artillery observers and forward air controlers. Scoutships are a part of the Intelligence, surveillance, target acquisition, and reconnaissance system (ISTAR).
They might be only lightly armed with self-defense weapons and "stealth", or they might be heavily armed for reconnaissance-in-force and killing enemy scoutships.
In a realistic solar-system-based no-FTL ain't-no-stealth-in-space universe, there is little or no need for scoutships. Constellations of observer satellites scattered over the entire solar system and the occasional drone recon missile will suffice. The astromilitaries of all space-faring nations will spread their recon satts near anything they consider to be of strategic importance.
In a science-fictional universe with either cloaking devices or faster-than-light starships, they may or may not be useful. It depends upon the limitations of the scifi technology.
Offhand, scoutships would be a useful anti-cloaking device measure if the cloak had a range limit. Meaning that a scoutship might be able to detect a hostile cloaked spaceship if the scout can get close enough.
Currently radars can be jammed if the enemy sends back fake radar returns on the same frequency. The radar can "burn through" the jamming by increasing the strength of the radar beam. Instead of turning up the power to the radar emitter, one can also increase the strength by reducing the distance between the radar emitter and the target (that pesky old inverse-square law strikes again). Thus a scoutship in advance of the task force would be closer to cloaked ships, and thus might be able to burn through the cloak while the distant task force was still blinded. If you make the assumption that the handwaving cloaking technology operates like radar jamming.
Obviously it would be useful for a task force to send scoutships ahead, if the scouts could spot cloaked ships and give advanced warning.
With respect to faster-than-light starships, the handwaving FTL technology would have to allow the existence of strategic FTL sensors in order to make scoutships worthwhile.
Remember the light-speed lag. Light moves quickly, but not at infinite speed. It takes about eight minutes to travel one astronomical unit. So if you are in orbit around Terra and you observe a spacecraft near the Sun with a telescope or radar, you are actually are seeing where the ship was eight minutes ago. By the same token, if you change course it will be eight minutes until the Sun-grazer ship will know.
James Huff is experimenting with plotting something similar to a Longscan display. He is trying to make a "probability plot" of where to aim your guns, given the target's acceleration, maneuvers, and lightspeed lag due to the range to the target. Mr. Huff generated these plots with a custom C++ program he wrote for generating iterated function systems.
Tactical combat sensors work at close range in a battle, guiding your weapons to the enemy targets (a "firing solution"), detecting incoming enemy weapons, and analyzing the enemy for weakness.
- Detection: ability to distinguish an object from the background
- Recognition: ability to classify the object class (metor, asteroid, spacecraft...)
- Identification: ability to describe the object in detail (HMS Repugnant battlecruiser of the royal Callistonian navy). Specifically is this an enemy spacecraft or other military asset?
- Firing Solution: enough information to calculate where to train your guns so you can shoot the snot out of the target
Merely detecting the presence of an enemy spacecraft is not enough. If you want to actually shoot it, you need to obtain enough sensor data to get a firing solution. These sensors are part of the ship gun fire-control system
These sensors give advanced warning to the space warship about incoming hostile weapons fire. Some will be mounted on point defense systems so said systems can shoot down incoming enemy missiles. Some will feed data displays in the ship's combat information center; suggesting to the captain that they might want to order the ship to, you know, dodge the missile?
These sensors are very similar to ordinary non-combat tactical sensors. Except they are much more sophisticated. As a general rule, meteors do not use stealth technology nor do they home in on you. Hostile weapons do.
When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.
In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the Doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.
This means that painting the ship with camouflage in an attempt to disguise its identity is pretty pointless.
During a battle, sensors also give "intelligence". That is, for example, if you fire your lasers at the target, and suddenly two of the target's nuclear power reactors have a drop in temperature, you've probably scragged them and the target's power budget has been substantially reduced. Your ship's captain will alter their battle tactics accordingly.
In a similar manner, a spectroscope can be used on any plumes of gas vented by the stricken target. If it is hydrogen, you probably punctured a propellant tank. If it contains oxygen, you probably holed the habitat module. If the target is antimatter powered and you suddenly detect a drastic increase in 511 keV gamma rays, turtle up quick cause she's gonna blow!