View Full Version : Active sensors in space

Phoenix Dragon
06-07-2001, 06:59 AM
Okay... I've been working on a story that revolves around realistic space combat, and I've run into a little stumbling block. It's not a -major- thing, but one of those little aspects that's neat to have... That, and I'm picky about making every little detail as accurate as I possibly can, coupled with the fact that I'm insanely curious about just about everything ;)

The little stumbling block I've come into is in what would be used for active sensors in long-ranged space combat. I've searched all over for information relating to this, but it seems it's not a very common question. I know that the wavelength for radar (Or other active sensors) has an effect on its abilities; Millimeter wave radar is excelent at picking out ground targets from the rest of the ground clutter for example, if I recall correctly. What kind of wavelength would be best for space-based combat, though?

For specifics on the way combat engages then, an "average" detection range is probably around 1-5 AU (Though not immediate, it could take an hour or two to detect a ship that is in range, just because of how long it takes to scan once through that hemesphere), though very powerfull arrays (Large orbital defence arrays or the rather rare heavy battleship) could detect a ship at further ranges (For an EXTREME range, some huge arrays might be able to pick out a ship-sized object at as much as 40 AU, though very rarely). Combat ships are fairly cylindrical and generally around 100 meters long, 35 or so wide (This is what most opposing ships will see), and about the same structural build/density as a modern jet fighter.

What wavelength would be best for an active sensor used in a situation like that? I know they each have their advantages... Gamma is very high-energy, so it might be easier to see reflected energy at long range, but it would take more to reflect it back to the sensor, as well... Etc. Right now I'm thinking of just sticking to normal radar, or maybe an IR-based system, but I figured that if there were any place I could ask with any chance of getting an educated answer... This would be the place :)

06-07-2001, 08:34 AM
At such distances, it seems that it would be easier to detect incoming signals rather than sending out an active "ping" to bounce off enemy ships. Unless you already knew the exact location of the ship, you would have to send out a broad signal. That signal would quickly weaken as it spread out over that vast distance (you're talking about 100's of millions of miles), bounced off the ship and made the journey back to your detector.

But maybe some EM expert here can give you a direct answer.

Jet Jaguar
06-07-2001, 08:35 AM
They use radar to map asteroids from Earth, you might want to look around some NASA/JPL web sites to see what they're using. A cursory search found they use something called delay-doppler radar, but I didn't find anything about the frequencies they're using.

06-07-2001, 08:35 AM
There are radar systems that work over such distances, such as the Arecibo planetary radar (http://www.alumni.caltech.edu/~nolan/radar/) which uses a 12.6cm wavelength radio wave.

But such radar systems are for detailed observation of known targets, not looking for unknown targets. They have very narrow beams, and it would take a long time to scan the sky.

In fact, I don't think any active system is practical over such a large distance. In an active system, you need to first illuminate the target with some type of radiation. The farther the target, the weaker the illumination - inverse square law. You then have to detect the reflection off the target - again, the farther the target, the weaker the reflection that reaches you. Another inverse square law. So the reflected radiation is actually inverse 4th power of distance - twice the distance equals 1/16 the signal strength. Also, stealth technology will make it even harder to detect.

I think a passive IR sensor would be the best bet. Any machine that use energy produces excess heat which need to be dumped into space. It's hard to dump it in a specific direction to avoid detection.

06-07-2001, 08:42 AM
I should also explain that scanning the sky with a narrow radar beam is not just a matter of making a good motorized mount that can swing the antenna quickly. Detection of a signal is not instantaneous; you need to dwell on one position for some time so you can accumulate enough signal. It's like photography - when it's dark, you need a longer exposure time.

Cap'n Crude
06-07-2001, 11:09 AM
I have no real-world input on what sort of sensors you'd use. This is more of a friendly suggestion, one writer to another, that you may be going in the wrong direction with your inquiry.

I realize that you may have considered some or all of the following already, and may not welcome my input. Also, if my voice seems muffled it's because I'm talking out of my ass.

You're writing a piece that's clearly science fiction, or at least speculative fiction. Consider the following:

1. After all this time, I don't believe that any of our unmanned probes have managed to get 40 AU away from Earth. I could be wrong about that, but that's an awfully long distance.

2. Since you're talking about combat over such a distance, one of the following must be the case (I'm assuming our own environment for ease of consideration):
(a) some future human settlement is in conflict with Earth.
(b) Some manner of extra-solar enemies arrive in-system and engage us, and we have the means to fight back.
In either case, the story needs a lead time of lots and lots of years, probably a century or more, before such a thing could be practical from the human end of things.

3. Hi, Opal. (did I do that right?)

4. For combat to be practical in these cases, you not only need sensors that can detect over such distance in a timely fashion, but weapons that can reach that far in a timely fashion, and communications systems and propulsion systems to match. Consider that, at present NASA space vehicle speeds, it would take 2-3 years to cover 1 AU of distance, not considering perturbation maneuvers to increase speed or fuel efficiency.

With that in mind, especially the factors of time and needing to tell a compelling story, I respectfully suggest that you fudge. Even the best hard-SF writers take liberties with the mechanics of their environment to make a story work.

Since you want to keep it realistic, you'll probably skip FTL travel for now -- but maybe not. If you have FTL, your sensor system could be augmented by whatever means are used to let ships cover incredible distances in practical timespans. Most FTL relies on a warp drive or hyperspace of some kind; why not speed up sensor emissions and returns by projecting them through that? If there's no FTL, then you may want to fall back on some newly-discovered feature of (relativity, quantum mechanics, unified theory of quantum gravity) that allows near-instantaneous detection of remote events.

At any rate, I don't want to tell you what to write or how to write it. I'm just saying that you wouldn't be the first author to kill a good story idea by trying to get all the details "accurate." Let the story flow, and fill in the technobabble later. Whatever direction you choose to go with your piece, good luck. Post a link to the boards when you're done.

06-07-2001, 01:42 PM
Right at the beginning of Peter F. Hamilton's Neutronium Alchemist there's one of the best hard-sci-fi (*cringing in anticipation of the SF-police*) space battle I've ever read.

Basically, the two ships stay as quiet as possible. Sending out a radar ping would betray their possition. They are battleships, with fancy heat-management systems to prevent leaving an IR trace. It's interstellar space which, contrary to intuition, is dark.

Really lovely scene. I highly recommend that you check it out.

Delay-doppler radar, by the way, is a means of mapping an object--it gives no advantages for detection. They send out the signal, and as it returns, they make a recording of it, chopping it up into small time segments.

For each time segment, you know how far the radar pulse traveled: divide the delay time by the speed of light and you get the distance between the telescope and the asteroid times two. This gives you the depth dimension, since some parts of the asteroid are closer to you, and some are farther away. You also know the doppler shift of each segment. The asteroid rotates, so parts of it are moving toward the telescope, so they're redshifted, and parts are moving away, so they're blueshifted. This gives you the "horizontal" dimension (perpendicular to the spin axis.)

So you can get lots of spatial resolution, even though the radar beam is much larger than the target! Pretty keen stuff.

06-07-2001, 02:53 PM
Before getting into the literature, might I say that active sensors may not be too smart. First of all, emissions of any kind in space will give you away, even if it's a laser beam, your target will still see you. Your best defense is your invisibility, and if you throw that away you're hosed.

Secondly, with an active sensor you have a round trip delay, rather than just the one way delay with an active sensor. In combat, over distances of light-seconds, this could give a critical advantage to the combatant who can successfuly apply passive sensors.

That said, you may want to invent something exotic, if a long-range sensor is really imporant to your story. Use an exotic particle like neutrinos or tachyons. Or perhaps something that senses gravitational distortions of the space-time fabric. As Cap'n Crude said, it's important that your story take precedence over the science.

Now - on to the prior art. If you're writing a work you hope to publish, then you really have to have read what's already been done. Forgive me if you already have.

I agree with Podkayne, you should read Hamilton's Neutronium Alchemist. But be warned- it's the third (thick) volume in a trilogy - called the "Night's Dawn" trilogy, iirc. Well worth reading though... I wouldn't exactly call it "hard SF," but it's very smartly done and very convincing.

I also recommend Startide Rising by David Brin. There are several excellent space combat scenes there depicting various hi-tech alien races fighting each other to be the first to catch the poor little newbie spacefaring Terrans who are in possession of a valuable "religious" artifact. And the Terrans use several terrific low-tech tricks to foil the aliens!

If you want the best example of long-range space combat, though, you need to read The Gripping Hand, by Niven and Pournelle. (It's the sequel to Mote in God's Eye) It's been a while, but I'm pretty certain I'm remembering the right title.

Niven and Pournelle observe just about every limitation you would expect in a hard-SF depiction of space combat at ranges of light-minutes (even light-hours!): light propogation delays, difficulty of detection, heat dissipation problems, microgravity problems, the works.

For example, one weapon is a laser beam. There are all kinds of problems. At these ranges, you see your target where he was minutes or hours ago. Where is he now? Is he evading or not? OK, make a prediction, take careful aim, and light him up. He's a couple of AU away, so it's going to be a half an hour until you even know you hit him. And even then, no laser is perfectly collimated; at this range the spot has spread out so much you're going to have to keep the laser trained on him to heat him up enough to do any damage at all.

And in The Gripping Hand, there's an all out war between fleets that gets done this way. It's very, very well done, with the authors taking advantage of these problems and the suspense of light-propagation delays to add tension and suspense. It reminded me a lot of submarine combat movies.

Definitely not Star Wars!

06-07-2001, 03:32 PM
Whoops, I guess I mean Reality Dysfunction: Emergence. . . you don't have to read the whole thing--the battle I was thinking of is right at the very beginning.

By the end of Emergence it's true, the series does depart from hard sci-fi to la-la land, but I was very impressed by Hamilton's grasp of physics and his use of bleeding-edge planetary science.

(For the confused, apparently Hamilton wrote two monstrously long novels, The Reality Dysfunction and The Neutronium Alchemist, which were split into two novels each for publication.)

Some Guy
06-07-2001, 05:37 PM
I'm not going to be the first to say it, but I'll say it again: in a hard/realistic setting, over the distances that you're talking about, active sensors are going to be a Bad Thing. That doesn't mean that they won't be tacically advantageous at short range, only that they will work against you at long range. Consider:

1. Whatever you're using as a sensing medium obviously has to be able to travel the distances involved, interact with matter out in space, return, and be detectable above background noise upon its return. An obvious point, which has been covered above, is that your active sensor can be passively detected - and in only half the time. Your enemy will know that yo're out there before you do. On a scale of several AUs, this gives the enemy (whom you've courteously warned) a big, big tactical advantage, as we'll see.

2. Again, on a scale of several AUs, it will be much quicker for your enemy to move away from its current position (by a safe margin that's easily calculated re: distance and known spread of your weapons) than it will be to receive your sensor information. Assuming competent command of the enemy ship, the only information that you have is a single straight vector that will not contain the enemy ship, at least, not any longer.

3. Since you will have to remain in the same place long enough to receive sensor data, you have just given the enemy a direct target vector for their weapons. At least some of these will be comparable in speed to your fastest sensors (most likely at or near c, of course). This means that, not only will the ship no be where you detected it, they can fire on you for a nearly guaranteed direct hit, and still have plenty of time to dodge before you have any idea what's up. This scenario pretty much spoils any possibility of using active sensors in a known hostile situation.

There are plenty of uses for active sensors in peacetime surveying, or even in short-range fighting. At long range, though, I agree with what's been said above: you want to be minimally detectable, much more so than you want to be able to detect things.

06-07-2001, 06:22 PM
Other options for detection in planetary systems:
- a network of passive sensors, e.g. in the Oort cloud or asteroid belt.
- Remote active sensors - the spaceship flies by Pluto when suddenly it is 'pinged' by a radar station on Charon. The enemy ship blows up the unmanned station, but it is too late - emissions reflect off the enemy ship and are detected by Our Hero in orbit around Mars.

These mainly work when defending a system. Note that the passive detectors could use a laser to 'call home', avoiding detection. IR dissipation by such a station is tricky, however.

When attacking a system, a decoy vessel can enter the system from one direction, pinging like crazy, while the real attack force is hiding behind a comet or something with a little antenna sticking out to detect reflections from the decoy.

Another attack system would be a swarm of passive sensors, again using a laser to call home. The danger is that a captured sensor probe could reveal the location of the ship that it is communicating with. Perhaps use of an extra communications ship, like a modern day E-3 or AWACS aircraft could be used - the sensors talk to the AWACS, the system dwellers see the distant AWACS but do not see the attack fleet that AWACS controls with laser communications.

Just a few thoughts - enjoy...

06-07-2001, 06:45 PM
Originally posted by Podkayne
For the confused, apparently Hamilton wrote two monstrously long novels, The Reality Dysfunction and The Neutronium Alchemist, which were split into two novels each for publication.
It's even more confusing than that:

IIRC, in the US, the two thick UK editions were split into three books for hardback release, and each of these were divided into two books for paperback publication.

I guess the US publishers think we Yanks are intimidated by anything longer than 500 pages. (When in fact, I love extremely long novels.)

I know for a fact that if you want to buy the whole story in the US in paperback, you have to buy SIX books.

Phoenix Dragon
06-07-2001, 10:53 PM
Sorry i couldn't reply earlier, this board is, unfortunatly, very slow for me early in the day, so I couldn't get here to post a follow-up.

Yes, active sensors are generally a "bad thing" that gives away your possition. That's something I already know :) Normal ship-based sensors center around thermal/IR-band passive sensors, used to detect the heat radiating from a ship, or at much longer ranges, the near star-hot jet of vented reaction material (These ships use fussion process to super-heat a reaction mass of water into plasma, at high pressure, and vent it out the rear of the ship for propulsion at extremely high velocity.. Gives good thrust, but it's about as stealthy as broadcasting your possitions over the radio). They also have a full spectrum of other sensors, usefull for detecting, among other things, trace radiation or EM fields emitted from opperating electronics or powerplants. That's the primary way of detecting a ship. Active sensors are a 'backup', and usually used an already-known contacts to get exact data on them, such as range (Passive sensors only give signal strength, not range, unless you have another observation point to triangulate from).

I havn't read that book, I'm afraid. The closest work of fiction I can think of is the depictions of space combat in Albedo, which is the only realistic portrayal of space combat that I've seen before...

Basically, the best comparison I can think of is modern-day sub-vs-sub combat. Passive sensors (In this example, hydrophones) watch carefully for an opponent, while trying to stay stealthy enough to not be seen. Sometimes, active sensors have to be used, though. A ship armed with -only- passive sensors would find itself at a fair disadvantage if the other ship had both, much as a sub without active sonar could be, if it finds a time it needs it.

In this case, though, the delay between the signal going out and returning is still very, very short compared to travel times. Combat in this case is measured not on a scale of minutes or even hours, but days and weeks. A ship generally "jumps" in-system at aproximatly 40-50 AU out, and then accelerates toward its objective... Usually, it will spend 1-5 days accelerating (No way to counter the force of inertia, such as in ST, so it has to be kept down to crew sustainable levels), then shut down and coast most of the way (Hopefully finding and destroying any targets before the ship has to turn around and decelerate, unless it's just a hit-and-run mission). Normally, it takes a couple weeks for a ship to get from jump-in to the biozone of a star, or even longer if the ship needs to conserve fuel (They have a fairly limited supply). At that scale, manuvering to give 'false' active sensor reports is rather futile. The time delay is taken into account, and even then, while an incomming ship may be able to approach at speeds up to a few thousand kilometers per second, light (And radiation) travels many times faster; At 5 AU, a ship could add a mere 7KPS to its velocity in one direction (Assuming a sustained 1.5 gee acceleration, which is rather tough on the crew) before the signal arrived at the other ship, or a "mere" 3750 kilometers distance. A normal targeted "ping" pattern would therefor account for this, and scan at least that wide of an arc... However, in doing so, the other ship would be venting -plasma- out to one side (As seen by the other ship); Not only did the other ship get a precice range with the "ping", as well as a velocity if it was a long ping or a couple rapid ones, or it's a dopler-style system, but the other ship is now lit up brighter than any possible background emission. The emitting ship now has all possible data it needs to get an extremely precice firing solution, while the other ship only has a bearing and signal strength/type to go by (Unless, of course, it has another ship or outboard sensor to triangulate with, or in the process of changing its vector, triangulates from regular updates on the passive track -- coincidentally, the exact same thing done in submarine combat)

So while they're a backup system, they can be quite usefull at times, especially if the ship you're facing is particularly stealthy. It's not something they're going to go pinging away with just for the heck of it, though, since its detection abilities are rather low compared to the passive sensors. Also, when a ship decides to go active, they are likely to follow up the initial ping with a few more, if it doesn't see the ship emitting (Manuvering) after that, just to make sure. The risk of attack from "pinging" is also low; At such range, energy-based attacks will have dispersed much too much to do damage. The main ship-to-ship weapons are long-range kinetic-kill missiles (Which at the multiple-thousand KPS speed of an incomming ship, PLUS the speed of the missile, is just about a guaranteed kill on anything it hits). Those missiles have both passive and active sensors, as well.

Trust me, I've gone through a lot of it, comparing all the tactical strengths and weaknesses already; When they'd be used, what they would be capable of, ways to counter them, etc. I've already got the story down, in rough form, and I'm just trying to figure out some of the more technical (And less important) aspects at this point... Like what wavelength to use :)

Some Guy
06-07-2001, 11:27 PM
Okay, I'll admit that you seem to have thought about this a bit :)

"Best",however, is subjective in this case, since there are plenty of different efficiencies you could be ging for at the expense of other things. The only hard-and-fast rule I can think of is that you shouldn't use photons that match your hull materials' standard absorption spectra, since you'll never get them back (duh).

If you're trying to make your sensors stealthy, the best bet is probably to match their emissions to the cosmic background radiation (there's a graph of it here (http://map.gsfc.nasa.gov/html/cbr.html) ). That should give you enough "noise" to cover up the signal as best as it can be - but of course it also means that your own sensors will need to be very precise to detect the small , instantaneous variations involved, and your actual signal will probably have to be stronger than it would otherwise.

A ship on the "defensive" (inside a star system) could do even better by mimicking part of the star's emission spectrum. An approaching ship would have a hard time detecting this, since it would look exactly the way they expect the stellar radiation to look.

If you don't especially care about your ship being detected, you could always use beams of highly energized electrons, then watching for the photon emission when they hit a ship's hull (this does depend on knowing what materials are likely to be present in the hull, though.)

Ah, well. I'm sure someone else will have more coherent ideas. It's off to bed for me :p

Phoenix Dragon
06-08-2001, 02:10 AM
Originally posted by Some Guy
A ship on the "defensive" (inside a star system) could do even better by mimicking part of the star's emission spectrum.

Also works for planetary bodies and the like too, to a lesser degree. The only trick is to find some way to dampen your normal emissions and emit those background emissions, without making a high-energy source that would give away your position. In effect, it'd most likely be better to just concentrate on the first step (Hiding your own emissions), and skipping on the rest :)

Though active stealth is an option, though it requires very highly-precice equipment to do it... When you get "pinged", the system sends back an EM emission of the same frequency, but with the -opposite- wavelength, hiding the source. Unfortunatly, it has some problems that make it hard to work. It doesn't work too well for a single, unexpected ping, nor one that jumps frequencies or comes in irregular intervals, but it might have uses. From what I understand, it's being tested for millitary aircraft right now.. :)

06-08-2001, 08:48 AM
Along with the other posters who recommended books, I'd look at "Downbelow Station" by C.J. Cherry. IIRC it won a Hugo award.

Because the spaceships moved at near light speeds (1/4 to 1/2 C) their sensors were of limited value. Computers would calculate a best guess as to the enemy position so long range firing was fairly inefficient. The descriptions weren't very detailed but it gave a good view of the difficulties involved.