How would you make "Sensors" for a starship?

[Sea Skimmer]

You might be able to use a web of lasers, matched to laser reflectors (not just random bounces off the hull) to constantly triangulate the relative positions of all ships in real time. I don’t know if lasers can ever realistically reach nanometer accuracy, but it seems to me that given sufficient technology we should be able to account for just about all factors that would affect the timing delay.

Every ship would also have at least one towed array, which would be mainly for tracking active emissions, but it could also have a telescope on its end. Even without interferometry towing a telescope 10km behind the ship would be a major advantage and as long as you don’t change course it should be straight.

[aimless]

I sort of handwaved the location and processing issues because they're studying putting a 6 telescope space array out there already, the Robin Laurance interferometer for DARWIN. Figured in a couple hundred years we'd be able to swing setting up a fleet.

[Darth Wong]

I sort of handwaved the location and processing issues because they're studying putting a 6 telescope space array out there already, the Robin Laurance interferometer for DARWIN. Figured in a couple hundred years we'd be able to swing setting up a fleet.

Keep in mind that a space array works largely because it is essentially static: you put the objects up there and then carefully align them, and you have plenty of time to do so. Moreover, the components themselves are also much more inert than a sci-fi spacecraft would be: inside a spacecraft you have people walking around, large amounts of power being generated, etc. Also, the difficulty increases as the distance between nodes increases.

Having said that, it's a cool idea for a sci-fi story, because it creates an extra plot wrinkle: in order to detect distant objects, your ships must slow down, become as still as possible, and essentially lose all ability to maneuver. They must also have a pre-existing notion of where to look, because it takes so much time to collect enough photons for a good image, and it's difficult and time-consuming to sweep over any appreciable part of the sky because the entire array must re-align itself. So in attempting to increase your detection ability, you also sacrifice your ability to move and do a lot of other things. Simply achieving angular alignment among the ships in the array is going to be a huge pain in the ass, especially since they're so massive. It will take a long time to line up all the ships with sufficient precision to use them as an interferometry array.

This whole discussion does, however, lead to the conclusion that fixed-location installations have a substantial advantage, since they would presumably have this kind of equipment set up already, and they would not need to jury-rig it on the fly, as a fleet of ships would.

[Starglider]

I sort of handwaved the location and processing issues because they're studying putting a 6 telescope space array out there already, the Robin Laurance interferometer for DARWIN. Figured in a couple hundred years we'd be able to swing setting up a fleet.

Having said that, it's a cool idea for a sci-fi story, because it creates an extra plot wrinkle: in order to detect distant objects, your ships must slow down, become as still as possible, and essentially lose all ability to maneuver.

Not necessarily. You could deploy a satellite array comparable to the one Aimless mentioned from your ship(s), and receive the observations via narrow-beam telemetry (this idea was in fact improvised in Schlock Mercenary as the Very Dangerous Array). This also solves the local inteference problem. If necessary you could leave them behind, go off and engage in combat, and then come back later to pick them up. This might not be a viable option for little patrol ships but the huge cruisers seen in say B5 could carry hundreds of semi-expendable sensor platforms, deployable individually or in arrays. The limiting factor there would be adding lightspeed comms lag on top of lightspeed detection lag.

P.S. Can anyone say at what range detecting the neutron output from large spaceborne fission and fusion reactors is viable, both in theory and in practice?

[Darth Wong]

That would be cool, although it suffers from the same problem: it must be aligned before use (I suppose you could assume that it's all automated, although there would still be some delay while the system aligns everything to the desired precision), it focuses on a very narrow detection cone, and it is extremely slow to sweep across the sky because of the need to stay focused on one place for a long time just to gather enough light for a decent image.

[Starglider]

That would be cool, although it suffers from the same problem: it must be aligned before use (I suppose you could assume that it's all automated, although there would still be some delay while the system aligns everything to the desired precision), it focuses on a very narrow detection cone, and it is extremely slow to sweep across the sky because of the need to stay focused on one place for a long time just to gather enough light for a decent image.

That's true, which is why you need to survey for interesting (e.g. accelerating, or inexplicably hot) dots with conventional telescopes first. This is another home-field advantage; on top of having an emplaced sensor network, the inhabitants of the system will likely have a full and regularly updated catalogue of local asteroids and comets, while an intruding party likely won't.

[Darth Wong]

I'd be curious just what kind of alignment time one might realistically expect, especially for something that just got dumped out the back of a starship. As a general rule, positioning time increases with the desired accuracy and the mass of the object (and the fact that the system basically has zero damping would not help). You want each telescope to be as large as possible in order to pick things up without having to wait until you're old, but the bigger it is, the longer it would take to line it up to the desired precision.

[sketerpot]

Some more complexity: if you want to get a decent picture of what's in front of your virtual telescope, you're going to need a lot more than just two telescopes for aperture synthesis. You really do need an array of telescopes, or it's not going to be very useful even if you know where to point the thing.

By the way, this is something that Schlock Mercenary gets right, with their Very Dangerous Array of faster-than-light torpedoes with sensors on them. The thing uses a vast number of telescopes, plus faster-than-light travel and communications to get long baselines, and even then it's mostly useless unless you know where to look.

[Darth Wong]

Yes, that was mentioned before: with two telescopes, all you can do is get a width.

[Darth Wong]

I hope Crossroads finds this thread useful. It's actually one of the more informative threads we've had, in terms of writing ideas for sci-fi, if only because we've discussed things that most sci-fi authors pay NO attention to whatsoever. Towed and detachable arrays, the need for time-consuming precision alignment, the limitations of sweeping a narrow detection cone across the sky, the need to achieve image stability to acquire pictures despite sensitivity limits, etc ... these require more thought than the standard sci-fi "sensors have picked up..." dialogue, but they would make it seem much more real. Now if only we could get sci-fi authors to actually pay attention to this stuff.

[Wyrm]

I certainly found the thread useful in double-checking my own sci-fi aperture synthesis trick. I got things mostly right the first time, and this thread allowed me to close up the few things I missed.

[rhoenix]

I hope Crossroads finds this thread useful. It's actually one of the more informative threads we've had, in terms of writing ideas for sci-fi, if only because we've discussed things that most sci-fi authors pay NO attention to whatsoever. Towed and detachable arrays, the need for time-consuming precision alignment, the limitations of sweeping a narrow detection cone across the sky, the need to achieve image stability to acquire pictures despite sensitivity limits, etc ... these require more thought than the standard sci-fi "sensors have picked up..." dialogue, but they would make it seem much more real. Now if only we could get sci-fi authors to actually pay attention to this stuff.

I know I certainly am.

As all of my Fun With... threads indicated, I've been tinkering with various scifi concepts toward the end of writing my own books. Honestly, if Crossroads hadn't made this thread, there likely would be a Fun With Sensors thread in SLAM within a month.

Specifically, the angle I was going for was that the typical "the sensors show x!" always sounded lame to me, as after even learning basics of electromagnetism and other related fields of physics, I knew that some ubiquitous magical sensor array was just as much a McGuffin as FTL or other related fields of scifi writing assumptions. This also makes it appear to the reader as if these magical sensors are a one-shot: if they detect something, they give lots of info. Otherwise, they're blind.

An excellent example would be the old arms race of stealth versus sensors - stealth works by fooling the sensors into thinking that little is amiss, and the sensors (of whatever type) escalate in power and resolution and capability to counter this. Because of this fact, having multiple ways of sensing remote objects as part of a sensor array suite makes sense; it also allows the author to illustrate competing forms of technologies in this area.

[aimless]

Further questions...Atomic Rocket has this for maximum detection range for a ship on 'silent'

The maximum range a ship running silent with engines shut down can be detected with current technology is:

Rd = 13.4 * sqrt(A) * T2
where:
Rd = detection range (km)
A = spacecraft projected area (m2 )
T = surface temperature (Kelvin, room temperature is about 285-290 K)

What does it mean by current technology and where does that 13.4 come from? Is that formula assuming a certain size of telescope?

[Ford Prefect]

I hope Crossroads finds this thread useful. It's actually one of the more informative threads we've had, in terms of writing ideas for sci-fi, if only because we've discussed things that most sci-fi authors pay NO attention to whatsoever.

It's actually interesting how infrequently this sort of thing comes up in sci-fi. Even in a story which deals with the consequences of creating huge amounts of waste heat, or even cares about mass and effects on acceleration (and this is also quite rare), the limitations of sensor technology rarely comes up. Apart from the link Starglider posted, the only other example of sensor drawbacks I am aware of comes from, of all things, a Mobile Suit Gundam film, where limited sensor resolution is used tactically by a number of characters.

[Count Chocula]

Assuming we're limited to Newton and Einstein, we're stuck with EM sensors. Again assuming our ships are of the Atomic Rocket type, we would have an abundant source of energy to power sensors. However, there are thermal limitations that I haven't seen addressed very well in sci-fi. Vacuum's a great thermal insulator, so I'm guessing that radiative cooling a la big external panels would not be an effective way to dissipate the heat generated by an active radar or ladar set. A radar that could detect objects even a few light seconds away would have to be narrowly focused and high-powered. Using several megawatts of power in what's effectively the inner liner of a Thermos bottle creates issues we haven't had to deal with yet. The ship's chemical fuel (water for example) would be the best heat sink, but overuse of active sensors would also raise YOUR thermal profile as the water warms up. From a defender's POV, this would argue in favor of a large fleet of relatively small ships, rather than a few large ones.

All of a sudden, the idea of space warfare most resembling submarine warfare makes a lot of sense. It also makes the opening scene of Attack of the Clones a little more believable

[Formless]

Assuming we're limited to Newton and Einstein, we're stuck with EM sensors. Again assuming our ships are of the Atomic Rocket type, we would have an abundant source of energy to power sensors. However, there are thermal limitations that I haven't seen addressed very well in sci-fi. Vacuum's a great thermal insulator, so I'm guessing that radiative cooling a la big external panels would not be an effective way to dissipate the heat generated by an active radar or ladar set. A radar that could detect objects even a few light seconds away would have to be narrowly focused and high-powered. Using several megawatts of power in what's effectively the inner liner of a Thermos bottle creates issues we haven't had to deal with yet. The ship's chemical fuel (water for example) would be the best heat sink, but overuse of active sensors would also raise YOUR thermal profile as the water warms up. From a defender's POV, this would argue in favor of a large fleet of relatively small ships, rather than a few large ones.

All of a sudden, the idea of space warfare most resembling submarine warfare makes a lot of sense. It also makes the opening scene of Attack of the Clones a little more believable

*sigh* There exists only three ways to get rid of waste heat: convection, conduction, and radiation. Since there is no atmosphere in space, the only way to get rid of waste heat for good up there is by radiation. You can try to store your waste heat in a heat sink all you want, but sooner or later you will need to radiate it away or you will cook your ship from within. A heat sink is just a stopgap solution best reserved for when the shooting starts and you must protect your radiators, but heat sinks cannot replace radiators. Plus, a larger ship can have a larger heat sink (up to and including orbital forts housed on asteroids and moons), so there goes your massed fleets of small vessels.

There is a reason space combat is nothing like submarine warfare; even if stealth is possible its not going to look like anything like terrestrial stealth techniques. Besides, I'm sure using active sensors like radar/ladar couldn't possibly give your position away to anyone looking for you.

This isn't rocket science This shouldn't be that hard to understand. Have you ever given Atomic Rockets a serious read, Count Chocula? Because it doesn't sound like you even understand what makes stealth in space so difficult in the first place.

[PeZook]

Stealth in space would be a worthwhile thing to pursue: not because it will make you invisible, but if it will throw off the enemy's calculations of your trajectory or reduce the range from which he can guide his weapons to you, or inrease his reaction time, then you get an advantage.

Remember, detecting the Shuttle's engines in orbit around Pluto is just one part of an engagement (this is a ludicrous claim, BTW: we could only do that if we knew where to look and for what phenomenon), you need to figure out where the bastard's going in order to actually shoot him.

[Fingolfin_Noldor]

Theoretically, you can use thermoelectrics to generate electricity and also remove heat. I remember reading of one plan to use thermoelectrics to derive power from a nuclear reactor, but the reactor still required huge arrays of cooling fins.

[Stark]

Thermocouples use heat to generate electricty, but the heat isn't 'removed'. If you don't radiate it into the environment, you're fucked (unless we're talking about low-duration drones or something that store their heat in sinks and dump it after brief trips).

With regard to sensor alignment to create an image, is there scope there for interefering with these systems? Could you beam noise into one of the sensors, or interfere with it's sensor elements, or push it slightly with lasers (or whatever) so that only once they generate their image to they discover one of the component sensors is compromised? Would the complex and accurate measurement systems that align them in the first place detect that sort of thing anyway?

[Surlethe]

Stark makes a good point. If you generate electricity with heat, that heat will be deposited throughout your ship as warmed-up resistors.

[TheLostVikings]

With regard to sensor alignment to create an image, is there scope there for interfering with these systems? Could you beam noise into one of the sensors, or interfere with it's sensor elements, or push it slightly with lasers (or whatever) so that only once they generate their image to they discover one of the component sensors is compromised? Would the complex and accurate measurement systems that align them in the first place detect that sort of thing anyway?

Well at the ranges we're currently talking about you'd need your own sensor array (or a large planet bound one) just in order to pinpoint their sensors, and due to light delay they aren't guaranteed to be in the same place when your beam appears. And if they at that point are able to track the beam back to its origin...
(though anyone juggling multiple sensor platforms in order to jury rig an inferometer array would admittedly have to follow a steady, and thus predictable, course)

Of course, since inferometer arrays can only scan a narrow part at the sky at a time it's entirely possibly to approach one without being spotted, by if they have a couple of wide angle IR telescopes sweeping back and forth (in order to scan for suspicious anomalies for the array to focus on) the chance of getting "close" undetected drops rapidly.

Not to mention that active jamming would have rather large problems with beam dispersal at interstellar ranges, so your laser jammers would pretty much have to be death rays, just currently operating outside their effective range.

(Also +1 to the sentiment of this thread being all sorts of useful and interesting)

[Starglider]

Could you beam noise into one of the sensors, or interfere with it's sensor elements, or push it slightly with lasers (or whatever) so that only once they generate their image to they discover one of the component sensors is compromised?

Probably not from a useful distance. Even if you know where the enemy array is, if you're close enough to do that, you're almost certainly close enough that the enemy doesn't need an interferometric array to detect you. If you've got weapons grade lasers on your ship you could just dazzle it, but that inherently gives away your position and the fact that you're a warship.

Would the complex and accurate measurement systems that align them in the first place detect that sort of thing anyway?

Yes. They already have to compensate for solar wind and micrometeorite impacts.

Thermocouples use heat to generate electricty, but the heat isn't 'removed'. If you don't radiate it into the environment, you're fucked (unless we're talking about low-duration drones or something that store their heat in sinks and dump it after brief trips).

Maybe he was thinking of Peltier heat pumps for the 'remove heat' part. That may help with controlling which direction the heat gets radiated in, but Peltiers have very poor energy efficiency compared to fluid-based heat pumps, so I doubt they'd make sense on a spacecraft.

[Kuroneko]

What does it mean by current technology and where does that 13.4 come from? Is that formula assuming a certain size of telescope?

It represents a telescope that can accurately detect objects while working with an irradiance of 25pW/m². It's certain that telescopes can resolve objects a dozen orders of magnitude fainter, but they are huge and typically stare at the same patch of sky for a long time. I've no idea what particular assumptions led to 25pW/m².

Having said that, it's a cool idea for a sci-fi story, because it creates an extra plot wrinkle: in order to detect distant objects, your ships must slow down, ...

Rather than simply stop accelerating? I suppose that at highly relativistic speeds, aberration and Doppler shift would present a problem, since it would concentrate nearly the entire sky into a bright patch in front of the ship. A small dot would become an even smaller dot (in front of the ship, about 1/(2γ) the size) surrounded by closely packed stars many orders of magnitude brighter. That would take some versatile sensors that work across pretty much all bands, since the overall Doppler shift between the ship and the potential target isn't known.

[Fingolfin_Noldor]

Stark makes a good point. If you generate electricity with heat, that heat will be deposited throughout your ship as warmed-up resistors.

So here's a couple of things. The heat has to go somewhere. To reduce the black body radiation, you have to lower the overall temperature. If we can somehow spread the heat out, you reduce the overall temperature of the black body and then reduce the IR emission. REmember that a high temperature device is a beacon showing up on an IR detector. If you smear it out, it's not so bad. A peltier, as some say, is not the best device, but for a lot of devices, like electronics and all, you reduce the heat emissions and you get to cool the device as well. For reactors, that's trickier of course and maybe one has to use some kind of internal reflectors/heat pipes/etc. or something to somehow spread out the heat.

[Stark]

Probably not from a useful distance. Even if you know where the enemy array is, if you're close enough to do that, you're almost certainly close enough that the enemy doesn't need an interferometric array to detect you. If you've got weapons grade lasers on your ship you could just dazzle it, but that inherently gives away your position and the fact that you're a warship.

It's no good for an incoming fleet, but if a single small object can disrupt the efforts of a large array to detect the larger/hotter/more obvious fleet incoming, it could be worth it. Certainly a 'right place right time' thing. I'm curious about the idea of 'electronic deception' in this sort of situation, trying to introduce junk data into an image that they won't be able to look at until after a complex procedure thus wasting their time.